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/*
* Copyright (C) 2014 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
#include "ssa_phi_elimination.h"
namespace art {
void SsaDeadPhiElimination::Run() {
// Add to the worklist phis referenced by non-phi instructions.
for (HReversePostOrderIterator it(*graph_); !it.Done(); it.Advance()) {
HBasicBlock* block = it.Current();
for (HInstructionIterator it(block->GetPhis()); !it.Done(); it.Advance()) {
HPhi* phi = it.Current()->AsPhi();
for (HUseIterator<HInstruction> it(phi->GetUses()); !it.Done(); it.Advance()) {
HUseListNode<HInstruction>* current = it.Current();
HInstruction* user = current->GetUser();
if (!user->IsPhi()) {
worklist_.Add(phi);
phi->SetLive();
break;
} else {
phi->SetDead();
}
}
}
}
// Process the worklist by propagating liveness to phi inputs.
while (!worklist_.IsEmpty()) {
HPhi* phi = worklist_.Pop();
for (HInputIterator it(phi); !it.Done(); it.Advance()) {
HInstruction* input = it.Current();
if (input->IsPhi() && input->AsPhi()->IsDead()) {
worklist_.Add(input->AsPhi());
input->AsPhi()->SetLive();
}
}
}
// Remove phis that are not live. Visit in post order so that phis
// that are not inputs of loop phis can be removed when they have
// no users left (dead phis might use dead phis).
for (HPostOrderIterator it(*graph_); !it.Done(); it.Advance()) {
HBasicBlock* block = it.Current();
HInstruction* current = block->GetFirstPhi();
HInstruction* next = nullptr;
while (current != nullptr) {
next = current->GetNext();
if (current->AsPhi()->IsDead()) {
if (current->HasUses()) {
for (HUseIterator<HInstruction> it(current->GetUses()); !it.Done(); it.Advance()) {
HUseListNode<HInstruction>* user_node = it.Current();
HInstruction* user = user_node->GetUser();
DCHECK(user->IsLoopHeaderPhi());
DCHECK(user->AsPhi()->IsDead());
// Just put itself as an input. The phi will be removed in this loop anyway.
user->SetRawInputAt(user_node->GetIndex(), user);
current->RemoveUser(user, user_node->GetIndex());
}
}
if (current->HasEnvironmentUses()) {
for (HUseIterator<HEnvironment> it(current->GetEnvUses()); !it.Done(); it.Advance()) {
HUseListNode<HEnvironment>* user_node = it.Current();
HEnvironment* user = user_node->GetUser();
user->SetRawEnvAt(user_node->GetIndex(), nullptr);
current->RemoveEnvironmentUser(user, user_node->GetIndex());
}
}
block->RemovePhi(current->AsPhi());
}
current = next;
}
}
}
void SsaRedundantPhiElimination::Run() {
// Add all phis in the worklist.
for (HReversePostOrderIterator it(*graph_); !it.Done(); it.Advance()) {
HBasicBlock* block = it.Current();
for (HInstructionIterator it(block->GetPhis()); !it.Done(); it.Advance()) {
worklist_.Add(it.Current()->AsPhi());
}
}
while (!worklist_.IsEmpty()) {
HPhi* phi = worklist_.Pop();
// If the phi has already been processed, continue.
if (!phi->IsInBlock()) {
continue;
}
// Find if the inputs of the phi are the same instruction.
HInstruction* candidate = phi->InputAt(0);
// A loop phi cannot have itself as the first phi. Note that this
// check relies on our simplification pass ensuring the pre-header
// block is first in the list of predecessors of the loop header.
DCHECK(!phi->IsLoopHeaderPhi() || phi->GetBlock()->IsLoopPreHeaderFirstPredecessor());
DCHECK_NE(phi, candidate);
for (size_t i = 1; i < phi->InputCount(); ++i) {
HInstruction* input = phi->InputAt(i);
// For a loop phi, if the input is the phi, the phi is still candidate for
// elimination.
if (input != candidate && input != phi) {
candidate = nullptr;
break;
}
}
// If the inputs are not the same, continue.
if (candidate == nullptr) {
continue;
}
if (phi->IsInLoop()) {
// Because we're updating the users of this phi, we may have new
// phis candidate for elimination if this phi is in a loop. Add phis that
// used this phi to the worklist.
for (HUseIterator<HInstruction> it(phi->GetUses()); !it.Done(); it.Advance()) {
HUseListNode<HInstruction>* current = it.Current();
HInstruction* user = current->GetUser();
if (user->IsPhi()) {
worklist_.Add(user->AsPhi());
}
}
}
phi->ReplaceWith(candidate);
phi->GetBlock()->RemovePhi(phi);
}
}
} // namespace art