// 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())
{
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 (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;
}
bool
Sink(MIRGenerator* mir, MIRGraph& graph)
{
TempAllocator& alloc = graph.alloc();
bool sinkEnabled = mir->optimizationInfo().sinkEnabled();
for (PostorderIterator block = graph.poBegin(); block != graph.poEnd(); block++) {
if (mir->shouldCancel("Sink"))
return false;
for (MInstructionReverseIterator iter = block->rbegin(); iter != block->rend(); ) {
MInstruction* ins = *iter++;
// Only instructions which can be recovered on bailout can be moved
// into the bailout paths.
if (ins->isGuard() || ins->isGuardRangeBailouts() ||
ins->isRecoveredOnBailout() || !ins->canRecoverOnBailout())
{
continue;
}
// Compute a common dominator for all uses of the current
// instruction.
bool hasLiveUses = false;
bool hasUses = false;
MBasicBlock* usesDominator = nullptr;
for (MUseIterator i(ins->usesBegin()), e(ins->usesEnd()); i != e; i++) {
hasUses = true;
MNode* consumerNode = (*i)->consumer();
if (consumerNode->isResumePoint())
continue;
MDefinition* consumer = consumerNode->toDefinition();
if (consumer->isRecoveredOnBailout())
continue;
hasLiveUses = true;
// If the instruction is a Phi, then we should dominate the
// predecessor from which the value is coming from.
MBasicBlock* consumerBlock = consumer->block();
if (consumer->isPhi())
consumerBlock = consumerBlock->getPredecessor(consumer->indexOf(*i));
usesDominator = CommonDominator(usesDominator, consumerBlock);
if (usesDominator == *block)
break;
}
// Leave this instruction for DCE.
if (!hasUses)
continue;
// We have no uses, so sink this instruction in all the bailout
// paths.
if (!hasLiveUses) {
MOZ_ASSERT(!usesDominator);
ins->setRecoveredOnBailout();
JitSpewDef(JitSpew_Sink, " No live uses, recover the instruction on bailout\n", ins);
continue;
}
// This guard is temporarly moved here as the above code deals with
// Dead Code elimination, which got moved into this Sink phase, as
// the Dead Code elimination used to move instructions with no-live
// uses to the bailout path.
if (!sinkEnabled)
continue;
// To move an effectful instruction, we would have to verify that the
// side-effect is not observed. In the mean time, we just inhibit
// this optimization on effectful instructions.
if (ins->isEffectful())
continue;
// If all the uses are under a loop, we might not want to work
// against LICM by moving everything back into the loop, but if the
// loop is it-self inside an if, then we still want to move the
// computation under this if statement.
while (block->loopDepth() < usesDominator->loopDepth()) {
MOZ_ASSERT(usesDominator != usesDominator->immediateDominator());
usesDominator = usesDominator->immediateDominator();
}
// Only move instructions if there is a branch between the dominator
// of the uses and the original instruction. This prevent moving the
// computation of the arguments into an inline function if there is
// no major win.
MBasicBlock* lastJoin = usesDominator;
while (*block != lastJoin && lastJoin->numPredecessors() == 1) {
MOZ_ASSERT(lastJoin != lastJoin->immediateDominator());
MBasicBlock* next = lastJoin->immediateDominator();
if (next->numSuccessors() > 1)
break;
lastJoin = next;
}
if (*block == lastJoin)
continue;
// Skip to the next instruction if we cannot find a common dominator
// for all the uses of this instruction, or if the common dominator
// correspond to the block of the current instruction.
if (!usesDominator || usesDominator == *block)
continue;
// Only instruction which can be recovered on bailout and which are
// sinkable can be moved into blocks which are below while filling
// the resume points with a clone which is recovered on bailout.
// If the instruction has live uses and if it is clonable, then we
// can clone the instruction for all non-dominated uses and move the
// instruction into the block which is dominating all live uses.
if (!ins->canClone())
continue;
// If the block is a split-edge block, which is created for folding
// test conditions, then the block has no resume point and has
// multiple predecessors. In such case, we cannot safely move
// bailing instruction to these blocks as we have no way to bailout.
if (!usesDominator->entryResumePoint() && usesDominator->numPredecessors() != 1)
continue;
JitSpewDef(JitSpew_Sink, " Can Clone & Recover, sink instruction\n", ins);
JitSpew(JitSpew_Sink, " into Block %u", usesDominator->id());
// Copy the arguments and clone the instruction.
MDefinitionVector operands(alloc);
for (size_t i = 0, end = ins->numOperands(); i < end; i++) {
if (!operands.append(ins->getOperand(i)))
return false;
}
MInstruction* clone = ins->clone(alloc, operands);
ins->block()->insertBefore(ins, clone);
clone->setRecoveredOnBailout();
// We should not update the producer of the entry resume point, as
// it cannot refer to any instruction within the basic block excepts
// for Phi nodes.
MResumePoint* entry = usesDominator->entryResumePoint();
// Replace the instruction by its clone in all the resume points /
// recovered-on-bailout instructions which are not in blocks which
// are dominated by the usesDominator block.
for (MUseIterator i(ins->usesBegin()), e(ins->usesEnd()); i != e; ) {
MUse* use = *i++;
MNode* consumer = use->consumer();
// If the consumer is a Phi, then we look for the index of the
// use to find the corresponding predecessor block, which is
// then used as the consumer block.
MBasicBlock* consumerBlock = consumer->block();
if (consumer->isDefinition() && consumer->toDefinition()->isPhi()) {
consumerBlock = consumerBlock->getPredecessor(
consumer->toDefinition()->toPhi()->indexOf(use));
}
// Keep the current instruction for all dominated uses, except
// for the entry resume point of the block in which the
// instruction would be moved into.
if (usesDominator->dominates(consumerBlock) &&
(!consumer->isResumePoint() || consumer->toResumePoint() != entry))
{
continue;
}
use->replaceProducer(clone);
}
// As we move this instruction in a different block, we should
// verify that we do not carry over a resume point which would refer
// to an outdated state of the control flow.
if (ins->resumePoint())
ins->clearResumePoint();
// Now, that all uses which are not dominated by usesDominator are
// using the cloned instruction, we can safely move the instruction
// into the usesDominator block.
MInstruction* at = usesDominator->safeInsertTop(nullptr, MBasicBlock::IgnoreRecover);
block->moveBefore(at, ins);
}
}
return true;
}
bool
jit::ReorderInstructions(MIRGraph& graph)
{
// Renumber all instructions in the graph as we go.
size_t nextId = 0;
// List of the headers of any loops we are in.
Vector<MBasicBlock*, 4, SystemAllocPolicy> loopHeaders;
for (ReversePostorderIterator block(graph.rpoBegin()); block != graph.rpoEnd(); block++) {
// Renumber all definitions inside the basic blocks.
for (MPhiIterator iter(block->phisBegin()); iter != block->phisEnd(); iter++)
iter->setId(nextId++);
for (MInstructionIterator iter(block->begin()); iter != block->end(); iter++)
iter->setId(nextId++);
// Don't reorder instructions within entry blocks, which have special requirements.
if (*block == graph.entryBlock() || *block == graph.osrBlock())
continue;
if (block->isLoopHeader()) {
if (!loopHeaders.append(*block))
return false;
}
MBasicBlock* innerLoop = loopHeaders.empty() ? nullptr : loopHeaders.back();
MInstruction* top = block->safeInsertTop();
MInstructionReverseIterator rtop = ++block->rbegin(top);
for (MInstructionIterator iter(block->begin(top)); iter != block->end(); ) {
MInstruction* ins = *iter;
// Filter out some instructions which are never reordered.
if (ins->isEffectful() ||
!ins->isMovable() ||
ins->resumePoint() ||
ins == block->lastIns())
{
iter++;
continue;
}
// Move constants with a single use in the current block to the
// start of the block. Constants won't be reordered by the logic
// below, as they have no inputs. Moving them up as high as
// possible can allow their use to be moved up further, though,
// and has no cost if the constant is emitted at its use.
if (ins->isConstant() &&
ins->hasOneUse() &&
ins->usesBegin()->consumer()->block() == *block &&
!IsFloatingPointType(ins->type()))
{
iter++;
MInstructionIterator targetIter = block->begin();
while (targetIter->isConstant() || targetIter->isInterruptCheck()) {
if (*targetIter == ins)
break;
targetIter++;
}
MoveBefore(*block, *targetIter, ins);
continue;
}
// Look for inputs where this instruction is the last use of that
// input. If we move this instruction up, the input's lifetime will
// be shortened, modulo resume point uses (which don't need to be
// stored in a register, and can be handled by the register
// allocator by just spilling at some point with no reload).
Vector<MDefinition*, 4, SystemAllocPolicy> lastUsedInputs;
for (size_t i = 0; i < ins->numOperands(); i++) {
MDefinition* input = ins->getOperand(i);
if (!input->isConstant() && IsLastUse(ins, input, innerLoop)) {
if (!lastUsedInputs.append(input))
return false;
}
}
// Don't try to move instructions which aren't the last use of any
// of their inputs (we really ought to move these down instead).
if (lastUsedInputs.length() < 2) {
iter++;
continue;
}
MInstruction* target = ins;
for (MInstructionReverseIterator riter = ++block->rbegin(ins); riter != rtop; riter++) {
MInstruction* prev = *riter;
if (prev->isInterruptCheck())
break;
// The instruction can't be moved before any of its uses.
bool isUse = false;
for (size_t i = 0; i < ins->numOperands(); i++) {
if (ins->getOperand(i) == prev) {
isUse = true;
break;
}
}
if (isUse)
break;
// The instruction can't be moved before an instruction that
// stores to a location read by the instruction.
if (prev->isEffectful() &&
(ins->getAliasSet().flags() & prev->getAliasSet().flags()) &&
ins->mightAlias(prev) != MDefinition::AliasType::NoAlias)
{
break;
}
// Make sure the instruction will still be the last use of one
// of its inputs when moved up this far.
for (size_t i = 0; i < lastUsedInputs.length(); ) {
bool found = false;
for (size_t j = 0; j < prev->numOperands(); j++) {
if (prev->getOperand(j) == lastUsedInputs[i]) {
found = true;
break;
}
}
if (found) {
lastUsedInputs[i] = lastUsedInputs.back();
lastUsedInputs.popBack();
} else {
i++;
}
}
if (lastUsedInputs.length() < 2)
break;
// We can move the instruction before this one.
target = prev;
}
iter++;
MoveBefore(*block, target, ins);
}
if (block->isLoopBackedge())
loopHeaders.popBack();
}
return true;
}
