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/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
* vim: set ts=8 sts=2 et sw=2 tw=80:
* 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
#include "jit/ValueNumbering.h"
#include "jit/IonAnalysis.h"
#include "jit/JitSpewer.h"
#include "jit/MIRGenerator.h"
#include "jit/MIRGraph.h"
using namespace js;
using namespace js::jit;
/*
* [SMDOC] IonMonkey Value Numbering
*
* 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) {}
// 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 ImplicitlyUsed 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,
ImplicitUseOption implicitUseOption) {
if (IsDiscardable(def)) {
values_.forget(def);
if (!deadDefs_.append(def)) {
return false;
}
} else {
if (implicitUseOption == SetImplicitUse) {
def->setImplicitlyUsedUnchecked();
}
}
return true;
}
// Discard |def| and anything in its use-def subtree which is no longer needed.
bool ValueNumberer::discardDefsRecursively(MDefinition* def,
AllowEffectful allowEffectful) {
MOZ_ASSERT(deadDefs_.empty(), "deadDefs_ not cleared");
return discardDef(def, allowEffectful) && 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 ImplicitlyUsed 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, SetImplicitUse)) {
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, DontSetImplicitUse)) {
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, DontSetImplicitUse)) {
return false;
}
}
return true;
}
// Discard |def| and mine its operands for any subsequently dead defs.
bool ValueNumberer::discardDef(MDefinition* def,
AllowEffectful allowEffectful) {
#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(allowEffectful == AllowEffectful::Yes
? IsDiscardableAllowEffectful(def)
: 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) {
// 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::NewFakeLoopPredecessor(graph_, block);
if (!fake) {
return false;
}
fake->setImmediateDominator(fake);
fake->addNumDominated(1);
fake->setDomIndex(fake->id());
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, DontSetImplicitUse) || !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.
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) {
#ifdef DEBUG
if (sim->isObjectKeysLength() && def->isArrayLength()) {
// /!\ Exception: MArrayLength::foldsTo replaces a sequence of
// instructions containing an effectful instruction by an effectful
// instruction.
} else {
// Otherwise, a new |sim| node mustn't be effectful when |def| wasn't
// effectful.
MOZ_ASSERT_IF(sim->isEffectful(), def->isEffectful());
}
#endif
// If both instructions are effectful, |sim| must have stolen the resume
// point of |def| when it's a new instruction.
MOZ_ASSERT_IF(def->isEffectful() && sim->isEffectful(),
!def->toInstruction()->resumePoint() &&
sim->toInstruction()->resumePoint());
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 (sim->bailoutKind() == BailoutKind::Unknown) {
sim->setBailoutKind(def->bailoutKind());
}
// Discard |def| if it's now unused. Similar to guards, we allow to replace
// effectful instructions when the node's foldsTo method said |def| can be
// replaced.
if (DeadIfUnusedAllowEffectful(def)) {
if (!discardDefsRecursively(def, AllowEffectful::Yes)) {
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->isPhi()) {
MOZ_ASSERT(def->isPhi());
rep->toPhi()->updateForReplacement(def->toPhi());
}
#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");
}
}
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)) {
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),
// Initialize the value set. It's tempting to pass in a length that is a
// function of graph_.getNumInstructionIds(). But 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.
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::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;
}