/*
* 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"
#include "base/arena_containers.h"
#include "base/arena_bit_vector.h"
#include "base/bit_vector-inl.h"
namespace art {
void SsaDeadPhiElimination::Run() {
MarkDeadPhis();
EliminateDeadPhis();
}
void SsaDeadPhiElimination::MarkDeadPhis() {
// Phis are constructed live and should not be revived if previously marked
// dead. This algorithm temporarily breaks that invariant but we DCHECK that
// only phis which were initially live are revived.
ArenaSet<HPhi*> initially_live(graph_->GetArena()->Adapter(kArenaAllocSsaPhiElimination));
// Add to the worklist phis referenced by non-phi instructions.
for (HReversePostOrderIterator it(*graph_); !it.Done(); it.Advance()) {
HBasicBlock* block = it.Current();
for (HInstructionIterator inst_it(block->GetPhis()); !inst_it.Done(); inst_it.Advance()) {
HPhi* phi = inst_it.Current()->AsPhi();
if (phi->IsDead()) {
continue;
}
bool keep_alive = (graph_->IsDebuggable() && phi->HasEnvironmentUses());
if (!keep_alive) {
for (const HUseListNode<HInstruction*>& use : phi->GetUses()) {
if (!use.GetUser()->IsPhi()) {
keep_alive = true;
break;
}
}
}
if (keep_alive) {
worklist_.push_back(phi);
} else {
phi->SetDead();
if (kIsDebugBuild) {
initially_live.insert(phi);
}
}
}
}
// Process the worklist by propagating liveness to phi inputs.
while (!worklist_.empty()) {
HPhi* phi = worklist_.back();
worklist_.pop_back();
for (HInputIterator it(phi); !it.Done(); it.Advance()) {
HPhi* input = it.Current()->AsPhi();
if (input != nullptr && input->IsDead()) {
// Input is a dead phi. Revive it and add to the worklist. We make sure
// that the phi was not dead initially (see definition of `initially_live`).
DCHECK(ContainsElement(initially_live, input));
input->SetLive();
worklist_.push_back(input);
}
}
}
}
void SsaDeadPhiElimination::EliminateDeadPhis() {
// 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;
HPhi* phi;
while (current != nullptr) {
phi = current->AsPhi();
next = current->GetNext();
if (phi->IsDead()) {
// Make sure the phi is only used by other dead phis.
if (kIsDebugBuild) {
for (const HUseListNode<HInstruction*>& use : phi->GetUses()) {
HInstruction* user = use.GetUser();
DCHECK(user->IsLoopHeaderPhi());
DCHECK(user->AsPhi()->IsDead());
}
}
// Remove the phi from use lists of its inputs.
for (size_t i = 0, e = phi->InputCount(); i < e; ++i) {
phi->RemoveAsUserOfInput(i);
}
// Remove the phi from environments that use it.
for (const HUseListNode<HEnvironment*>& use : phi->GetEnvUses()) {
HEnvironment* user = use.GetUser();
user->SetRawEnvAt(use.GetIndex(), nullptr);
}
// Delete it from the instruction list.
block->RemovePhi(phi, /*ensure_safety=*/ false);
}
current = next;
}
}
}
void SsaRedundantPhiElimination::Run() {
// Add all phis in the worklist. Order does not matter for correctness, and
// neither will necessarily converge faster.
for (HReversePostOrderIterator it(*graph_); !it.Done(); it.Advance()) {
HBasicBlock* block = it.Current();
for (HInstructionIterator inst_it(block->GetPhis()); !inst_it.Done(); inst_it.Advance()) {
worklist_.push_back(inst_it.Current()->AsPhi());
}
}
ArenaBitVector visited_phis_in_cycle(graph_->GetArena(),
graph_->GetCurrentInstructionId(),
/* expandable */ false,
kArenaAllocSsaPhiElimination);
ArenaVector<HPhi*> cycle_worklist(graph_->GetArena()->Adapter(kArenaAllocSsaPhiElimination));
while (!worklist_.empty()) {
HPhi* phi = worklist_.back();
worklist_.pop_back();
// If the phi has already been processed, continue.
if (!phi->IsInBlock()) {
continue;
}
// If the phi is dead, we know we won't revive it and it will be removed,
// so don't process it.
if (phi->IsDead()) {
continue;
}
HInstruction* candidate = nullptr;
visited_phis_in_cycle.ClearAllBits();
cycle_worklist.clear();
cycle_worklist.push_back(phi);
visited_phis_in_cycle.SetBit(phi->GetId());
bool catch_phi_in_cycle = phi->IsCatchPhi();
bool irreducible_loop_phi_in_cycle = phi->IsIrreducibleLoopHeaderPhi();
// First do a simple loop over inputs and check if they are all the same.
for (size_t j = 0; j < phi->InputCount(); ++j) {
HInstruction* input = phi->InputAt(j);
if (input == phi) {
continue;
} else if (candidate == nullptr) {
candidate = input;
} else if (candidate != input) {
candidate = nullptr;
break;
}
}
// If we haven't found a candidate, check for a phi cycle. Note that we need to detect
// such cycles to avoid having reference and non-reference equivalents. We check this
// invariant in the graph checker.
if (candidate == nullptr) {
// We iterate over the array as long as it grows.
for (size_t i = 0; i < cycle_worklist.size(); ++i) {
HPhi* current = cycle_worklist[i];
DCHECK(!current->IsLoopHeaderPhi() ||
current->GetBlock()->IsLoopPreHeaderFirstPredecessor());
for (size_t j = 0; j < current->InputCount(); ++j) {
HInstruction* input = current->InputAt(j);
if (input == current) {
continue;
} else if (input->IsPhi()) {
if (!visited_phis_in_cycle.IsBitSet(input->GetId())) {
cycle_worklist.push_back(input->AsPhi());
visited_phis_in_cycle.SetBit(input->GetId());
catch_phi_in_cycle |= input->AsPhi()->IsCatchPhi();
irreducible_loop_phi_in_cycle |= input->IsIrreducibleLoopHeaderPhi();
} else {
// Already visited, nothing to do.
}
} else if (candidate == nullptr) {
candidate = input;
} else if (candidate != input) {
candidate = nullptr;
// Clear the cycle worklist to break out of the outer loop.
cycle_worklist.clear();
break;
}
}
}
}
if (candidate == nullptr) {
continue;
}
if (irreducible_loop_phi_in_cycle && !candidate->IsConstant()) {
// For irreducible loops, we need to keep the phis to satisfy our linear scan
// algorithm.
// There is one exception for constants, as the type propagation requires redundant
// cyclic phis of a constant to be removed. This is ok for the linear scan as it
// has to deal with constants anyway, and they can trivially be rematerialized.
continue;
}
for (HPhi* current : cycle_worklist) {
// The candidate may not dominate a phi in a catch block: there may be non-throwing
// instructions at the beginning of a try range, that may be the first input of
// catch phis.
// TODO(dbrazdil): Remove this situation by moving those non-throwing instructions
// before the try entry.
if (catch_phi_in_cycle) {
if (!candidate->StrictlyDominates(current)) {
continue;
}
} else {
DCHECK(candidate->StrictlyDominates(current));
}
// Because we're updating the users of this phi, we may have new candidates
// for elimination. Add phis that use this phi to the worklist.
for (const HUseListNode<HInstruction*>& use : current->GetUses()) {
HInstruction* user = use.GetUser();
if (user->IsPhi() && !visited_phis_in_cycle.IsBitSet(user->GetId())) {
worklist_.push_back(user->AsPhi());
}
}
DCHECK(candidate->StrictlyDominates(current));
current->ReplaceWith(candidate);
current->GetBlock()->RemovePhi(current);
}
}
}
} // namespace art