SILGenPattern.cpp

//===--- SILGenPattern.cpp - Pattern matching codegen ---------------------===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2014 - 2017 Apple Inc. and the Swift project authors
// Licensed under Apache License v2.0 with Runtime Library Exception
//
// See https://swift.org/LICENSE.txt for license information
// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "patternmatch-silgen"
#include "Cleanup.h"
#include "ExitableFullExpr.h"
#include "Initialization.h"
#include "LValue.h"
#include "RValue.h"
#include "SILGen.h"
#include "Scope.h"
#include "swift/AST/ASTWalker.h"
#include "swift/AST/DiagnosticsSIL.h"
#include "swift/AST/Pattern.h"
#include "swift/AST/SILOptions.h"
#include "swift/AST/SubstitutionMap.h"
#include "swift/AST/Types.h"
#include "swift/Basic/Assertions.h"
#include "swift/Basic/Defer.h"
#include "swift/Basic/ProfileCounter.h"
#include "swift/Basic/STLExtras.h"
#include "swift/SIL/DynamicCasts.h"
#include "swift/SIL/SILArgument.h"
#include "swift/SIL/SILUndef.h"
#include "swift/SIL/TypeLowering.h"
#include "llvm/ADT/MapVector.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/FormattedStream.h"

using namespace swift;
using namespace Lowering;

//===----------------------------------------------------------------------===//
//                             Pattern Utilities
//===----------------------------------------------------------------------===//

// TODO: These routines should probably be refactored into their own file since
// they have nothing to do with the implementation of SILGenPattern
// specifically.

/// Shallow-dump a pattern node one level deep for debug purposes.
static void dumpPattern(const Pattern *p, llvm::raw_ostream &os) {
  if (!p) {
    // We use null to represent a synthetic wildcard.
    os << '_';
    return;
  }
  p = p->getSemanticsProvidingPattern();
  switch (p->getKind()) {
  case PatternKind::Any:
    os << '_';
    return;
  case PatternKind::Expr:
    os << "<expr>";
    return;
  case PatternKind::Named:
    os << "var " << cast<NamedPattern>(p)->getBoundName();
    return;
  case PatternKind::Tuple: {
    unsigned numFields = cast<TuplePattern>(p)->getNumElements();
    if (numFields == 0)
      os << "()";
    else if (numFields == 1)
      os << "(_)";
    else {
      os << '(';
      for (unsigned i = 0; i < numFields - 1; ++i)
        os << ',';
      os << ')';
    }
    return;
  }
  case PatternKind::Is:
    os << "is ";
    cast<IsPattern>(p)->getCastType()->print(os);
    break;
  case PatternKind::EnumElement: {
    auto eep = cast<EnumElementPattern>(p);
    os << '.' << eep->getName();
    return;
  }

  case PatternKind::OptionalSome:
    os << ".some";
    return;

  case PatternKind::Bool:
    os << (cast<BoolPattern>(p)->getValue() ? "true" : "false");
    return;

  case PatternKind::Paren:
  case PatternKind::Typed:
  case PatternKind::Binding:
    llvm_unreachable("not semantic");
  }
}

/// Is the given specializable pattern directly refutable, as opposed
/// to containing some refutability in a nested position?
static bool isDirectlyRefutablePattern(const Pattern *p) {
  if (!p) return false;

  switch (p->getKind()) {
  case PatternKind::Any:
  case PatternKind::Named:
  case PatternKind::Expr:
    llvm_unreachable("non-specializable patterns");
  
  // Tuple and nominal-type patterns are not themselves directly refutable.
  case PatternKind::Tuple:
    return false;

  // isa and enum-element patterns are refutable, at least in theory.
  case PatternKind::Is:
  case PatternKind::EnumElement:
  case PatternKind::OptionalSome:
  case PatternKind::Bool:
    return true;

  // Recur into simple wrapping patterns.
  case PatternKind::Paren:
  case PatternKind::Typed:
  case PatternKind::Binding:
    return isDirectlyRefutablePattern(p->getSemanticsProvidingPattern());
  }  
  llvm_unreachable("bad pattern");
}

const unsigned AlwaysRefutable = ~0U;

/// Return the number of times a pattern must be specialized
/// before becoming irrefutable.
///
/// \return AlwaysRefutable if the pattern is never irrefutable
static unsigned getNumSpecializationsRecursive(const Pattern *p, unsigned n) {
  // n is partially here to make simple cases tail-recursive, but it
  // also gives us a simple opportunity to bail out early when we see
  // an always-refutable pattern.
  if (n == AlwaysRefutable) return n;

  switch (p->getKind()) {
  // True wildcards.
  case PatternKind::Any:
  case PatternKind::Named:
    return n;

  // Expressions are always-refutable wildcards.
  case PatternKind::Expr:
    return AlwaysRefutable;
  
  // Tuple and nominal-type patterns are not themselves directly refutable.
  case PatternKind::Tuple: {
    auto tuple = cast<TuplePattern>(p);
    for (auto &elt : tuple->getElements())
      n = getNumSpecializationsRecursive(elt.getPattern(), n);
    return n;
  }
  
  // isa and enum-element patterns are refutable, at least in theory.
  case PatternKind::Is: {
    auto isa = cast<IsPattern>(p);
    ++n;
    if (auto sub = isa->getSubPattern())
      return getNumSpecializationsRecursive(sub, n);
    return n;
  }
  case PatternKind::EnumElement: {
    auto en = cast<EnumElementPattern>(p);
    ++n;
    if (en->hasSubPattern())
      n = getNumSpecializationsRecursive(en->getSubPattern(), n);
    return n;
  }
  case PatternKind::OptionalSome: {
    auto en = cast<OptionalSomePattern>(p);
    return getNumSpecializationsRecursive(en->getSubPattern(), n+1);
  }
  case PatternKind::Bool:
    return n+1;

  // Recur into simple wrapping patterns.
  case PatternKind::Paren:
  case PatternKind::Typed:
  case PatternKind::Binding:
    return getNumSpecializationsRecursive(p->getSemanticsProvidingPattern(), n);
  }  
  llvm_unreachable("bad pattern");
}

/// Return the number of times a pattern must be specialized
/// before becoming irrefutable.
///
/// \return AlwaysRefutable if the pattern is never irrefutable
static unsigned getNumSpecializations(const Pattern *p) {
  return (p ? getNumSpecializationsRecursive(p, 0) : 0);
}

/// True if a pattern is a wildcard, meaning it matches any value. '_' and
/// variable patterns are wildcards. We also consider ExprPatterns to be
/// wildcards; we test the match expression as a guard outside of the normal
/// pattern clause matrix. When destructuring wildcard patterns, we also use
/// nullptr to represent newly-constructed wildcards.
static bool isWildcardPattern(const Pattern *p) {
  if (!p)
    return true;

  switch (p->getKind()) {
  // Simple wildcards.
  case PatternKind::Any:
  case PatternKind::Expr:
  case PatternKind::Named:
    return true;
  
  // Non-wildcards.
  case PatternKind::Tuple:
  case PatternKind::Is:
  case PatternKind::EnumElement:
  case PatternKind::OptionalSome:
  case PatternKind::Bool:
    return false;

  // Recur into simple wrapping patterns.
  case PatternKind::Paren:
  case PatternKind::Typed:
  case PatternKind::Binding:
    return isWildcardPattern(p->getSemanticsProvidingPattern());
  }

  llvm_unreachable("Unhandled PatternKind in switch.");
}

/// Check to see if the given pattern is a specializing pattern,
/// and return a semantic pattern for it.
Pattern *getSpecializingPattern(Pattern *p) {
  // Empty entries are basically AnyPatterns.
  if (!p) return nullptr;

  p = p->getSemanticsProvidingPattern();
  return (isWildcardPattern(p) ? nullptr : p);
}

/// Given a pattern stored in a clause matrix, check to see whether it
/// can be specialized the same way as the first one.
static Pattern *getSimilarSpecializingPattern(Pattern *p, Pattern *first) {
  // Empty entries are basically AnyPatterns.
  if (!p) return nullptr;

  assert(first && getSpecializingPattern(first) == first);

  // Map down to the semantics-providing pattern.
  p = p->getSemanticsProvidingPattern();

  // If the patterns are exactly the same kind, we might be able to treat them
  // similarly.
  switch (p->getKind()) {
  case PatternKind::EnumElement:
  case PatternKind::OptionalSome: {
    // If one is an OptionalSomePattern and one is an EnumElementPattern, then
    // they are the same since the OptionalSomePattern is just sugar for
    // .Some(x).
    if ((isa<OptionalSomePattern>(p) && isa<EnumElementPattern>(first)) ||
        (isa<OptionalSomePattern>(first) && isa<EnumElementPattern>(p)))
      return p;
    LLVM_FALLTHROUGH;
  }
  case PatternKind::Tuple:
  case PatternKind::Named:
  case PatternKind::Any:
  case PatternKind::Bool:
  case PatternKind::Expr: {
    // These kinds are only similar to the same kind.
    if (p->getKind() == first->getKind())
      return p;
    return nullptr;
  }
  case PatternKind::Is: {
    auto pIs = cast<IsPattern>(p);
    // 'is' patterns are only similar to matches to the same type.
    if (auto firstIs = dyn_cast<IsPattern>(first)) {
      if (firstIs->getCastType()->isEqual(pIs->getCastType()))
        return p;
    }
    return nullptr;
  }
    
  case PatternKind::Paren:
  case PatternKind::Binding:
  case PatternKind::Typed:
    llvm_unreachable("not semantic");
  }

  llvm_unreachable("Unhandled PatternKind in switch.");
}

//===----------------------------------------------------------------------===//
//                           SILGenPattern Emission
//===----------------------------------------------------------------------===//

namespace {

/// A row which we intend to specialize.
struct RowToSpecialize {
  /// The pattern from this row which we are specializing upon.
  swift::Pattern *Pattern;

  /// The index of the target row.
  unsigned RowIndex;

  /// Whether the row will be irrefutable after this specialization.
  bool Irrefutable;

  /// Profile Count of hte row we intend to specialize.
  ProfileCounter Count;
};

/// Changes that we wish to apply to a row which we have specialized.
struct SpecializedRow {
  /// The patterns which should replace the specialized pattern.
  SmallVector<Pattern *, 4> Patterns;

  /// The index of the target row.
  unsigned RowIndex;
};

/// An array of arguments.
using ArgArray = ArrayRef<ConsumableManagedValue>;

/// A callback which dispatches a failure case.
using FailureHandler =
  std::function<void(SILLocation failureLoc)>;

/// A callback which redispatches a set of specialized rows.
using SpecializationHandler =
  std::function<void(ArgArray values, ArrayRef<SpecializedRow> rowChanges,
                     const FailureHandler &contDest)>;

class ClauseMatrix;
class ClauseRow;

/// A class controlling the emission of the decision tree for a pattern match
/// statement (switch, if/let, or while/let condition).
///
/// The value cleanup rules during pattern match emission are complicated
/// because we're trying to allow as much borrowing/forwarding of
/// values as possible, so that we only need to actually copy/retain
/// values as late as possible.  This means we end up having to do
/// a pretty delicate dance to manage the active set of cleanups.
///
/// We split values into three categories:
///   - TakeAlways (which are owned by the current portion of the
///     decision tree)
///   - CopyOnSuccess (which are not owned at all by the current
///     portion of the decision tree)
///   - TakeOnSuccess (which are owned only if the decision tree
///     actually passes all guards and enters a case block)
/// In particular, it is important that a TakeOnSuccess value not be
/// destructively modified unless success is assured.
///
/// Whenever the decision tree branches, it must forward values down
/// correctly.  A TakeAlways value becomes TakeOnSuccess for all but
/// last branch of the tree.
///
/// Values should be forwarded down the decision tree with the
/// appropriate cleanups.  CopyOnSuccess values should not have
/// attached cleanups.  TakeAlways or TakeOnSuccess values should have
/// cleanups when their types are non-trivial.  When a value is
/// forwarded down into a branch of the decision tree, its cleanup
/// might be deactivated within that subtree; to protect against the
/// cleanup being removed when this happens, the cleanup must be first
/// put in the PersistentlyActive state before the emission of the
/// subtree, then restored to its current state when the subtree is
/// finished.
///
/// The set of active cleanups should always be instantaneously
/// consistent: that is, there should always be exactly one cleanup
/// tracking a +1 value.  It's okay to deactivate a cleanup for a
/// TakeOnSuccess value and then introduce new cleanups for all of its
/// subobjects.  Jumps outside of the decision tree entirely will be
/// fine: the jump will simply destroy the subobjects instead of the
/// aggregate.  However, jumps to somewhere else within the decision
/// tree require careful attention if the jump could lead to a
/// cleanups depth outside the subobject cleanups (causing them to be
/// run) but inside the old cleanup (in which case it will be
/// reactivated).  Therefore, such borrowings must be "unforwarded"
/// during the emission of such jumps by disabling the new cleanups
/// and re-enabling the outer cleanup.  It's okay to re-enable
/// cleanups like this because these jumps only occur when a branch of
/// the decision tree fails with a non-exhaustive match, which means
/// the value should have been passed down as TakeOnSuccess, and the
/// decision tree is not allowed to destructively modify objects that
/// are TakeOnSuccess when failure is still a possibility.
class PatternMatchEmission {
  PatternMatchEmission(const PatternMatchEmission &) = delete;
  PatternMatchEmission &operator=(const PatternMatchEmission &) = delete;

  SILGenFunction &SGF;
  
  /// PatternMatchStmt - The 'switch', or do-catch statement that we're emitting
  /// this pattern match for.
  Stmt *PatternMatchStmt;
  CleanupsDepth PatternMatchStmtDepth;
  llvm::MapVector<CaseStmt*, std::pair<SILBasicBlock*, bool>> SharedCases;
  llvm::SmallVector<std::tuple<CaseStmt*, Pattern*, SILBasicBlock*>, 4>
    DestructiveCases;
  CleanupsDepth EndNoncopyableBorrowDest = CleanupsDepth::invalid();
  ValueOwnership NoncopyableMatchOwnership = ValueOwnership::Default;
  ManagedValue NoncopyableConsumableValue;

  llvm::DenseMap<VarDecl*, SILValue> Temporaries;

public:
  using CompletionHandlerTy =
    llvm::function_ref<void(PatternMatchEmission &, ArgArray, ClauseRow &)>;
    
private:
  CompletionHandlerTy CompletionHandler;
  
public:
  
  PatternMatchEmission(SILGenFunction &SGF, Stmt *S,
                       CompletionHandlerTy completionHandler)
    : SGF(SGF), PatternMatchStmt(S),
      CompletionHandler(completionHandler) {}

  std::optional<SILLocation> getSubjectLocationOverride(SILLocation loc) const {
    if (auto *Switch = dyn_cast<SwitchStmt>(PatternMatchStmt))
      if (!Switch->isImplicit())
        return SILLocation(Switch->getSubjectExpr());
    return std::nullopt;
  }

  void emitDispatch(ClauseMatrix &matrix, ArgArray args,
                    const FailureHandler &failure);

  void initSharedCaseBlockDest(CaseStmt *caseBlock, bool hasFallthroughTo);

  void emitAddressOnlyAllocations();

  void emitAddressOnlyInitialization(VarDecl *dest, SILValue value);

  void addDestructiveCase(CaseStmt *caseStmt, Pattern *casePattern) {
    // Save the case pattern to emit later.
    DestructiveCases.emplace_back(caseStmt, casePattern, SGF.B.getInsertionBB());
    // Clear the insertion point to ensure we don't leave detritus in the current
    // block.
    SGF.B.clearInsertionPoint();
  }
  void emitDestructiveCaseBlocks();

  JumpDest getSharedCaseBlockDest(CaseStmt *caseStmt);
  void emitSharedCaseBlocks(ValueOwnership ownership,
                            llvm::function_ref<void(CaseStmt *)> bodyEmitter);

  void emitCaseBody(CaseStmt *caseBlock);

  SILValue getAddressOnlyTemporary(VarDecl *decl) {
    auto found = Temporaries.find(decl);
    assert(found != Temporaries.end());
    return found->second;
  }
  
  // Set up match emission to borrow a noncopyable subject value.
  void setNoncopyableBorrowingOwnership() {
    NoncopyableMatchOwnership = ValueOwnership::Shared;
  }
  // Set up match emission to mutate a noncopyable subject value.
  // The matching phase will still be performed on an immutable borrow up to
  // the point a case is finally chosen, and then components of the value will
  // be bound to variables in the pattern to be modified.
  //
  // The `endBorrowDest` parameter sets the cleanups depth of when the borrow
  // of the subject began, which we will pop up to in order to re-project and
  // consume components.
  void setNoncopyableMutatingOwnership(CleanupsDepth endBorrowDest,
                                       ManagedValue mutatedAddress) {
    assert(mutatedAddress.isLValue());
    NoncopyableMatchOwnership = ValueOwnership::InOut;
    EndNoncopyableBorrowDest = endBorrowDest;
    NoncopyableConsumableValue = mutatedAddress;
  }
  // Set up match emission to consume a noncopyable subject value.
  // The matching phase will still be performed on a borrow up to the point a
  // case is finally chosen, and then components of the value will be consumed
  // to bind variables in the pattern.
  //
  // The `endBorrowDest` parameter sets the cleanups depth of when the borrow
  // of the subject began, which we will pop up to in order to re-project and
  // consume components.
  void setNoncopyableConsumingOwnership(CleanupsDepth endBorrowDest,
                                        ManagedValue ownedValue) {
    assert(ownedValue.isPlusOne(SGF));
    NoncopyableMatchOwnership = ValueOwnership::Owned;
    EndNoncopyableBorrowDest = endBorrowDest;
    NoncopyableConsumableValue = ownedValue;
  }

  std::optional<ValueOwnership> getNoncopyableOwnership() const {
    if (NoncopyableMatchOwnership == ValueOwnership::Default) {
      return std::nullopt;
    }
    return NoncopyableMatchOwnership;
  }

  CleanupsDepth getEndNoncopyableBorrowDest() const {
    assert(NoncopyableMatchOwnership >= ValueOwnership::InOut);
    return EndNoncopyableBorrowDest;
  }

private:
  void emitWildcardDispatch(ClauseMatrix &matrix, ArgArray args, unsigned row,
                            const FailureHandler &failure);

  void bindRefutablePatterns(const ClauseRow &row, ArgArray args,
                             const FailureHandler &failure);

  void emitGuardBranch(SILLocation loc, Expr *guard,
                       const FailureHandler &failure,
                       const ClauseRow &row, ArgArray args);

  // Bind copyable variable bindings as independent variables.
  void bindIrrefutablePatterns(const ClauseRow &row, ArgArray args,
                               bool forIrrefutableRow, bool hasMultipleItems);
                               
  // Bind noncopyable variable bindings as borrows.
  void bindIrrefutableBorrows(const ClauseRow &row, ArgArray args,
                               bool forIrrefutableRow, bool hasMultipleItems);
  
  // End the borrow of the subject and derived values during a move-only match.
  void unbindAndEndBorrows(const ClauseRow &row, ArgArray args);

  void bindVariable(Pattern *pattern, VarDecl *var,
                    ConsumableManagedValue value, bool isIrrefutable,
                    bool hasMultipleItems);

  void bindBorrow(Pattern *pattern, VarDecl *var,
                  ConsumableManagedValue value);

  void emitSpecializedDispatch(ClauseMatrix &matrix, ArgArray args,
                               unsigned &lastRow, unsigned column,
                               const FailureHandler &failure);
  void emitTupleObjectDispatch(ArrayRef<RowToSpecialize> rows,
                               ConsumableManagedValue src,
                               const SpecializationHandler &handleSpec,
                               const FailureHandler &failure);
  void emitTupleDispatch(ArrayRef<RowToSpecialize> rows,
                         ConsumableManagedValue src,
                         const SpecializationHandler &handleSpec,
                         const FailureHandler &failure);
  void emitIsDispatch(ArrayRef<RowToSpecialize> rows,
                      ConsumableManagedValue src,
                      const SpecializationHandler &handleSpec,
                      const FailureHandler &failure);
  void emitEnumElementObjectDispatch(ArrayRef<RowToSpecialize> rows,
                                     ConsumableManagedValue src,
                                     const SpecializationHandler &handleSpec,
                                     const FailureHandler &failure,
                                     ProfileCounter defaultCaseCount);
  void emitEnumElementDispatch(ArrayRef<RowToSpecialize> rows,
                               ConsumableManagedValue src,
                               const SpecializationHandler &handleSpec,
                               const FailureHandler &failure,
                               ProfileCounter defaultCaseCount);
  void emitBoolDispatch(ArrayRef<RowToSpecialize> rows,
                        ConsumableManagedValue src,
                        const SpecializationHandler &handleSpec,
                        const FailureHandler &failure);
};

/// A handle to a row in a clause matrix. Does not own memory; use of the
/// ClauseRow must be dominated by its originating ClauseMatrix.
///
/// TODO: This should be refactored into a more general formulation that uses a
/// child template pattern to inject our logic. This will then allow us to
/// inject "mock" objects in a unittest file.
class ClauseRow {
  friend class ClauseMatrix;
  
  Stmt *ClientData;
  Pattern *CasePattern;
  Expr *CaseGuardExpr;
  
  
  /// HasFallthroughTo - True if there is a fallthrough into this case.
  bool HasFallthroughTo;


  /// The number of remaining specializations until this row becomes
  /// irrefutable.
  unsigned NumRemainingSpecializations;

  SmallVector<Pattern*, 4> Columns;

public:
  ClauseRow(Stmt *clientData, Pattern *CasePattern, Expr *CaseGuardExpr,
            bool HasFallthroughTo)
    : ClientData(clientData),
      CasePattern(CasePattern),
      CaseGuardExpr(CaseGuardExpr),
      HasFallthroughTo(HasFallthroughTo) {
    Columns.push_back(CasePattern);
    if (CaseGuardExpr)
      NumRemainingSpecializations = AlwaysRefutable;
    else 
      NumRemainingSpecializations = getNumSpecializations(Columns[0]);
  }

  template<typename T>
  T *getClientData() const {
    return static_cast<T*>(ClientData);
  }

  Pattern *getCasePattern() const { return CasePattern; }
  Expr *getCaseGuardExpr() const { return CaseGuardExpr; }
  bool hasFallthroughTo() const { return HasFallthroughTo; }
  
  ArrayRef<Pattern *> getColumns() const {
    return Columns;
  }
  MutableArrayRef<Pattern *> getColumns() {
    return Columns;
  }

  /// Specialize the given column to the given array of new columns.
  ///
  /// Places the new columns using the column-specialization algorithm.
  void specializeInPlace(unsigned column, ArrayRef<Pattern *> newColumns) {
    // We assume that this method always removes one level of pattern
    // and replacing it with its direct sub-patterns.  Therefore, we
    // can adjust the number of remaining specializations very easily.
    //
    // We don't need to test whether NumRemainingSpecializations is
    // AlwaysRefutable before decrementing because we only ever test
    // this value against zero.
    if (isDirectlyRefutablePattern(Columns[column]))
      --NumRemainingSpecializations;

    if (newColumns.size() == 1) {
      Columns[column] = newColumns[0];
    } else if (newColumns.empty()) {
      if (column + 1 == Columns.size()) {
        Columns.pop_back();
      } else {
        Columns[column] = Columns.pop_back_val();
      }
    } else {
      Columns[column] = newColumns[0];
      Columns.append(newColumns.begin() + 1, newColumns.end());
    }
  }

  /// Is this row currently irrefutable?
  bool isIrrefutable() const {
    return NumRemainingSpecializations == 0;
  }

  /// Will this row be irrefutable after we single-step specialize the
  /// given column?
  bool isIrrefutableAfterSpecializing(unsigned column) const {
    if (NumRemainingSpecializations == 1)
      return isDirectlyRefutablePattern(Columns[column]);
    return NumRemainingSpecializations == 0;
  }
  
  Pattern * const *begin() const {
    return getColumns().begin();
  }
  Pattern * const *end() const {
    return getColumns().end();
  }
  
  Pattern **begin() {
    return getColumns().begin();
  }
  Pattern **end() {
    return getColumns().end();
  }
  
  Pattern *operator[](unsigned column) const {
    return getColumns()[column];
  }
  Pattern *&operator[](unsigned column) {
    return getColumns()[column];
  }
  unsigned columns() const {
    return Columns.size();
  }

  LLVM_ATTRIBUTE_USED void dump() const { return print(llvm::errs()); }
  void print(llvm::raw_ostream &out) const;
};

/// A clause matrix. This matrix associates subpattern rows to their
/// corresponding guard expressions, and associates destination basic block
/// and columns to their associated subject value.
class ClauseMatrix {
  SmallVector<ClauseRow *, 4> Rows;

  ClauseMatrix(const ClauseMatrix &) = delete;
  ClauseMatrix &operator=(const ClauseMatrix &) = delete;
  ClauseMatrix() = default;
public:
  /// Create a clause matrix from the given pattern-row storage.
  /// (actively matched values) and enough initial capacity for the
  /// given number of rows. The clause matrix will be initialized with zero rows
  /// and a column for every occurrence. Rows can be added using addRows.
  explicit ClauseMatrix(MutableArrayRef<ClauseRow> rows) {
    for (ClauseRow &row : rows) {
      Rows.push_back(&row);
    }
  }

  ClauseMatrix(ClauseMatrix &&) = default;
  ClauseMatrix &operator=(ClauseMatrix &&) = default;
  
  unsigned rows() const { return Rows.size(); }

  ClauseRow &operator[](unsigned row) {
    return *Rows[row];
  }
  const ClauseRow &operator[](unsigned row) const {
    return *Rows[row];
  }

  /// Destructively specialize the rows of this clause matrix.  The
  /// rows should not be used in this matrix afterwards.
  ClauseMatrix specializeRowsInPlace(unsigned column,
                                     ArrayRef<SpecializedRow> newRows) {
    assert(!newRows.empty() && "specializing for an empty set of rows?");

    ClauseMatrix innerMatrix;
    for (unsigned i = 0, e = newRows.size(); i != e; ++i) {
      assert((i == 0 || newRows[i - 1].RowIndex < newRows[i].RowIndex) &&
             "specialized rows are out of order?");

      ClauseRow *rowData = Rows[newRows[i].RowIndex];
      rowData->specializeInPlace(column, newRows[i].Patterns);
      innerMatrix.Rows.push_back(rowData);
    }
    return innerMatrix;
  }

  LLVM_ATTRIBUTE_USED void dump() const { return print(llvm::errs()); }
  void print(llvm::raw_ostream &out) const;
};

} // end anonymous namespace

void ClauseRow::print(llvm::raw_ostream &out) const {
  out << "[ ";
  for (const Pattern *column : *this) {
    dumpPattern(column, out);
    out << ' ';
  }
  out << "]\n";
}

void ClauseMatrix::print(llvm::raw_ostream &out) const {
  if (Rows.empty()) { return; }

  // Tabulate the strings for each column, row-major.
  // We need to pad the strings out like a real matrix.
  SmallVector<std::vector<std::string>, 4> patternStrings;
  SmallVector<size_t, 4> columnSizes;

  patternStrings.resize(Rows.size());
    
  llvm::formatted_raw_ostream fos(out);
    
  for (unsigned r = 0, rend = rows(); r < rend; ++r) {
    const ClauseRow &row = (*this)[r];
    auto &rowStrings = patternStrings[r];

    // Make sure that column sizes has an entry for all our columns.
    if (row.columns() > columnSizes.size())
      columnSizes.resize(row.columns(), 0);
    rowStrings.reserve(row.columns());

    for (unsigned c = 0, cend = row.columns(); c < cend; ++c) {
      rowStrings.push_back("");
      std::string &str = rowStrings.back();
      {
        llvm::raw_string_ostream ss(str);
        dumpPattern(row[c], ss);
        ss.flush();
      }

      columnSizes[c] = std::max(columnSizes[c], str.size());
    }
  }

  for (unsigned r = 0, rend = rows(); r < rend; ++r) {
    fos << "[ ";
    for (unsigned c = 0, cend = patternStrings[r].size(); c < cend; ++c) {
      unsigned start = fos.getColumn();
      fos << patternStrings[r][c];
      fos.PadToColumn(start + columnSizes[c] + 1);
    }
    fos << "]\n";
  }
  fos.flush();
}

/// Forward a value down into a branch of the decision tree that may
/// fail and lead back to other branch(es).
///
/// Essentially equivalent to forwardIntoIrrefutableSubtree, except it
/// converts AlwaysTake to TakeOnSuccess.
static ConsumableManagedValue
forwardIntoSubtree(SILGenFunction &SGF, SILLocation loc,
                   CleanupStateRestorationScope &scope,
                   ConsumableManagedValue outerCMV) {
  loc.markAutoGenerated();
  ManagedValue outerMV = outerCMV.getFinalManagedValue();
  if (!outerMV.hasCleanup()) return outerCMV;

  auto consumptionKind = outerCMV.getFinalConsumption();
  (void)consumptionKind;

  // If we have an object and it is take always, we need to borrow the value
  // since our subtree does not own the value.
  if (outerMV.getType().isObject()) {
    assert(consumptionKind == CastConsumptionKind::TakeAlways &&
           "Object without cleanup that is not take_always?!");
    return {outerMV.borrow(SGF, loc), CastConsumptionKind::BorrowAlways};
  }

  // Only address only values use TakeOnSuccess.
  assert(outerMV.getType().isAddressOnly(SGF.F) &&
         "TakeOnSuccess can only be used with address only values");

  assert((consumptionKind == CastConsumptionKind::TakeAlways ||
          consumptionKind == CastConsumptionKind::TakeOnSuccess) &&
         "non-+1 consumption with a cleanup?");
  scope.pushCleanupState(outerMV.getCleanup(),
                         CleanupState::PersistentlyActive);

  // Success means that we won't end up in the other branch,
  // but failure doesn't.
  return {outerMV, CastConsumptionKind::TakeOnSuccess};
}

/// Forward a value down into an irrefutable branch of the decision tree.
///
/// Essentially equivalent to forwardIntoSubtree, except it preserves
/// AlwaysTake consumption.
static void forwardIntoIrrefutableSubtree(SILGenFunction &SGF,
                                          CleanupStateRestorationScope &scope,
                                          ConsumableManagedValue outerCMV) {
  ManagedValue outerMV = outerCMV.getFinalManagedValue();
  if (!outerMV.hasCleanup()) return;

  assert(outerCMV.getFinalConsumption() != CastConsumptionKind::CopyOnSuccess
         && "copy-on-success value with cleanup?");
  scope.pushCleanupState(outerMV.getCleanup(),
                         CleanupState::PersistentlyActive);

}

namespace {

class ArgForwarderBase {
  SILGenFunction &SGF;
  CleanupStateRestorationScope Scope;

protected:
  ArgForwarderBase(SILGenFunction &SGF) : SGF(SGF), Scope(SGF.Cleanups) {}

  ConsumableManagedValue forward(ConsumableManagedValue value,
                                 SILLocation loc) {
    return forwardIntoSubtree(SGF, loc, Scope, value);
  }

  void forwardIntoIrrefutable(ConsumableManagedValue value) {
    return forwardIntoIrrefutableSubtree(SGF, Scope, value);
  }
};

/// A RAII-ish object for forwarding a bunch of arguments down to one
/// side of a branch.
class ArgForwarder : private ArgForwarderBase {
  ArgArray OuterArgs;
  SmallVector<ConsumableManagedValue, 4> ForwardedArgsBuffer;

public:
  ArgForwarder(SILGenFunction &SGF, ArgArray outerArgs, SILLocation loc,
               bool isFinalUse)
      : ArgForwarderBase(SGF), OuterArgs(outerArgs) {
    // If this is a final use along this path, we don't need to change
    // any of the args.  However, we do need to make sure that the
    // cleanup state gets restored later, because being final on this
    // path isn't the same as being final along all paths.
    if (isFinalUse) {
      for (auto &outerArg : outerArgs)
        forwardIntoIrrefutable(outerArg);
    } else {
      ForwardedArgsBuffer.reserve(outerArgs.size());
      for (auto &outerArg : outerArgs)
        ForwardedArgsBuffer.push_back(forward(outerArg, loc));
    }
  }

  ArgArray getForwardedArgs() const {
    if (didForwardArgs()) return ForwardedArgsBuffer;
    return OuterArgs;
  }

private:
  bool didForwardArgs() const { return !ForwardedArgsBuffer.empty(); }
};

/// A RAII-ish object for forwarding a bunch of arguments down to one
/// side of a branch.
class SpecializedArgForwarder : private ArgForwarderBase {
  ArgArray OuterArgs;
  bool IsFinalUse;
  SmallVector<ConsumableManagedValue, 4> ForwardedArgsBuffer;

public:
  /// Construct a specialized arg forwarder for a (locally) successful
  /// dispatch.
  SpecializedArgForwarder(SILGenFunction &SGF, ArgArray outerArgs,
                          unsigned column, ArgArray newArgs, SILLocation loc,
                          bool isFinalUse)
      : ArgForwarderBase(SGF), OuterArgs(outerArgs), IsFinalUse(isFinalUse) {
    assert(column < outerArgs.size());

    ForwardedArgsBuffer.reserve(outerArgs.size() - 1 + newArgs.size());

    // Place the new columns with the column-specialization algorithm:
    //  - place the first new column (if any) in the same position as the
    //    original column;
    //  - if there are no new columns, and the removed column was not
    //    the last column, the last column is moved to the removed column.

    // The outer columns before the specialized column.
    for (unsigned i = 0, e = column; i != e; ++i)
      ForwardedArgsBuffer.push_back(forward(outerArgs[i], loc));

    // The specialized column.
    if (!newArgs.empty()) {
      ForwardedArgsBuffer.push_back(newArgs[0]);
      newArgs = newArgs.slice(1);
    } else if (column + 1 < outerArgs.size()) {
      ForwardedArgsBuffer.push_back(forward(outerArgs.back(), loc));
      outerArgs = outerArgs.slice(0, outerArgs.size() - 1);
    }

    // The rest of the outer columns.
    for (unsigned i = column + 1, e = outerArgs.size(); i != e; ++i)
      ForwardedArgsBuffer.push_back(forward(outerArgs[i], loc));

    // The rest of the new args.
    ForwardedArgsBuffer.append(newArgs.begin(), newArgs.end());
  }

  /// Returns the forward arguments.  The new rows are placed using
  /// the column-specialization algorithm.
  ArgArray getForwardedArgs() const {
    return ForwardedArgsBuffer;
  }

private:
  ConsumableManagedValue forward(ConsumableManagedValue value,
                                 SILLocation loc) {
    if (IsFinalUse) {
      ArgForwarderBase::forwardIntoIrrefutable(value);
      return value;
    } else {
      return ArgForwarderBase::forward(value, loc);
    }
  }
};

/// A RAII-ish object for undoing the forwarding of cleanups along a
/// failure path.
class ArgUnforwarder {
  SILGenFunction &SGF;
  CleanupStateRestorationScope Scope;
public:
  ArgUnforwarder(SILGenFunction &SGF) : SGF(SGF), Scope(SGF.Cleanups) {}

  static bool requiresUnforwarding(SILGenFunction &SGF,
                                   ConsumableManagedValue operand) {
    return operand.hasCleanup() &&
           operand.getFinalConsumption()
             == CastConsumptionKind::TakeOnSuccess;
  }

  /// Given that an aggregate was divided into a set of borrowed
  /// values which are now being tracked individually, temporarily
  /// disable all of the borrowed-value cleanups and restore the
  /// aggregate cleanup.
  void unforwardBorrowedValues(ConsumableManagedValue aggregate,
                               ArgArray subobjects) {
    if (!requiresUnforwarding(SGF, aggregate))
      return;
    Scope.pushCleanupState(aggregate.getCleanup(), CleanupState::Active);
    for (auto &subobject : subobjects) {
      if (subobject.hasCleanup())
        Scope.pushCleanupState(subobject.getCleanup(), CleanupState::Dormant);
    }
  }
};

} // end anonymous namespace

/// Return the dispatchable length of the given column.
static unsigned getConstructorPrefix(const ClauseMatrix &matrix,
                                     unsigned firstRow, unsigned column) {
  assert(firstRow < matrix.rows() &&
         "getting column constructor prefix in matrix with no rows remaining?");

  // Require the first row to be a non-wildcard.
  auto first = getSpecializingPattern(matrix[firstRow][column]);
  if (!first) return 0;

  // Then count the number of rows with the same kind of pattern.
  unsigned row = firstRow + 1;
  for (unsigned rend = matrix.rows(); row < rend; ++row) {
    if (!getSimilarSpecializingPattern(matrix[row][column], first))
      break;
  }
  return row - firstRow;
}

/// Select the "necessary column", Maranget's term for the column
/// most likely to give an optimal decision tree.
///
/// \return std::nullopt if we didn't find a meaningful necessary column
static std::optional<unsigned> chooseNecessaryColumn(const ClauseMatrix &matrix,
                                                     unsigned firstRow) {
  assert(firstRow < matrix.rows() &&
         "choosing necessary column of matrix with no rows remaining?");

  // First of all, if we have zero or one columns, this is trivial
  // to decide.
  auto numColumns = matrix[firstRow].columns();
  if (numColumns <= 1) {
    if (numColumns == 1 && !isWildcardPattern(matrix[firstRow][0])) {
      return 0;
    }
    return std::nullopt;
  }

  // Use the "constructor prefix" heuristic from Maranget to pick the
  // necessary column. The column with the most pattern nodes prior to a
  // wildcard turns out to be a good and cheap-to-calculate heuristic for
  // generating an optimal decision tree.  We ignore patterns that aren't
  // similar to the head pattern.
  std::optional<unsigned> bestColumn;
  unsigned longestConstructorPrefix = 0;
  for (unsigned c = 0; c != numColumns; ++c) {
    unsigned constructorPrefix = getConstructorPrefix(matrix, firstRow, c);
    if (constructorPrefix > longestConstructorPrefix) {
      longestConstructorPrefix = constructorPrefix;
      bestColumn = c;
    }
  }

  return bestColumn;
}

/// Recursively emit a decision tree from the given pattern matrix.
void PatternMatchEmission::emitDispatch(ClauseMatrix &clauses, ArgArray args,
                                        const FailureHandler &outerFailure) {
  if (clauses.rows() == 0) {
    SGF.B.createUnreachable(SILLocation(PatternMatchStmt));
    return;
  }

  unsigned firstRow = 0;
  while (true) {
    // If there are no rows remaining, then we fail.
    if (firstRow == clauses.rows()) {
      outerFailure(clauses[clauses.rows() - 1].getCasePattern());
      return;
    }
    
    // Try to find a "necessary column".
    std::optional<unsigned> column = chooseNecessaryColumn(clauses, firstRow);

    // Emit the subtree in its own scope.
    ExitableFullExpr scope(SGF, CleanupLocation(PatternMatchStmt));
    auto innerFailure = [&](SILLocation loc) {
      if (firstRow == clauses.rows()) return outerFailure(loc);
      SGF.Cleanups.emitBranchAndCleanups(scope.getExitDest(), loc);
    };

    // If there is no necessary column, just emit the first row.
    if (!column) {
      unsigned wildcardRow = firstRow++;
      emitWildcardDispatch(clauses, args, wildcardRow, innerFailure);
    } else {
      // Otherwise, specialize on the necessary column.
      emitSpecializedDispatch(clauses, args, firstRow, column.value(),
                              innerFailure);
    }

    assert(!SGF.B.hasValidInsertionPoint());
    SILBasicBlock *contBB = scope.exit();
    // If the continuation block has no uses, ...
    if (contBB->pred_empty()) {
      // If we have no more rows to emit, clear the IP and destroy the
      // continuation block.
      if (firstRow == clauses.rows()) {
        SGF.B.clearInsertionPoint();
        SGF.eraseBasicBlock(contBB);
        return;
      }

      // Otherwise, if there is no fallthrough, then the next row is
      // unreachable: emit a dead code diagnostic if:
      // 1) It's for a 'default' case (since Space Engine already handles
      //    unreachable enum case patterns) or it's for a enum case which
      //    has expression patterns since redundancy checking for such
      //    patterns isn't sufficiently done by the Space Engine.
      // 2) It's for a case statement in a do-catch.
      if (!clauses[firstRow].hasFallthroughTo()) {
        SourceLoc Loc;
        bool isDefault = false;
        bool isParentDoCatch = false;
        bool caseHasExprPattern = false;
        if (auto *S = clauses[firstRow].getClientData<Stmt>()) {
          Loc = S->getStartLoc();
          if (auto *CS = dyn_cast<CaseStmt>(S)) {
            caseHasExprPattern = llvm::any_of(
                CS->getCaseLabelItems(), [&](const CaseLabelItem item) {
                  return item.getPattern()->getKind() == PatternKind::Expr;
                });
            isParentDoCatch = CS->getParentKind() == CaseParentKind::DoCatch;
            isDefault = CS->isDefault() && !CS->hasUnknownAttr();
          }
        } else {
          Loc = clauses[firstRow].getCasePattern()->getStartLoc();
        }
        if (isParentDoCatch || isDefault || caseHasExprPattern) {
          SGF.SGM.diagnose(Loc, diag::unreachable_case, isDefault);
        }
      }
    }
  }
}

/// Emit the decision tree for a row containing only non-specializing
/// patterns.
///
/// \param matrixArgs - appropriate for the entire clause matrix, not
///   just this one row
void PatternMatchEmission::emitWildcardDispatch(ClauseMatrix &clauses,
                                                ArgArray matrixArgs,
                                                unsigned row,
                                                const FailureHandler &failure) {
  // Get appropriate arguments.
  ArgForwarder forwarder(SGF, matrixArgs, clauses[row].getCasePattern(),
                         /*isFinalUse*/ row + 1 == clauses.rows());
  ArgArray args = forwarder.getForwardedArgs();

  // Bind all the refutable patterns first.  We want to do this first
  // so that we can treat the rest of the bindings as inherently
  // successful if we don't have a guard.  This approach assumes that
  // expression patterns can't refer to bound arguments.
  bindRefutablePatterns(clauses[row], args, failure);

  // Okay, the rest of the bindings are irrefutable if there isn't a guard.
  Expr *guardExpr = clauses[row].getCaseGuardExpr();
  bool hasGuard = guardExpr != nullptr;
  assert(!hasGuard || !clauses[row].isIrrefutable());

  auto stmt = clauses[row].getClientData<Stmt>();
  assert(isa<CaseStmt>(stmt));

  auto *caseStmt = dyn_cast<CaseStmt>(stmt);
  bool hasMultipleItems =
      caseStmt && (clauses[row].hasFallthroughTo() ||
                   caseStmt->getCaseLabelItems().size() > 1);

  if (auto ownership = getNoncopyableOwnership()) {
    // A noncopyable pattern match always happens over a borrow first.
    // If the final pattern match is only borrowing as well,
    // we can bind the variables immediately here too.
    if (*ownership <= ValueOwnership::Shared) {
      bindIrrefutableBorrows(clauses[row], args,
                             !hasGuard, hasMultipleItems);
    }
    
    if (hasGuard) {
      // The guard will bind borrows locally if necessary.
      this->emitGuardBranch(guardExpr, guardExpr, failure,
                            clauses[row], args);
    }
    
    if (*ownership > ValueOwnership::Shared) {
      unbindAndEndBorrows(clauses[row], args);
    }
  } else {
    // Bind the rest of the patterns.
    // For noncopyable bindings, this will bind them as borrows initially if there
    // is a guard expression, since we don't want to consume or modify the value
    // until we commit to a pattern match.
    bindIrrefutablePatterns(clauses[row], args, !hasGuard, hasMultipleItems);

    // Emit the guard branch, if it exists.
    if (guardExpr) {
      this->emitGuardBranch(guardExpr, guardExpr, failure,
                            clauses[row], args);
    }
  }

  // Enter the row.
  CompletionHandler(*this, args, clauses[row]);
  assert(!SGF.B.hasValidInsertionPoint());
}

/// Bind all the refutable patterns in the given row.
void PatternMatchEmission::
bindRefutablePatterns(const ClauseRow &row, ArgArray args,
                      const FailureHandler &failure) {
  assert(row.columns() == args.size());
  for (unsigned i = 0, e = args.size(); i != e; ++i) {
    if (!row[i]) // We use null patterns to mean artificial AnyPatterns
      continue;

    Pattern *pattern = row[i]->getSemanticsProvidingPattern();
    switch (pattern->getKind()) {
    // Irrefutable patterns that we'll handle in a later pass.
    case PatternKind::Any:
      break;
    case PatternKind::Named:
      break;

    case PatternKind::Expr: {
      ExprPattern *exprPattern = cast<ExprPattern>(pattern);
      DebugLocOverrideRAII LocOverride{SGF.B,
                                       getSubjectLocationOverride(pattern)};
      FullExpr scope(SGF.Cleanups, CleanupLocation(pattern));
      bindVariable(pattern, exprPattern->getMatchVar(), args[i],
                   /*isForSuccess*/ false, /* hasMultipleItems */ false);
      emitGuardBranch(pattern, exprPattern->getMatchExpr(), failure,
                      row, args);
      break;
    }
    default:
      llvm_unreachable("bad pattern kind");
    }
  }
}

/// Bind all the irrefutable patterns in the given row, which is nothing
/// but wildcard patterns.
///
/// Note that forIrrefutableRow can be true even if !row.isIrrefutable()
/// because we might have already bound all the refutable parts.
void PatternMatchEmission::bindIrrefutablePatterns(const ClauseRow &row,
                                                   ArgArray args,
                                                   bool forIrrefutableRow,
                                                   bool hasMultipleItems) {
  assert(row.columns() == args.size());
  for (unsigned i = 0, e = args.size(); i != e; ++i) {
    if (!row[i]) // We use null patterns to mean artificial AnyPatterns
      continue;

    Pattern *pattern = row[i]->getSemanticsProvidingPattern();
    switch (pattern->getKind()) {
    case PatternKind::Any: // We can just drop Any values.
      break;
    case PatternKind::Expr: // Ignore expression patterns, which we should have
                            // bound in an earlier pass.
      break;
    case PatternKind::Named: {
      NamedPattern *named = cast<NamedPattern>(pattern);
      bindVariable(pattern, named->getDecl(), args[i], forIrrefutableRow,
                   hasMultipleItems);
      break;
    }
    default:
      llvm_unreachable("bad pattern kind");
    }
  }
}

void PatternMatchEmission::bindIrrefutableBorrows(const ClauseRow &row,
                                                  ArgArray args,
                                                  bool forIrrefutableRow,
                                                  bool hasMultipleItems) {
  assert(row.columns() == args.size());
  for (unsigned i = 0, e = args.size(); i != e; ++i) {
    if (!row[i]) // We use null patterns to mean artificial AnyPatterns
      continue;

    Pattern *pattern = row[i]->getSemanticsProvidingPattern();
    switch (pattern->getKind()) {
    case PatternKind::Any: // We can just drop Any values.
      break;
    case PatternKind::Expr: // Ignore expression patterns, which we should have
                            // bound in an earlier pass.
      break;
    case PatternKind::Named: {
      NamedPattern *named = cast<NamedPattern>(pattern);
      // If the subpattern matches a copyable type, and the match isn't
      // explicitly `borrowing`, then we can bind it as a normal copyable
      // value.
      if (named->getDecl()->getIntroducer() != VarDecl::Introducer::Borrowing
          && !named->getType()->isNoncopyable()) {
        bindVariable(pattern, named->getDecl(), args[i], forIrrefutableRow,
                     hasMultipleItems);
      } else {
        bindBorrow(pattern, named->getDecl(), args[i]);
      }
      break;
    }
    default:
      llvm_unreachable("bad pattern kind");
    }
  }
}

void
PatternMatchEmission::unbindAndEndBorrows(const ClauseRow &row,
                                          ArgArray args) {
  assert(*getNoncopyableOwnership() > ValueOwnership::Shared);
  
  // Unbind the pattern variables since their borrow will be invalidated.
  for (auto column : row) {
    if (!column) // We use null patterns to mean artificial AnyPatterns
      continue;

    Pattern *pattern = column->getSemanticsProvidingPattern();
    switch (pattern->getKind()) {
    case PatternKind::Any: // We can just drop Any values.
      break;
    case PatternKind::Expr: // Ignore expression patterns, which we should have
                            // bound in an earlier pass.
      break;
    case PatternKind::Named: {
      NamedPattern *named = cast<NamedPattern>(pattern);
      SGF.VarLocs.erase(named->getDecl());
      break;
    }
    default:
      llvm_unreachable("bad pattern kind");
    }
  }
  
  // Stop borrowing the value by popping up to the scope outside the borrow.
  SGF.Cleanups.endNoncopyablePatternMatchBorrow(EndNoncopyableBorrowDest,
                                                PatternMatchStmt);
}


/// Should we take control of the mang
static bool shouldTake(ConsumableManagedValue value, bool isIrrefutable) {
  switch (value.getFinalConsumption()) {
  case CastConsumptionKind::TakeAlways: return true;
  case CastConsumptionKind::TakeOnSuccess: return isIrrefutable;
  case CastConsumptionKind::CopyOnSuccess: return false;
  case CastConsumptionKind::BorrowAlways: return false;
  }
  llvm_unreachable("bad consumption kind");
}

/// Bind a variable into the current scope.
void PatternMatchEmission::bindVariable(Pattern *pattern, VarDecl *var,
                                        ConsumableManagedValue value,
                                        bool isIrrefutable,
                                        bool hasMultipleItems) {
  // If this binding is one of multiple patterns, each individual binding
  // will just be let, and then the chosen value will get forwarded into
  // a var box in the final shared case block.
  bool immutable = var->isLet() || hasMultipleItems;

  // Initialize the variable value.
  InitializationPtr init = SGF.emitInitializationForVarDecl(var, immutable);

  // Do not emit debug descriptions at this stage.
  //
  // If there are multiple let bindings, the value is forwarded to the case
  // block via a phi. Emitting duplicate debug values for the incoming values
  // leads to bogus debug info -- we must emit the debug value only on the phi.
  //
  // If there's only one let binding, we still want to wait until we can nest
  // the scope for the case body under the scope for the pattern match.
  init->setEmitDebugValueOnInit(false);

  auto mv = value.getFinalManagedValue();
  if (shouldTake(value, isIrrefutable)) {
    mv.forwardInto(SGF, pattern, init.get());
  } else {
    mv.copyInto(SGF, pattern, init.get());
  }
}

/// Bind a borrow binding into the current scope.
void PatternMatchEmission::bindBorrow(Pattern *pattern, VarDecl *var,
                                      ConsumableManagedValue value) {
  assert(value.getFinalConsumption() == CastConsumptionKind::BorrowAlways);
  
  auto bindValue = value.asBorrowedOperand2(SGF, pattern).getFinalManagedValue();

  // Borrow bindings of copyable type should still be no-implicit-copy.
  if (!bindValue.getType().isMoveOnly()) {
    if (bindValue.getType().isAddress()) {
      bindValue = ManagedValue::forBorrowedAddressRValue(
        SGF.B.createCopyableToMoveOnlyWrapperAddr(pattern, bindValue.getValue()));
    } else {
      bindValue =
        SGF.B.createGuaranteedCopyableToMoveOnlyWrapperValue(pattern, bindValue);
    }
  }

  if (bindValue.getType().isObject()) {
    // Create a notional copy for the borrow checker to use.
    bindValue = bindValue.copy(SGF, pattern);
  } else {
    bindValue = SGF.B.createOpaqueBorrowBeginAccess(pattern, bindValue);
  }
  // We mark the borrow check as "strict" because we don't want to allow
  // consumes through the binding, even if the original value manages to be
  // stack promoted during AllocBoxToStack or anything like that.
  bindValue = SGF.B.createMarkUnresolvedNonCopyableValueInst(pattern, bindValue,
              MarkUnresolvedNonCopyableValueInst::CheckKind::NoConsumeOrAssign,
              MarkUnresolvedNonCopyableValueInst::IsStrict);

  SGF.VarLocs[var] = SILGenFunction::VarLoc(bindValue.getValue(),
                                            SILAccessEnforcement::Unknown);
}

/// Evaluate a guard expression and, if it returns false, branch to
/// the given destination.
void PatternMatchEmission::emitGuardBranch(SILLocation loc, Expr *guard,
                                           const FailureHandler &failure,
                                           const ClauseRow &row, ArgArray args){
  SILBasicBlock *falseBB = SGF.B.splitBlockForFallthrough();
  SILBasicBlock *trueBB = SGF.B.splitBlockForFallthrough();

  // Emit the match test.
  SILValue testBool;
  {
    FullExpr scope(SGF.Cleanups, CleanupLocation(guard));
    
    // If the final pattern match is destructive, then set up borrow bindings
    // to evaluate the guard expression without allowing it to destruct the
    // subject yet.
    if (auto ownership = getNoncopyableOwnership()) {
      if (*ownership > ValueOwnership::Shared) {
        bindIrrefutableBorrows(row, args,
                               /*irrefutable*/ false,
                               /*multiple items*/ false);
      }
    }
    testBool = SGF.emitRValueAsSingleValue(guard).getUnmanagedValue();
  }

  // Extract the i1 from the Bool struct.
  auto i1Value = SGF.emitUnwrapIntegerResult(loc, testBool);
  SGF.B.createCondBranch(loc, i1Value, trueBB, falseBB);

  SGF.B.setInsertionPoint(falseBB);
  failure(loc);

  SGF.B.setInsertionPoint(trueBB);
}

/// Perform specialized dispatch on the particular column.
///
/// \param matrixArgs - appropriate for the entire clause matrix, not
///   just these specific rows
void PatternMatchEmission::emitSpecializedDispatch(ClauseMatrix &clauses,
                                                   ArgArray matrixArgs,
                                                   unsigned &lastRow,
                                                   unsigned column,
                                               const FailureHandler &failure) {
  // HEY! LISTEN!
  //
  // When a pattern specializes its submatrix (like an 'as' or enum element
  // pattern), it *must* chain the FailureHandler for its inner submatrixes
  // through our `failure` handler if it manipulates any cleanup state.
  // Here's an example from emitEnumElementDispatch:
  //
  //       const FailureHandler *innerFailure = &failure;
  //       FailureHandler specializedFailure = [&](SILLocation loc) {
  //         ArgUnforwarder unforwarder(SGF);
  //         unforwarder.unforwardBorrowedValues(src, origCMV);
  //         failure(loc);
  //       };
  //
  //       if (ArgUnforwarder::requiresUnforwarding(src))
  //         innerFailure = &specializedFailure;
  //
  // Note that the inner failure handler either is exactly the outer failure
  // or performs the work necessary to clean up after the failed specialized
  // decision tree immediately before chaining onto the outer failure.
  // It is specifically NOT correct to do something like this:
  //
  //       /* DON'T DO THIS */
  //       ExitableFullExpr scope;
  //       FailureHandler innerFailure = [&](SILLocation loc) {
  //         emitBranchAndCleanups(scope, loc);
  //       };
  //       ...
  //       /* DON'T DO THIS */
  //       scope.exit();
  //       ArgUnforwarder unforwarder(SGF);
  //       unforwarder.unforwardBorrowedValues(src, origCMV);
  //       failure(loc);
  //       /* DON'T DO THIS */
  //
  // since the cleanup state changes performed by ArgUnforwarder will
  // occur too late.
  
  unsigned firstRow = lastRow;

  // Collect the rows to specialize.
  SmallVector<RowToSpecialize, 4> rowsToSpecialize;
  auto addRowToSpecialize = [&](Pattern *pattern, unsigned rowIndex) {
    assert(getSpecializingPattern(clauses[rowIndex][column]) == pattern);
    bool irrefutable = clauses[rowIndex].isIrrefutableAfterSpecializing(column);
    auto caseBlock = clauses[rowIndex].getClientData<CaseStmt>();
    ProfileCounter count = ProfileCounter();
    if (caseBlock) {
      count = SGF.loadProfilerCount(caseBlock);
    }
    rowsToSpecialize.push_back({pattern, rowIndex, irrefutable, count});
  };

  ProfileCounter defaultCaseCount = ProfileCounter();
  Pattern *firstSpecializer = getSpecializingPattern(clauses[firstRow][column]);
  assert(firstSpecializer && "specializing unspecializable row?");
  addRowToSpecialize(firstSpecializer, firstRow);

  // Take a prefix of rows that share the same semantic kind of pattern.
  for (++lastRow; lastRow != clauses.rows(); ++lastRow) {
    Pattern *specializer =
      getSimilarSpecializingPattern(clauses[lastRow][column], firstSpecializer);
    if (!specializer) {
      auto caseBlock = clauses[lastRow].getClientData<CaseStmt>();
      if (caseBlock) {
        defaultCaseCount = SGF.loadProfilerCount(caseBlock);
      }
      break;
    }
    addRowToSpecialize(specializer, lastRow);
  }
  assert(lastRow - firstRow == rowsToSpecialize.size());

  // Forward just the specialized argument right now.  We'll forward
  // the rest in the handler.
  bool isFinalUse = (lastRow == clauses.rows());
  ArgForwarder outerForwarder(SGF, matrixArgs[column], firstSpecializer,
                              isFinalUse);
  auto arg = outerForwarder.getForwardedArgs()[0];

  SpecializationHandler handler = [&](ArrayRef<ConsumableManagedValue> newArgs,
                                      ArrayRef<SpecializedRow> rows,
                                      const FailureHandler &innerFailure) {
    // These two operations must follow the same rules for column
    // placement because 'arguments' are parallel to the matrix columns.
    // We use the column-specialization algorithm described in
    // specializeInPlace.
    ClauseMatrix innerClauses = clauses.specializeRowsInPlace(column, rows);

    SpecializedArgForwarder innerForwarder(SGF, matrixArgs, column, newArgs,
                                           firstSpecializer, isFinalUse);
    ArgArray innerArgs = innerForwarder.getForwardedArgs();

    emitDispatch(innerClauses, innerArgs, innerFailure);
  };

  switch (firstSpecializer->getKind()) {
  case PatternKind::Any:
  case PatternKind::Expr:
  case PatternKind::Named:
    llvm_unreachable("cannot specialize wildcard pattern");

  case PatternKind::Paren:
  case PatternKind::Typed:
  case PatternKind::Binding:
    llvm_unreachable("non-semantic pattern kind!");
  
  case PatternKind::Tuple:
    return emitTupleDispatch(rowsToSpecialize, arg, handler, failure);
  case PatternKind::Is:
    return emitIsDispatch(rowsToSpecialize, arg, handler, failure);
  case PatternKind::EnumElement:
  case PatternKind::OptionalSome:
    return emitEnumElementDispatch(rowsToSpecialize, arg, handler, failure,
                                   defaultCaseCount);
  case PatternKind::Bool:
    return emitBoolDispatch(rowsToSpecialize, arg, handler, failure);
  }
  llvm_unreachable("bad pattern kind");
}

/// Given that we've broken down a source value into this subobject,
/// and that we were supposed to use the given consumption rules on
/// it, construct an appropriate managed value.
static ConsumableManagedValue
getManagedSubobject(SILGenFunction &SGF, SILValue value,
                    const TypeLowering &valueTL,
                    CastConsumptionKind consumption) {
  switch (consumption) {
  case CastConsumptionKind::BorrowAlways:
  case CastConsumptionKind::CopyOnSuccess:
    return {ManagedValue::forBorrowedRValue(value), consumption};
  case CastConsumptionKind::TakeAlways:
  case CastConsumptionKind::TakeOnSuccess:
    return {SGF.emitManagedRValueWithCleanup(value, valueTL), consumption};
  }
  llvm_unreachable("covered switch");
}

/// Given that we've broken down a source value into this subobject,
/// and that we were supposed to use the given consumption rules on
/// it, construct an appropriate managed value.
static ConsumableManagedValue
getManagedSubobject(SILGenFunction &SGF, ManagedValue value,
                    CastConsumptionKind consumption) {
  switch (consumption) {
  case CastConsumptionKind::BorrowAlways:
  case CastConsumptionKind::CopyOnSuccess:
    return {value.unmanagedBorrow(), consumption};
  case CastConsumptionKind::TakeAlways:
  case CastConsumptionKind::TakeOnSuccess: {
    auto loc = RegularLocation::getAutoGeneratedLocation();
    return {value.ensurePlusOne(SGF, loc), consumption};
  }
  }
  llvm_unreachable("covered switch");
}

static ConsumableManagedValue
emitReabstractedSubobject(SILGenFunction &SGF, SILLocation loc,
                          ConsumableManagedValue value,
                          const TypeLowering &valueTL,
                          AbstractionPattern abstraction,
                          CanType substFormalType) {
  // Return if there's no abstraction.  (The first condition is just
  // a fast path.)
  if (value.getType().getASTType() == substFormalType ||
      value.getType() == SGF.getLoweredType(substFormalType))
    return value;

  // Otherwise, turn to +1 and re-abstract.
  ManagedValue mv = SGF.getManagedValue(loc, value);
  return ConsumableManagedValue::forOwned(
           SGF.emitOrigToSubstValue(loc, mv, abstraction, substFormalType));
}

void PatternMatchEmission::emitTupleObjectDispatch(
    ArrayRef<RowToSpecialize> rows, ConsumableManagedValue src,
    const SpecializationHandler &handleCase,
    const FailureHandler &outerFailure) {
  // Construct the specialized rows.
  SmallVector<SpecializedRow, 4> specializedRows;
  specializedRows.resize(rows.size());
  for (unsigned i = 0, e = rows.size(); i != e; ++i) {
    specializedRows[i].RowIndex = rows[i].RowIndex;

    auto pattern = cast<TuplePattern>(rows[i].Pattern);
    for (auto &elt : pattern->getElements()) {
      specializedRows[i].Patterns.push_back(elt.getPattern());
    }
  }

  auto firstPat = rows[0].Pattern;
  SILLocation loc = firstPat;

  // Final consumption here will be either BorrowAlways or TakeAlways.
  ManagedValue v = src.getFinalManagedValue();

  SmallVector<ConsumableManagedValue, 8> destructured;
  SGF.B.emitDestructureValueOperation(
      loc, v, [&](unsigned index, ManagedValue v) {
        destructured.push_back({v, src.getFinalConsumption()});
      });

  // Since we did all of our work at +0, we just send down the outer failure.
  handleCase(destructured, specializedRows, outerFailure);
}

/// Perform specialized dispatch for tuples.
///
/// This is simple; all the tuples have the same structure.
void PatternMatchEmission::
emitTupleDispatch(ArrayRef<RowToSpecialize> rows, ConsumableManagedValue src,
                  const SpecializationHandler &handleCase,
                  const FailureHandler &outerFailure) {
  auto firstPat = rows[0].Pattern;
  SILLocation loc = firstPat;

  // If our source is an address that is loadable, perform a load_borrow.
  if (src.getType().isAddress() && src.getType().isLoadable(SGF.F)) {
    // We should only see take_on_success if we have a base type that is address
    // only.
    assert(src.getFinalConsumption() != CastConsumptionKind::TakeOnSuccess &&
           "Can only occur if base type is address only?!");
    src = {SGF.B.createLoadBorrow(loc, src.getFinalManagedValue()),
           CastConsumptionKind::BorrowAlways};
  }

  // Then if we have an object...
  if (src.getType().isObject()) {
    // Make sure that if we have a copy_on_success, non-trivial value that we do
    // not have a value with @owned ownership.
    assert((!src.getType().isTrivial(SGF.F) ||
            src.getFinalConsumption() != CastConsumptionKind::CopyOnSuccess ||
            src.getOwnershipKind() != OwnershipKind::Owned) &&
           "@owned value without cleanup + copy_on_success");

    // We should only see take_on_success if we have a base type that is address
    // only.
    assert(src.getFinalConsumption() != CastConsumptionKind::TakeOnSuccess &&
           "Can only occur if base type is address only?!");

    // Then perform a forward or reborrow destructure on the object.
    return emitTupleObjectDispatch(rows, src, handleCase, outerFailure);
  }

  // Construct the specialized rows.
  SmallVector<SpecializedRow, 4> specializedRows;
  specializedRows.resize(rows.size());
  for (unsigned i = 0, e = rows.size(); i != e; ++i) {
    specializedRows[i].RowIndex = rows[i].RowIndex;

    auto pattern = cast<TuplePattern>(rows[i].Pattern);
    for (auto &elt : pattern->getElements()) {
      specializedRows[i].Patterns.push_back(elt.getPattern());
    }
  }

  // At this point we know that we must have an address only type, since we
  // would have loaded it earlier.
  SILValue v = src.getFinalManagedValue().forward(SGF);
  assert(v->getType().isAddressOnly(SGF.F) &&
         "Loadable values were handled earlier");

  // The destructured tuple that we pass off to our sub pattern. This may
  // contain values that we have performed a load_borrow from subsequent to
  // "performing a SILGenPattern borrow".
  SmallVector<ConsumableManagedValue, 4> subPatternArgs;

  // An array of values that have the same underlying values as our
  // subPatternArgs, but may have a different cleanup and final consumption
  // kind. These are at +1 and are unforwarded.
  SmallVector<ConsumableManagedValue, 4> unforwardArgs;

  // Break down the values.
  auto tupleSILTy = v->getType();
  for (unsigned i : range(tupleSILTy.castTo<TupleType>()->getNumElements())) {
    SILType fieldTy = tupleSILTy.getTupleElementType(i);
    auto &fieldTL = SGF.getTypeLowering(fieldTy);

    SILValue member = SGF.B.createTupleElementAddr(loc, v, i, fieldTy);
    // Inline constructor.
    auto memberCMV = ([&]() -> ConsumableManagedValue {
      if (!fieldTL.isLoadable()) {
        // If we have an address only type, just get the managed
        // subobject.
        return getManagedSubobject(SGF, member, fieldTL,
                                   src.getFinalConsumption());
      }

      // If we have a loadable type, then we have a loadable sub-type of the
      // underlying address only tuple.
      auto memberMV = ManagedValue::forBorrowedAddressRValue(member);
      switch (src.getFinalConsumption()) {
      case CastConsumptionKind::TakeAlways: {
        // If our original source value is take always, perform a load [take].
        return {SGF.B.createLoadTake(loc, memberMV),
                CastConsumptionKind::TakeAlways};
      }
      case CastConsumptionKind::TakeOnSuccess: {
        // If we have a take_on_success, we propagate down the member as a +1
        // address value and do not load.
        //
        // DISCUSSION: Unforwarding objects violates ownership since
        // unforwarding relies on forwarding an aggregate into subvalues and
        // on failure disabling the subvalue cleanups and re-enabling the
        // cleanup for the aggregate (which was already destroyed). So we are
        // forced to use an address here so we can forward/unforward this
        // value. We maintain our invariants that loadable types are always
        // loaded and are never take on success by passing down to our
        // subPattern a borrow of this value. See below.
        return getManagedSubobject(SGF, member, fieldTL,
                                   src.getFinalConsumption());
      }
      case CastConsumptionKind::CopyOnSuccess:
      case CastConsumptionKind::BorrowAlways: {
        // We translate copy_on_success => borrow_always.
        auto memberMV = ManagedValue::forBorrowedAddressRValue(member);
        return {SGF.B.createLoadBorrow(loc, memberMV),
                CastConsumptionKind::BorrowAlways};
      }
      }
      llvm_unreachable("covered switch");
    }());

    // If we aren't loadable, add to the unforward array.
    if (!fieldTL.isLoadable()) {
      unforwardArgs.push_back(memberCMV);
    } else {
      // If we have a loadable type that we didn't load, we must have had a
      // take_on_success address. This means that our parent cleanup is
      // currently persistently active, so we needed to propagate an active +1
      // cleanup on our address so we can take if we actually succeed. That
      // being said, we do not want to pass objects with take_on_success into
      // the actual subtree. So we perform a load_borrow at this point. This
      // will ensure that we will always finish the end_borrow before we jumped
      // to a failure point, but at the same time the original +1 value will be
      // appropriately destroyed/forwarded around.
      if (memberCMV.getType().isAddress()) {
        unforwardArgs.push_back(memberCMV);
        auto val = memberCMV.getFinalManagedValue();
        memberCMV = {SGF.B.createLoadBorrow(loc, val),
                     CastConsumptionKind::BorrowAlways};
      }
    }
    subPatternArgs.push_back(memberCMV);
  }

  // Maybe revert to the original cleanups during failure branches.
  const FailureHandler *innerFailure = &outerFailure;
  FailureHandler specializedFailure = [&](SILLocation loc) {
    ArgUnforwarder unforwarder(SGF);
    unforwarder.unforwardBorrowedValues(src, unforwardArgs);
    outerFailure(loc);
  };
  if (ArgUnforwarder::requiresUnforwarding(SGF, src))
    innerFailure = &specializedFailure;

  // Recurse.
  handleCase(subPatternArgs, specializedRows, *innerFailure);
}

static CanType getTargetType(const RowToSpecialize &row) {
  auto type = cast<IsPattern>(row.Pattern)->getCastType();
  return type->getCanonicalType();
}

static ConsumableManagedValue
emitCastOperand(SILGenFunction &SGF, SILLocation loc,
                      ConsumableManagedValue src, CanType sourceType,
                      CanType targetType,
                      SmallVectorImpl<ConsumableManagedValue> &borrowedValues) {
  // Reabstract to the most general abstraction, and put it into a
  // temporary if necessary.

  // Figure out if we need the value to be in a temporary.
  bool requiresAddress =
    !canSILUseScalarCheckedCastInstructions(SGF.SGM.M, sourceType, targetType);

  AbstractionPattern abstraction = SGF.SGM.M.Types.getMostGeneralAbstraction();
  auto &srcAbstractTL = SGF.getTypeLowering(abstraction, sourceType);

  bool hasAbstraction = (src.getType() != srcAbstractTL.getLoweredType());

  // Fast path: no re-abstraction required.
  if (!hasAbstraction && (!requiresAddress || src.getType().isAddress())) {
    return src;
  }

  // We know that we must have a loadable type at this point since address only
  // types do not need reabstraction and are addresses. So we should have exited
  // above already.
  assert(src.getType().isLoadable(SGF.F) &&
         "Should have a loadable value at this point");

  // Since our finalValue is loadable, we could not have had a take_on_success
  // here.
  assert(src.getFinalConsumption() != CastConsumptionKind::TakeOnSuccess &&
         "Loadable types can not have take_on_success?!");

  std::unique_ptr<TemporaryInitialization> init;
  SGFContext ctx;
  if (requiresAddress) {
    init = SGF.emitTemporary(loc, srcAbstractTL);
    ctx = SGFContext(init.get());
  }

  // This will always produce a +1 take always value no matter what src's
  // ownership is.
  ManagedValue finalValue = SGF.getManagedValue(loc, src);
  if (hasAbstraction) {
    // Reabstract the value if we need to. This should produce a +1 value as
    // well.
    finalValue =
        SGF.emitSubstToOrigValue(loc, finalValue, abstraction, sourceType, ctx);
  }
  assert(finalValue.isPlusOneOrTrivial(SGF));

  // If we at this point do not require an address, return final value. We know
  // that it is a +1 take always value.
  if (!requiresAddress) {
    return ConsumableManagedValue::forOwned(finalValue);
  }

  // At this point, we know that we have a non-address only type since we are
  // materializing an object into memory and addresses can not be stored into
  // memory.
  SGF.B.emitStoreValueOperation(loc, finalValue.forward(SGF),
                                init->getAddress(),
                                StoreOwnershipQualifier::Init);
  init->finishInitialization(SGF);

  // We know that either our initial value was already take_always or we made a
  // copy of the underlying value. In either case, we now have a take_always +1
  // value.
  return ConsumableManagedValue::forOwned(init->getManagedAddress());
}

/// Perform specialized dispatch for a sequence of IsPatterns.
void PatternMatchEmission::emitIsDispatch(ArrayRef<RowToSpecialize> rows,
                                      ConsumableManagedValue src,
                                      const SpecializationHandler &handleCase,
                                      const FailureHandler &failure) {
  CanType sourceType = rows[0].Pattern->getType()->getCanonicalType();
  CanType targetType = getTargetType(rows[0]);

  // Make any abstraction modifications necessary for casting.
  SmallVector<ConsumableManagedValue, 4> borrowedValues;
  ConsumableManagedValue operand = emitCastOperand(
      SGF, rows[0].Pattern, src, sourceType, targetType, borrowedValues);

  // Emit the 'is' check.

  // Build the specialized-rows array.
  SmallVector<SpecializedRow, 4> specializedRows;
  specializedRows.reserve(rows.size());
  for (auto &row : rows) {
    assert(getTargetType(row) == targetType
           && "can only specialize on one type at a time");
    auto is = cast<IsPattern>(row.Pattern);
    specializedRows.push_back({});
    specializedRows.back().RowIndex = row.RowIndex;
    specializedRows.back().Patterns.push_back(is->getSubPattern());
  }

  SILLocation loc = rows[0].Pattern;

  ConsumableManagedValue castOperand = operand.asBorrowedOperand(SGF, loc);

  // Chain inner failures onto the outer failure.
  const FailureHandler *innerFailure = &failure;
  FailureHandler specializedFailure = [&](SILLocation loc) {
    ArgUnforwarder unforwarder(SGF);
    unforwarder.unforwardBorrowedValues(src, borrowedValues);
    failure(loc);
  };
  if (ArgUnforwarder::requiresUnforwarding(SGF, src))
    innerFailure = &specializedFailure;
  
  // Perform a conditional cast branch.
  SGF.emitCheckedCastBranch(
      loc, castOperand, sourceType, targetType, SGFContext(),
      // Success block: recurse.
      [&](ManagedValue castValue) {
        handleCase(ConsumableManagedValue::forOwned(castValue), specializedRows,
                   *innerFailure);
        assert(!SGF.B.hasValidInsertionPoint() && "did not end block");
      },
      // Failure block: branch out to the continuation block.
      [&](std::optional<ManagedValue> mv) { (*innerFailure)(loc); },
      rows[0].Count);
}

namespace {
  struct CaseInfo {
    SmallVector<SpecializedRow, 2> SpecializedRows;
    Pattern *FirstMatcher;
    bool Irrefutable = false;
  };

  class CaseBlocks {
    // These vectors are completely parallel, but the switch instructions want
    // only the first two, so we split them up.
    SmallVector<std::pair<EnumElementDecl *, SILBasicBlock *>, 4> CaseBBs;
    SmallVector<ProfileCounter, 4> CaseCounts;
    SmallVector<CaseInfo, 4> CaseInfos;
    SILBasicBlock *DefaultBB = nullptr;

  public:
    /// Create destination blocks for switching over the cases in an enum
    /// defined by \p rows.
    CaseBlocks(SILGenFunction &SGF,
               ArrayRef<RowToSpecialize> rows,
               CanType sourceType,
               SILBasicBlock *curBB);

    ArrayRef<std::pair<EnumElementDecl *, SILBasicBlock *>>
    getCaseBlocks() const {
      return CaseBBs;
    }

    ArrayRef<ProfileCounter> getCounts() const { return CaseCounts; }

    SILBasicBlock *getDefaultBlock() const { return DefaultBB; }

    void forEachCase(llvm::function_ref<void(EnumElementDecl *,
                                             SILBasicBlock *,
                                             const CaseInfo &)> op) const {
      for_each(CaseBBs, CaseInfos,
               [op](std::pair<EnumElementDecl *, SILBasicBlock *> casePair,
                    const CaseInfo &info) {
        op(casePair.first, casePair.second, info);
      });
    }

    bool hasAnyRefutableCase() const {
      return llvm::any_of(CaseInfos, [](const CaseInfo &info) {
        return !info.Irrefutable;
      });
    }
  };
} // end anonymous namespace

CaseBlocks::CaseBlocks(
    SILGenFunction &SGF,
    ArrayRef<RowToSpecialize> rows,
    CanType sourceType,
    SILBasicBlock *curBB) {

  CaseBBs.reserve(rows.size());
  CaseInfos.reserve(rows.size());
  CaseCounts.reserve(rows.size());

  auto enumDecl = sourceType.getEnumOrBoundGenericEnum();

  llvm::SmallDenseMap<EnumElementDecl *, unsigned, 16> caseToIndex;
  for (auto &row : rows) {
    EnumElementDecl *formalElt;
    Pattern *subPattern = nullptr;
    if (auto eep = dyn_cast<EnumElementPattern>(row.Pattern)) {
      formalElt = eep->getElementDecl();
      subPattern = eep->getSubPattern();
    } else {
      auto *osp = cast<OptionalSomePattern>(row.Pattern);
      formalElt = osp->getElementDecl();
      subPattern = osp->getSubPattern();
    }
    assert(formalElt->getParentEnum() == enumDecl);

    unsigned index = CaseInfos.size();
    auto insertionResult = caseToIndex.insert({formalElt, index});
    if (!insertionResult.second) {
      index = insertionResult.first->second;
    } else {
      curBB = SGF.createBasicBlockAfter(curBB);
      CaseBBs.push_back({formalElt, curBB});
      CaseInfos.push_back(CaseInfo());
      CaseInfos.back().FirstMatcher = row.Pattern;
      CaseCounts.push_back(row.Count);
    }
    assert(caseToIndex[formalElt] == index);
    assert(CaseBBs[index].first == formalElt);

    auto &info = CaseInfos[index];
    info.Irrefutable = (info.Irrefutable || row.Irrefutable);
    info.SpecializedRows.push_back(SpecializedRow());
    auto &specRow = info.SpecializedRows.back();
    specRow.RowIndex = row.RowIndex;

    // Use the row pattern, if it has one.
    if (subPattern) {
      specRow.Patterns.push_back(subPattern);
      // It's also legal to write:
      //   case .Some { ... }
      // which is an implicit wildcard.
    } else {
      specRow.Patterns.push_back(nullptr);
    }
  }

  assert(CaseBBs.size() == CaseInfos.size());

  // Check to see if the enum may have values beyond the cases we can see
  // at compile-time. This includes future cases (for resilient enums) and
  // random values crammed into C enums.
  bool canAssumeExhaustive =
      enumDecl->isEffectivelyExhaustive(SGF.getModule().getSwiftModule(),
                                        SGF.F.getResilienceExpansion());
  if (canAssumeExhaustive) {
    // Check that Sema didn't let any cases slip through.
    canAssumeExhaustive = llvm::all_of(enumDecl->getAllElements(),
                                       [&](const EnumElementDecl *elt) {
      return caseToIndex.count(elt);
    });
  }

  if (!canAssumeExhaustive)
    DefaultBB = SGF.createBasicBlockAfter(curBB);
}

/// Perform specialized dispatch for a sequence of EnumElementPattern or an
/// OptionalSomePattern.
void PatternMatchEmission::emitEnumElementObjectDispatch(
    ArrayRef<RowToSpecialize> rows, ConsumableManagedValue src,
    const SpecializationHandler &handleCase, const FailureHandler &outerFailure,
    ProfileCounter defaultCastCount) {
  assert(src.getFinalConsumption() != CastConsumptionKind::TakeOnSuccess &&
         "SIL ownership does not support TakeOnSuccess");

  CanType sourceType = rows[0].Pattern->getType()->getCanonicalType();

  // Collect the cases and specialized rows.
  CaseBlocks blocks{SGF, rows, sourceType, SGF.B.getInsertionBB()};

  RegularLocation loc(PatternMatchStmt, rows[0].Pattern, SGF.SGM.M);
  SILValue srcValue = src.getFinalManagedValue().forward(SGF);
  auto *sei = SGF.B.createSwitchEnum(loc, srcValue, blocks.getDefaultBlock(),
                                     blocks.getCaseBlocks(), blocks.getCounts(),
                                     defaultCastCount);

  // Okay, now emit all the cases.
  blocks.forEachCase([&](EnumElementDecl *elt, SILBasicBlock *caseBB,
                         const CaseInfo &caseInfo) {
    SILLocation loc = caseInfo.FirstMatcher;
    auto &specializedRows = caseInfo.SpecializedRows;

    SGF.B.setInsertionPoint(caseBB);

    // We're in conditionally-executed code; enter a scope.
    Scope scope(SGF.Cleanups, CleanupLocation(loc));

    // Create a BB argument or 'unchecked_take_enum_data_addr'
    // instruction to receive the enum case data if it has any.

    SILType eltTy;
    bool hasNonVoidAssocValue = false;
    bool hasAssocValue = elt->hasAssociatedValues();
    ManagedValue caseResult;
    auto caseConsumption = CastConsumptionKind::BorrowAlways;
    if (hasAssocValue) {
      eltTy = src.getType().getEnumElementType(elt, SGF.SGM.M,
                                               SGF.getTypeExpansionContext());
      hasNonVoidAssocValue = !eltTy.getASTType()->isVoid();

      caseResult = SGF.B.createForwardedTermResult(eltTy);

      // The consumption kind of a switch enum's source and its case result can
      // differ. For example, a TakeAlways source may have no ownership because
      // it holds a trivial value, but its nontrivial result may be
      // Guaranteed. For valid OSSA, we reconcile it with the case result
      // value's ownership here.
      if (caseResult.getOwnershipKind() == OwnershipKind::Owned)
        caseConsumption = CastConsumptionKind::TakeAlways;
    }

    ConsumableManagedValue eltCMV;

    // Void (i.e. empty) cases.
    //
    if (!hasNonVoidAssocValue) {
      // Inline constructor.
      eltCMV = [&]() -> ConsumableManagedValue {
        // If we have an associated value, rather than no payload at all, we
        // still need to create the argument. So do that instead of creating the
        // empty-tuple. Otherwise, we need to create undef or the empty-tuple.
        if (hasAssocValue) {
          return {caseResult, caseConsumption};
        }

        // Otherwise, try to avoid making an empty tuple value if it's obviously
        // going to be ignored. This assumes that we won't even try to touch the
        // value in such cases, although we may touch the cleanup (enough to see
        // that it's not present).
        bool hasNonAny =
            llvm::any_of(specializedRows, [&](const SpecializedRow &row) {
              auto *p = row.Patterns[0];
              return p && !isa<AnyPattern>(p->getSemanticsProvidingPattern());
            });
        if (hasNonAny) {
          return ConsumableManagedValue::forUnmanaged(SGF.emitEmptyTuple(loc));
        }

        return ConsumableManagedValue::forUnmanaged(
            SILUndef::get(SGF.F, SGF.SGM.Types.getEmptyTupleType()));
      }();

      // Okay, specialize on the argument.
    } else {
      auto *eltTL = &SGF.getTypeLowering(eltTy);

      eltCMV = {caseResult, caseConsumption};

      // If the payload is boxed, project it.
      if (elt->isIndirect() || elt->getParentEnum()->isIndirect()) {
        ManagedValue boxedValue =
            SGF.B.createProjectBox(loc, eltCMV.getFinalManagedValue(), 0);
        eltTL = &SGF.getTypeLowering(boxedValue.getType());
        if (eltTL->isLoadable() || !SGF.silConv.useLoweredAddresses()) {
          boxedValue = SGF.B.createLoadBorrow(loc, boxedValue);
          eltCMV = {boxedValue, CastConsumptionKind::BorrowAlways};
        } else {
          // Otherwise, we have an address only payload and we use
          // copy on success instead.
          eltCMV = {boxedValue, CastConsumptionKind::CopyOnSuccess};
        }
      }

      // Reabstract to the substituted type, if needed.
      CanType substEltTy =
          sourceType
              ->getTypeOfMember(elt, elt->getPayloadInterfaceType())
              ->getCanonicalType();

      AbstractionPattern origEltTy =
          (elt->getParentEnum()->isOptionalDecl()
               ? AbstractionPattern(substEltTy)
               : SGF.SGM.M.Types.getAbstractionPattern(elt));

      // If we reabstracted, we may have a +1 value returned. We are ok with
      // that as long as it is TakeAlways.
      eltCMV = emitReabstractedSubobject(SGF, loc, eltCMV, *eltTL, origEltTy,
                                         substEltTy);
    }

    handleCase(eltCMV, specializedRows, outerFailure);
    assert(!SGF.B.hasValidInsertionPoint() && "did not end block");
  });

  // Emit the default block if we needed one.
  if (SILBasicBlock *defaultBB = blocks.getDefaultBlock()) {
    SGF.B.setInsertionPoint(defaultBB);
    ManagedValue::forForwardedRValue(SGF, sei->createDefaultResult());
    outerFailure(rows.back().Pattern);
  }
}

/// Perform specialized dispatch for a sequence of EnumElementPattern or an
/// OptionalSomePattern.
void PatternMatchEmission::emitEnumElementDispatch(
    ArrayRef<RowToSpecialize> rows, ConsumableManagedValue src,
    const SpecializationHandler &handleCase, const FailureHandler &outerFailure,
    ProfileCounter defaultCaseCount) {
  // Why do we need to do this here (I just cargo-culted this).
  RegularLocation loc(PatternMatchStmt, rows[0].Pattern, SGF.SGM.M);

  // If our source is an address that is loadable, perform a load_borrow.
  if (src.getType().isAddress() && src.getType().isLoadable(SGF.F)) {
    assert(src.getFinalConsumption() != CastConsumptionKind::TakeOnSuccess &&
           "Can only have take_on_success with address only values");
    src = {SGF.B.createLoadBorrow(loc, src.getFinalManagedValue()),
           CastConsumptionKind::BorrowAlways};
  }

  // If we have an object...
  if (src.getType().isObject()) {
    // Do a quick assert that we do not have take_on_success. This should only
    // be passed take_on_success if src is an address only type.
    assert(src.getFinalConsumption() != CastConsumptionKind::TakeOnSuccess &&
           "Can only have take_on_success with address only values");
    if (src.getType().isAddressOnly(SGF.F) &&
        src.getOwnershipKind() == OwnershipKind::Guaranteed) {
      // If it's an opaque value with guaranteed ownership, we need to copy.
      src = src.copy(SGF, PatternMatchStmt);
    }

    // Finally perform the enum element dispatch.
    return emitEnumElementObjectDispatch(rows, src, handleCase, outerFailure,
                                         defaultCaseCount);
  }

  // After this point we now that we must have an address only type.
  assert(src.getType().isAddressOnly(SGF.F) &&
         "Should have an address only type here");
  assert(!UncheckedTakeEnumDataAddrInst::isDestructive(src.getType().getEnumOrBoundGenericEnum(),
                                                       SGF.getModule()) &&
         "address only enum projection should never be destructive");

  CanType sourceType = rows[0].Pattern->getType()->getCanonicalType();

  // Collect the cases and specialized rows.
  CaseBlocks blocks{SGF, rows, sourceType, SGF.B.getInsertionBB()};

  // We (used to) lack a SIL instruction to nondestructively project data from an
  // address-only enum, so we can only do so in place if we're allowed to take
  // the source always. Copy the source if we can't.
  //
  // TODO: This should no longer be necessary now that we guarantee that
  // potentially address-only enums never use spare bit optimization.
  switch (src.getFinalConsumption()) {
  case CastConsumptionKind::TakeAlways:
  case CastConsumptionKind::CopyOnSuccess:
  case CastConsumptionKind::BorrowAlways:
    // No change to src necessary.
    break;

  case CastConsumptionKind::TakeOnSuccess:
    // If any of the specialization cases is refutable, we must copy.
    if (!blocks.hasAnyRefutableCase())
      break;

    src = ConsumableManagedValue(
        ManagedValue::forUnmanagedOwnedValue(src.getValue()),
        CastConsumptionKind::CopyOnSuccess);
    break;
  }

  // Emit the switch_enum_addr instruction.
  SGF.B.createSwitchEnumAddr(loc, src.getValue(), blocks.getDefaultBlock(),
                             blocks.getCaseBlocks(), blocks.getCounts(),
                             defaultCaseCount);

  // Okay, now emit all the cases.
  blocks.forEachCase([&](EnumElementDecl *eltDecl, SILBasicBlock *caseBB,
                         const CaseInfo &caseInfo) {
    SILLocation loc = caseInfo.FirstMatcher;
    auto &specializedRows = caseInfo.SpecializedRows;

    SGF.B.setInsertionPoint(caseBB);

    // We need to make sure our cleanup stays around long enough for us to emit
    // our destroy, so setup a cleanup state restoration scope for each case.
    CleanupStateRestorationScope srcScope(SGF.Cleanups);
    forwardIntoIrrefutableSubtree(SGF, srcScope, src);

    // We're in conditionally-executed code; enter a scope.
    Scope scope(SGF.Cleanups, CleanupLocation(loc));

    // Create a BB argument or 'unchecked_take_enum_data_addr'
    // instruction to receive the enum case data if it has any.
    SILType eltTy;
    bool hasElt = false;
    if (eltDecl->hasAssociatedValues()) {
      eltTy = src.getType().getEnumElementType(eltDecl, SGF.SGM.M,
                                               SGF.getTypeExpansionContext());
      hasElt = !eltTy.getASTType()->isVoid();
    }

    ConsumableManagedValue eltCMV, origCMV;

    // Empty cases.  Try to avoid making an empty tuple value if it's
    // obviously going to be ignored.  This assumes that we won't even
    // try to touch the value in such cases, although we may touch the
    // cleanup (enough to see that it's not present).
    if (!hasElt) {
      bool hasNonAny = false;
      for (auto &specRow : specializedRows) {
        auto pattern = specRow.Patterns[0];
        if (pattern &&
            !isa<AnyPattern>(pattern->getSemanticsProvidingPattern())) {
          hasNonAny = true;
          break;
        }
      }

      // Forward src along this path so we don't emit a destroy_addr on our
      // subject value for this case.
      //
      // FIXME: Do we actually want to do this? SILGen tests today assume this
      // pattern. It might be worth leaving the destroy_addr there to create
      // additional liveness information. For now though, we maintain the
      // current behavior.
      src.getFinalManagedValue().forward(SGF);

      SILValue result;
      if (hasNonAny) {
        result = SGF.emitEmptyTuple(loc);
      } else {
        result = SILUndef::get(&SGF.F, SGF.SGM.Types.getEmptyTupleType());
      }
      origCMV = ConsumableManagedValue::forUnmanaged(result);
      eltCMV = origCMV;

    // Okay, specialize on the argument.
    } else {
      auto *eltTL = &SGF.getTypeLowering(eltTy);

      // Normally we'd just use the consumption of the source
      // because the difference between TakeOnSuccess and TakeAlways
      // doesn't matter for irrefutable rows.  But if we need to
      // re-abstract, we'll see a lot of benefit from figuring out
      // that we can use TakeAlways here.
      auto eltConsumption = src.getFinalConsumption();
      if (caseInfo.Irrefutable &&
          eltConsumption == CastConsumptionKind::TakeOnSuccess) {
        eltConsumption = CastConsumptionKind::TakeAlways;
      }

      ManagedValue eltValue;
      // We can only project destructively from an address-only enum, so
      // copy the value if we can't consume it.
      // TODO: Copying should be avoidable now that we guarantee that address-
      // only enums never use spare bit optimization.
      switch (eltConsumption) {
      case CastConsumptionKind::TakeAlways: {
        auto finalValue = src.getFinalManagedValue();
        eltValue = SGF.B.createUncheckedTakeEnumDataAddr(loc, finalValue,
                                                         eltDecl, eltTy);
        break;
      }
      case CastConsumptionKind::BorrowAlways: {
        eltValue = ManagedValue::forBorrowedAddressRValue(
          SGF.B.createUncheckedTakeEnumDataAddr(loc, src.getValue(),
                                                eltDecl, eltTy));
        break;
      }
      case CastConsumptionKind::CopyOnSuccess: {
        auto temp = SGF.emitTemporary(loc, SGF.getTypeLowering(src.getType()));
        SGF.B.createCopyAddr(loc, src.getValue(), temp->getAddress(), IsNotTake,
                             IsInitialization);
        temp->finishInitialization(SGF);

        // We can always take from the copy.
        eltConsumption = CastConsumptionKind::TakeAlways;
        eltValue = SGF.B.createUncheckedTakeEnumDataAddr(
            loc, temp->getManagedAddress(), eltDecl, eltTy);
        break;
      }

      // We can't conditionally take, since UncheckedTakeEnumDataAddr
      // invalidates the enum.
      case CastConsumptionKind::TakeOnSuccess:
        llvm_unreachable("not allowed");
      }

      // If we have a loadable payload despite the enum being address only, load
      // the value. This invariant makes it easy to specialize code for
      // ownership.
      if (eltTL->isLoadable()) {
        // If we do not have a loadable value, just use getManagedSubobject
        // Load a loadable data value.
        switch (eltConsumption) {
        case CastConsumptionKind::CopyOnSuccess:
          eltValue = SGF.B.createLoadBorrow(loc, eltValue);
          eltConsumption = CastConsumptionKind::BorrowAlways;
          break;

        case CastConsumptionKind::TakeAlways:
          eltValue = SGF.B.createLoadTake(loc, eltValue);
          break;
          
        case CastConsumptionKind::BorrowAlways:
          eltValue = SGF.B.createLoadBorrow(loc, eltValue);
          break;
          
        case CastConsumptionKind::TakeOnSuccess:
          llvm_unreachable("not possible");
        }
        origCMV = {eltValue, eltConsumption};
      } else {
        origCMV = getManagedSubobject(SGF, eltValue, eltConsumption);
      }

      eltCMV = origCMV;

      // If the payload is boxed, project it.
      if (eltDecl->isIndirect() || eltDecl->getParentEnum()->isIndirect()) {
        ManagedValue boxedValue =
          SGF.B.createProjectBox(loc, origCMV.getFinalManagedValue(), 0);
        eltTL = &SGF.getTypeLowering(boxedValue.getType());
        if (eltTL->isLoadable()) {
          boxedValue = SGF.B.createLoadBorrow(loc, boxedValue);
          eltCMV = {boxedValue, CastConsumptionKind::BorrowAlways};
        } else {
          // The boxed value may be shared, so we always have to copy it.
          eltCMV = getManagedSubobject(SGF, boxedValue.getValue(), *eltTL,
                                       CastConsumptionKind::CopyOnSuccess);
        }
      }

      // Reabstract to the substituted type, if needed.
      CanType substEltTy =
        sourceType->getTypeOfMember(eltDecl,
                                    eltDecl->getPayloadInterfaceType())
                  ->getCanonicalType();

      AbstractionPattern origEltTy =
          (eltDecl->getParentEnum()->isOptionalDecl()
               ? AbstractionPattern(substEltTy)
               : SGF.SGM.M.Types.getAbstractionPattern(eltDecl));

      eltCMV = emitReabstractedSubobject(SGF, loc, eltCMV, *eltTL,
                                         origEltTy, substEltTy);
    }

    const FailureHandler *innerFailure = &outerFailure;
    FailureHandler specializedFailure = [&](SILLocation loc) {
      ArgUnforwarder unforwarder(SGF);
      unforwarder.unforwardBorrowedValues(src, origCMV);
      outerFailure(loc);
    };
    if (ArgUnforwarder::requiresUnforwarding(SGF, src))
      innerFailure = &specializedFailure;

    handleCase(eltCMV, specializedRows, *innerFailure);
    assert(!SGF.B.hasValidInsertionPoint() && "did not end block");
  });

  // Emit the default block if we needed one.
  if (SILBasicBlock *defaultBB = blocks.getDefaultBlock()) {
    SGF.B.setInsertionPoint(defaultBB);
    outerFailure(rows.back().Pattern);
  }
}

/// Perform specialized dispatch for a sequence of EnumElementPattern or an
/// OptionalSomePattern.
void PatternMatchEmission::
emitBoolDispatch(ArrayRef<RowToSpecialize> rows, ConsumableManagedValue src,
                 const SpecializationHandler &handleCase,
                 const FailureHandler &outerFailure) {

  struct CaseInfo {
    Pattern *FirstMatcher;
    bool Irrefutable = false;
    SmallVector<SpecializedRow, 2> SpecializedRows;
  };

  SILBasicBlock *curBB = SGF.B.getInsertionBB();
  auto &Context = SGF.getASTContext();

  // Collect the cases and specialized rows.
  //
  // These vectors are completely parallel, but the switch
  // instructions want only the first information, so we split them up.
  SmallVector<std::pair<SILValue, SILBasicBlock*>, 4> caseBBs;
  SmallVector<CaseInfo, 4> caseInfos;
  SILBasicBlock *defaultBB = nullptr;

  caseBBs.reserve(rows.size());
  caseInfos.reserve(rows.size());

  // Create destination blocks for all the cases.
  unsigned caseToIndex[2] = { ~0U, ~0U };
  for (auto &row : rows) {
    bool isTrue = cast<BoolPattern>(row.Pattern)->getValue();

    unsigned index = caseInfos.size();
    if (caseToIndex[isTrue] != ~0U) {
      // We already had an entry for this bool value.
      index = caseToIndex[isTrue];
    } else {
      caseToIndex[isTrue] = index;
    
      curBB = SGF.createBasicBlockAfter(curBB);
      auto *IL = SGF.B.createIntegerLiteral(PatternMatchStmt,
                                    SILType::getBuiltinIntegerType(1, Context),
                                            isTrue ? 1 : 0);
      caseBBs.push_back({SILValue(IL), curBB});
      caseInfos.resize(caseInfos.size() + 1);
      caseInfos.back().FirstMatcher = row.Pattern;
    }

    auto &info = caseInfos[index];
    info.Irrefutable = (info.Irrefutable || row.Irrefutable);
    info.SpecializedRows.resize(info.SpecializedRows.size() + 1);
    auto &specRow = info.SpecializedRows.back();
    specRow.RowIndex = row.RowIndex;

    specRow.Patterns.push_back(nullptr);
  }

  assert(caseBBs.size() == caseInfos.size());

  // Check to see if we need a default block.
  if (caseBBs.size() < 2)
    defaultBB = SGF.createBasicBlockAfter(curBB);

  // Emit the switch_value
  RegularLocation loc(PatternMatchStmt, rows[0].Pattern, SGF.SGM.M);
  SILValue srcValue = src.getFinalManagedValue().forward(SGF);

  // Extract the i1 from the Bool struct.
  auto i1Value = SGF.emitUnwrapIntegerResult(loc, srcValue);
  SGF.B.createSwitchValue(loc, i1Value, defaultBB, caseBBs);

  // Okay, now emit all the cases.
  for (unsigned i = 0, e = caseInfos.size(); i != e; ++i) {
    auto &caseInfo = caseInfos[i];
    auto &specializedRows = caseInfo.SpecializedRows;

    SILBasicBlock *caseBB = caseBBs[i].second;
    SGF.B.setInsertionPoint(caseBB);

    // We're in conditionally-executed code; enter a scope.
    Scope scope(SGF.Cleanups, CleanupLocation(loc));

    SILValue result = SILUndef::get(&SGF.F, SGF.SGM.Types.getEmptyTupleType());
    ConsumableManagedValue CMV =
      ConsumableManagedValue::forUnmanaged(result);

    handleCase(CMV, specializedRows, outerFailure);
    assert(!SGF.B.hasValidInsertionPoint() && "did not end block");
  }

  // Emit the default block if we needed one.
  if (defaultBB) {
    SGF.B.setInsertionPoint(defaultBB);
    outerFailure(rows.back().Pattern);
  }
}

/// Emit destructive case blocks.
void PatternMatchEmission::emitDestructiveCaseBlocks() {
  // We must not be in the middle of emitting any block at this point.
  assert(!SGF.B.hasValidInsertionPoint());

  // Unwind the borrow scope for the subject. We're going to start picking it
  // apart in the case bodies now.
  SGF.Cleanups.endNoncopyablePatternMatchBorrow(EndNoncopyableBorrowDest,
                                                PatternMatchStmt,
                                                /* pop cleanups*/ true);
  
  // Visitor to consume a value while binding pattern variables out of it.
  // The conditions tested by the pattern are assumed to be true because
  // the match has already happened at this point.
  class ConsumingPatternBindingVisitor
    : public PatternVisitor<ConsumingPatternBindingVisitor, void, ManagedValue>
  {
    PatternMatchEmission &emission;
    CaseStmt *stmt;
  public:
    ConsumingPatternBindingVisitor(PatternMatchEmission &emission, CaseStmt *stmt)
      : emission(emission), stmt(stmt)
    {}
    
    VarDecl *getMatchingCaseVarDecl(VarDecl *patternVar) {
      for (auto *caseVar : stmt->getCaseBodyVariables()) {
        if (patternVar->hasName()
            && patternVar->getName() == caseVar->getName()) {
          return caseVar;
        }
      }
      return nullptr;
    }
  
    void visitNamedPattern(NamedPattern *p, ManagedValue mv) {
      // Find the matching case block variable for the pattern variable.
      auto caseVar = getMatchingCaseVarDecl(p->getDecl());
      if (!caseVar) {
        return;
      }
      
      // Set up a variable binding for it.
      // TODO: Handle multiple case pattern blocks.
      emission.bindVariable(p, caseVar,
                   ConsumableManagedValue(mv, CastConsumptionKind::TakeAlways),
                   /*irrefutable*/ true, /*hasMultipleItems*/ false);

      // Emit a debug description for the variable, nested within a scope
      // for the pattern match.
      SILDebugVariable dbgVar(caseVar->isLet(), /*ArgNo=*/0);
      emission.SGF.B
        .emitDebugDescription(caseVar, emission.SGF.VarLocs[caseVar].value,
                              dbgVar);
    }
    
    void visitTuplePattern(TuplePattern *p, ManagedValue mv) {
      auto &SGF = emission.SGF;
      // Destructure the tuple and bind its components.
      if (mv.getType().isObject()) {
        auto destructure = SGF.B.createDestructureTuple(p, mv.forward(SGF));
        
        for (unsigned i = 0, e = p->getNumElements(); i < e; ++i) {
          auto elementVal = destructure->getAllResults()[i];
          visit(p->getElement(i).getPattern(),
                SGF.emitManagedRValueWithCleanup(elementVal));
        }
      } else {
        auto baseAddr = mv.forward(emission.SGF);
        for (unsigned i = 0, e = p->getNumElements(); i < e; ++i) {
          SILValue element = SGF.B.createTupleElementAddr(p, baseAddr, i);
          if (element->getType().isLoadable(SGF.F)) {
            element = SGF.B.createLoad(p, element, LoadOwnershipQualifier::Take);
          }
          visit(p->getElement(i).getPattern(),
                SGF.emitManagedRValueWithCleanup(element));
        }
      }
    }
    
    void visitIsPattern(IsPattern *p, ManagedValue mv) {
      // TODO
      llvm_unreachable("cast pattern in noncopyable pattern match not implemented");
    }
    
    void visitEnumProjection(ManagedValue mv,
                             EnumElementDecl *enumCase,
                             SILLocation loc,
                             Pattern *subPattern) {
      if (!enumCase->hasAssociatedValues()) {
        // Nothing to do if there's no payload to match.
        return;
      }
                             
      auto &SGF = emission.SGF;

      // Force-project the enum payload. We can assume that the case tag
      // matches because the pattern match has already happened.
      if (mv.getType().isObject()) {
        auto payload = SGF.B.createUncheckedEnumData(loc, mv.forward(SGF),
                                                     enumCase);
        visit(subPattern,
              SGF.emitManagedRValueWithCleanup(payload));
      } else {
        SILValue payload = SGF.B.createUncheckedTakeEnumDataAddr(loc,
                                                          mv.forward(SGF),
                                                          enumCase);
        if (payload->getType().isLoadable(SGF.F)) {
          payload = SGF.B.createLoad(loc, payload,
                                     payload->getType().isTrivial(SGF.F)
                                         ? LoadOwnershipQualifier::Trivial
                                         : LoadOwnershipQualifier::Take);
        }
        visit(subPattern,
              SGF.emitManagedRValueWithCleanup(payload));
      }
    }
    
    void visitEnumElementPattern(EnumElementPattern *p, ManagedValue mv) {
      visitEnumProjection(mv, p->getElementDecl(), p, p->getSubPattern());
    }
    
    void visitOptionalSomePattern(OptionalSomePattern *p, ManagedValue mv) {
      visitEnumProjection(mv, p->getElementDecl(), p, p->getSubPattern());
    }
    
    // Drop subpatterns that can't bind anything.

    void visitExprPattern(ExprPattern *P, ManagedValue mv) {
      // Drop the value. The expr pattern conditions were already checked and
      // there's nothing to bind.
    }
    void visitBoolPattern(BoolPattern *P, ManagedValue mv) {
      // No bindings.
    }
    void visitAnyPattern(AnyPattern *P, ManagedValue mv) {
      // Drop the value.
    }

    // Pass through decorative patterns.
    
    void visitParenPattern(ParenPattern *P, ManagedValue mv) {
      return visit(P->getSubPattern(), mv);
    }
    void visitBindingPattern(BindingPattern *P, ManagedValue mv) {
      return visit(P->getSubPattern(), mv);
    }
    void visitTypedPattern(TypedPattern *P, ManagedValue mv) {
      return visit(P->getSubPattern(), mv);
    }
  };
  
  // Now we can start destructively binding the value.
  for (auto &casePattern : DestructiveCases) {
    CaseStmt *stmt;
    Pattern *pattern;
    SILBasicBlock *bb;
    std::tie(stmt, pattern, bb) = casePattern;

    SGF.B.setInsertionPoint(bb);
    SGF.emitProfilerIncrement(stmt);
    
    // Restore the original subject's cleanup when we're done with this block so
    // it can be destructured again in other blocks.
    CleanupStateRestorationScope restoreSubject(SGF.Cleanups);
    if (NoncopyableConsumableValue.hasCleanup()) {
      restoreSubject.pushCleanupState(NoncopyableConsumableValue.getCleanup(),
                                      CleanupState::PersistentlyActive);
    }
    
    // Create a scope to break down the subject value.
    Scope caseScope(SGF, pattern);
    
    ManagedValue subject;
    if (NoncopyableConsumableValue.getType().isAddress()) {
      // If the subject value is in memory, enter a deinit access for the memory.
      // This saves the move-only-checker from trying to analyze the payload
      // decomposition as a potential partial consume. We always fully consume
      // the subject on this path.
      subject = SGF.B.createOpaqueConsumeBeginAccess(pattern,
                                                     NoncopyableConsumableValue);
    } else {
      // Clone the original subject's cleanup state so that it will be reliably
      // consumed in this scope, while leaving the original for other case
      // blocks to re-consume.
      subject = SGF.emitManagedRValueWithCleanup(
                                      NoncopyableConsumableValue.forward(SGF));
    }
    

    // TODO: handle fallthroughs and multiple cases bindings
    // In those cases we'd need to forward bindings through the shared case
    // destination blocks.
    if (stmt->hasFallthroughDest()
        || stmt->getCaseLabelItems().size() != 1) {
      // This should already have been diagnosed as unsupported, so just emit
      // an unreachable here.
      SGF.B.createUnreachable(stmt);
      continue;
    }

    // Bind variables from the pattern.
    if (stmt->hasCaseBodyVariables()) {
      ConsumingPatternBindingVisitor(*this, stmt)
        .visit(pattern, subject);
    }

    // Emit the case body.
    emitCaseBody(stmt);
  }
}

/// Emit the body of a case statement at the current insertion point.
void PatternMatchEmission::emitCaseBody(CaseStmt *caseBlock) {
  SGF.emitStmt(caseBlock->getBody());

  // Implicitly break out of the pattern match statement.
  if (SGF.B.hasValidInsertionPoint()) {
    // Case blocks without trailing braces have a line location of the last
    // instruction in the case block.
    SILLocation cleanupLoc =
            RegularLocation::getAutoGeneratedLocation(caseBlock->getEndLoc());
    if (auto *braces = dyn_cast<BraceStmt>(caseBlock->getBody()))
      if (braces->getNumElements() == 1 &&
          dyn_cast_or_null<DoStmt>(braces->getFirstElement().dyn_cast<Stmt *>()))
        cleanupLoc = CleanupLocation(caseBlock);
    SGF.emitBreakOutOf(cleanupLoc, PatternMatchStmt);
  }
}

void PatternMatchEmission::initSharedCaseBlockDest(CaseStmt *caseBlock,
                                                   bool hasFallthroughTo) {
  auto result = SharedCases.insert({caseBlock, {nullptr, hasFallthroughTo}});
  assert(result.second);

  auto *block = SGF.createBasicBlock();
  result.first->second.first = block;

  // Add args for any pattern variables if we have any.
  for (auto *vd : caseBlock->getCaseBodyVariablesOrEmptyArray()) {
    if (!vd->hasName())
      continue;

    // We don't pass address-only values in basic block arguments.
    SILType ty = SGF.getLoweredType(vd->getTypeInContext());
    if (ty.isAddressOnly(SGF.F))
      continue;
    block->createPhiArgument(ty, OwnershipKind::Owned, vd);
  }
}

/// Retrieve the jump destination for a shared case block.
JumpDest PatternMatchEmission::getSharedCaseBlockDest(CaseStmt *caseBlock) {
  auto result = SharedCases.find(caseBlock);
  assert(result != SharedCases.end());

  auto *block = result->second.first;
  assert(block);

  return JumpDest(block, PatternMatchStmtDepth,
                  CleanupLocation(PatternMatchStmt));
}

void PatternMatchEmission::emitAddressOnlyAllocations() {
  for (auto &entry : SharedCases) {
    CaseStmt *caseBlock = entry.first;

    // If we have a shared case with bound decls, setup the arguments for the
    // shared block by emitting the temporary allocation used for the arguments
    // of the shared block.
    for (auto *vd : caseBlock->getCaseBodyVariablesOrEmptyArray()) {
      if (!vd->hasName())
        continue;

      SILType ty = SGF.getLoweredType(vd->getTypeInContext());
      if (!ty.isAddressOnly(SGF.F))
        continue;
      assert(!Temporaries[vd]);
      // Don't generate debug info for the temporary, as another debug_value
      // will be created in the body, with the same scope and a different type
      // Not sure if this is the best way to avoid that?
      Temporaries[vd] = SGF.emitTemporaryAllocation(
        vd, ty, DoesNotHaveDynamicLifetime, IsNotLexical, IsNotFromVarDecl,
        /* generateDebugInfo = */ false);
    }
  }

  // Now we have all of our cleanups entered, so we can record the
  // depth.
  PatternMatchStmtDepth = SGF.getCleanupsDepth();
}

void PatternMatchEmission::
emitAddressOnlyInitialization(VarDecl *dest, SILValue value) {
  auto found = Temporaries.find(dest);
  assert(found != Temporaries.end());
  if (SGF.useLoweredAddresses()) {
    SGF.B.createCopyAddr(dest, value, found->second, IsNotTake,
                         IsInitialization);
    return;
  }
  auto copy = SGF.B.createCopyValue(dest, value);
  SGF.B.createStore(dest, copy, found->second, StoreOwnershipQualifier::Init);
}

/// Emit all the shared case statements.
void PatternMatchEmission::emitSharedCaseBlocks(
    ValueOwnership ownership,
    llvm::function_ref<void(CaseStmt *)> bodyEmitter) {
  if (ownership >= ValueOwnership::Shared
      && !SharedCases.empty()) {
    SGF.SGM.diagnose(SharedCases.front().first,
                     diag::noncopyable_shared_case_block_unimplemented);
    
    for (auto &entry : SharedCases) {
      SILBasicBlock *caseBB = entry.second.first;
      SGF.B.setInsertionPoint(caseBB);
      SGF.B.createUnreachable(entry.first);
    }
    
    return;
  }
  for (auto &entry : SharedCases) {
    CaseStmt *caseBlock = entry.first;
    SILBasicBlock *caseBB = entry.second.first;
    bool hasFallthroughTo = entry.second.second;
    assert(caseBB->empty());

    // If this case can only have one predecessor, then merge it into that
    // predecessor.  We rely on the SIL CFG here, because unemitted shared case
    // blocks might fallthrough into this one.
    if (!hasFallthroughTo && caseBlock->getCaseLabelItems().size() == 1) {
      SILBasicBlock *predBB = caseBB->getSinglePredecessorBlock();
      assert(predBB && "Should only have 1 predecessor because it isn't shared");
      assert(isa<BranchInst>(predBB->getTerminator()) &&
             "Should have uncond branch to shared block");
      predBB->getTerminator()->eraseFromParent();
      caseBB->eraseFromParent();

      // Emit the case body into the predecessor's block.
      SGF.B.setInsertionPoint(predBB);
      
    } else {
      // If we did not need a shared case block, we shouldn't have emitted one.
      assert(!caseBB->pred_empty() &&
             "Shared case block without predecessors?!");

      // Otherwise, move the block to after the first predecessor.
      auto predBB = *caseBB->pred_begin();
      SGF.F.moveBlockAfter(caseBB, predBB);

      // Then emit the case body into the caseBB.
      SGF.B.setInsertionPoint(caseBB);
    }

    // Make sure that before/after we emit the case body we have emitted all
    // cleanups we created within.
    assert(SGF.getCleanupsDepth() == PatternMatchStmtDepth);
    SWIFT_DEFER { assert(SGF.getCleanupsDepth() == PatternMatchStmtDepth); };

    if (!caseBlock->hasCaseBodyVariables()) {
      emitCaseBody(caseBlock);
      continue;
    }

    // If we have a shared case with bound decls, then the case stmt pattern has
    // the order of variables that are the incoming BB arguments. Setup the
    // VarLocs to point to the incoming args and setup initialization so any
    // args needing Cleanup will get that as well.
    LexicalScope scope(SGF, CleanupLocation(caseBlock));
    unsigned argIndex = 0;
    for (auto *vd : caseBlock->getCaseBodyVariables()) {
      if (!vd->hasName())
        continue;

      SILType ty = SGF.getLoweredType(vd->getTypeInContext());

      // Initialize mv at +1. We always pass values in at +1 for today into
      // shared blocks.
      ManagedValue mv;
      if (ty.isAddressOnly(SGF.F)) {
        // There's no basic block argument, since we don't allow basic blocks
        // to have address arguments.
        //
        // Instead, we map the variable to a temporary alloc_stack in
        // emitAddressOnlyAllocations(), and store into it at each
        // predecessor block.
        //
        // There's nothing to do here, since the value should already have
        // been initialized on entry.
        auto found = Temporaries.find(vd);
        assert(found != Temporaries.end());
        mv = SGF.emitManagedRValueWithCleanup(found->second);
      } else {
        SILValue arg = caseBB->getArgument(argIndex++);
        assert(arg->getOwnershipKind() == OwnershipKind::Owned ||
               arg->getOwnershipKind() == OwnershipKind::None);
        mv = SGF.emitManagedRValueWithCleanup(arg);
      }

      // Emit a debug description of the incoming arg, nested within the scope
      // for the pattern match.
      SILDebugVariable dbgVar(vd->isLet(), /*ArgNo=*/0);
      SGF.B.emitDebugDescription(vd, mv.getValue(), dbgVar);

      if (vd->isLet()) {
        // Just emit a let and leave the cleanup alone.
        SGF.VarLocs[vd].value = mv.getValue();
        continue;
      }

      // Otherwise, the pattern variables were all emitted as lets and one got
      // passed in. Since we have a var, alloc a box for the var and forward in
      // the chosen value.
      SGF.VarLocs.erase(vd);
      auto newVar = SGF.emitInitializationForVarDecl(vd, vd->isLet());
      newVar->copyOrInitValueInto(SGF, vd, mv, /*isInit*/ true);
      newVar->finishInitialization(SGF);
    }

    // Now that we have setup all of the VarLocs correctly, emit the shared case
    // body.
    bodyEmitter(caseBlock);
  }
}

/// Context info used to emit FallthroughStmts.
/// Since fallthrough-able case blocks must not bind variables, they are always
/// emitted in the outermost scope of the switch.
class Lowering::PatternMatchContext {
public:
  PatternMatchEmission &Emission;
};

namespace {

struct UnexpectedEnumCaseInfo {
  CanType subjectTy;
  ManagedValue metatype;
  ManagedValue rawValue;
  NullablePtr<const EnumDecl> singleObjCEnum;

  UnexpectedEnumCaseInfo(CanType subjectTy, ManagedValue metatype,
                         ManagedValue rawValue, const EnumDecl *singleObjCEnum)
      : subjectTy(subjectTy), metatype(metatype), rawValue(rawValue),
        singleObjCEnum(singleObjCEnum) {
    assert(isa<MetatypeInst>(metatype));
    assert(bool(rawValue) && isa<UncheckedTrivialBitCastInst>(rawValue));
    assert(singleObjCEnum->hasRawType());
  }

  UnexpectedEnumCaseInfo(CanType subjectTy, ManagedValue valueMetatype)
      : subjectTy(subjectTy), metatype(valueMetatype), rawValue(),
        singleObjCEnum() {
    assert(isa<ValueMetatypeInst>(valueMetatype));
  }

  bool isSingleObjCEnum() const { return singleObjCEnum.isNonNull(); }

  void cleanupInstsIfUnused() {
    auto f = [](SILValue v) {
      if (!v->use_empty())
        return;
      cast<SingleValueInstruction>(v)->eraseFromParent();
    };
    f(metatype.getValue());
    if (rawValue)
      f(rawValue.getValue());
  }
};

} // end anonymous namespace

static void emitDiagnoseOfUnexpectedEnumCaseValue(SILGenFunction &SGF,
                                                  SILLocation loc,
                                                  UnexpectedEnumCaseInfo ueci) {
  ASTContext &ctx = SGF.getASTContext();
  auto diagnoseFailure = ctx.getDiagnoseUnexpectedEnumCaseValue();
  if (!diagnoseFailure) {
    SGF.B.createUnconditionalFail(loc, "unexpected enum case");
    return;
  }

  auto genericSig = diagnoseFailure->getGenericSignature();
  auto subs = SubstitutionMap::get(
      genericSig,
      [&](SubstitutableType *type) -> Type {
        auto genericParam = cast<GenericTypeParamType>(type);
        assert(genericParam->getDepth() == 0);
        assert(genericParam->getIndex() < 2);
        switch (genericParam->getIndex()) {
        case 0:
          return ueci.subjectTy;

        case 1:
          return ueci.singleObjCEnum.get()->getRawType();

        default:
          llvm_unreachable("wrong generic signature for expected case value");
        }
      },
      LookUpConformanceInModule());

  SGF.emitApplyOfLibraryIntrinsic(
      loc, diagnoseFailure, subs,
      {ueci.metatype, ueci.rawValue.materialize(SGF, loc)}, SGFContext());
}

static void emitDiagnoseOfUnexpectedEnumCase(SILGenFunction &SGF,
                                             SILLocation loc,
                                             UnexpectedEnumCaseInfo ueci) {
  if (ueci.subjectTy->isNoncopyable()) {
    // TODO: The DiagnoseUnexpectedEnumCase intrinsic currently requires a
    // Copyable parameter. For noncopyable enums it should be impossible to
    // reach an unexpected case statically, so just emit a trap for now.
    SGF.B.createUnconditionalFail(loc, "unexpected enum case");
    return;
  }
  ASTContext &ctx = SGF.getASTContext();
  auto diagnoseFailure = ctx.getDiagnoseUnexpectedEnumCase();
  if (!diagnoseFailure) {
    SGF.B.createUnconditionalFail(loc, "unexpected enum case");
    return;
  }

  auto diagnoseSignature = diagnoseFailure->getGenericSignature();
  auto genericArgsMap = SubstitutionMap::get(
      diagnoseSignature,
      [&](SubstitutableType *type) -> Type { return ueci.subjectTy; },
      LookUpConformanceInModule());

  SGF.emitApplyOfLibraryIntrinsic(loc, diagnoseFailure, genericArgsMap,
                                  ueci.metatype, SGFContext());
}

static void switchCaseStmtSuccessCallback(SILGenFunction &SGF,
                                          PatternMatchEmission &emission,
                                          ArgArray argArray, ClauseRow &row) {
  auto caseBlock = row.getClientData<CaseStmt>();
  SGF.emitProfilerIncrement(caseBlock);

  // Certain case statements can be entered along multiple paths, either because
  // they have multiple labels or because of fallthrough. When we need multiple
  // entrance path, we factor the paths with a shared block.
  //
  // If we don't have a fallthrough or a multi-pattern 'case', we can emit the
  // body inline. Emit the statement here and bail early.
  if (!row.hasFallthroughTo() && caseBlock->getCaseLabelItems().size() == 1) {
    // Debug values for case body variables must be nested within a scope for
    // the case block to avoid name conflicts.
    DebugScope scope(SGF, CleanupLocation(caseBlock));

    // If we have case body vars, set them up to point at the matching var
    // decls.
    if (caseBlock->hasCaseBodyVariables()) {
      // Since we know that we only have one case label item, grab its pattern
      // vars and use that to update expected with the right SILValue.
      //
      // TODO: Do we need a copy here?
      SmallVector<VarDecl *, 4> patternVars;
      row.getCasePattern()->collectVariables(patternVars);
      for (auto *expected : caseBlock->getCaseBodyVariables()) {
        if (!expected->hasName())
          continue;
        for (auto *vd : patternVars) {
          if (!vd->hasName() || vd->getName() != expected->getName()) {
            continue;
          }

          // Ok, we found a match. Update the VarLocs for the case block.
          auto &expectedLoc = SGF.VarLocs[expected];
          auto vdLoc = SGF.VarLocs.find(vd);
          assert(vdLoc != SGF.VarLocs.end());
          expectedLoc = SILGenFunction::VarLoc(vdLoc->second.value,
                                               vdLoc->second.access,
                                               vdLoc->second.box);

          // Emit a debug description for the variable, nested within a scope
          // for the pattern match.
          SILDebugVariable dbgVar(vd->isLet(), /*ArgNo=*/0);
          SGF.B.emitDebugDescription(vd, vdLoc->second.value, dbgVar);
        }
      }
    }
    emission.emitCaseBody(caseBlock);
    return;
  }

  // Ok, at this point we know that we have a multiple entrance block. Grab our
  // shared destination in preparation for branching to it.
  //
  // NOTE: We do not emit anything yet, since we will emit the shared block
  // later.
  JumpDest sharedDest = emission.getSharedCaseBlockDest(caseBlock);

  // If we do not have any bound decls, we do not need to setup any
  // variables. Just jump to the shared destination.
  if (!caseBlock->hasCaseBodyVariables()) {
    // Don't emit anything yet, we emit it at the cleanup level of the switch
    // statement.
    JumpDest sharedDest = emission.getSharedCaseBlockDest(caseBlock);
    SGF.Cleanups.emitBranchAndCleanups(sharedDest, caseBlock);
    return;
  }

  // Generate the arguments from this row's pattern in the case block's expected
  // order, and keep those arguments from being cleaned up, as we're passing the
  // +1 along to the shared case block dest. (The cleanups still happen, as they
  // are threaded through here messily, but the explicit retains here counteract
  // them, and then the retain/release pair gets optimized out.)
  SmallVector<SILValue, 4> args;
  SmallVector<VarDecl *, 4> patternVars;
  row.getCasePattern()->collectVariables(patternVars);
  for (auto *expected : caseBlock->getCaseBodyVariables()) {
    if (!expected->hasName())
      continue;
    for (auto *var : patternVars) {
      if (!var->hasName() || var->getName() != expected->getName())
        continue;

      SILValue value = SGF.VarLocs[var].value;
      SILType type = value->getType();

      // If we have an address-only type, initialize the temporary
      // allocation. We're not going to pass the address as a block
      // argument.
      if (type.isAddressOnly(SGF.F)) {
        emission.emitAddressOnlyInitialization(expected, value);
        break;
      }

      // If we have a loadable address, perform a load [copy].
      SILLocation loc(var);
      loc.markAutoGenerated();
      if (type.isAddress()) {
        value = SGF.B.emitLoadValueOperation(loc, value,
                                             LoadOwnershipQualifier::Copy);
        args.push_back(value);
        break;
      }

      value = SGF.B.emitCopyValueOperation(loc, value);
      args.push_back(value);
      break;
    }
  }

  // Now that we have initialized our arguments, branch to the shared dest.
  SGF.Cleanups.emitBranchAndCleanups(sharedDest, caseBlock, args);
}

// TODO: Integrate this with findStorageExprForMoveOnly, which does almost the
// same check.
static bool isBorrowableSubject(SILGenFunction &SGF,
                                Expr *subjectExpr) {
  // Look through forwarding expressions.
  for (;;) {
    subjectExpr = subjectExpr->getValueProvidingExpr();
    
    // Look through loads.
    if (auto load = dyn_cast<LoadExpr>(subjectExpr)) {
      subjectExpr = load->getSubExpr();
      continue;
    }
    
    // Look through optional force-projections.
    // We can't look through optional evaluations here because wrapping the
    // value up in an Optional at the end needs a copy/move to create the
    // temporary optional.
    if (auto force = dyn_cast<ForceValueExpr>(subjectExpr)) {
      subjectExpr = force->getSubExpr();
      continue;
    }

    // Look through parens.
    if (auto paren = dyn_cast<ParenExpr>(subjectExpr)) {
      subjectExpr = paren->getSubExpr();
      continue;
    }

    // Look through `try`, `await`, and `unsafe`.
    if (auto tryExpr = dyn_cast<TryExpr>(subjectExpr)) {
      subjectExpr = tryExpr->getSubExpr();
      continue;
    }
    if (auto awaitExpr = dyn_cast<AwaitExpr>(subjectExpr)) {
      subjectExpr = awaitExpr->getSubExpr();
      continue;
    }
    if (auto unsafeExpr = dyn_cast<UnsafeExpr>(subjectExpr)) {
      subjectExpr = unsafeExpr->getSubExpr();
      continue;
    }

    break;
  }
  
  // An explicit `borrow` expression requires us to do a borrowing access.
  if (isa<BorrowExpr>(subjectExpr)) {
    return true;
  }
  
  AbstractStorageDecl *storage;
  AccessSemantics access;
  
  // A subject is potentially borrowable if it's some kind of storage reference.
  if (auto declRef = dyn_cast<DeclRefExpr>(subjectExpr)) {
    storage = dyn_cast<AbstractStorageDecl>(declRef->getDecl());
    access = declRef->getAccessSemantics();
  } else if (auto memberRef = dyn_cast<MemberRefExpr>(subjectExpr)) {
    storage = dyn_cast<AbstractStorageDecl>(memberRef->getMember().getDecl());
    access = memberRef->getAccessSemantics();
  } else if (auto subscript = dyn_cast<SubscriptExpr>(subjectExpr)) {
    storage = dyn_cast<AbstractStorageDecl>(subscript->getMember().getDecl());
    access = subscript->getAccessSemantics();
  } else {
    return false;
  }
  
  // If the member being referenced isn't storage, there's no benefit to
  // borrowing it.
  if (!storage) {
    return false;
  }
  
  // Check the access strategy used to read the storage.
  auto strategy =
      storage->getAccessStrategy(access, AccessKind::Read, SGF.SGM.SwiftModule,
                                 SGF.F.getResilienceExpansion(),
                                 /*useOldABI=*/false);

  switch (strategy.getKind()) {
  case AccessStrategy::Kind::Storage:
    // Accessing storage directly benefits from borrowing.
    return true;
  case AccessStrategy::Kind::DirectToAccessor:
  case AccessStrategy::Kind::DispatchToAccessor:
    // If we use an accessor, the kind of accessor affects whether we get
    // a reference we can borrow or a temporary that will be consumed.
    switch (strategy.getAccessor()) {
    case AccessorKind::Get:
    case AccessorKind::DistributedGet:
      // Get returns an owned value.
      return false;
    case AccessorKind::Read:
    case AccessorKind::Read2:
    case AccessorKind::Modify:
    case AccessorKind::Modify2:
    case AccessorKind::Address:
    case AccessorKind::MutableAddress:
      // Read, modify, and addressors yield a borrowable reference.
      return true;
    case AccessorKind::Init:
    case AccessorKind::Set:
    case AccessorKind::WillSet:
    case AccessorKind::DidSet:
      llvm_unreachable("should not be involved in a read");
    }
    llvm_unreachable("switch not covered?");
    
  case AccessStrategy::Kind::MaterializeToTemporary:
  case AccessStrategy::Kind::DispatchToDistributedThunk:
    return false;
  }
  llvm_unreachable("switch not covered?");
}

void SILGenFunction::emitSwitchStmt(SwitchStmt *S) {
  LLVM_DEBUG(llvm::dbgs() << "emitting switch stmt\n";
             S->dump(llvm::dbgs());
             llvm::dbgs() << '\n');

  auto subjectExpr = S->getSubjectExpr();
  auto subjectTy = subjectExpr->getType();
  auto subjectLoweredTy = getLoweredType(subjectTy);
  auto subjectLoweredAddress =
    silConv.useLoweredAddresses() && subjectLoweredTy.isAddressOnly(F);

  // If the subject expression is uninhabited, we're already dead.
  // Emit an unreachable in place of the switch statement.
  if (subjectTy->isStructurallyUninhabited()) {
    emitIgnoredExpr(S->getSubjectExpr());
    B.createUnreachable(S);
    return;
  }

  // If the subject is noncopyable, then we have to know beforehand whether the
  // final match is going to consume the value. Since we can't copy it during
  // the match, we'll have to perform the initial match as a borrow and then
  // reproject the value to consume it.
  auto ownership = ValueOwnership::Default;
  if (subjectTy->isNoncopyable()) {
    // Determine the overall ownership behavior of the switch, based on the
    // subject expression and the patterns' ownership behavior.
    
    // If the subject expression is borrowable, then perform the switch as
    // a borrow. (A `consume` expression would render the expression
    // non-borrowable.) Otherwise, perform it as a consume.
    ownership = isBorrowableSubject(*this, subjectExpr)
      ? ValueOwnership::Shared
      : ValueOwnership::Owned;
    for (auto caseLabel : S->getCases()) {
      for (auto item : caseLabel->getCaseLabelItems()) {
        ownership = std::max(ownership, item.getPattern()->getOwnership());
      }
      // To help migrate early adopters of the feature, if the case body is
      // obviously trying to return out one of the pattern bindings, which
      // would necessitate a consuming switch, perform the switch as a consume,
      // and warn that this will need to be made explicit in the future.
      if (ownership == ValueOwnership::Shared) {
        if (auto ret = dyn_cast_or_null<ReturnStmt>(
              caseLabel->getBody()->getSingleActiveElement()
                       .dyn_cast<Stmt*>())) {
          if (ret->hasResult()) {
            Expr *result = ret->getResult()->getSemanticsProvidingExpr();
            if (result->getType()->isNoncopyable()) {
              while (auto conv = dyn_cast<ImplicitConversionExpr>(result)) {
                result = conv->getSubExpr()->getSemanticsProvidingExpr();
              }
              if (auto dr = dyn_cast<DeclRefExpr>(result)) {
                if (std::find(caseLabel->getCaseBodyVariables().begin(),
                              caseLabel->getCaseBodyVariables().end(),
                              dr->getDecl())
                      != caseLabel->getCaseBodyVariables().end()) {
                  SGM.diagnose(result->getLoc(),
                               diag::return_borrowing_switch_binding);
                  ownership = ValueOwnership::Owned;
                }
              }
            }
          }
        }
      }
    }
  }

  // For copyable subjects, or borrowing matches of noncopyable subjects,
  // we immediately emit the case body once we get a match.
  auto immutableCompletionHandler = [this](PatternMatchEmission &emission,
                                           ArgArray argArray, ClauseRow &row) {
    return switchCaseStmtSuccessCallback(*this, emission, argArray, row);
  };
  // If we're doing a
  // consuming match, then we delay emission of the case blocks until we've
  // finished emitting the dispatch tree, so that the borrow scope during the
  // match can be closed out before consuming.
  auto destructiveCompletionHandler = [](PatternMatchEmission &emission,
                                         ArgArray argArray, ClauseRow &row) {
    emission.addDestructiveCase(row.getClientData<CaseStmt>(),
                                row.getCasePattern());
  };
  PatternMatchEmission::CompletionHandlerTy completionHandler;
  if (ownership <= ValueOwnership::Shared) {
    completionHandler = immutableCompletionHandler;
  } else {
    completionHandler = destructiveCompletionHandler;
  }
  PatternMatchEmission emission(*this, S, completionHandler);

  // Add a row for each label of each case.
  SmallVector<ClauseRow, 8> clauseRows;
  clauseRows.reserve(S->getCases().size());
  bool hasFallthrough = false;
  for (auto caseBlock : S->getCases()) {
    // If the previous block falls through into this block or we have multiple
    // case label items, create a shared case block to generate the shared
    // block.
    if (hasFallthrough || caseBlock->getCaseLabelItems().size() > 1) {
      emission.initSharedCaseBlockDest(caseBlock, hasFallthrough);
    }

    for (auto &labelItem : caseBlock->getCaseLabelItems()) {
      clauseRows.emplace_back(caseBlock,
                              const_cast<Pattern*>(labelItem.getPattern()),
                              const_cast<Expr*>(labelItem.getGuardExpr()),
                              hasFallthrough);
    }

    hasFallthrough = caseBlock->hasFallthroughDest();
  }

  // Emit alloc_stacks for address-only variables appearing in
  // multiple-entry case blocks.
  emission.emitAddressOnlyAllocations();

  SILBasicBlock *contBB = createBasicBlock();
  // Depending on the switch ownership behavior, we want to either:
  //
  // - allow the subject to (potentially or explicitly) be forwarded into the
  //   case blocks. In this case we want to include the cleanups for the subject
  //   in the case blocks so that the subject and its components can be
  //   consumed by each case block.
  //
  // or:
  //
  // - evaluate the subject under a formal access scope (a borrow or inout).
  //   In this case the lifetime of the access should cover all the way to the
  //   exits out of the switch.
  //
  // When we break out of a case block, we take the subject's remnants with us
  // in the former case, but not the latter.q
  CleanupsDepth subjectDepth = Cleanups.getCleanupsDepth();
  LexicalScope switchScope(*this, CleanupLocation(S));
  std::optional<FormalEvaluationScope> switchFormalAccess;

  bool subjectUndergoesFormalAccess;
  switch (ownership) {
  // For normal copyable subjects, we allow the value to be forwarded into
  // the cases, since we can copy it as needed to evaluate the pattern match.
  case ValueOwnership::Default:
  // Similarly, if the subject is an explicitly consumed noncopyable value,
  // we can forward ownership of the subject's parts into matching case blocks.
  case ValueOwnership::Owned:
    subjectUndergoesFormalAccess = false;
    break;

  // Borrowed and inout pattern matches both undergo a formal access.
  case ValueOwnership::Shared:
  case ValueOwnership::InOut:
    subjectUndergoesFormalAccess = true;
    break;
  }

  PatternMatchContext switchContext = { emission };
  SwitchStack.push_back(&switchContext);

  // Emit the subject value.
  ManagedValue subjectMV;
  switch (ownership) {
  case ValueOwnership::Default: {
    // A regular copyable pattern match. Emit as a regular rvalue.
    // If at +1, dispatching will consume it. If it is at +0, we just forward
    // down borrows.
    subjectMV = emitRValueAsSingleValue(
        S->getSubjectExpr(),
        SGFContext::AllowGuaranteedPlusZero);
    break;
  }
  case ValueOwnership::Shared: {
    // A borrowing pattern match. See if we can emit the subject under a read
    // formal access.
    switchFormalAccess.emplace(*this);
    auto subjectExpr = S->getSubjectExpr();
    if (auto subjectLVExpr = findStorageReferenceExprForMoveOnly(subjectExpr,
                                       StorageReferenceOperationKind::Borrow)) {
      LValue sharedLV = emitLValue(subjectLVExpr,
        subjectLoweredAddress ? SGFAccessKind::BorrowedAddressRead
                              : SGFAccessKind::BorrowedObjectRead);
      subjectMV = emitBorrowedLValue(S->getSubjectExpr(), std::move(sharedLV));
    } else {
      // Emit the value as an allowed-+0 rvalue if it doesn't have special
      // lvalue treatment.
      subjectMV = emitRValueAsSingleValue(S->getSubjectExpr(),
                                          SGFContext::AllowGuaranteedPlusZero);
    }
    break;
  }
  case ValueOwnership::InOut: {
    // A mutating pattern match. Emit the subject under a modify access.
    llvm_unreachable("not yet implemented");
  }
  case ValueOwnership::Owned: {
    // A consuming pattern match. Emit as a +1 rvalue.
    subjectMV = emitRValueAsSingleValue(S->getSubjectExpr());
    break;
  }
  }
  
  // Inline constructor for subject.
  auto subject = ([&]() -> ConsumableManagedValue {
    // TODO: Move-only-wrapped subjects should also undergo a noncopying switch.
    if (subjectMV.getType().isMoveOnly(/*or wrapped*/ false)) {
      assert(ownership > ValueOwnership::Default);
      // Based on the ownership behavior, prepare the subject.
      // The pattern match itself will always be performed on a borrow, to
      // ensure that the order of pattern evaluation doesn't prematurely
      // consume or modify the value until we commit to a match. But if the
      // match consumes the value, then we need a +1 value to go back to in
      // order to consume the parts we match to, so we force a +1 value then
      // borrow that for the pattern match.
      switch (ownership) {
      case ValueOwnership::Default:
        llvm_unreachable("invalid");
      
      case ValueOwnership::Shared:
        emission.setNoncopyableBorrowingOwnership();
        if (subjectMV.getType().isAddress()) {
          // Initiate a read access on the memory, to ensure that even
          // if the underlying memory is mutable or consumable, the pattern
          // match is not allowed to modify it.
          subjectMV = B.createOpaqueBorrowBeginAccess(S, subjectMV);
          if (subjectMV.getType().isLoadable(F)) {
            // Load a borrow if the type is loadable.
            subjectMV = subjectUndergoesFormalAccess
              ? B.createFormalAccessLoadBorrow(S, subjectMV)
              : B.createLoadBorrow(S, subjectMV);
          }
        } else {
          // Initiate a fixed borrow on the subject, so that it's treated as
          // opaque by the move checker.
          subjectMV =
              subjectUndergoesFormalAccess
                  ? B.createFormalAccessBeginBorrow(
                        S, subjectMV, IsNotLexical, BeginBorrowInst::IsFixed)
                  : B.createBeginBorrow(S, subjectMV, IsNotLexical,
                                        BeginBorrowInst::IsFixed);
        }
        return {subjectMV, CastConsumptionKind::BorrowAlways};
        
      case ValueOwnership::InOut:
        // TODO: mutating switches
        llvm_unreachable("not implemented");
        
      case ValueOwnership::Owned:
        // Make sure we own the subject value.
        subjectMV = subjectMV.ensurePlusOne(*this, S);
        if (subjectMV.getType().isAddress() &&
            subjectMV.getType().isLoadable(F)) {
          // Move the value into memory if it's loadable.
          subjectMV = B.createLoadTake(S, subjectMV);
        }
        
        // Unwind cleanups to this point when we're ready to reproject
        // the value for consumption or mutation.
        emission.setNoncopyableConsumingOwnership(
          Cleanups.getCleanupsDepth(),
          subjectMV);

        // Perform the pattern match on an opaque borrow or read access of the
        // subject.
        if (subjectMV.getType().isAddress()) {
          subjectMV = B.createOpaqueBorrowBeginAccess(S, subjectMV);
        } else {
          subjectMV = B.createBeginBorrow(S, subjectMV, IsNotLexical,
                                          BeginBorrowInst::IsFixed);
        }
        return {subjectMV, CastConsumptionKind::BorrowAlways};
      }
      llvm_unreachable("unhandled value ownership");
    }
    
    // TODO: Move-only-wrapped subjects should also undergo a noncopying switch.
    // For now, unwrap them and perform a normal switch over them.
    if (subjectMV.getType().isMoveOnlyWrapped()) {
      if (subjectMV.getType().isAddress()) {
        auto isPlusOne = subjectMV.isPlusOne(*this);
        auto subjectAddr
          = B.createMoveOnlyWrapperToCopyableAddr(S, subjectMV.forward(*this));

        if (isPlusOne) {
          subjectMV = emitManagedRValueWithCleanup(subjectAddr);
        } else {
          subjectMV = ManagedValue::forBorrowedAddressRValue(subjectAddr);
        }
      } else {
        if (subjectMV.isPlusOne(*this)) {
          subjectMV
            = B.createOwnedMoveOnlyWrapperToCopyableValue(S, subjectMV);
        } else {
          subjectMV
            = B.createGuaranteedMoveOnlyWrapperToCopyableValue(S, subjectMV);
        }
      }
    }

    // If we have a plus one value...
    if (subjectMV.isPlusOne(*this)) {
      // And we have an address that is loadable, perform a load [take].
      if (subjectMV.getType().isAddress() &&
          subjectMV.getType().isLoadable(F)) {
        subjectMV = B.createLoadTake(S, subjectMV);
      }
      return {subjectMV, CastConsumptionKind::TakeAlways};
    }

    // If we have a loadable address and +0, perform a load borrow.
    if (subjectMV.getType().isAddress() &&
        subjectMV.getType().isLoadable(F)) {
      subjectMV = B.createLoadBorrow(S, subjectMV);
    }

    // If then we have an object, return it at +0.
    // For opaque values, return at +1
    if (subjectMV.getType().isObject()) {
      if (subjectMV.getType().isAddressOnly(F)) {
        return {subjectMV.copy(*this, S), CastConsumptionKind::TakeAlways};
      }
      return {subjectMV, CastConsumptionKind::BorrowAlways};
    }

    // If we have an address only type returned without a cleanup, we
    // need to do a copy just to be safe. So for efficiency we pass it
    // down take_always.
    return {subjectMV.copy(*this, S), CastConsumptionKind::TakeAlways};
  }());

  CleanupsDepth caseBodyDepth = Cleanups.getCleanupsDepth();
  std::optional<LexicalScope> caseBodyScope;
  if (subjectUndergoesFormalAccess) {
    caseBodyScope.emplace(*this, CleanupLocation(S));
  }

  // Enter a break/continue scope. As discussed above, the depth we jump to
  // depends on whether the subject is under a formal access.
  JumpDest contDest(contBB,
                    subjectUndergoesFormalAccess ? caseBodyDepth
                                                 : subjectDepth,
                    CleanupLocation(S));
  BreakContinueDestStack.push_back({S, contDest, JumpDest(S)});

  // If we need to diagnose an unexpected enum case or unexpected enum case
  // value, we need access to a value metatype for the subject. Emit this state
  // now before we emit the actual switch to ensure that the subject has not
  // been consumed.
  auto unexpectedEnumCaseInfo = ([&]() -> UnexpectedEnumCaseInfo {
    SILLocation loc = RegularLocation::getAutoGeneratedLocation();
    CanType canSubjectTy = subjectTy->getCanonicalType();
    CanType metatypeType = MetatypeType::get(canSubjectTy)->getCanonicalType();
    SILType loweredMetatypeType =
        getLoweredType(AbstractionPattern::getOpaque(), metatypeType);
    ManagedValue value = subject.getFinalManagedValue();

    if (auto *singleEnumDecl = canSubjectTy->getEnumOrBoundGenericEnum()) {
      if (singleEnumDecl->isObjC()) {
        auto metatype = ManagedValue::forObjectRValueWithoutOwnership(
            B.createMetatype(loc, loweredMetatypeType));

        // Bitcast the enum value to its raw type. (This is only safe for @objc
        // enums.)
        SILType loweredRawType = getLoweredType(singleEnumDecl->getRawType());
        assert(loweredRawType.isTrivial(F));
        assert(loweredRawType.isObject());
        auto rawValue =
            B.createUncheckedTrivialBitCast(loc, value, loweredRawType);
        return {canSubjectTy, metatype, rawValue, singleEnumDecl};
      }
    }

    return {canSubjectTy,
            B.createValueMetatype(loc, loweredMetatypeType, value)};
  }());

  auto failure = [&](SILLocation location) {
    // If we fail to match anything, we trap. This can happen with a switch
    // over an @objc enum, which may contain any value of its underlying type,
    // or a switch over a non-frozen Swift enum when the user hasn't written a
    // catch-all case.
    SWIFT_DEFER { B.createUnreachable(location); };

    // Special case: if it's a single @objc enum, we can print the raw value.
    if (unexpectedEnumCaseInfo.isSingleObjCEnum()) {
      emitDiagnoseOfUnexpectedEnumCaseValue(*this, location,
                                            unexpectedEnumCaseInfo);
      return;
    }
    emitDiagnoseOfUnexpectedEnumCase(*this, location, unexpectedEnumCaseInfo);
  };

  // Set up an initial clause matrix.
  ClauseMatrix clauses(clauseRows);

  // Recursively specialize and emit the clause matrix.
  emission.emitDispatch(clauses, subject, failure);

  // If we're doing a consuming pattern match, end the borrow phase and emit
  // the consuming case blocks now.
  if (ownership > ValueOwnership::Shared) {
    emission.emitDestructiveCaseBlocks();
  }

  assert(!B.hasValidInsertionPoint());
  // Disable the cleanups for values that should be consumed by the case
  // bodies.
  caseBodyScope.reset();
  // If the subject isn't under a formal access, that includes the subject
  // itself.
  if (!subjectUndergoesFormalAccess) {
    switchScope.pop();
  }

  // Then emit the case blocks shared by multiple pattern cases.
  emission.emitSharedCaseBlocks(ownership,
      [&](CaseStmt *caseStmt) { emission.emitCaseBody(caseStmt); });

  // Bookkeeping.
  SwitchStack.pop_back();
  BreakContinueDestStack.pop_back();

  // If the continuation block has no predecessors, this
  // point is not reachable.
  if (contBB->pred_empty()) {
    eraseBasicBlock(contBB);
  } else {
    B.emitBlock(contBB);
  }
  
  // End the formal access to the subject now (if there was one).
  if (subjectUndergoesFormalAccess) {
    switchFormalAccess.reset();
    switchScope.pop();
  }

  // Now that we have emitted everything, see if our unexpected enum case info
  // metatypes were actually used. If not, delete them.
  unexpectedEnumCaseInfo.cleanupInstsIfUnused();
}

void SILGenFunction::emitSwitchFallthrough(FallthroughStmt *S) {
  assert(!SwitchStack.empty() && "fallthrough outside of switch?!");
  PatternMatchContext *context = SwitchStack.back();

  // Get the destination block.
  CaseStmt *destCaseStmt = S->getFallthroughDest();
  JumpDest sharedDest = context->Emission.getSharedCaseBlockDest(destCaseStmt);

  // If our destination case doesn't have any bound decls, there is no rebinding
  // to do. Just jump to the shared dest.
  if (!destCaseStmt->hasCaseBodyVariables()) {
    Cleanups.emitBranchAndCleanups(sharedDest, S);
    return;
  }

  // Generate branch args to pass along current vars to fallthrough case.
  SmallVector<SILValue, 4> args;
  CaseStmt *fallthroughSourceStmt = S->getFallthroughSource();

  for (auto *expected : destCaseStmt->getCaseBodyVariables()) {
    if (!expected->hasName())
      continue;

    // The type checker enforces that if our destination case has variables then
    // our fallthrough source must as well.
    for (auto *var : fallthroughSourceStmt->getCaseBodyVariables()) {
      if (!var->hasName() || var->getName() != expected->getName()) {
        continue;
      }

      auto &varLoc = VarLocs[var];
      SILValue value = varLoc.value;
      SILValue box = varLoc.box;

      if (value->getType().isAddressOnly(F)) {
        context->Emission.emitAddressOnlyInitialization(expected, value);
        break;
      }

      SILLocation loc(var);
      loc.markAutoGenerated();
      if (box) {
        SILValue argValue =
            B.emitLoadValueOperation(loc, value, LoadOwnershipQualifier::Copy);
        args.push_back(argValue);
        break;
      }

      auto argValue = B.emitCopyValueOperation(loc, value);
      args.push_back(argValue);
      break;
    }
  }
  Cleanups.emitBranchAndCleanups(sharedDest, S, args);
}

void SILGenFunction::emitCatchDispatch(DoCatchStmt *S, ManagedValue exn,
                                       ArrayRef<CaseStmt *> clauses,
                                       JumpDest catchFallthroughDest) {

  auto completionHandler = [&](PatternMatchEmission &emission,
                               ArgArray argArray, ClauseRow &row) {
    auto clause = row.getClientData<CaseStmt>();
    emitProfilerIncrement(clause);

    // Certain catch clauses can be entered along multiple paths because they
    // have multiple labels. When we need multiple entrance path, we factor the
    // paths with a shared block.
    //
    // If we don't have a multi-pattern 'catch', we can emit the
    // body inline. Emit the statement here and bail early.
    if (clause->getCaseLabelItems().size() == 1) {
      // Debug values for catch clause variables must be nested within a scope for
      // the catch block to avoid name conflicts.
      DebugScope scope(*this, CleanupLocation(clause));

      // If we have case body vars, set them up to point at the matching var
      // decls.
      if (clause->hasCaseBodyVariables()) {
        // Since we know that we only have one case label item, grab its pattern
        // vars and use that to update expected with the right SILValue.
        //
        // TODO: Do we need a copy here?
        SmallVector<VarDecl *, 4> patternVars;
        row.getCasePattern()->collectVariables(patternVars);
        for (auto *expected : clause->getCaseBodyVariables()) {
          if (!expected->hasName())
            continue;
          for (auto *vd : patternVars) {
            if (!vd->hasName() || vd->getName() != expected->getName()) {
              continue;
            }

            // Ok, we found a match. Update the VarLocs for the case block.
            auto &expectedLoc = VarLocs[expected];
            auto vdLoc = VarLocs.find(vd);
            assert(vdLoc != VarLocs.end());
            expectedLoc = VarLoc(vdLoc->second.value,
                                 vdLoc->second.access,
                                 vdLoc->second.box);

            // Emit a debug description of the incoming arg, nested within the scope
            // for the pattern match.
            SILDebugVariable dbgVar(vd->isLet(), /*ArgNo=*/0);
            B.emitDebugDescription(vd, vdLoc->second.value, dbgVar);
          }
        }
      }

      emitStmt(clause->getBody());

      // If we fell out of the catch clause, branch to the fallthrough dest.
      if (B.hasValidInsertionPoint()) {
        Cleanups.emitBranchAndCleanups(catchFallthroughDest, clause->getBody());
      }
      return;
    }

    // Ok, at this point we know that we have a multiple entrance block. Grab
    // our shared destination in preparation for branching to it.
    //
    // NOTE: We do not emit anything yet, since we will emit the shared block
    // later.
    JumpDest sharedDest = emission.getSharedCaseBlockDest(clause);

    // If we do not have any bound decls, we do not need to setup any
    // variables. Just jump to the shared destination.
    if (!clause->hasCaseBodyVariables()) {
      // Don't emit anything yet, we emit it at the cleanup level of the switch
      // statement.
      JumpDest sharedDest = emission.getSharedCaseBlockDest(clause);
      Cleanups.emitBranchAndCleanups(sharedDest, clause);
      return;
    }

    // Generate the arguments from this row's pattern in the case block's
    // expected order, and keep those arguments from being cleaned up, as we're
    // passing the +1 along to the shared case block dest. (The cleanups still
    // happen, as they are threaded through here messily, but the explicit
    // retains here counteract them, and then the retain/release pair gets
    // optimized out.)
    SmallVector<SILValue, 4> args;
    SmallVector<VarDecl *, 4> patternVars;
    row.getCasePattern()->collectVariables(patternVars);
    for (auto *expected : clause->getCaseBodyVariables()) {
      if (!expected->hasName())
        continue;
      for (auto *var : patternVars) {
        if (!var->hasName() || var->getName() != expected->getName())
          continue;

        SILValue value = VarLocs[var].value;
        SILType type = value->getType();

        // If we have an address-only type, initialize the temporary
        // allocation. We're not going to pass the address as a block
        // argument.
        if (type.isAddressOnly(F)) {
          emission.emitAddressOnlyInitialization(expected, value);
          break;
        }

        // If we have a loadable address, perform a load [copy].
        SILLocation loc(var);
        loc.markAutoGenerated();
        if (type.isAddress()) {
          value = B.emitLoadValueOperation(loc, value,
                                           LoadOwnershipQualifier::Copy);
          args.push_back(value);
          break;
        }

        value = B.emitCopyValueOperation(loc, value);
        args.push_back(value);
        break;
      }
    }

    // Now that we have initialized our arguments, branch to the shared dest.
    Cleanups.emitBranchAndCleanups(sharedDest, clause, args);
  };

  LLVM_DEBUG(llvm::dbgs() << "emitting catch dispatch\n"; S->dump(llvm::dbgs());
             llvm::dbgs() << '\n');

  PatternMatchEmission emission(*this, S, completionHandler);

  // Add a row for each label of each case.
  SmallVector<ClauseRow, 8> clauseRows;
  clauseRows.reserve(S->getCatches().size());
  for (auto caseBlock : S->getCatches()) {
    // If we have multiple case label items, create a shared case block to
    // generate the shared block.
    if (caseBlock->getCaseLabelItems().size() > 1) {
      emission.initSharedCaseBlockDest(caseBlock, /*hasFallthrough*/ false);
    }

    for (auto &labelItem : caseBlock->getCaseLabelItems()) {
      clauseRows.emplace_back(caseBlock,
                              const_cast<Pattern *>(labelItem.getPattern()),
                              const_cast<Expr *>(labelItem.getGuardExpr()),
                              /*hasFallthrough*/ false);
    }
  }

  // Emit alloc_stacks for address-only variables appearing in
  // multiple-entry case blocks.
  emission.emitAddressOnlyAllocations();

  Scope stmtScope(Cleanups, CleanupLocation(S));

  auto consumptionKind = exn.getType().isObject()
      ? CastConsumptionKind::BorrowAlways
      : CastConsumptionKind::CopyOnSuccess;

  // Our model is that sub-cases get the exception at +0 and the throw (if we
  // need to rethrow the exception) gets the exception at +1 since we need to
  // trampoline it's ownership to our caller.
  ConsumableManagedValue subject = {exn.borrow(*this, S), consumptionKind};

  auto failure = [&](SILLocation location) {
    // If we fail to match anything, just rethrow the exception.

    // If the throw destination is not valid, then the PatternMatchEmission
    // logic is emitting an unreachable block but didn't prune the failure BB.
    // Mark it as such.
    if (!ThrowDest.isValid()) {
      B.createUnreachable(S);
      return;
    }

    // Since we borrowed exn before sending it to our subcases, we know that it
    // must be at +1 at this point. That being said, SILGenPattern will
    // potentially invoke this for each of the catch statements, so we need to
    // copy before we pass it into the throw.
    CleanupStateRestorationScope scope(Cleanups);
    if (exn.hasCleanup()) {
      scope.pushCleanupState(exn.getCleanup(),
                             CleanupState::PersistentlyActive);
    }
    emitThrow(S, exn);
  };
  // Set up an initial clause matrix.
  ClauseMatrix clauseMatrix(clauseRows);

  // Recursively specialize and emit the clause matrix.
  emission.emitDispatch(clauseMatrix, subject, failure);
  assert(!B.hasValidInsertionPoint());

  stmtScope.pop();

  // Then emit the case blocks shared by multiple pattern cases.
  emission.emitSharedCaseBlocks(ValueOwnership::Default,
  [&](CaseStmt *caseStmt) {
    emitStmt(caseStmt->getBody());

    // If we fell out of the catch clause, branch to the fallthrough dest.
    if (B.hasValidInsertionPoint()) {
      Cleanups.emitBranchAndCleanups(catchFallthroughDest, caseStmt->getBody());
    }
  });
}