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;
}
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;
}
