PartialApplySimplification.cpp

//===--- PartialApplySimplification.cpp - Lower partial applications ------===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2014 - 2021 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
//
//===----------------------------------------------------------------------===//
///
/// \file
///
/// Reduces all partial application functions into explicit closure
/// constructions.
///
/// \c partial_apply is a useful high-level representation for optimization
/// passes like inlining, but it abstracts over many details of how closures
/// are constructed. In order to make IRGen lowering simpler, and provide some
/// opportunity for other passes to optimize closure construction.
///
/// When a closure implementation function is private, and is only referenced by
/// partial applications all of the same shape, then we can replace the function
/// with one that takes a closure box instead of the partially applied
/// arguments. Otherwise, a partial application forwarder function is generated
/// as a shim between the closure entry point, which takes the box, and the
/// original function, which takes the loaded arguments.
///
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "sil-partial-apply-simplification"

#include "llvm/Support/Debug.h"
#include "llvm/ADT/Statistic.h"
#include "swift/Basic/Assertions.h"
#include "swift/SIL/SILCloner.h"
#include "swift/SIL/SILInstruction.h"
#include "swift/SIL/TypeSubstCloner.h"
#include "swift/SILOptimizer/PassManager/Passes.h"
#include "swift/SILOptimizer/PassManager/Transforms.h"
#include "swift/SILOptimizer/Utils/SILOptFunctionBuilder.h"
#include "swift/SILOptimizer/Utils/SpecializationMangler.h"

STATISTIC(NumInvocationFunctionsChanged,
          "Number of invocation functions rewritten");
STATISTIC(NumUnsupportedChangesToInvocationFunctions,
          "Number of invocation functions that could be rewritten, but aren't yet");
STATISTIC(NumPartialApplyCalleesWithNonApplyUses,
          "Number of invocation functions with non-apply uses");
STATISTIC(NumPartialApplyCalleesWithEscapingAndApplyUses,
          "Number of invocation functions with both escaping and full apply uses");
STATISTIC(NumPartialApplyCalleesPossiblyUsedExternally,
          "Number of invocation functions possibly used externally");
STATISTIC(NumPartialApplyCalleesDeclarationOnly,
          "Number of invocation functions that are declaration-only");
STATISTIC(NumPartialApplyCalleesWithMismatchedPartialApplies,
          "Number of invocation functions that have mismatched partial_apply sites");
STATISTIC(NumDynamicPartialApplicationForwarders,
          "Number of dynamic partial application forwarder thunks generated");

using namespace swift;

//===----------------------------------------------------------------------===//
//                            Top Level Entrypoint
//===----------------------------------------------------------------------===//

namespace {

struct KnownCallee {
  /// The set of function_refs to the callee.
  llvm::SetVector<FunctionRefInst *> FunctionRefs;
  /// The set of partial application sites.
  llvm::SetVector<PartialApplyInst *> PartialApplications;
  /// The set of full application sites.
  llvm::SetVector<FullApplySite> FullApplications;
  /// If the callee has a non-partial-apply, non-apply use, this points to an
  /// arbitrary one, for logging purposes.
  SILInstruction *NonApplyUse = nullptr;
};

class PartialApplySimplificationPass : public SILModuleTransform {
  /// The entry point to the transformation.
  void run() override {
    // Scan all partial applications in the module so we know what to work with.
    llvm::DenseMap<SILFunction *, KnownCallee> knownCallees;
    llvm::SetVector<swift::PartialApplyInst *> dynamicCallees;
    for (auto &f : *getModule()) {
      scanFunction(&f, knownCallees, dynamicCallees);
    }

    for (auto &knownCallee : knownCallees) {
      processKnownCallee(knownCallee.first, knownCallee.second);
    }
    
    for (auto *dynamicPA : dynamicCallees) {
      processDynamicCallee(dynamicPA);
    }
  }

  void scanFunction(SILFunction *f,
                    llvm::DenseMap<SILFunction *,
                                   KnownCallee> &knownCallees,
                    llvm::SetVector<PartialApplyInst *> &dynamicCallees);
  
  void processKnownCallee(SILFunction *callee,
                          const KnownCallee &pa);
  
  void processDynamicCallee(PartialApplyInst *pa);
  
  void generateForwardingThunksForKnownCallee();
  void rewriteKnownCalleeConventionOnly(SILFunction *callee,
                                        const KnownCallee &pa,
                                        PartialApplyInst *examplePA,
                                        CanSILFunctionType newCalleeTy);
  void rewriteKnownCalleeWithExplicitContext(SILFunction *callee,
                                             const KnownCallee &pa,
                                             PartialApplyInst *examplePA);
};

}

/// True if the partial application is in a form that can be trivially
/// lowered.
///
/// This is true if:
/// - the callee has convention(method)
/// - one argument is applied
/// - the callee is either not generic, or can read its generic environment
///   out of the single applied argument
/// - if the partial application is noescape:
///   - the argument is word-sized or smaller
///   - the argument is either trivial, or passed with a net +0 convention
///     (guaranteed, unowned, in_guaranteed, inout)
/// - if the partial application is escapable:
///   - the argument is either a single Swift-refcounted word, or trivial and
///     sized strictly less than one word
///   - the argument ownership convention matches the callee convention of the
///     resulting function
static bool isSimplePartialApply(SILModule &M,
                                 CanSILFunctionType calleeTy,
                                 TypeExpansionContext context,
                                 ParameterConvention calleeConvention,
                                 unsigned numPartiallyAppliedArgs,
                                 bool isOnStack) {
  if (calleeTy->isPolymorphic()) {
    // TODO: Check if the "self" parameter provides the generic environment
    return false;
  }
  
  if (calleeTy->getRepresentation() != SILFunctionTypeRepresentation::Method) {
    return false;
  }

  // TODO: could discount empty captured values here
  if (numPartiallyAppliedArgs != 1) {
    return false;
  }

  auto contextParam = calleeTy->getSelfParameter();

  auto argTy = contextParam.getArgumentType(M, calleeTy, context);
  if (isOnStack) {
    switch (contextParam.getConvention()) {
    case ParameterConvention::Indirect_Inout:
    case ParameterConvention::Indirect_In_Guaranteed:
    case ParameterConvention::Indirect_InoutAliasable:
    case ParameterConvention::Pack_Inout:
    case ParameterConvention::Pack_Guaranteed:
    case ParameterConvention::Pack_Owned:
      // Indirect and pack arguments are trivially word sized.
      return true;
        
    case ParameterConvention::Direct_Guaranteed:
    case ParameterConvention::Direct_Unowned:
      return SILType::getPrimitiveObjectType(argTy)
        .isPointerSizeAndAligned(M, context.getResilienceExpansion());
      // TODO: If we're running as an IRGen pass, use IRGen's version of
      // `isPointerSizeAndAligned` as a more accurate check.
    
    // +1 arguments need a thunk to stage a copy for the callee to consume.
    case ParameterConvention::Direct_Owned:
    case ParameterConvention::Indirect_In_CXX:
    case ParameterConvention::Indirect_In:
      return false;
    }
  } else {
    if (contextParam.isFormalIndirect()) {
      return false;
    }
    
    // The context parameter's convention must match the callee convention of
    // the resulting closure.
    if (contextParam.getConvention() != calleeConvention) {
      return false;
    }
    
    // The context type must consist of only a swift-refcounted object
    // reference.
    return SILType::getPrimitiveObjectType(argTy)
      .isSingleSwiftRefcounted(M, context.getResilienceExpansion());
  }
  
  return true;
}

static bool isSimplePartialApply(PartialApplyInst *i) {
  return isSimplePartialApply(i->getModule(),
                              i->getCallee()->getType().castTo<SILFunctionType>(),
                              i->getFunction()->getTypeExpansionContext(),
                              i->getFunctionType()->getCalleeConvention(),
                              i->getNumArguments(),
                              i->isOnStack());
}

void PartialApplySimplificationPass::scanFunction(SILFunction *f,
                         llvm::DenseMap<SILFunction *,
                                        KnownCallee> &knownCallees,
                         llvm::SetVector<PartialApplyInst *> &dynamicCallees) {
  // Consider all partial_apply instructions.
  for (auto &block : *f) {
    for (auto &inst : block) {
      // Examine the uses of static function refs.
      if (auto *fr = dyn_cast<FunctionRefInst>(&inst)) {
        auto &knownCallee = knownCallees[fr->getReferencedFunction()];
        knownCallee.FunctionRefs.insert(fr);
        
        for (auto *frUse : fr->getUses()) {
          // Collect partial applications for further transformation.
          if (auto pa = dyn_cast<PartialApplyInst>(frUse->getUser())) {
            knownCallee.PartialApplications.insert(pa);
            continue;
          }
          
          // Collect full apply sites for potential transformation as well.
          if (auto fa = FullApplySite::isa(frUse->getUser())) {
            knownCallee.FullApplications.insert(fa);
            continue;
          }
          
          // Record if the function has uses that aren't partial applies.
          knownCallee.NonApplyUse = frUse->getUser();
        }
      }
      
      if (auto *pa = dyn_cast<PartialApplyInst>(&inst)) {
        // Static callees get handled when we see the function_ref.
        if (isa<FunctionRefInst>(pa->getCallee())) {
          continue;
        }

        // If the callee isn't static, then we'll need to create a dynamic
        // forwarder thunk to simplify this partial application.
        dynamicCallees.insert(pa);
      }
    }
  }
}

void PartialApplySimplificationPass::processKnownCallee(SILFunction *callee,
                                                        const KnownCallee &pa) {
  // Skip functions with no partial application uses.
  if (pa.PartialApplications.empty())
    return;

  LLVM_DEBUG(llvm::dbgs() << "***** Processing known partial_apply callee "
                          << callee->getName() << " *****\n");
  
  // If the subject of the partial application has other uses that aren't
  // partial applications, then thunk it.
  if (pa.NonApplyUse) {
    LLVM_DEBUG(llvm::dbgs() << "Callee has non-apply uses; thunking\n";
               pa.NonApplyUse->print(llvm::dbgs()));
    ++NumPartialApplyCalleesWithNonApplyUses;
    return generateForwardingThunksForKnownCallee();
  }

  // If the subject of the partial application might have external references,
  // or is itself an external reference, we can't change the existing function
  // signature. We'll always use forwarding thunks in this case.
  if (callee->isPossiblyUsedExternally()) {
    LLVM_DEBUG(llvm::dbgs() << "Callee is possibly used externally; thunking\n");
    ++NumPartialApplyCalleesPossiblyUsedExternally;
    return generateForwardingThunksForKnownCallee();
  }
  if (callee->empty()) {
    LLVM_DEBUG(llvm::dbgs() << "Callee is a declaration only; thunking\n");
    ++NumPartialApplyCalleesDeclarationOnly;
    return generateForwardingThunksForKnownCallee();
  }
  
  // Look at the set of all partial applications on this callee to figure
  // out what to do.
  // If all of the partial applications are identical (same number of arguments,
  // same convention, same escapiness, etc.), then we'll alter the invocation
  // function directly (or leave it alone, if the partial apply is simple
  // enough already.)
  
  // Take an arbitrary partial application as an example to compare the others.
  auto examplePA = pa.PartialApplications.front();
  for (auto i = pa.PartialApplications.begin() + 1,
            e = pa.PartialApplications.end();
       i != e;
       ++i) {
    auto thisPA = *i;
    if (examplePA->getNumArguments() != thisPA->getNumArguments()
        || examplePA->getFunctionType()->getCalleeConvention()
            != thisPA->getFunctionType()->getCalleeConvention()
        || !examplePA->getFunctionType()->getExtInfo()
            .isEqualTo(thisPA->getFunctionType()->getExtInfo(), true)) {
      LLVM_DEBUG(llvm::dbgs() << "Mismatched partial application arguments; thunking:\n";
                 thisPA->print(llvm::dbgs());
                 examplePA->print(llvm::dbgs()));
      ++NumPartialApplyCalleesWithMismatchedPartialApplies;
      return generateForwardingThunksForKnownCallee();
    }
  }
  
  // OK, all the partial applications look the same.
  LLVM_DEBUG(llvm::dbgs() << "All partial applications look like this:\n";
             examplePA->print(llvm::dbgs()));
  
  // If they're simple already, then we don't need to do anything.
  if (isSimplePartialApply(examplePA)) {
    LLVM_DEBUG(llvm::dbgs() << "And they're already simple, don't need to do anything!\n");
    return;
  }

  // Would the partial application become simple with a mere convention change?
  auto calleeTyAsMethod = callee->getLoweredFunctionType()
    ->getWithRepresentation(SILFunctionTypeRepresentation::Method);
  if (isSimplePartialApply(callee->getModule(),
                           calleeTyAsMethod,
                           examplePA->getFunction()->getTypeExpansionContext(),
                           examplePA->getFunctionType()->getCalleeConvention(),
                           examplePA->getNumArguments(),
                           examplePA->isOnStack())) {
    return rewriteKnownCalleeConventionOnly(callee, pa, examplePA,
                                            calleeTyAsMethod);
  }
  // TODO: We could also look at whether a ownership convention change on the
  // argument(s) might make it into a simple partial_apply.

  // If the partial applications form escaping closures, and there are also
  // full application sites, then we don't want to burden those full
  // application sites with having to allocate a box for the captured arguments.
  // Emit a thunk for the partial application sites.
  //
  // TODO: Evaluate if stack-allocating the escapable box is acceptable.
  if (!examplePA->isOnStack() && !pa.FullApplications.empty()) {
    LLVM_DEBUG(llvm::dbgs() << "Callee has mix of escaping partial_apply and full application sites; thunking:\n";
               pa.FullApplications.front().getInstruction()->print(llvm::dbgs()));
    ++NumPartialApplyCalleesWithEscapingAndApplyUses;
    return generateForwardingThunksForKnownCallee();
  }
    
  // Rewrite the function type to take the captures in box form.
  rewriteKnownCalleeWithExplicitContext(callee, pa, examplePA);

}

void PartialApplySimplificationPass::processDynamicCallee(PartialApplyInst *pa){
  // TODO
  ++NumDynamicPartialApplicationForwarders;
}

void PartialApplySimplificationPass::generateForwardingThunksForKnownCallee() {
  LLVM_DEBUG(llvm::dbgs() << "TODO: create forwarding thunk here\n");
  return;
}

void PartialApplySimplificationPass::
rewriteKnownCalleeConventionOnly(SILFunction *callee,
                                 const KnownCallee &pa,
                                 PartialApplyInst *examplePA,
                                 CanSILFunctionType newCalleeTy) {
  // Rewrite the type of the invocation function.
  callee->rewriteLoweredTypeUnsafe(newCalleeTy);
  
  // Rewrite the apply sites using the new function type.
  auto rewriteApplySite = [&](ApplySite site) {
    SILBuilder B(*site.getFunction());
    B.setInsertionPoint(site.getInstruction());
    auto loc = site.getLoc();
    auto fr = B.createFunctionRef(loc, callee);
    SILInstruction *newInst;
    
    SmallVector<SILValue, 4> args;
    args.append(site.getArguments().begin(),
                site.getArguments().end());
    
    switch (site.getKind()) {
    case ApplySiteKind::PartialApplyInst: {
      auto pa = cast<PartialApplyInst>(site.getInstruction());
      newInst = B.createPartialApply(loc, fr, site.getSubstitutionMap(), args,
                                     pa->getCalleeConvention(),
                                     pa->getResultIsolation(),
                                     pa->isOnStack());
      break;
    }
    case ApplySiteKind::ApplyInst:
      newInst = B.createApply(loc, fr, site.getSubstitutionMap(), args);
      break;
    case ApplySiteKind::BeginApplyInst:
      newInst = B.createBeginApply(loc, fr, site.getSubstitutionMap(), args);
      break;
    case ApplySiteKind::TryApplyInst: {
      auto tryApply = cast<TryApplyInst>(site.getInstruction());
      newInst = B.createTryApply(loc, fr, site.getSubstitutionMap(), args,
                                 tryApply->getNormalBB(),
                                 tryApply->getErrorBB());
      break;
    }
    }
    site.getInstruction()->replaceAllUsesPairwiseWith(newInst);
    site.getInstruction()->eraseFromParent();
  };
  
  for (auto paSite : pa.PartialApplications) {
    rewriteApplySite(paSite);
  }
  for (auto faSite : pa.FullApplications) {
    rewriteApplySite(faSite);
  }
  
  // Once all the applications have been rewritten, then the original
  // function refs with the old function type should all be unused. Delete
  // them, since they are no longer valid.
  for (auto fr : pa.FunctionRefs) {
    fr->eraseFromParent();
  }
}

void PartialApplySimplificationPass::
rewriteKnownCalleeWithExplicitContext(SILFunction *callee,
                                      const KnownCallee &pa,
                                      PartialApplyInst *examplePA) {
  auto &C = callee->getASTContext();

  auto origTy = callee->getLoweredFunctionType();
  auto paResultTy = cast<SILFunctionType>(examplePA->getType().getASTType());
  // The box captures the generic context and the values of the arguments that
  // were partially applied. The invocation function is modified to take
  // a single partially-applied argument for the box, and unload the
  // elements of the box inside the function.
  SmallVector<SILField, 4> boxFields;
  
  unsigned numUnapplied
    = origTy->getParameters().size() - examplePA->getArguments().size();
  auto partiallyAppliedParams = origTy->getParameters().slice(numUnapplied);
  for (auto param : partiallyAppliedParams) {
    switch (param.getConvention()) {
    // Conventions where a copy of the argument is captured.
    case ParameterConvention::Direct_Guaranteed:
    case ParameterConvention::Direct_Owned:
    case ParameterConvention::Direct_Unowned:
    case ParameterConvention::Indirect_In:
    case ParameterConvention::Indirect_In_Guaranteed:
    case ParameterConvention::Indirect_In_CXX:
    case ParameterConvention::Pack_Guaranteed:
    case ParameterConvention::Pack_Owned:
      boxFields.push_back(SILField(param.getInterfaceType(), /*mutable*/false));
      break;
    
    // Conventions where an address to the argument is captured.
    case ParameterConvention::Indirect_Inout:
    case ParameterConvention::Indirect_InoutAliasable:
    case ParameterConvention::Pack_Inout:
      // Put a RawPointer in the box, which we can turn back into an address
      // in the function
      boxFields.push_back(SILField(C.TheRawPointerType, /*mutable*/false));
      break;
    }
  }
  
  // The new signature carries over the unapplied arguments.
  SmallVector<SILParameterInfo, 4> newParams;
  for (unsigned i = 0; i < numUnapplied; ++i) {
    newParams.push_back(origTy->getParameters()[i]);
  }
  
  // Instead of the applied arguments, we receive a box containing the
  // values for those arguments. Work out what that box type is.
  // TODO: We need a representation of boxes that
  // capture the generic environment to represent partial applications in
  // their full generality.
  if (origTy->getInvocationGenericSignature()) {
    LLVM_DEBUG(llvm::dbgs() << "TODO: generic partial_apply not yet implemented\n");
    ++NumUnsupportedChangesToInvocationFunctions;
    return;
  }

  // TODO: SILBoxType is only implemented for a single field right now, and we
  // don't yet have a corresponding type for nonescaping captures, so
  // represent the captures as a tuple for now.
  CanType tupleTy;

  if (boxFields.size() == 1) {
    tupleTy = boxFields[0].getLoweredType();
  } else {
    llvm::SmallVector<TupleTypeElt, 4> tupleElts;
    for (auto field : boxFields) {
      tupleElts.push_back(TupleTypeElt(field.getLoweredType()));
    }
    tupleTy = TupleType::get(tupleElts, C)->getCanonicalType();
  }
  
  CanType contextTy;
  SILParameterInfo contextParam;
  
  bool isNoEscape = examplePA->getFunctionType()->isNoEscape();
  if (isNoEscape) {
    contextTy = tupleTy;
    // Nonescaping closures borrow their context from the outer frame.
    contextParam = SILParameterInfo(contextTy,
                                 ParameterConvention::Indirect_In_Guaranteed);
  } else {
    SILField tupleField(tupleTy, /*mutable*/ false);
    auto newBoxLayout = SILLayout::get(C,
                                       origTy->getInvocationGenericSignature(),
                                       tupleField,
                                       /*capturesGenerics*/ false);
    SubstitutionMap identitySubstitutionMap;
    if (auto origSig = origTy->getInvocationGenericSignature()) {
      identitySubstitutionMap = origSig->getIdentitySubstitutionMap();
    }
    contextTy = SILBoxType::get(C, newBoxLayout, identitySubstitutionMap);
    contextParam = SILParameterInfo(contextTy,
                                    paResultTy->getCalleeConvention());
  }
  
  newParams.push_back(contextParam);
  
  auto newExtInfo = origTy->getExtInfo()
    .withRepresentation(SILFunctionTypeRepresentation::Method);
  
  auto newTy = SILFunctionType::get(origTy->getInvocationGenericSignature(),
                                    newExtInfo, origTy->getCoroutineKind(),
                                    origTy->getCalleeConvention(),
                                    newParams,
                                    origTy->getYields(),
                                    origTy->getResults(),
                                    origTy->getOptionalErrorResult(),
                                    origTy->getPatternSubstitutions(),
                                    origTy->getInvocationSubstitutions(),
                                    C);
  
  LLVM_DEBUG(llvm::dbgs() << "Changing invocation function signature to\n";
             newTy->print(llvm::dbgs());
             llvm::dbgs() << '\n');
  
  // Change the invocation function to use the new type, and unbox the
  // captures in its entry block.
  callee->rewriteLoweredTypeUnsafe(newTy);
  
  // Update the entry block.
  {
    SILBuilder B(*callee);
    auto &entry = *callee->begin();
    
    // Insert an argument for the context before the originally applied args.
    auto contextArgTy = callee->mapTypeIntoContext(
                                 SILType::getPrimitiveObjectType(contextTy));
    if (isIndirectFormalParameter(contextParam.getConvention())) {
      contextArgTy = contextArgTy.getAddressType();
    }
    
    ValueOwnershipKind contextOwnership(*callee, contextArgTy,
                         SILArgumentConvention(contextParam.getConvention()));

    auto numUnappliedArgs = numUnapplied + origTy->getNumIndirectFormalResults();
    
    auto contextArg = entry.insertFunctionArgument(numUnappliedArgs,
                                               contextArgTy,
                                               contextOwnership);
    auto appliedBBArgs = entry.getArguments().slice(numUnappliedArgs + 1);

    // Replace the original arguments applied by the partial_apply, by
    // projections out of the box.
    SmallVector<AllocStackInst *, 4> AddedStackAllocs;
    B.setInsertionPoint(&entry, entry.begin());
    auto loc = examplePA->getLoc();
    for (unsigned i = 0; i < appliedBBArgs.size(); ++i) {
      auto appliedArg = appliedBBArgs[i];
      auto param = partiallyAppliedParams[i];

      SILValue proj;
      if (isNoEscape) {
        proj = contextArg;
      } else {
        proj = B.createProjectBox(loc, contextArg, 0);
      }
      if (boxFields.size() > 1) {
        proj = B.createTupleElementAddr(loc, proj, i);
      }
      // Load the value out of the context according to the current ownership
      // mode of the function and the calling convention for the parameter.
      SILValue projectedArg;
      if (callee->hasOwnership()) {
        switch (auto conv = param.getConvention()) {
        case ParameterConvention::Direct_Unowned:
          // Load an unowned image of the value from the box.
          projectedArg = B.createLoadUnowned(loc, proj, IsNotTake);
          break;
        case ParameterConvention::Direct_Guaranteed:
          // Load a borrow of the value from the box.
          projectedArg = B.createLoadBorrow(loc, proj);
          break;
        case ParameterConvention::Direct_Owned:
          // Load a copy of the value from the box.
          projectedArg = B.createLoad(loc, proj, LoadOwnershipQualifier::Copy);
          break;
        case ParameterConvention::Indirect_In_CXX:
        case ParameterConvention::Indirect_In: {
          // Allocate space for a copy of the value that can be consumed by the
          // function body. We'll need to deallocate the stack slot after the
          // cloned body.
          auto copySlot = B.createAllocStack(loc,
                                            proj->getType().getAddressType());
          AddedStackAllocs.push_back(copySlot);
          B.createCopyAddr(loc, proj, copySlot, IsNotTake, IsInitialization);
          projectedArg = copySlot;
          break;
        }
        case ParameterConvention::Indirect_In_Guaranteed:
          // We can borrow the value in-place in the box.
          projectedArg = proj;
          break;
        case ParameterConvention::Indirect_Inout:
        case ParameterConvention::Indirect_InoutAliasable: {
          // The box capture is a RawPointer with the value of the capture
          // address.
          auto ptrVal = B.createLoad(loc, proj, LoadOwnershipQualifier::Trivial);
          projectedArg = B.createPointerToAddress(loc, ptrVal,
                      appliedArg->getType(),
                      /*strict*/ conv == ParameterConvention::Indirect_Inout);
          break;
        }
        case ParameterConvention::Pack_Guaranteed:
        case ParameterConvention::Pack_Owned:
        case ParameterConvention::Pack_Inout:
          llvm_unreachable("unsupported!");
          break;
        }
      } else {
        switch (auto conv = param.getConvention()) {
        case ParameterConvention::Direct_Unowned:
          // Load an unowned image of the value from the box.
          projectedArg = B.createLoad(loc, proj, LoadOwnershipQualifier::Unqualified);
          break;
        case ParameterConvention::Direct_Guaranteed:
          // Load a borrow of the value from the box.
          projectedArg = B.createLoad(loc, proj, LoadOwnershipQualifier::Unqualified);
          break;
        case ParameterConvention::Direct_Owned:
          // Load a copy of the value from the box.
          projectedArg = B.createLoad(loc, proj, LoadOwnershipQualifier::Unqualified);
          B.createRetainValue(loc, projectedArg, Atomicity::Atomic);
          break;
        case ParameterConvention::Indirect_In_CXX:
        case ParameterConvention::Indirect_In: {
          // Allocate space for a copy of the value that can be consumed by the
          // function body. We'll need to deallocate the stack slot after the
          // cloned body.
          auto copySlot = B.createAllocStack(loc,
                                             proj->getType().getAddressType());
          AddedStackAllocs.push_back(copySlot);
          B.createCopyAddr(loc, proj, copySlot, IsNotTake, IsInitialization);
          projectedArg = copySlot;
          break;
        }
        case ParameterConvention::Indirect_In_Guaranteed:
          // We can borrow the value in-place in the box.
          projectedArg = proj;
          break;
        case ParameterConvention::Indirect_Inout:
        case ParameterConvention::Indirect_InoutAliasable: {
          // The box capture is a RawPointer with the value of the capture
          // address.
          auto ptrVal = B.createLoad(loc, proj, LoadOwnershipQualifier::Unqualified);
          projectedArg = B.createPointerToAddress(loc, ptrVal,
                      appliedArg->getType(),
                      /*strict*/ conv == ParameterConvention::Indirect_Inout);
          break;
        }
        case ParameterConvention::Pack_Guaranteed:
        case ParameterConvention::Pack_Owned:
        case ParameterConvention::Pack_Inout:
          llvm_unreachable("unsupported!");
          break;
        }
      }
      
      // Replace the original bb arg with the applied arg.
      appliedArg->replaceAllUsesWith(projectedArg);
    }
    
    // If the box is callee-consumed, we can release it now.
    if (contextParam.getConvention() == ParameterConvention::Direct_Owned) {
      if (callee->hasOwnership()) {
        B.createDestroyValue(loc, contextArg);
      } else {
        B.createStrongRelease(loc, contextArg, Atomicity::Atomic);
      }
    }
    
    // Erase the original applied arguments.
    for (unsigned i = 0; i < appliedBBArgs.size(); ++i) {
      entry.eraseArgument(numUnappliedArgs + 1);
    }
    
    // If we needed to introduce any stack slots to consume copies of
    // Indirect_In arguments, then balance them with deallocations on all
    // function exits.
    if (!AddedStackAllocs.empty()) {
      llvm_unreachable("todo");
    }
  }
  
  // Rewrite partial applications to partially apply the new clone.
  auto rewriteApplySite = [&](ApplySite site) {
    auto caller = site->getFunction();
    SILBuilder B(*caller);
    auto loc = site->getLoc();
    B.setInsertionPoint(site.getInstruction());
    
    auto newFunctionRef = B.createFunctionRef(loc, callee);
    SILValue contextBuffer, contextProj;
    auto contextStorageTy = SILType::getPrimitiveAddressType(contextTy)
      .subst(getModule()->Types, site.getSubstitutionMap());
    if (isNoEscape) {
      auto contextAlloc = B.createAllocStack(loc, contextStorageTy);
      contextBuffer = contextProj = contextAlloc;
      
      // We'll need to deallocate the context buffer after we don't need it.
      // For a partial_apply, that's after the partial_apply itself is
      // deallocated.
      if (auto ppa = dyn_cast<PartialApplyInst>(site.getInstruction())) {
        auto deallocStackUses = ppa->getUsersOfType<DeallocStackInst>();
        assert(deallocStackUses.begin() != deallocStackUses.end());
        for (auto use : deallocStackUses) {
          B.setInsertionPoint(use->getNextInstruction());
          B.createDeallocStack(loc, contextBuffer);
        }
      // For a full application, we're done immediately after the call.
      // If the apply site is a terminator, dealloc in all the successor
      // blocks.
      } else if (auto term = dyn_cast<TermInst>(site.getInstruction())) {
        for (auto successor : term->getSuccessorBlocks()) {
          B.setInsertionPoint(successor->begin());
          B.createDeallocStack(loc, contextBuffer);
        }
      // If the apply site is a normal instruction, dealloc after it.
      } else {
        B.setInsertionPoint(site.getInstruction()->getNextInstruction());
        B.createDeallocStack(loc, contextBuffer);
      }
      // Continue emitting code to populate the context.
      B.setInsertionPoint(contextAlloc->getNextInstruction());
    } else {
      contextBuffer = B.createAllocBox(
          loc, contextStorageTy.castTo<SILBoxType>(),
          /*debug variable*/ std::nullopt, DoesNotHaveDynamicLifetime,
          /*reflection*/ true);
      contextProj = B.createProjectBox(loc, contextBuffer, 0);
    }
    
    // Transfer the formerly partially-applied arguments into the box.
    SmallVector<SILValue, 4> newArgs;
    // Carry over non-partial-applied arguments, if any.
    auto appliedArgs = site.getArguments();
    auto paArgsOffset = appliedArgs.size() - boxFields.size();
    for (unsigned i = 0; i < paArgsOffset; ++i) {
      newArgs.push_back(appliedArgs[i]);
    }
    for (unsigned i = 0; i < boxFields.size(); ++i) {
      auto arg = appliedArgs[i + paArgsOffset];
      SILValue proj = contextProj;
      if (boxFields.size() > 1) {
        proj = B.createTupleElementAddr(loc, proj, i);
      }
      auto param = partiallyAppliedParams[i];

      switch (param.getConvention()) {
      case ParameterConvention::Direct_Owned:
      case ParameterConvention::Direct_Unowned:
      case ParameterConvention::Direct_Guaranteed:
        // Move the value into the box.
        if (caller->hasOwnership()) {
          B.createStore(loc, arg, proj, StoreOwnershipQualifier::Init);
        } else {
          B.createStore(loc, arg, proj, StoreOwnershipQualifier::Unqualified);
        }
        break;
          
      case ParameterConvention::Indirect_In_Guaranteed:
      case ParameterConvention::Indirect_In:
      case ParameterConvention::Indirect_In_CXX:
        // Move the value from its current memory location to the box.
        B.createCopyAddr(loc, arg, proj, IsTake, IsInitialization);
        break;
        
      case ParameterConvention::Indirect_InoutAliasable:
      case ParameterConvention::Indirect_Inout: {
        // Pass a pointer to the argument into the box.
        auto p = B.createAddressToPointer(loc, arg,
                                          SILType::getRawPointerType(C),
                                          /*needsStackProtection=*/ false);
        if (caller->hasOwnership()) {
          B.createStore(loc, p, proj, StoreOwnershipQualifier::Trivial);
        } else {
          B.createStore(loc, p, proj, StoreOwnershipQualifier::Unqualified);
        }
        break;
      }

      case ParameterConvention::Pack_Guaranteed:
      case ParameterConvention::Pack_Owned:
      case ParameterConvention::Pack_Inout:
        llvm_unreachable("unsupported!");
        break;
      }
    }
    
    // Transform the application to use the context instead of the original
    // arguments.
    newArgs.push_back(contextBuffer);
    SILInstruction *newInst;
    switch (site.getKind()) {
    case ApplySiteKind::PartialApplyInst: {
      auto oldPA = cast<PartialApplyInst>(site.getInstruction());
      auto paIsolation = oldPA->getResultIsolation();
      auto paConvention = isNoEscape ? ParameterConvention::Direct_Guaranteed
                                     : contextParam.getConvention();
      auto paOnStack = isNoEscape ? PartialApplyInst::OnStack
                                  : PartialApplyInst::NotOnStack;
      auto newPA = B.createPartialApply(loc, newFunctionRef,
                                     site.getSubstitutionMap(),
                                     newArgs,
                                     paConvention,
                                     paIsolation,
                                     paOnStack);
      assert(isSimplePartialApply(newPA)
             && "partial apply wasn't simple after transformation?");
      newInst = newPA;
      break;
    }
    case ApplySiteKind::ApplyInst:
      newInst = B.createApply(loc, newFunctionRef,
                              site.getSubstitutionMap(), newArgs);
      break;
    case ApplySiteKind::BeginApplyInst:
      newInst = B.createBeginApply(loc, newFunctionRef,
                                   site.getSubstitutionMap(), newArgs);
      break;
    case ApplySiteKind::TryApplyInst: {
      auto tai = cast<TryApplyInst>(site.getInstruction());
      newInst = B.createTryApply(loc, newFunctionRef,
                                 site.getSubstitutionMap(), newArgs,
                                 tai->getNormalBB(),
                                 tai->getErrorBB());
      break;
    }
    }
    site.getInstruction()->replaceAllUsesPairwiseWith(newInst);
    site.getInstruction()->eraseFromParent();
  };
  
  for (auto paSite : pa.PartialApplications) {
    rewriteApplySite(paSite);
  }
  
  // Rewrite full application sites to package up the partially applied
  // arguments as well.
  for (auto fa : pa.FullApplications) {
    rewriteApplySite(fa);
  }
  
  // Once all the applications have been rewritten, then the original
  // function refs with the old function type should all be unused. Delete
  // them, since they are no longer valid.
  for (auto fr : pa.FunctionRefs) {
    fr->eraseFromParent();
  }

  ++NumInvocationFunctionsChanged;
  return;
}

SILTransform *swift::createPartialApplySimplification() {
  return new PartialApplySimplificationPass();
}