v8/src/hydrogen-store-elimination.cc

122 lines
3.8 KiB
C++

// Copyright 2013 the V8 project authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#include "src/hydrogen-instructions.h"
#include "src/hydrogen-store-elimination.h"
namespace v8 {
namespace internal {
#define TRACE(x) if (FLAG_trace_store_elimination) PrintF x
// Performs a block-by-block local analysis for removable stores.
void HStoreEliminationPhase::Run() {
GVNFlagSet flags; // Use GVN flags as an approximation for some instructions.
flags.RemoveAll();
flags.Add(kArrayElements);
flags.Add(kArrayLengths);
flags.Add(kStringLengths);
flags.Add(kBackingStoreFields);
flags.Add(kDoubleArrayElements);
flags.Add(kDoubleFields);
flags.Add(kElementsPointer);
flags.Add(kInobjectFields);
flags.Add(kExternalMemory);
flags.Add(kStringChars);
flags.Add(kTypedArrayElements);
for (int i = 0; i < graph()->blocks()->length(); i++) {
unobserved_.Rewind(0);
HBasicBlock* block = graph()->blocks()->at(i);
if (!block->IsReachable()) continue;
for (HInstructionIterator it(block); !it.Done(); it.Advance()) {
HInstruction* instr = it.Current();
if (instr->CheckFlag(HValue::kIsDead)) continue;
// TODO(titzer): eliminate unobserved HStoreKeyed instructions too.
switch (instr->opcode()) {
case HValue::kStoreNamedField:
// Remove any unobserved stores overwritten by this store.
ProcessStore(HStoreNamedField::cast(instr));
break;
case HValue::kLoadNamedField:
// Observe any unobserved stores on this object + field.
ProcessLoad(HLoadNamedField::cast(instr));
break;
default:
ProcessInstr(instr, flags);
break;
}
}
}
}
void HStoreEliminationPhase::ProcessStore(HStoreNamedField* store) {
HValue* object = store->object()->ActualValue();
int i = 0;
while (i < unobserved_.length()) {
HStoreNamedField* prev = unobserved_.at(i);
if (aliasing_->MustAlias(object, prev->object()->ActualValue()) &&
prev->CanBeReplacedWith(store)) {
// This store is guaranteed to overwrite the previous store.
prev->DeleteAndReplaceWith(NULL);
TRACE(("++ Unobserved store S%d overwritten by S%d\n",
prev->id(), store->id()));
unobserved_.Remove(i);
} else {
// TODO(titzer): remove map word clearing from folded allocations.
i++;
}
}
// Only non-transitioning stores are removable.
if (!store->has_transition()) {
TRACE(("-- Might remove store S%d\n", store->id()));
unobserved_.Add(store, zone());
}
}
void HStoreEliminationPhase::ProcessLoad(HLoadNamedField* load) {
HValue* object = load->object()->ActualValue();
int i = 0;
while (i < unobserved_.length()) {
HStoreNamedField* prev = unobserved_.at(i);
if (aliasing_->MayAlias(object, prev->object()->ActualValue()) &&
load->access().Equals(prev->access())) {
TRACE(("-- Observed store S%d by load L%d\n", prev->id(), load->id()));
unobserved_.Remove(i);
} else {
i++;
}
}
}
void HStoreEliminationPhase::ProcessInstr(HInstruction* instr,
GVNFlagSet flags) {
if (unobserved_.length() == 0) return; // Nothing to do.
if (instr->CanDeoptimize()) {
TRACE(("-- Observed stores at I%d (%s might deoptimize)\n",
instr->id(), instr->Mnemonic()));
unobserved_.Rewind(0);
return;
}
if (instr->CheckChangesFlag(kNewSpacePromotion)) {
TRACE(("-- Observed stores at I%d (%s might GC)\n",
instr->id(), instr->Mnemonic()));
unobserved_.Rewind(0);
return;
}
if (instr->DependsOnFlags().ContainsAnyOf(flags)) {
TRACE(("-- Observed stores at I%d (GVN flags of %s)\n",
instr->id(), instr->Mnemonic()));
unobserved_.Rewind(0);
return;
}
}
} } // namespace v8::internal