PrunedLiveness.cpp

//===--- PrunedLiveness.cpp - Compute liveness from selected uses ---------===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2014 - 2020 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
//
//===----------------------------------------------------------------------===//

#include "swift/SIL/PrunedLiveness.h"
#include "swift/SIL/BasicBlockDatastructures.h"
#include "swift/SIL/BasicBlockUtils.h"
#include "swift/SIL/OwnershipUtils.h"

using namespace swift;

/// Mark blocks live during a reverse CFG traversal from one specific block
/// containing a user.
void PrunedLiveBlocks::computeUseBlockLiveness(SILBasicBlock *userBB) {
  // If we are visiting this block, then it is not already LiveOut. Mark it
  // LiveWithin to indicate a liveness boundary within the block.
  markBlockLive(userBB, LiveWithin);

  SmallVector<SILBasicBlock *, 8> predBBWorklist({userBB});
  while (!predBBWorklist.empty()) {
    SILBasicBlock *bb = predBBWorklist.pop_back_val();

    // The popped `bb` is live; now mark all its predecessors LiveOut.
    //
    // Traversal terminates at any previously visited block, including the
    // blocks initialized as definition blocks.
    for (auto *predBB : bb->getPredecessorBlocks()) {
      switch (getBlockLiveness(predBB)) {
      case Dead:
        predBBWorklist.push_back(predBB);
        LLVM_FALLTHROUGH;
      case LiveWithin:
        markBlockLive(predBB, LiveOut);
        break;
      case LiveOut:
        break;
      }
    }
  }
}

/// Update the current def's liveness based on one specific use instruction.
///
/// Return the updated liveness of the \p use block (LiveOut or LiveWithin).
///
/// Terminators are not live out of the block.
PrunedLiveBlocks::IsLive PrunedLiveBlocks::updateForUse(SILInstruction *user) {
  SWIFT_ASSERT_ONLY(seenUse = true);

  auto *bb = user->getParent();
  switch (getBlockLiveness(bb)) {
  case LiveOut:
    return LiveOut;
  case LiveWithin:
    return LiveWithin;
  case Dead: {
    // This use block has not yet been marked live. Mark it and its predecessor
    // blocks live.
    computeUseBlockLiveness(bb);
    return getBlockLiveness(bb);
  }
  }
  llvm_unreachable("covered switch");
}

//===----------------------------------------------------------------------===//
//                            MARK: PrunedLiveness
//===----------------------------------------------------------------------===//

void PrunedLiveness::updateForUse(SILInstruction *user, bool lifetimeEnding) {
  auto useBlockLive = liveBlocks.updateForUse(user);
  // Record all uses of blocks on the liveness boundary. For blocks marked
  // LiveWithin, the boundary is considered to be the last use in the block.
  //
  // FIXME: Why is nonLifetimeEndingUsesInLiveOut inside PrunedLiveness, and
  // what does it mean? Blocks may transition to LiveOut later. Or they may
  // already be LiveOut from a previous use. After computing liveness, clients
  // should check uses that are in PrunedLivenessBoundary.
  if (!lifetimeEnding && useBlockLive == PrunedLiveBlocks::LiveOut) {
    if (nonLifetimeEndingUsesInLiveOut)
      nonLifetimeEndingUsesInLiveOut->insert(user);
    return;
  }
  // Note that a user may use the current value from multiple operands. If any
  // of the uses are non-lifetime-ending, then we must consider the user
  // itself non-lifetime-ending; it cannot be a final destroy point because
  // the value of the non-lifetime-ending operand must be kept alive until the
  // end of the user. Consider a call that takes the same value using
  // different conventions:
  //
  //   apply %f(%val, %val) : $(@guaranteed, @owned) -> ()
  //
  // This call is not considered the end of %val's lifetime. The @owned
  // argument must be copied.
  auto iterAndSuccess = users.insert({user, lifetimeEnding});
  if (!iterAndSuccess.second)
    iterAndSuccess.first->second &= lifetimeEnding;
}

bool PrunedLiveness::updateForBorrowingOperand(Operand *op) {
  assert(op->getOperandOwnership() == OperandOwnership::Borrow);

  // A nested borrow scope is considered a use-point at each scope ending
  // instruction.
  //
  // TODO: Handle reborrowed copies by considering the extended borrow
  // scope. Temporarily bail-out on reborrows because we can't handle uses
  // that aren't dominated by currentDef.
  if (!BorrowingOperand(op).visitScopeEndingUses([this](Operand *end) {
        if (end->getOperandOwnership() == OperandOwnership::Reborrow) {
          return false;
        }
        updateForUse(end->getUser(), /*lifetimeEnding*/ false);
        return true;
      })) {
    return false;
  }
  return true;
}

void PrunedLiveness::extendAcrossLiveness(PrunedLiveness &otherLivesness) {
  // update this liveness for all the interesting users in otherLivesness.
  for (std::pair<SILInstruction *, bool> userAndEnd : otherLivesness.users) {
    updateForUse(userAndEnd.first, userAndEnd.second);
  }
}

bool PrunedLiveness::isWithinBoundary(SILInstruction *inst) const {
  SILBasicBlock *block = inst->getParent();
  switch (getBlockLiveness(block)) {
  case PrunedLiveBlocks::Dead:
    return false;
  case PrunedLiveBlocks::LiveWithin:
    break;
  case PrunedLiveBlocks::LiveOut:
    return true;
  }
  // The boundary is within this block. This instruction is before the boundary
  // iff any interesting uses occur after it.
  for (SILInstruction &inst :
         make_range(std::next(inst->getIterator()), block->end())) {
    switch (isInterestingUser(&inst)) {
    case PrunedLiveness::NonUser:
      break;
    case PrunedLiveness::NonLifetimeEndingUse:
    case PrunedLiveness::LifetimeEndingUse:
      return true;
    }
  }
  return false;
}

bool PrunedLiveness::areUsesWithinBoundary(ArrayRef<Operand *> uses,
                                           DeadEndBlocks *deadEndBlocks) const {
  auto checkDeadEnd = [deadEndBlocks](SILInstruction *inst) {
    return deadEndBlocks && deadEndBlocks->isDeadEnd(inst->getParent());
  };

  for (auto *use : uses) {
    auto *user = use->getUser();
    if (!isWithinBoundary(user) && !checkDeadEnd(user))
      return false;
  }
  return true;
}

bool PrunedLiveness::areUsesOutsideBoundary(
    ArrayRef<Operand *> uses, DeadEndBlocks *deadEndBlocks) const {
  auto checkDeadEnd = [deadEndBlocks](SILInstruction *inst) {
    return deadEndBlocks && deadEndBlocks->isDeadEnd(inst->getParent());
  };

  for (auto *use : uses) {
    auto *user = use->getUser();
    if (isWithinBoundary(user) || checkDeadEnd(user))
      return false;
  }
  return true;
}

// An SSA def meets all the criteria for pruned liveness--def dominates all uses
// with no holes in the liverange. The lifetime-ending uses are also
// recorded--destroy_value or end_borrow. However destroy_values may not
// jointly-post dominate if dead-end blocks are present.
void PrunedLiveness::computeSSALiveness(SILValue def) {
  initializeDefBlock(def->getParentBlock());
  for (Operand *use : def->getUses()) {
    updateForUse(use->getUser(), use->isLifetimeEnding());
  }
}

void PrunedLivenessBoundary::visitInsertionPoints(
    llvm::function_ref<void(SILBasicBlock::iterator insertPt)> visitor,
    DeadEndBlocks *deBlocks) {
  for (SILInstruction *user : lastUsers) {
    if (!isa<TermInst>(user)) {
      visitor(std::next(user->getIterator()));
      continue;
    }
    auto *predBB = user->getParent();
    for (SILBasicBlock *succ : predBB->getSuccessors()) {
      if (deBlocks && deBlocks->isDeadEnd(succ))
        continue;

      assert(succ->getSinglePredecessorBlock() == predBB);
      visitor(succ->begin());
    }
  }
  for (SILBasicBlock *edge : boundaryEdges) {
    if (deBlocks && deBlocks->isDeadEnd(edge))
      continue;

    visitor(edge->begin());
  }
}

// Use \p liveness to find the last use in \p bb and add it to \p
// boundary.lastUsers.
static void findLastUserInBlock(SILBasicBlock *bb,
                                PrunedLivenessBoundary &boundary,
                                const PrunedLiveness &liveness) {
  for (auto instIter = bb->rbegin(), endIter = bb->rend(); instIter != endIter;
       ++instIter) {
    auto *inst = &*instIter;
    if (liveness.isInterestingUser(inst) == PrunedLiveness::NonUser)
      continue;

    boundary.lastUsers.push_back(inst);
    return;
  }
  llvm_unreachable("No user in LiveWithin block");
}

void PrunedLivenessBoundary::compute(const PrunedLiveness &liveness) {
  for (SILBasicBlock *bb : liveness.getDiscoveredBlocks()) {
    // Process each block that has not been visited and is not LiveOut.
    switch (liveness.getBlockLiveness(bb)) {
    case PrunedLiveBlocks::LiveOut:
      for (SILBasicBlock *succBB : bb->getSuccessors()) {
        if (liveness.getBlockLiveness(succBB) == PrunedLiveBlocks::Dead) {
          boundaryEdges.push_back(succBB);
        }
      }
      break;
    case PrunedLiveBlocks::LiveWithin: {
      // The liveness boundary is inside this block. Insert a final destroy
      // inside the block if it doesn't already have one.
      findLastUserInBlock(bb, *this, liveness);
      break;
    }
    case PrunedLiveBlocks::Dead:
      llvm_unreachable("All discovered blocks must be live");
    }
  }
}

void PrunedLivenessBoundary::compute(const PrunedLiveness &liveness,
                                     ArrayRef<SILBasicBlock *> postDomBlocks) {
  if (postDomBlocks.empty())
    return; // all paths must be dead-ends or infinite loops

  BasicBlockWorklist blockWorklist(postDomBlocks[0]->getParent());

  // Visit each post-dominating block as the starting point for a
  // backward CFG traversal.
  for (auto *bb : postDomBlocks) {
    blockWorklist.push(bb);
  }
  while (auto *bb = blockWorklist.pop()) {
    // Process each block that has not been visited and is not LiveOut.
    switch (liveness.getBlockLiveness(bb)) {
    case PrunedLiveBlocks::LiveOut:
      // A lifetimeEndBlock may be determined to be LiveOut after analyzing the
      // extended liveness. It is irrelevant for finding the boundary.
      break;
    case PrunedLiveBlocks::LiveWithin: {
      // The liveness boundary is inside this block. Insert a final destroy
      // inside the block if it doesn't already have one.
      findLastUserInBlock(bb, *this, liveness);
      break;
    }
    case PrunedLiveBlocks::Dead:
      // Continue searching upward to find the pruned liveness boundary.
      for (auto *predBB : bb->getPredecessorBlocks()) {
        if (liveness.getBlockLiveness(predBB) == PrunedLiveBlocks::LiveOut) {
          boundaryEdges.push_back(bb);
        } else {
          blockWorklist.pushIfNotVisited(predBB);
        }
      }
      break;
    }
  }
}