/*
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* Copyright (C) 2015 The Android Open Source Project
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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#include "licm.h"
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#include "side_effects_analysis.h"
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namespace art {
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static bool IsPhiOf(HInstruction* instruction, HBasicBlock* block) {
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return instruction->IsPhi() && instruction->GetBlock() == block;
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}
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/**
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* Returns whether `instruction` has all its inputs and environment defined
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* before the loop it is in.
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*/
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static bool InputsAreDefinedBeforeLoop(HInstruction* instruction) {
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DCHECK(instruction->IsInLoop());
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HLoopInformation* info = instruction->GetBlock()->GetLoopInformation();
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for (const HInstruction* input : instruction->GetInputs()) {
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HLoopInformation* input_loop = input->GetBlock()->GetLoopInformation();
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// We only need to check whether the input is defined in the loop. If it is not
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// it is defined before the loop.
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if (input_loop != nullptr && input_loop->IsIn(*info)) {
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return false;
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}
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}
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for (HEnvironment* environment = instruction->GetEnvironment();
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environment != nullptr;
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environment = environment->GetParent()) {
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for (size_t i = 0, e = environment->Size(); i < e; ++i) {
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HInstruction* input = environment->GetInstructionAt(i);
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if (input != nullptr) {
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HLoopInformation* input_loop = input->GetBlock()->GetLoopInformation();
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if (input_loop != nullptr && input_loop->IsIn(*info)) {
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// We can move an instruction that takes a loop header phi in the environment:
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// we will just replace that phi with its first input later in `UpdateLoopPhisIn`.
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bool is_loop_header_phi = IsPhiOf(input, info->GetHeader());
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if (!is_loop_header_phi) {
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return false;
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}
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}
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}
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}
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}
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return true;
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}
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/**
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* If `environment` has a loop header phi, we replace it with its first input.
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*/
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static void UpdateLoopPhisIn(HEnvironment* environment, HLoopInformation* info) {
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for (; environment != nullptr; environment = environment->GetParent()) {
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for (size_t i = 0, e = environment->Size(); i < e; ++i) {
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HInstruction* input = environment->GetInstructionAt(i);
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if (input != nullptr && IsPhiOf(input, info->GetHeader())) {
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environment->RemoveAsUserOfInput(i);
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HInstruction* incoming = input->InputAt(0);
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environment->SetRawEnvAt(i, incoming);
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incoming->AddEnvUseAt(environment, i);
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}
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}
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}
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}
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bool LICM::Run() {
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bool didLICM = false;
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DCHECK(side_effects_.HasRun());
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// Only used during debug.
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ArenaBitVector* visited = nullptr;
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if (kIsDebugBuild) {
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visited = new (graph_->GetAllocator()) ArenaBitVector(graph_->GetAllocator(),
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graph_->GetBlocks().size(),
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false,
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kArenaAllocLICM);
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}
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// Post order visit to visit inner loops before outer loops.
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for (HBasicBlock* block : graph_->GetPostOrder()) {
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if (!block->IsLoopHeader()) {
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// Only visit the loop when we reach the header.
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continue;
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}
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HLoopInformation* loop_info = block->GetLoopInformation();
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SideEffects loop_effects = side_effects_.GetLoopEffects(block);
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HBasicBlock* pre_header = loop_info->GetPreHeader();
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for (HBlocksInLoopIterator it_loop(*loop_info); !it_loop.Done(); it_loop.Advance()) {
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HBasicBlock* inner = it_loop.Current();
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DCHECK(inner->IsInLoop());
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if (inner->GetLoopInformation() != loop_info) {
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// Thanks to post order visit, inner loops were already visited.
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DCHECK(visited->IsBitSet(inner->GetBlockId()));
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continue;
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}
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if (kIsDebugBuild) {
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visited->SetBit(inner->GetBlockId());
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}
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if (loop_info->ContainsIrreducibleLoop()) {
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// We cannot licm in an irreducible loop, or in a natural loop containing an
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// irreducible loop.
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continue;
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}
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DCHECK(!loop_info->IsIrreducible());
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// We can move an instruction that can throw only as long as it is the first visible
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// instruction (throw or write) in the loop. Note that the first potentially visible
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// instruction that is not hoisted stops this optimization. Non-throwing instructions,
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// on the other hand, can still be hoisted.
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bool found_first_non_hoisted_visible_instruction_in_loop = !inner->IsLoopHeader();
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for (HInstructionIterator inst_it(inner->GetInstructions());
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!inst_it.Done();
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inst_it.Advance()) {
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HInstruction* instruction = inst_it.Current();
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bool can_move = false;
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if (instruction->CanBeMoved() && InputsAreDefinedBeforeLoop(instruction)) {
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if (instruction->CanThrow()) {
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if (!found_first_non_hoisted_visible_instruction_in_loop) {
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DCHECK(instruction->GetBlock()->IsLoopHeader());
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if (instruction->IsClinitCheck()) {
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// clinit is only done once, and since all visible instructions
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// in the loop header so far have been hoisted out, we can hoist
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// the clinit check out also.
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can_move = true;
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} else if (!instruction->GetSideEffects().MayDependOn(loop_effects)) {
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can_move = true;
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}
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}
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} else if (!instruction->GetSideEffects().MayDependOn(loop_effects)) {
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can_move = true;
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}
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}
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if (can_move) {
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// We need to update the environment if the instruction has a loop header
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// phi in it.
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if (instruction->NeedsEnvironment()) {
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UpdateLoopPhisIn(instruction->GetEnvironment(), loop_info);
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} else {
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DCHECK(!instruction->HasEnvironment());
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}
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instruction->MoveBefore(pre_header->GetLastInstruction());
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MaybeRecordStat(stats_, MethodCompilationStat::kLoopInvariantMoved);
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didLICM = true;
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}
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if (!can_move && (instruction->CanThrow() || instruction->DoesAnyWrite())) {
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// If `instruction` can do something visible (throw or write),
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// we cannot move further instructions that can throw.
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found_first_non_hoisted_visible_instruction_in_loop = true;
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}
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}
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}
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}
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return didLICM;
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}
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} // namespace art
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