PhiStorageOptimizer.cpp

//===--- PhiStorageOptimizer.cpp - Phi storage optimizer ------------------===//
//
// 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
//
//===----------------------------------------------------------------------===//
///
/// PhiStorageOptimizer implements an analysis used by AddressLowering
/// to reuse storage across block arguments.
///
/// In OSSA, phi operands can often be coalesced because they are
/// consuming--they end the lifetime of their operand. This optimization may
/// fail to coalesce an operand for two major reasons:
///
/// 1. This phi operand is already coalesced with other storage, possibly of a
/// different type:
///
///     %field = struct_extract %struct : $Struct<T>, #field
///     br bb(%field : $T)
///
///   bb(%phi : @owned $T):
///     ...
///
/// 2. This phi operand interferes with another coalesced phi operand.
///
/// Only one of the call results below, either %get0 or %get1, can be coalesced
/// with %phi. The %phi will itself be coalesced with this function's indirect
/// @out argument.
///
///   sil [ossa] @function : $@convention(thin) <T> () -> @out T {
///   bb0:
///     %get0 = apply %get<T>() : $@convention(thin) <τ_0_0>() -> @out τ_0_0
///     %get1 = apply %get<T>() : $@convention(thin) <τ_0_0>() -> @out τ_0_0
///     cond_br undef, bb2, bb1
///
///   bb1:
///     destroy_value %get0 : $T
///     br bb3(%get1 : $T)
///
///   bb2:
///     destroy_value %get1 : $T
///     br bb3(%get0 : $T)
///
///   bb3(%phi : @owned $T):
///     return %phi : $T
///
/// TODO: Liveness is currently recorded at the block level. This could be
/// extended to handle operand with nonoverlapping liveness in the same
/// block. In this case, %get0 and %get1 could both be coalesced with a bit of
/// extra book-keeping:
///
///   bb0:
///     %get0 = apply %get<T>() : $@convention(thin) <τ_0_0>() -> @out τ_0_0
///     cond_br undef, bb1, bb2
///
///   bb1:
///     destroy_value %get0 : $T
///     %get1 = apply %get<T>() : $@convention(thin) <τ_0_0>() -> @out τ_0_0
///     br bb3(%get1 : $T)
///
///   bb2:
///     br bb3(%get0 : $T)
///
///   bb3(%phi : @owned $T):
///
/// TODO: This does not yet coalesce the copy_value instructions that produce a
/// phi operand. Such a copy implies that both the operand and phi value are
/// live past the phi. Nonetheless, they could still be coalesced as
/// follows... First coalesce all direct phi operands. Then transitively
/// coalesce copies by checking if the copy's source is coalesceable, then
/// redoing the liveness traversal from the uses of the copy.
///
/// TODO: This approach uses on-the-fly liveness discovery for all incoming
/// values at once. It requires no storage for liveness. Hopefully this is
/// sufficient for -Onone. At -O, we could explore implementing strong phi
/// elimination. However, that depends the ability to perform interference
/// checks between arbitrary storage locations, which requires computing and
/// storing liveness per-storage location.
///
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "address-lowering"

#include "PhiStorageOptimizer.h"
#include "swift/Basic/Assertions.h"
#include "swift/SIL/BasicBlockDatastructures.h"
#include "swift/SIL/Dominance.h"
#include "swift/SIL/NodeDatastructures.h"
#include "swift/SIL/SILBasicBlock.h"
#include "swift/SIL/SILInstruction.h"

using namespace swift;

namespace swift {

/// An analysis used by AddressLowering to reuse phi storage.
///
/// Populates CoalescedPhi::coalescedOperands with all phi operands that can
/// reuse the phi's storage.
class PhiStorageOptimizer {
  PhiValue phi;
  const ValueStorageMap &valueStorageMap;
  DominanceInfo *domInfo;

  CoalescedPhi &coalescedPhi;

  BasicBlockSet occupiedBlocks;

public:
  PhiStorageOptimizer(PhiValue phi, const ValueStorageMap &valueStorageMap,
                      CoalescedPhi &coalescedPhi, DominanceInfo *domInfo)
      : phi(phi), valueStorageMap(valueStorageMap), domInfo(domInfo),
        coalescedPhi(coalescedPhi), occupiedBlocks(getFunction()) {}

  SILFunction *getFunction() const { return phi.phiBlock->getParent(); }

  void optimize();

protected:
  using ProjectedValues = StackList<SILValue>;
  bool findUseProjections(SILValue value, SmallVectorImpl<Operand *> &operands);
  bool findDefProjections(SILValue value,
                          SmallVectorImpl<SILValue> *projecteds);
  SILBasicBlock *canCoalesceValue(SILValue incomingVal,
                                  ProjectedValues *projectedValues);
  void tryCoalesceOperand(SILBasicBlock *incomingPred);
  bool recordUseLiveness(ProjectedValues &incomingVals,
                         SILBasicBlock *dominatingBlock,
                         BasicBlockSetVector &liveBlocks);
  SILBasicBlock *getDominatingBlock(SILValue incomingVal,
                                    ProjectedValues *projectedValues);
};

} // namespace swift

void CoalescedPhi::coalesce(PhiValue phi,
                            const ValueStorageMap &valueStorageMap,
                            DominanceInfo *domInfo) {
  assert(empty() && "attempt to recoalesce the same phi");

  PhiStorageOptimizer(phi, valueStorageMap, *this, domInfo).optimize();
}

/// Optimize phi storage by coalescing phi operands.
///
/// Finds all non-interfering phi operands and adds them to the result's
/// coalescedOperands. The algorithm can be described in the abstract as follows
/// (assuming no critical edges):
///
/// All blocks are in one of three states at any point:
/// - clean (not present in the live or occupied set)
/// - live
/// - occupied
///
/// All blocks start clean.
///
/// For each incoming value:
///
///   For all uses of the current incoming value:
///
///     Scan the CFG backward following predecessors.
///     If the current block is:
///
///       Clean: mark it live and continue scanning.
///
///       Live: stop scanning and continue with the next use.
///
///       Occupied: record interference, stop scanning, continue to next use.
///
///   If no occupied blocks were reached, mark this phi operand coalesced. It's
///   storage can be projected from the phi storage.
///
///   Mark all live blocks occupied.
///
/// In the end, we have a set of non-interfering incoming values that can reuse
/// the phi's storage.
void PhiStorageOptimizer::optimize() {
  // The single incoming value case always projects storage.
  if (auto *predecessor = phi.phiBlock->getSinglePredecessorBlock()) {
    if (canCoalesceValue(phi.getValue()->getIncomingPhiValue(predecessor),
                         /*projectedValues=*/nullptr)) {
      // Storage will always be allocated for the phi.  The optimization
      // attempts to let incoming values reuse the phi's storage.  This isn't
      // always possible, even in the single incoming value case.  For example,
      // it isn't possible when the incoming value is a function argument or
      // when the incoming value is already a projection.
      coalescedPhi.coalescedOperands.push_back(phi.getOperand(predecessor));
    }
    return;
  }
  for (auto *incomingPred : phi.phiBlock->getPredecessorBlocks()) {
    tryCoalesceOperand(incomingPred);
  }
}

/// Return \p value's defining instruction's operand which is a composing use
/// projection into \p defInst's storage, or nullptr if none exists.
///
/// Precondition: \p value is defined by an instruction.
bool PhiStorageOptimizer::findUseProjections(
    SILValue value, SmallVectorImpl<Operand *> &operands) {
  auto *defInst = value->getDefiningInstruction();
  auto ordinal = valueStorageMap.getOrdinal(value);
  bool found = false;
  for (Operand &oper : defInst->getAllOperands()) {
    if (valueStorageMap.isComposingUseProjection(&oper) &&
        valueStorageMap.getStorage(oper.get()).projectedStorageID == ordinal) {
      found = true;
      operands.push_back(&oper);
    }
  }
  return found;
}

bool PhiStorageOptimizer::findDefProjections(
    SILValue value, SmallVectorImpl<SILValue> *projecteds) {
  bool found = false;
  for (auto user : value->getUsers()) {
    for (auto result : user->getResults()) {
      auto *storage = valueStorageMap.getStorageOrNull(result);
      if (!storage)
        continue;
      if (storage->isDefProjection) {
        assert(storage->projectedStorageID ==
               valueStorageMap.getOrdinal(value));
        projecteds->push_back(result);
        found = true;
      }
    }
  }
  return found;
}

// Determine whether storage reuse is possible with \p incomingVal.  Ignore
// possible interference.  If reuse is possible, return the block which
// dominates all defs whose storage is projected out of incomingVal.
//
// Precondition: \p incomingVal is an operand of this phi.
SILBasicBlock *
PhiStorageOptimizer::canCoalesceValue(SILValue incomingVal,
                                      ProjectedValues *projectedValues) {
  // A Phi must not project from storage that was initialized on a path that
  // reaches the phi because other uses of the storage may interfere with the
  // phi. A phi may, however, be a composing use projection.
  assert(!valueStorageMap.getStorage(phi.getValue()).isDefProjection
         && !valueStorageMap.getStorage(phi.getValue()).isPhiProjection());

  auto &incomingStorage = valueStorageMap.getStorage(incomingVal);

  // If the incoming use directly reuses its def storage, projects out of its
  // def storage, or is pre-allocated, then it can't be coalesced. When incoming
  // storage is directly reused, isAllocated() is false. isProjection() covers
  // the other cases.
  //
  // Coalescing use projections from incomingVal into its other non-phi uses
  // could be handled, but would require by recursively following uses across
  // projections when computing liveness.
  if (!incomingStorage.isAllocated() || incomingStorage.isProjection())
    return nullptr;

  if (PhiValue(incomingVal)) {
    // Do not coalesce a phi with other phis. This would require liveness
    // analysis of the whole phi web before coalescing phi operands.
    return nullptr;
  }

  // Don't coalesce an incoming value unless it's storage is from a stack
  // allocation, which can be replaced with another alloc_stack.
  if (!isa<AllocStackInst>(incomingStorage.storageAddress))
    return nullptr;

  return getDominatingBlock(incomingVal, projectedValues);
}

// Process a single incoming phi operand. Compute the value's liveness while
// checking for interference. If no interference exists, mark it coalesced.
void PhiStorageOptimizer::tryCoalesceOperand(SILBasicBlock *incomingPred) {
  Operand *incomingOper = phi.getOperand(incomingPred);
  SILValue incomingVal = incomingOper->get();

  ProjectedValues projectedValues(incomingPred->getFunction());
  auto *dominatingBlock = canCoalesceValue(incomingVal, &projectedValues);
  if (!dominatingBlock)
    return;

  BasicBlockSetVector liveBlocks(getFunction());
  if (!recordUseLiveness(projectedValues, dominatingBlock, liveBlocks))
    return;

  for (auto *block : liveBlocks) {
    occupiedBlocks.insert(block);
  }
  assert(occupiedBlocks.contains(incomingPred));
  coalescedPhi.coalescedOperands.push_back(incomingOper);
}

/// The block which dominates all the defs whose storage is projected out of the
/// storage for \p incomingVal.
///
/// Populates \p projectedValues with the values whose storage is recursively
/// projected out of the storage for incomingVal.
SILBasicBlock *
PhiStorageOptimizer::getDominatingBlock(SILValue incomingVal,
                                        ProjectedValues *projectedValues) {
  assert(domInfo);

  // Recursively find the set of value definitions and the dominating LCA.
  ValueWorklist values(incomingVal->getFunction());

  values.push(incomingVal);

  SILBasicBlock *lca = incomingVal->getParentBlock();
  auto updateLCA = [&](SILBasicBlock *other) {
    lca = domInfo->findNearestCommonDominator(lca, other);
  };

  while (auto value = values.pop()) {
    if (projectedValues)
      projectedValues->push_back(value);
    auto *defInst = value->getDefiningInstruction();
    if (!defInst) {
      assert(!PhiValue(value));
      updateLCA(value->getParentBlock());
      continue;
    }
    SmallVector<Operand *, 2> operands;
    if (findUseProjections(value, operands)) {
      assert(operands.size() > 0);
      // Any operand whose storage is a use-projection out of `value`'s storage
      // dominates `value` (i.e. `defInst`), so skip updating the LCA here.
      for (auto *operand : operands) {
        values.pushIfNotVisited(operand->get());
      }
      continue;
    }
    SmallVector<SILValue, 2> projecteds;
    if (findDefProjections(value, &projecteds)) {
      assert(projecteds.size() > 0);
      // Walking up the storage projection chain from this point, every
      // subsequent projection must be a def projection
      // [projection_chain_structure]. Every such projection is dominated by
      // `value` (i.e. defInst).
      //
      // If the walk were only updating the LCA, it could stop here.
      //
      // It is also collecting values whose storage is projected out of the
      // phi, however, so the walk must continue.
      updateLCA(defInst->getParent());
      for (auto projected : projecteds) {
        values.pushIfNotVisited(projected);
      }
      continue;
    }
    updateLCA(defInst->getParent());
  }
  return lca;
}

// Record liveness generated by uses of \p projectedVals.
//
// Return true if no interference was detected along the way.
bool PhiStorageOptimizer::recordUseLiveness(ProjectedValues &projectedVals,
                                            SILBasicBlock *dominatingBlock,
                                            BasicBlockSetVector &liveBlocks) {
  assert(liveBlocks.empty());

  for (auto projectedVal : projectedVals) {
    for (auto *use : projectedVal->getUses()) {
      StackList<SILBasicBlock *> liveBBWorklist(getFunction());

      // If \p liveBB is already occupied by another value, return
      // false. Otherwise, mark \p liveBB live and push it onto liveBBWorklist.
      auto visitLiveBlock = [&](SILBasicBlock *liveBB) {
        assert(liveBB != phi.phiBlock && "phi operands are consumed");

        if (occupiedBlocks.contains(liveBB))
          return false;

        // Stop liveness traversal at dominatingBlock.
        if (liveBlocks.insert(liveBB) && liveBB != dominatingBlock) {
          liveBBWorklist.push_back(liveBB);
        }
        return true;
      };
      if (!visitLiveBlock(use->getUser()->getParent()))
        return false;

      while (!liveBBWorklist.empty()) {
        auto *succBB = liveBBWorklist.pop_back_val();
        for (auto *predBB : succBB->getPredecessorBlocks()) {
          if (!visitLiveBlock(predBB))
            return false;
        }
      }
    }
  }
  return true;
}