// 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;
}
return true;
}
// If this instruction has a dependency() into an unreachable block, we'll
// need to update AliasAnalysis.
MInstruction* 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;
// If |sim| doesn't belong to a block, insert it next to |def|.
if (sim->block() == nullptr)
def->block()->insertAfter(def->toInstruction(), sim->toInstruction());
#ifdef DEBUG
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;
}
// Otherwise, procede to optimize with |sim| in place of |def|.
def = sim;
}
// 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 DEBUG
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;
}
