src/jit/ValueNumbering.cpp

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
 * vim: set ts=8 sts=4 et sw=4 tw=99:
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

#include "jit/ValueNumbering.h"

#include "jit/AliasAnalysis.h"
#include "jit/IonAnalysis.h"
#include "jit/JitSpewer.h"
#include "jit/MIRGenerator.h"

using namespace js;
using namespace js::jit;

/*
 * Some notes on the main algorithm here:
 *  - The SSA identifier id() is the value number. We do replaceAllUsesWith as
 *    we go, so there's always at most one visible value with a given number.
 *
 *  - Consequently, the GVN algorithm is effectively pessimistic. This means it
 *    is not as powerful as an optimistic GVN would be, but it is simpler and
 *    faster.
 *
 *  - We iterate in RPO, so that when visiting a block, we've already optimized
 *    and hashed all values in dominating blocks. With occasional exceptions,
 *    this allows us to do everything in a single pass.
 *
 *  - When we do use multiple passes, we just re-run the algorithm on the whole
 *    graph instead of doing sparse propagation. This is a tradeoff to keep the
 *    algorithm simpler and lighter on inputs that don't have a lot of
 *    interesting unreachable blocks or degenerate loop induction variables, at
 *    the expense of being slower on inputs that do. The loop for this always
 *    terminates, because it only iterates when code is or will be removed, so
 *    eventually it must stop iterating.
 *
 *  - Values are not immediately removed from the hash set when they go out of
 *    scope. Instead, we check for dominance after a lookup. If the dominance
 *    check fails, the value is removed.
 */

HashNumber ValueNumberer::VisibleValues::ValueHasher::hash(Lookup ins) {
  return ins->valueHash();
}

// Test whether two MDefinitions are congruent.
bool ValueNumberer::VisibleValues::ValueHasher::match(Key k, Lookup l) {
  // If one of the instructions depends on a store, and the other instruction
  // does not depend on the same store, the instructions are not congruent.
  if (k->dependency() != l->dependency()) return false;

  bool congruent =
      k->congruentTo(l);  // Ask the values themselves what they think.
#ifdef JS_JITSPEW
  if (congruent != l->congruentTo(k)) {
    JitSpew(
        JitSpew_GVN,
        "      congruentTo relation is not symmetric between %s%u and %s%u!!",
        k->opName(), k->id(), l->opName(), l->id());
  }
#endif
  return congruent;
}

void ValueNumberer::VisibleValues::ValueHasher::rekey(Key& k, Key newKey) {
  k = newKey;
}

ValueNumberer::VisibleValues::VisibleValues(TempAllocator& alloc)
    : set_(alloc) {}

// Initialize the set.
bool ValueNumberer::VisibleValues::init() { return set_.init(); }

// Look up the first entry for |def|.
ValueNumberer::VisibleValues::Ptr ValueNumberer::VisibleValues::findLeader(
    const MDefinition* def) const {
  return set_.lookup(def);
}

// Look up the first entry for |def|.
ValueNumberer::VisibleValues::AddPtr
ValueNumberer::VisibleValues::findLeaderForAdd(MDefinition* def) {
  return set_.lookupForAdd(def);
}

// Insert a value into the set.
bool ValueNumberer::VisibleValues::add(AddPtr p, MDefinition* def) {
  return set_.add(p, def);
}

// Insert a value onto the set overwriting any existing entry.
void ValueNumberer::VisibleValues::overwrite(AddPtr p, MDefinition* def) {
  set_.replaceKey(p, def);
}

// |def| will be discarded, so remove it from any sets.
void ValueNumberer::VisibleValues::forget(const MDefinition* def) {
  Ptr p = set_.lookup(def);
  if (p && *p == def) set_.remove(p);
}

// Clear all state.
void ValueNumberer::VisibleValues::clear() { set_.clear(); }

#ifdef DEBUG
// Test whether |def| is in the set.
bool ValueNumberer::VisibleValues::has(const MDefinition* def) const {
  Ptr p = set_.lookup(def);
  return p && *p == def;
}
#endif

// Call MDefinition::justReplaceAllUsesWith, and add some GVN-specific asserts.
static void ReplaceAllUsesWith(MDefinition* from, MDefinition* to) {
  MOZ_ASSERT(from != to, "GVN shouldn't try to replace a value with itself");
  MOZ_ASSERT(from->type() == to->type(), "Def replacement has different type");
  MOZ_ASSERT(!to->isDiscarded(),
             "GVN replaces an instruction by a removed instruction");

  // We don't need the extra setting of UseRemoved flags that the regular
  // replaceAllUsesWith does because we do it ourselves.
  from->justReplaceAllUsesWith(to);
}

// Test whether |succ| is a successor of |block|.
static bool HasSuccessor(const MControlInstruction* block,
                         const MBasicBlock* succ) {
  for (size_t i = 0, e = block->numSuccessors(); i != e; ++i) {
    if (block->getSuccessor(i) == succ) return true;
  }
  return false;
}

// Given a block which has had predecessors removed but is still reachable, test
// whether the block's new dominator will be closer than its old one and whether
// it will expose potential optimization opportunities.
static MBasicBlock* ComputeNewDominator(MBasicBlock* block, MBasicBlock* old) {
  MBasicBlock* now = block->getPredecessor(0);
  for (size_t i = 1, e = block->numPredecessors(); i < e; ++i) {
    MBasicBlock* pred = block->getPredecessor(i);
    // Note that dominators haven't been recomputed yet, so we have to check
    // whether now dominates pred, not block.
    while (!now->dominates(pred)) {
      MBasicBlock* next = now->immediateDominator();
      if (next == old) return old;
      if (next == now) {
        MOZ_ASSERT(block == old,
                   "Non-self-dominating block became self-dominating");
        return block;
      }
      now = next;
    }
  }
  MOZ_ASSERT(old != block || old != now,
             "Missed self-dominating block staying self-dominating");
  return now;
}

// Test for any defs which look potentially interesting to GVN.
static bool BlockHasInterestingDefs(MBasicBlock* block) {
  return !block->phisEmpty() || *block->begin() != block->lastIns();
}

// Walk up the dominator tree from |block| to the root and test for any defs
// which look potentially interesting to GVN.
static bool ScanDominatorsForDefs(MBasicBlock* block) {
  for (MBasicBlock* i = block;;) {
    if (BlockHasInterestingDefs(block)) return true;

    MBasicBlock* immediateDominator = i->immediateDominator();
    if (immediateDominator == i) break;
    i = immediateDominator;
  }
  return false;
}

// Walk up the dominator tree from |now| to |old| and test for any defs which
// look potentially interesting to GVN.
static bool ScanDominatorsForDefs(MBasicBlock* now, MBasicBlock* old) {
  MOZ_ASSERT(old->dominates(now),
             "Refined dominator not dominated by old dominator");

  for (MBasicBlock* i = now; i != old; i = i->immediateDominator()) {
    if (BlockHasInterestingDefs(i)) return true;
  }
  return false;
}

// Given a block which has had predecessors removed but is still reachable, test
// whether the block's new dominator will be closer than its old one and whether
// it will expose potential optimization opportunities.
static bool IsDominatorRefined(MBasicBlock* block) {
  MBasicBlock* old = block->immediateDominator();
  MBasicBlock* now = ComputeNewDominator(block, old);

  // If this block is just a goto and it doesn't dominate its destination,
  // removing its predecessors won't refine the dominators of anything
  // interesting.
  MControlInstruction* control = block->lastIns();
  if (*block->begin() == control && block->phisEmpty() && control->isGoto() &&
      !block->dominates(control->toGoto()->target())) {
    return false;
  }

  // We've computed block's new dominator. Test whether there are any
  // newly-dominating definitions which look interesting.
  if (block == old) return block != now && ScanDominatorsForDefs(now);
  MOZ_ASSERT(block != now, "Non-self-dominating block became self-dominating");
  return ScanDominatorsForDefs(now, old);
}

// |def| has just had one of its users release it. If it's now dead, enqueue it
// for discarding, otherwise just make note of it.
bool ValueNumberer::handleUseReleased(MDefinition* def,
                                      UseRemovedOption useRemovedOption) {
  if (IsDiscardable(def)) {
    values_.forget(def);
    if (!deadDefs_.append(def)) return false;
  } else {
    if (useRemovedOption == SetUseRemoved) def->setUseRemovedUnchecked();
  }
  return true;
}

// Discard |def| and anything in its use-def subtree which is no longer needed.
bool ValueNumberer::discardDefsRecursively(MDefinition* def) {
  MOZ_ASSERT(deadDefs_.empty(), "deadDefs_ not cleared");

  return discardDef(def) && processDeadDefs();
}

// Assuming |resume| is unreachable, release its operands.
// It might be nice to integrate this code with prepareForDiscard, however GVN
// needs it to call handleUseReleased so that it can observe when a definition
// becomes unused, so it isn't trivial to do.
bool ValueNumberer::releaseResumePointOperands(MResumePoint* resume) {
  for (size_t i = 0, e = resume->numOperands(); i < e; ++i) {
    if (!resume->hasOperand(i)) continue;
    MDefinition* op = resume->getOperand(i);
    resume->releaseOperand(i);

    // We set the UseRemoved flag when removing resume point operands,
    // because even though we may think we're certain that a particular
    // branch might not be taken, the type information might be incomplete.
    if (!handleUseReleased(op, SetUseRemoved)) return false;
  }
  return true;
}

// Assuming |phi| is dead, release and remove its operands. If an operand
// becomes dead, push it to the discard worklist.
bool ValueNumberer::releaseAndRemovePhiOperands(MPhi* phi) {
  // MPhi saves operands in a vector so we iterate in reverse.
  for (int o = phi->numOperands() - 1; o >= 0; --o) {
    MDefinition* op = phi->getOperand(o);
    phi->removeOperand(o);
    if (!handleUseReleased(op, DontSetUseRemoved)) return false;
  }
  return true;
}

// Assuming |def| is dead, release its operands. If an operand becomes dead,
// push it to the discard worklist.
bool ValueNumberer::releaseOperands(MDefinition* def) {
  for (size_t o = 0, e = def->numOperands(); o < e; ++o) {
    MDefinition* op = def->getOperand(o);
    def->releaseOperand(o);
    if (!handleUseReleased(op, DontSetUseRemoved)) return false;
  }
  return true;
}

// Discard |def| and mine its operands for any subsequently dead defs.
bool ValueNumberer::discardDef(MDefinition* def) {
#ifdef JS_JITSPEW
  JitSpew(JitSpew_GVN, "      Discarding %s %s%u",
          def->block()->isMarked() ? "unreachable" : "dead", def->opName(),
          def->id());
#endif
#ifdef DEBUG
  MOZ_ASSERT(def != nextDef_, "Invalidating the MDefinition iterator");
  if (def->block()->isMarked()) {
    MOZ_ASSERT(!def->hasUses(), "Discarding def that still has uses");
  } else {
    MOZ_ASSERT(IsDiscardable(def), "Discarding non-discardable definition");
    MOZ_ASSERT(!values_.has(def), "Discarding a definition still in the set");
  }
#endif

  MBasicBlock* block = def->block();
  if (def->isPhi()) {
    MPhi* phi = def->toPhi();
    if (!releaseAndRemovePhiOperands(phi)) return false;
    block->discardPhi(phi);
  } else {
    MInstruction* ins = def->toInstruction();
    if (MResumePoint* resume = ins->resumePoint()) {
      if (!releaseResumePointOperands(resume)) return false;
    }
    if (!releaseOperands(ins)) return false;
    block->discardIgnoreOperands(ins);
  }

  // If that was the last definition in the block, it can be safely removed
  // from the graph.
  if (block->phisEmpty() && block->begin() == block->end()) {
    MOZ_ASSERT(block->isMarked(),
               "Reachable block lacks at least a control instruction");

    // As a special case, don't remove a block which is a dominator tree
    // root so that we don't invalidate the iterator in visitGraph. We'll
    // check for this and remove it later.
    if (block->immediateDominator() != block) {
      JitSpew(JitSpew_GVN, "      Block block%u is now empty; discarding",
              block->id());
      graph_.removeBlock(block);
      blocksRemoved_ = true;
    } else {
      JitSpew(JitSpew_GVN,
              "      Dominator root block%u is now empty; will discard later",
              block->id());
    }
  }

  return true;
}

// Recursively discard all the defs on the deadDefs_ worklist.
bool ValueNumberer::processDeadDefs() {
  MDefinition* nextDef = nextDef_;
  while (!deadDefs_.empty()) {
    MDefinition* def = deadDefs_.popCopy();

    // Don't invalidate the MDefinition iterator. This is what we're going
    // to visit next, so we won't miss anything.
    if (def == nextDef) continue;

    if (!discardDef(def)) return false;
  }
  return true;
}

// Test whether |block|, which is a loop header, has any predecessors other than
// |loopPred|, the loop predecessor, which it doesn't dominate.
static bool hasNonDominatingPredecessor(MBasicBlock* block,
                                        MBasicBlock* loopPred) {
  MOZ_ASSERT(block->isLoopHeader());
  MOZ_ASSERT(block->loopPredecessor() == loopPred);

  for (uint32_t i = 0, e = block->numPredecessors(); i < e; ++i) {
    MBasicBlock* pred = block->getPredecessor(i);
    if (pred != loopPred && !block->dominates(pred)) return true;
  }
  return false;
}

// A loop is about to be made reachable only through an OSR entry into one of
// its nested loops. Fix everything up.
bool ValueNumberer::fixupOSROnlyLoop(MBasicBlock* block,
                                     MBasicBlock* backedge) {
  // Create an empty and unreachable(!) block which jumps to |block|. This
  // allows |block| to remain marked as a loop header, so we don't have to
  // worry about moving a different block into place as the new loop header,
  // which is hard, especially if the OSR is into a nested loop. Doing all
  // that would produce slightly more optimal code, but this is so
  // extraordinarily rare that it isn't worth the complexity.
  MBasicBlock* fake =
      MBasicBlock::New(graph_, block->info(), nullptr, MBasicBlock::NORMAL);
  if (fake == nullptr) return false;

  graph_.insertBlockBefore(block, fake);
  fake->setImmediateDominator(fake);
  fake->addNumDominated(1);
  fake->setDomIndex(fake->id());
  fake->setUnreachable();

  // Create zero-input phis to use as inputs for any phis in |block|.
  // Again, this is a little odd, but it's the least-odd thing we can do
  // without significant complexity.
  for (MPhiIterator iter(block->phisBegin()), end(block->phisEnd());
       iter != end; ++iter) {
    MPhi* phi = *iter;
    MPhi* fakePhi = MPhi::New(graph_.alloc(), phi->type());
    fake->addPhi(fakePhi);
    if (!phi->addInputSlow(fakePhi)) return false;
  }

  fake->end(MGoto::New(graph_.alloc(), block));

  if (!block->addPredecessorWithoutPhis(fake)) return false;

  // Restore |backedge| as |block|'s loop backedge.
  block->clearLoopHeader();
  block->setLoopHeader(backedge);

  JitSpew(JitSpew_GVN, "        Created fake block%u", fake->id());
  hasOSRFixups_ = true;
  return true;
}

// Remove the CFG edge between |pred| and |block|, after releasing the phi
// operands on that edge and discarding any definitions consequently made dead.
bool ValueNumberer::removePredecessorAndDoDCE(MBasicBlock* block,
                                              MBasicBlock* pred,
                                              size_t predIndex) {
  MOZ_ASSERT(
      !block->isMarked(),
      "Block marked unreachable should have predecessors removed already");

  // Before removing the predecessor edge, scan the phi operands for that edge
  // for dead code before they get removed.
  MOZ_ASSERT(nextDef_ == nullptr);
  for (MPhiIterator iter(block->phisBegin()), end(block->phisEnd());
       iter != end;) {
    MPhi* phi = *iter++;
    MOZ_ASSERT(!values_.has(phi),
               "Visited phi in block having predecessor removed");
    MOZ_ASSERT(!phi->isGuard());

    MDefinition* op = phi->getOperand(predIndex);
    phi->removeOperand(predIndex);

    nextDef_ = iter != end ? *iter : nullptr;
    if (!handleUseReleased(op, DontSetUseRemoved) || !processDeadDefs())
      return false;

    // If |nextDef_| became dead while we had it pinned, advance the
    // iterator and discard it now.
    while (nextDef_ && !nextDef_->hasUses() &&
           !nextDef_->isGuardRangeBailouts()) {
      phi = nextDef_->toPhi();
      iter++;
      nextDef_ = iter != end ? *iter : nullptr;
      if (!discardDefsRecursively(phi)) return false;
    }
  }
  nextDef_ = nullptr;

  block->removePredecessorWithoutPhiOperands(pred, predIndex);
  return true;
}

// Remove the CFG edge between |pred| and |block|, and if this makes |block|
// unreachable, mark it so, and remove the rest of its incoming edges too. And
// discard any instructions made dead by the entailed release of any phi
// operands.
bool ValueNumberer::removePredecessorAndCleanUp(MBasicBlock* block,
                                                MBasicBlock* pred) {
  MOZ_ASSERT(!block->isMarked(),
             "Removing predecessor on block already marked unreachable");

  // We'll be removing a predecessor, so anything we know about phis in this
  // block will be wrong.
  for (MPhiIterator iter(block->phisBegin()), end(block->phisEnd());
       iter != end; ++iter)
    values_.forget(*iter);

  // If this is a loop header, test whether it will become an unreachable
  // loop, or whether it needs special OSR-related fixups.
  bool isUnreachableLoop = false;
  if (block->isLoopHeader()) {
    if (block->loopPredecessor() == pred) {
      if (MOZ_UNLIKELY(hasNonDominatingPredecessor(block, pred))) {
        JitSpew(JitSpew_GVN,
                "      "
                "Loop with header block%u is now only reachable through an "
                "OSR entry into the middle of the loop!!",
                block->id());
      } else {
        // Deleting the entry into the loop makes the loop unreachable.
        isUnreachableLoop = true;
        JitSpew(JitSpew_GVN,
                "      "
                "Loop with header block%u is no longer reachable",
                block->id());
      }
#ifdef JS_JITSPEW
    } else if (block->hasUniqueBackedge() && block->backedge() == pred) {
      JitSpew(JitSpew_GVN, "      Loop with header block%u is no longer a loop",
              block->id());
#endif
    }
  }

  // Actually remove the CFG edge.
  if (!removePredecessorAndDoDCE(block, pred, block->getPredecessorIndex(pred)))
    return false;

  // We've now edited the CFG; check to see if |block| became unreachable.
  if (block->numPredecessors() == 0 || isUnreachableLoop) {
    JitSpew(JitSpew_GVN, "      Disconnecting block%u", block->id());

    // Remove |block| from its dominator parent's subtree. This is the only
    // immediately-dominated-block information we need to update, because
    // everything dominated by this block is about to be swept away.
    MBasicBlock* parent = block->immediateDominator();
    if (parent != block) parent->removeImmediatelyDominatedBlock(block);

    // Completely disconnect it from the CFG. We do this now rather than
    // just doing it later when we arrive there in visitUnreachableBlock
    // so that we don't leave a partially broken loop sitting around. This
    // also lets visitUnreachableBlock assert that numPredecessors() == 0,
    // which is a nice invariant.
    if (block->isLoopHeader()) block->clearLoopHeader();
    for (size_t i = 0, e = block->numPredecessors(); i < e; ++i) {
      if (!removePredecessorAndDoDCE(block, block->getPredecessor(i), i))
        return false;
    }

    // Clear out the resume point operands, as they can hold things that
    // don't appear to dominate them live.
    if (MResumePoint* resume = block->entryResumePoint()) {
      if (!releaseResumePointOperands(resume) || !processDeadDefs())
        return false;
      if (MResumePoint* outer = block->outerResumePoint()) {
        if (!releaseResumePointOperands(outer) || !processDeadDefs())
          return false;
      }
      MOZ_ASSERT(nextDef_ == nullptr);
      for (MInstructionIterator iter(block->begin()), end(block->end());
           iter != end;) {
        MInstruction* ins = *iter++;
        nextDef_ = iter != end ? *iter : nullptr;
        if (MResumePoint* resume = ins->resumePoint()) {
          if (!releaseResumePointOperands(resume) || !processDeadDefs())
            return false;
        }
      }
      nextDef_ = nullptr;
    } else {
#ifdef DEBUG
      MOZ_ASSERT(block->outerResumePoint() == nullptr,
                 "Outer resume point in block without an entry resume point");
      for (MInstructionIterator iter(block->begin()), end(block->end());
           iter != end; ++iter) {
        MOZ_ASSERT(iter->resumePoint() == nullptr,
                   "Instruction with resume point in block without entry "
                   "resume point");
      }
#endif
    }

    // Use the mark to note that we've already removed all its predecessors,
    // and we know it's unreachable.
    block->mark();
  }

  return true;
}

// Return a simplified form of |def|, if we can.
MDefinition* ValueNumberer::simplified(MDefinition* def) const {
  return def->foldsTo(graph_.alloc());
}

// If an equivalent and dominating value already exists in the set, return it.
// Otherwise insert |def| into the set and return it.
MDefinition* ValueNumberer::leader(MDefinition* def) {
  // If the value isn't suitable for eliminating, don't bother hashing it. The
  // convention is that congruentTo returns false for node kinds that wish to
  // opt out of redundance elimination.
  // TODO: It'd be nice to clean up that convention (bug 1031406).
  if (!def->isEffectful() && def->congruentTo(def)) {
    // Look for a match.
    VisibleValues::AddPtr p = values_.findLeaderForAdd(def);
    if (p) {
      MDefinition* rep = *p;
      if (!rep->isDiscarded() && rep->block()->dominates(def->block())) {
        // We found a dominating congruent value.
        return rep;
      }

      // The congruent value doesn't dominate. It never will again in this
      // dominator tree, so overwrite it.
      values_.overwrite(p, def);
    } else {
      // No match. Add a new entry.
      if (!values_.add(p, def)) return nullptr;
    }

#ifdef JS_JITSPEW
    JitSpew(JitSpew_GVN, "      Recording %s%u", def->opName(), def->id());
#endif
  }

  return def;
}

// Test whether |phi| is dominated by a congruent phi.
bool ValueNumberer::hasLeader(const MPhi* phi,
                              const MBasicBlock* phiBlock) const {
  if (VisibleValues::Ptr p = values_.findLeader(phi)) {
    const MDefinition* rep = *p;
    return rep != phi && rep->block()->dominates(phiBlock);
  }
  return false;
}

// Test whether there are any phis in |header| which are newly optimizable, as a
// result of optimizations done inside the loop. This is not a sparse approach,
// but restarting is rare enough in practice. Termination is ensured by
// discarding the phi triggering the iteration.
bool ValueNumberer::loopHasOptimizablePhi(MBasicBlock* header) const {
  // If the header is unreachable, don't bother re-optimizing it.
  if (header->isMarked()) return false;

  // Rescan the phis for any that can be simplified, since they may be reading
  // values from backedges.
  for (MPhiIterator iter(header->phisBegin()), end(header->phisEnd());
       iter != end; ++iter) {
    MPhi* phi = *iter;
    MOZ_ASSERT_IF(!phi->hasUses(), !DeadIfUnused(phi));

    if (phi->operandIfRedundant() || hasLeader(phi, header))
      return true;  // Phi can be simplified.
  }
  return false;
}

// Visit |def|.
bool ValueNumberer::visitDefinition(MDefinition* def) {
  // Nop does not fit in any of the previous optimization, as its only purpose
  // is to reduce the register pressure by keeping additional resume
  // point. Still, there is no need consecutive list of MNop instructions, and
  // this will slow down every other iteration on the Graph.
  if (def->isNop()) {
    MNop* nop = def->toNop();
    MBasicBlock* block = nop->block();

    // We look backward to know if we can remove the previous Nop, we do not
    // look forward as we would not benefit from the folding made by GVN.
    MInstructionReverseIterator iter = ++block->rbegin(nop);

    // This nop is at the beginning of the basic block, just replace the
    // resume point of the basic block by the one from the resume point.
    if (iter == block->rend()) {
      JitSpew(JitSpew_GVN, "      Removing Nop%u", nop->id());
      nop->moveResumePointAsEntry();
      block->discard(nop);
      return true;
    }

    // The previous instruction is also a Nop, no need to keep it anymore.
    MInstruction* prev = *iter;
    if (prev->isNop()) {
      JitSpew(JitSpew_GVN, "      Removing Nop%u", prev->id());
      block->discard(prev);
      return true;
    }

    // The Nop is introduced to capture the result and make sure the operands
    // are not live anymore when there are no further uses. Though when
    // all operands are still needed the Nop doesn't decrease the liveness
    // and can get removed.
    MResumePoint* rp = nop->resumePoint();
    if (rp && rp->numOperands() > 0 &&
        rp->getOperand(rp->numOperands() - 1) == prev &&
        !nop->block()->lastIns()->isThrow() &&
        !prev->isAssertRecoveredOnBailout()) {
      size_t numOperandsLive = 0;
      for (size_t j = 0; j < prev->numOperands(); j++) {
        for (size_t i = 0; i < rp->numOperands(); i++) {
          if (prev->getOperand(j) == rp->getOperand(i)) {
            numOperandsLive++;
            break;
          }
        }
      }

      if (numOperandsLive == prev->numOperands()) {
        JitSpew(JitSpew_GVN, "      Removing Nop%u", nop->id());
        block->discard(nop);
      }
    }

    return true;
  }

  // Skip optimizations on instructions which are recovered on bailout, to
  // avoid mixing instructions which are recovered on bailouts with
  // instructions which are not.
  if (def->isRecoveredOnBailout()) return true;

  // If this instruction has a dependency() into an unreachable block, we'll
  // need to update AliasAnalysis.
  MDefinition* dep = def->dependency();
  if (dep != nullptr && (dep->isDiscarded() || dep->block()->isDead())) {
    JitSpew(JitSpew_GVN, "      AliasAnalysis invalidated");
    if (updateAliasAnalysis_ && !dependenciesBroken_) {
      // TODO: Recomputing alias-analysis could theoretically expose more
      // GVN opportunities.
      JitSpew(JitSpew_GVN, "        Will recompute!");
      dependenciesBroken_ = true;
    }
    // Temporarily clear its dependency, to protect foldsTo, which may
    // wish to use the dependency to do store-to-load forwarding.
    def->setDependency(def->toInstruction());
  } else {
    dep = nullptr;
  }

  // Look for a simplified form of |def|.
  MDefinition* sim = simplified(def);
  if (sim != def) {
    if (sim == nullptr) return false;

    bool isNewInstruction = sim->block() == nullptr;

    // If |sim| doesn't belong to a block, insert it next to |def|.
    if (isNewInstruction)
      def->block()->insertAfter(def->toInstruction(), sim->toInstruction());

#ifdef JS_JITSPEW
    JitSpew(JitSpew_GVN, "      Folded %s%u to %s%u", def->opName(), def->id(),
            sim->opName(), sim->id());
#endif
    MOZ_ASSERT(!sim->isDiscarded());
    ReplaceAllUsesWith(def, sim);

    // The node's foldsTo said |def| can be replaced by |rep|. If |def| is a
    // guard, then either |rep| is also a guard, or a guard isn't actually
    // needed, so we can clear |def|'s guard flag and let it be discarded.
    def->setNotGuardUnchecked();

    if (def->isGuardRangeBailouts()) sim->setGuardRangeBailoutsUnchecked();

    if (DeadIfUnused(def)) {
      if (!discardDefsRecursively(def)) return false;

      // If that ended up discarding |sim|, then we're done here.
      if (sim->isDiscarded()) return true;
    }

    if (!rerun_ && def->isPhi() && !sim->isPhi()) {
      rerun_ = true;
      JitSpew(JitSpew_GVN,
              "      Replacing phi%u may have enabled cascading optimisations; "
              "will re-run",
              def->id());
    }

    // Otherwise, procede to optimize with |sim| in place of |def|.
    def = sim;

    // If the simplified instruction was already part of the graph, then we
    // probably already visited and optimized this instruction.
    if (!isNewInstruction) return true;
  }

  // Now that foldsTo is done, re-enable the original dependency. Even though
  // it may be pointing into a discarded block, it's still valid for the
  // purposes of detecting congruent loads.
  if (dep != nullptr) def->setDependency(dep);

  // Look for a dominating def which makes |def| redundant.
  MDefinition* rep = leader(def);
  if (rep != def) {
    if (rep == nullptr) return false;
    if (rep->updateForReplacement(def)) {
#ifdef JS_JITSPEW
      JitSpew(JitSpew_GVN, "      Replacing %s%u with %s%u", def->opName(),
              def->id(), rep->opName(), rep->id());
#endif
      ReplaceAllUsesWith(def, rep);

      // The node's congruentTo said |def| is congruent to |rep|, and it's
      // dominated by |rep|. If |def| is a guard, it's covered by |rep|,
      // so we can clear |def|'s guard flag and let it be discarded.
      def->setNotGuardUnchecked();

      if (DeadIfUnused(def)) {
        // discardDef should not add anything to the deadDefs, as the
        // redundant operation should have the same input operands.
        mozilla::DebugOnly<bool> r = discardDef(def);
        MOZ_ASSERT(
            r,
            "discardDef shouldn't have tried to add anything to the worklist, "
            "so it shouldn't have failed");
        MOZ_ASSERT(deadDefs_.empty(),
                   "discardDef shouldn't have added anything to the worklist");
      }
      def = rep;
    }
  }

  return true;
}

// Visit the control instruction at the end of |block|.
bool ValueNumberer::visitControlInstruction(MBasicBlock* block) {
  // Look for a simplified form of the control instruction.
  MControlInstruction* control = block->lastIns();
  MDefinition* rep = simplified(control);
  if (rep == control) return true;

  if (rep == nullptr) return false;

  MControlInstruction* newControl = rep->toControlInstruction();
  MOZ_ASSERT(!newControl->block(),
             "Control instruction replacement shouldn't already be in a block");
#ifdef JS_JITSPEW
  JitSpew(JitSpew_GVN, "      Folded control instruction %s%u to %s%u",
          control->opName(), control->id(), newControl->opName(),
          graph_.getNumInstructionIds());
#endif

  // If the simplification removes any CFG edges, update the CFG and remove
  // any blocks that become dead.
  size_t oldNumSuccs = control->numSuccessors();
  size_t newNumSuccs = newControl->numSuccessors();
  if (newNumSuccs != oldNumSuccs) {
    MOZ_ASSERT(newNumSuccs < oldNumSuccs,
               "New control instruction has too many successors");
    for (size_t i = 0; i != oldNumSuccs; ++i) {
      MBasicBlock* succ = control->getSuccessor(i);
      if (HasSuccessor(newControl, succ)) continue;
      if (succ->isMarked()) continue;
      if (!removePredecessorAndCleanUp(succ, block)) return false;
      if (succ->isMarked()) continue;
      if (!rerun_) {
        if (!remainingBlocks_.append(succ)) return false;
      }
    }
  }

  if (!releaseOperands(control)) return false;
  block->discardIgnoreOperands(control);
  block->end(newControl);
  if (block->entryResumePoint() && newNumSuccs != oldNumSuccs)
    block->flagOperandsOfPrunedBranches(newControl);
  return processDeadDefs();
}

// |block| is unreachable. Mine it for opportunities to delete more dead
// code, and then discard it.
bool ValueNumberer::visitUnreachableBlock(MBasicBlock* block) {
  JitSpew(JitSpew_GVN, "    Visiting unreachable block%u%s%s%s", block->id(),
          block->isLoopHeader() ? " (loop header)" : "",
          block->isSplitEdge() ? " (split edge)" : "",
          block->immediateDominator() == block ? " (dominator root)" : "");

  MOZ_ASSERT(block->isMarked(),
             "Visiting unmarked (and therefore reachable?) block");
  MOZ_ASSERT(block->numPredecessors() == 0,
             "Block marked unreachable still has predecessors");
  MOZ_ASSERT(block != graph_.entryBlock(), "Removing normal entry block");
  MOZ_ASSERT(block != graph_.osrBlock(), "Removing OSR entry block");
  MOZ_ASSERT(deadDefs_.empty(), "deadDefs_ not cleared");

  // Disconnect all outgoing CFG edges.
  for (size_t i = 0, e = block->numSuccessors(); i < e; ++i) {
    MBasicBlock* succ = block->getSuccessor(i);
    if (succ->isDead() || succ->isMarked()) continue;
    if (!removePredecessorAndCleanUp(succ, block)) return false;
    if (succ->isMarked()) continue;
    // |succ| is still reachable. Make a note of it so that we can scan
    // it for interesting dominator tree changes later.
    if (!rerun_) {
      if (!remainingBlocks_.append(succ)) return false;
    }
  }

  // Discard any instructions with no uses. The remaining instructions will be
  // discarded when their last use is discarded.
  MOZ_ASSERT(nextDef_ == nullptr);
  for (MDefinitionIterator iter(block); iter;) {
    MDefinition* def = *iter++;
    if (def->hasUses()) continue;
    nextDef_ = iter ? *iter : nullptr;
    if (!discardDefsRecursively(def)) return false;
  }

  nextDef_ = nullptr;
  MControlInstruction* control = block->lastIns();
  return discardDefsRecursively(control);
}

// Visit all the phis and instructions |block|.
bool ValueNumberer::visitBlock(MBasicBlock* block) {
  MOZ_ASSERT(!block->isMarked(), "Blocks marked unreachable during GVN");
  MOZ_ASSERT(!block->isDead(), "Block to visit is already dead");

  JitSpew(JitSpew_GVN, "    Visiting block%u", block->id());

  // Visit the definitions in the block top-down.
  MOZ_ASSERT(nextDef_ == nullptr);
  for (MDefinitionIterator iter(block); iter;) {
    if (!graph_.alloc().ensureBallast()) return false;
    MDefinition* def = *iter++;

    // Remember where our iterator is so that we don't invalidate it.
    nextDef_ = iter ? *iter : nullptr;

    // If the definition is dead, discard it.
    if (IsDiscardable(def)) {
      if (!discardDefsRecursively(def)) return false;
      continue;
    }

    if (!visitDefinition(def)) return false;
  }
  nextDef_ = nullptr;

  if (!graph_.alloc().ensureBallast()) return false;

  return visitControlInstruction(block);
}

// Visit all the blocks dominated by dominatorRoot.
bool ValueNumberer::visitDominatorTree(MBasicBlock* dominatorRoot) {
  JitSpew(JitSpew_GVN,
          "  Visiting dominator tree (with %" PRIu64
          " blocks) rooted at block%u%s",
          uint64_t(dominatorRoot->numDominated()), dominatorRoot->id(),
          dominatorRoot == graph_.entryBlock()
              ? " (normal entry block)"
              : dominatorRoot == graph_.osrBlock()
                    ? " (OSR entry block)"
                    : dominatorRoot->numPredecessors() == 0
                          ? " (odd unreachable block)"
                          : " (merge point from normal entry and OSR entry)");
  MOZ_ASSERT(dominatorRoot->immediateDominator() == dominatorRoot,
             "root is not a dominator tree root");

  // Visit all blocks dominated by dominatorRoot, in RPO. This has the nice
  // property that we'll always visit a block before any block it dominates,
  // so we can make a single pass through the list and see every full
  // redundance.
  size_t numVisited = 0;
  size_t numDiscarded = 0;
  for (ReversePostorderIterator iter(graph_.rpoBegin(dominatorRoot));;) {
    MOZ_ASSERT(iter != graph_.rpoEnd(), "Inconsistent dominator information");
    MBasicBlock* block = *iter++;
    // We're only visiting blocks in dominatorRoot's tree right now.
    if (!dominatorRoot->dominates(block)) continue;

    // If this is a loop backedge, remember the header, as we may not be able
    // to find it after we simplify the block.
    MBasicBlock* header =
        block->isLoopBackedge() ? block->loopHeaderOfBackedge() : nullptr;

    if (block->isMarked()) {
      // This block has become unreachable; handle it specially.
      if (!visitUnreachableBlock(block)) return false;
      ++numDiscarded;
    } else {
      // Visit the block!
      if (!visitBlock(block)) return false;
      ++numVisited;
    }

    // If the block is/was a loop backedge, check to see if the block that
    // is/was its header has optimizable phis, which would want a re-run.
    if (!rerun_ && header && loopHasOptimizablePhi(header)) {
      JitSpew(JitSpew_GVN,
              "    Loop phi in block%u can now be optimized; will re-run GVN!",
              header->id());
      rerun_ = true;
      remainingBlocks_.clear();
    }

    MOZ_ASSERT(numVisited <= dominatorRoot->numDominated() - numDiscarded,
               "Visited blocks too many times");
    if (numVisited >= dominatorRoot->numDominated() - numDiscarded) break;
  }

  totalNumVisited_ += numVisited;
  values_.clear();
  return true;
}

// Visit all the blocks in the graph.
bool ValueNumberer::visitGraph() {
  // Due to OSR blocks, the set of blocks dominated by a blocks may not be
  // contiguous in the RPO. Do a separate traversal for each dominator tree
  // root. There's always the main entry, and sometimes there's an OSR entry,
  // and then there are the roots formed where the OSR paths merge with the
  // main entry paths.
  for (ReversePostorderIterator iter(graph_.rpoBegin());;) {
    MOZ_ASSERT(iter != graph_.rpoEnd(), "Inconsistent dominator information");
    MBasicBlock* block = *iter;
    if (block->immediateDominator() == block) {
      if (!visitDominatorTree(block)) return false;

      // Normally unreachable blocks would be removed by now, but if this
      // block is a dominator tree root, it has been special-cased and left
      // in place in order to avoid invalidating our iterator. Now that
      // we've finished the tree, increment the iterator, and then if it's
      // marked for removal, remove it.
      ++iter;
      if (block->isMarked()) {
        JitSpew(JitSpew_GVN, "      Discarding dominator root block%u",
                block->id());
        MOZ_ASSERT(
            block->begin() == block->end(),
            "Unreachable dominator tree root has instructions after tree walk");
        MOZ_ASSERT(block->phisEmpty(),
                   "Unreachable dominator tree root has phis after tree walk");
        graph_.removeBlock(block);
        blocksRemoved_ = true;
      }

      MOZ_ASSERT(totalNumVisited_ <= graph_.numBlocks(),
                 "Visited blocks too many times");
      if (totalNumVisited_ >= graph_.numBlocks()) break;
    } else {
      // This block a dominator tree root. Proceed to the next one.
      ++iter;
    }
  }
  totalNumVisited_ = 0;
  return true;
}

bool ValueNumberer::insertOSRFixups() {
  ReversePostorderIterator end(graph_.end());
  for (ReversePostorderIterator iter(graph_.begin()); iter != end;) {
    MBasicBlock* block = *iter++;

    // Only add fixup block above for loops which can be reached from OSR.
    if (!block->isLoopHeader()) continue;

    // If the loop header is not self-dominated, then this loop does not
    // have to deal with a second entry point, so there is no need to add a
    // second entry point with a fixup block.
    if (block->immediateDominator() != block) continue;

    if (!fixupOSROnlyLoop(block, block->backedge())) return false;
  }

  return true;
}

// OSR fixups serve the purpose of representing the non-OSR entry into a loop
// when the only real entry is an OSR entry into the middle. However, if the
// entry into the middle is subsequently folded away, the loop may actually
// have become unreachable. Mark-and-sweep all blocks to remove all such code.
bool ValueNumberer::cleanupOSRFixups() {
  // Mark.
  Vector<MBasicBlock*, 0, JitAllocPolicy> worklist(graph_.alloc());
  unsigned numMarked = 2;
  graph_.entryBlock()->mark();
  graph_.osrBlock()->mark();
  if (!worklist.append(graph_.entryBlock()) ||
      !worklist.append(graph_.osrBlock()))
    return false;
  while (!worklist.empty()) {
    MBasicBlock* block = worklist.popCopy();
    for (size_t i = 0, e = block->numSuccessors(); i != e; ++i) {
      MBasicBlock* succ = block->getSuccessor(i);
      if (!succ->isMarked()) {
        ++numMarked;
        succ->mark();
        if (!worklist.append(succ)) return false;
      } else if (succ->isLoopHeader() && succ->loopPredecessor() == block &&
                 succ->numPredecessors() == 3) {
        // Unmark fixup blocks if the loop predecessor is marked after
        // the loop header.
        succ->getPredecessor(1)->unmarkUnchecked();
      }
    }

    // OSR fixup blocks are needed if and only if the loop header is
    // reachable from its backedge (via the OSR block) and not from its
    // original loop predecessor.
    //
    // Thus OSR fixup blocks are removed if the loop header is not
    // reachable, or if the loop header is reachable from both its backedge
    // and its original loop predecessor.
    if (block->isLoopHeader()) {
      MBasicBlock* maybeFixupBlock = nullptr;
      if (block->numPredecessors() == 2) {
        maybeFixupBlock = block->getPredecessor(0);
      } else {
        MOZ_ASSERT(block->numPredecessors() == 3);
        if (!block->loopPredecessor()->isMarked())
          maybeFixupBlock = block->getPredecessor(1);
      }

      if (maybeFixupBlock && !maybeFixupBlock->isMarked() &&
          maybeFixupBlock->numPredecessors() == 0) {
        MOZ_ASSERT(maybeFixupBlock->numSuccessors() == 1,
                   "OSR fixup block should have exactly one successor");
        MOZ_ASSERT(maybeFixupBlock != graph_.entryBlock(),
                   "OSR fixup block shouldn't be the entry block");
        MOZ_ASSERT(maybeFixupBlock != graph_.osrBlock(),
                   "OSR fixup block shouldn't be the OSR entry block");
        maybeFixupBlock->mark();
      }
    }
  }

  // And sweep.
  return RemoveUnmarkedBlocks(mir_, graph_, numMarked);
}

ValueNumberer::ValueNumberer(MIRGenerator* mir, MIRGraph& graph)
    : mir_(mir),
      graph_(graph),
      values_(graph.alloc()),
      deadDefs_(graph.alloc()),
      remainingBlocks_(graph.alloc()),
      nextDef_(nullptr),
      totalNumVisited_(0),
      rerun_(false),
      blocksRemoved_(false),
      updateAliasAnalysis_(false),
      dependenciesBroken_(false),
      hasOSRFixups_(false) {}

bool ValueNumberer::init() {
  // Initialize the value set. It's tempting to pass in a size here of some
  // function of graph_.getNumInstructionIds(), however if we start out with a
  // large capacity, it will be far larger than the actual element count for
  // most of the pass, so when we remove elements, it would often think it
  // needs to compact itself. Empirically, just letting the HashTable grow as
  // needed on its own seems to work pretty well.
  return values_.init();
}

bool ValueNumberer::run(UpdateAliasAnalysisFlag updateAliasAnalysis) {
  updateAliasAnalysis_ = updateAliasAnalysis == UpdateAliasAnalysis;

  JitSpew(JitSpew_GVN, "Running GVN on graph (with %" PRIu64 " blocks)",
          uint64_t(graph_.numBlocks()));

  // Adding fixup blocks only make sense iff we have a second entry point into
  // the graph which cannot be reached any more from the entry point.
  if (graph_.osrBlock()) {
    if (!insertOSRFixups()) return false;
  }

  // Top level non-sparse iteration loop. If an iteration performs a
  // significant change, such as discarding a block which changes the
  // dominator tree and may enable more optimization, this loop takes another
  // iteration.
  int runs = 0;
  for (;;) {
    if (!visitGraph()) return false;

    // Test whether any block which was not removed but which had at least
    // one predecessor removed will have a new dominator parent.
    while (!remainingBlocks_.empty()) {
      MBasicBlock* block = remainingBlocks_.popCopy();
      if (!block->isDead() && IsDominatorRefined(block)) {
        JitSpew(JitSpew_GVN,
                "  Dominator for block%u can now be refined; will re-run GVN!",
                block->id());
        rerun_ = true;
        remainingBlocks_.clear();
        break;
      }
    }

    if (blocksRemoved_) {
      if (!AccountForCFGChanges(mir_, graph_, dependenciesBroken_,
                                /* underValueNumberer = */ true))
        return false;

      blocksRemoved_ = false;
      dependenciesBroken_ = false;
    }

    if (mir_->shouldCancel("GVN (outer loop)")) return false;

    // If no further opportunities have been discovered, we're done.
    if (!rerun_) break;

    rerun_ = false;

    // Enforce an arbitrary iteration limit. This is rarely reached, and
    // isn't even strictly necessary, as the algorithm is guaranteed to
    // terminate on its own in a finite amount of time (since every time we
    // re-run we discard the construct which triggered the re-run), but it
    // does help avoid slow compile times on pathological code.
    ++runs;
    if (runs == 6) {
      JitSpew(JitSpew_GVN, "Re-run cutoff of %d reached. Terminating GVN!",
              runs);
      break;
    }

    JitSpew(JitSpew_GVN,
            "Re-running GVN on graph (run %d, now with %" PRIu64 " blocks)",
            runs, uint64_t(graph_.numBlocks()));
  }

  if (MOZ_UNLIKELY(hasOSRFixups_)) {
    if (!cleanupOSRFixups()) return false;
    hasOSRFixups_ = false;
  }

  return true;
}