bool
EagerSimdUnbox(MIRGenerator* mir, MIRGraph& graph)
{
const JitCompartment* jitCompartment = GetJitContext()->compartment->jitCompartment();
for (PostorderIterator block = graph.poBegin(); block != graph.poEnd(); block++) {
if (mir->shouldCancel("Eager Simd Unbox"))
return false;
for (MInstructionReverseIterator ins = block->rbegin(); ins != block->rend(); ins++) {
if (!ins->isSimdUnbox())
continue;
MSimdUnbox* unbox = ins->toSimdUnbox();
if (!unbox->input()->isPhi())
continue;
MPhi* phi = unbox->input()->toPhi();
if (!CanUnboxSimdPhi(jitCompartment, phi, unbox->simdType()))
continue;
UnboxSimdPhi(jitCompartment, graph, phi, unbox->simdType());
}
}
return true;
}
// Instructions are useless if they are unused and have no side effects.
// This pass eliminates useless instructions.
// The graph itself is unchanged.
bool
ion::EliminateDeadCode(MIRGenerator *mir, MIRGraph &graph)
{
// Traverse in postorder so that we hit uses before definitions.
// Traverse instruction list backwards for the same reason.
for (PostorderIterator block = graph.poBegin(); block != graph.poEnd(); block++) {
if (mir->shouldCancel("Eliminate Dead Code (main loop)"))
return false;
// Remove unused instructions.
for (MInstructionReverseIterator inst = block->rbegin(); inst != block->rend(); ) {
if (!inst->isEffectful() && !inst->hasUses() && !inst->isGuard() &&
!inst->isControlInstruction()) {
inst = block->discardAt(inst);
} else {
inst++;
}
}
}
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
ion::EliminatePhis(MIRGenerator *mir, MIRGraph &graph)
{
Vector<MPhi *, 16, SystemAllocPolicy> worklist;
// Add all observable phis to a worklist. We use the "in worklist" bit to
// mean "this phi is live".
for (PostorderIterator block = graph.poBegin(); block != graph.poEnd(); block++) {
if (mir->shouldCancel("Eliminate Phis (populate loop)"))
return false;
MPhiIterator iter = block->phisBegin();
while (iter != block->phisEnd()) {
// Flag all as unused, only observable phis would be marked as used
// when processed by the work list.
iter->setUnused();
// If the phi is redundant, remove it here.
if (MDefinition *redundant = IsPhiRedundant(*iter)) {
iter->replaceAllUsesWith(redundant);
iter = block->discardPhiAt(iter);
continue;
}
// Enqueue observable Phis.
if (IsPhiObservable(*iter)) {
iter->setInWorklist();
if (!worklist.append(*iter))
return false;
}
iter++;
}
}
// Iteratively mark all phis reachable from live phis.
while (!worklist.empty()) {
if (mir->shouldCancel("Eliminate Phis (worklist)"))
return false;
MPhi *phi = worklist.popCopy();
JS_ASSERT(phi->isUnused());
phi->setNotInWorklist();
// The removal of Phis can produce newly redundant phis.
if (MDefinition *redundant = IsPhiRedundant(phi)) {
// Add to the worklist the used phis which are impacted.
for (MUseDefIterator it(phi); it; it++) {
if (it.def()->isPhi()) {
MPhi *use = it.def()->toPhi();
if (!use->isUnused()) {
use->setUnusedUnchecked();
use->setInWorklist();
if (!worklist.append(use))
return false;
}
}
}
phi->replaceAllUsesWith(redundant);
} else {
// Otherwise flag them as used.
phi->setNotUnused();
}
// The current phi is/was used, so all its operands are used.
for (size_t i = 0; i < phi->numOperands(); i++) {
MDefinition *in = phi->getOperand(i);
if (!in->isPhi() || !in->isUnused() || in->isInWorklist())
continue;
in->setInWorklist();
if (!worklist.append(in->toPhi()))
return false;
}
}
// Sweep dead phis.
for (PostorderIterator block = graph.poBegin(); block != graph.poEnd(); block++) {
MPhiIterator iter = block->phisBegin();
while (iter != block->phisEnd()) {
if (iter->isUnused())
iter = block->discardPhiAt(iter);
else
iter++;
}
}
return true;
}
// Operands to a resume point which are dead at the point of the resume can be
// replaced with undefined values. This analysis supports limited detection of
// dead operands, pruning those which are defined in the resume point's basic
// block and have no uses outside the block or at points later than the resume
// point.
//
// This is intended to ensure that extra resume points within a basic block
// will not artificially extend the lifetimes of any SSA values. This could
// otherwise occur if the new resume point captured a value which is created
// between the old and new resume point and is dead at the new resume point.
bool
ion::EliminateDeadResumePointOperands(MIRGenerator *mir, MIRGraph &graph)
{
for (PostorderIterator block = graph.poBegin(); block != graph.poEnd(); block++) {
if (mir->shouldCancel("Eliminate Dead Resume Point Operands (main loop)"))
return false;
// The logic below can get confused on infinite loops.
if (block->isLoopHeader() && block->backedge() == *block)
continue;
for (MInstructionIterator ins = block->begin(); ins != block->end(); ins++) {
// No benefit to replacing constant operands with other constants.
if (ins->isConstant())
continue;
// Scanning uses does not give us sufficient information to tell
// where instructions that are involved in box/unbox operations or
// parameter passing might be live. Rewriting uses of these terms
// in resume points may affect the interpreter's behavior. Rather
// than doing a more sophisticated analysis, just ignore these.
if (ins->isUnbox() || ins->isParameter())
continue;
// If the instruction's behavior has been constant folded into a
// separate instruction, we can't determine precisely where the
// instruction becomes dead and can't eliminate its uses.
if (ins->isFolded())
continue;
// Check if this instruction's result is only used within the
// current block, and keep track of its last use in a definition
// (not resume point). This requires the instructions in the block
// to be numbered, ensured by running this immediately after alias
// analysis.
uint32_t maxDefinition = 0;
for (MUseDefIterator uses(*ins); uses; uses++) {
if (uses.def()->block() != *block ||
uses.def()->isBox() ||
uses.def()->isPassArg() ||
uses.def()->isPhi())
{
maxDefinition = UINT32_MAX;
break;
}
maxDefinition = Max(maxDefinition, uses.def()->id());
}
if (maxDefinition == UINT32_MAX)
continue;
// Walk the uses a second time, removing any in resume points after
// the last use in a definition.
for (MUseIterator uses(ins->usesBegin()); uses != ins->usesEnd(); ) {
if (uses->node()->isDefinition()) {
uses++;
continue;
}
MResumePoint *mrp = uses->node()->toResumePoint();
if (mrp->block() != *block ||
!mrp->instruction() ||
mrp->instruction() == *ins ||
mrp->instruction()->id() <= maxDefinition)
{
uses++;
continue;
}
// Store an undefined value in place of all dead resume point
// operands. Making any such substitution can in general alter
// the interpreter's behavior, even though the code is dead, as
// the interpreter will still execute opcodes whose effects
// cannot be observed. If the undefined value were to flow to,
// say, a dead property access the interpreter could throw an
// exception; we avoid this problem by removing dead operands
// before removing dead code.
MConstant *constant = MConstant::New(UndefinedValue());
block->insertBefore(*(block->begin()), constant);
uses = mrp->replaceOperand(uses, constant);
}
}
}
return true;
}
bool
ion::EliminatePhis(MIRGenerator *mir, MIRGraph &graph,
Observability observe)
{
// Eliminates redundant or unobservable phis from the graph. A
// redundant phi is something like b = phi(a, a) or b = phi(a, b),
// both of which can be replaced with a. An unobservable phi is
// one that whose value is never used in the program.
//
// Note that we must be careful not to eliminate phis representing
// values that the interpreter will require later. When the graph
// is first constructed, we can be more aggressive, because there
// is a greater correspondence between the CFG and the bytecode.
// After optimizations such as GVN have been performed, however,
// the bytecode and CFG may not correspond as closely to one
// another. In that case, we must be more conservative. The flag
// |conservativeObservability| is used to indicate that eliminate
// phis is being run after some optimizations have been performed,
// and thus we should use more conservative rules about
// observability. The particular danger is that we can optimize
// away uses of a phi because we think they are not executable,
// but the foundation for that assumption is false TI information
// that will eventually be invalidated. Therefore, if
// |conservativeObservability| is set, we will consider any use
// from a resume point to be observable. Otherwise, we demand a
// use from an actual instruction.
Vector<MPhi *, 16, SystemAllocPolicy> worklist;
// Add all observable phis to a worklist. We use the "in worklist" bit to
// mean "this phi is live".
for (PostorderIterator block = graph.poBegin(); block != graph.poEnd(); block++) {
if (mir->shouldCancel("Eliminate Phis (populate loop)"))
return false;
MPhiIterator iter = block->phisBegin();
while (iter != block->phisEnd()) {
// Flag all as unused, only observable phis would be marked as used
// when processed by the work list.
iter->setUnused();
// If the phi is redundant, remove it here.
if (MDefinition *redundant = IsPhiRedundant(*iter)) {
iter->replaceAllUsesWith(redundant);
iter = block->discardPhiAt(iter);
continue;
}
// Enqueue observable Phis.
if (IsPhiObservable(*iter, observe)) {
iter->setInWorklist();
if (!worklist.append(*iter))
return false;
}
iter++;
}
}
// Iteratively mark all phis reachable from live phis.
while (!worklist.empty()) {
if (mir->shouldCancel("Eliminate Phis (worklist)"))
return false;
MPhi *phi = worklist.popCopy();
JS_ASSERT(phi->isUnused());
phi->setNotInWorklist();
// The removal of Phis can produce newly redundant phis.
if (MDefinition *redundant = IsPhiRedundant(phi)) {
// Add to the worklist the used phis which are impacted.
for (MUseDefIterator it(phi); it; it++) {
if (it.def()->isPhi()) {
MPhi *use = it.def()->toPhi();
if (!use->isUnused()) {
use->setUnusedUnchecked();
use->setInWorklist();
if (!worklist.append(use))
return false;
}
}
}
phi->replaceAllUsesWith(redundant);
} else {
// Otherwise flag them as used.
phi->setNotUnused();
}
// The current phi is/was used, so all its operands are used.
for (size_t i = 0; i < phi->numOperands(); i++) {
MDefinition *in = phi->getOperand(i);
if (!in->isPhi() || !in->isUnused() || in->isInWorklist())
continue;
in->setInWorklist();
if (!worklist.append(in->toPhi()))
return false;
}
}
// Sweep dead phis.
for (PostorderIterator block = graph.poBegin(); block != graph.poEnd(); block++) {
MPhiIterator iter = block->phisBegin();
while (iter != block->phisEnd()) {
if (iter->isUnused())
iter = block->discardPhiAt(iter);
else
iter++;
}
}
return true;
}
