/*
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* Copyright (C) 2014 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 "dead_code_elimination.h"
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#include "base/array_ref.h"
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#include "base/bit_vector-inl.h"
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#include "base/scoped_arena_allocator.h"
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#include "base/scoped_arena_containers.h"
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#include "base/stl_util.h"
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#include "ssa_phi_elimination.h"
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namespace art {
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static void MarkReachableBlocks(HGraph* graph, ArenaBitVector* visited) {
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// Use local allocator for allocating memory.
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ScopedArenaAllocator allocator(graph->GetArenaStack());
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ScopedArenaVector<HBasicBlock*> worklist(allocator.Adapter(kArenaAllocDCE));
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constexpr size_t kDefaultWorlistSize = 8;
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worklist.reserve(kDefaultWorlistSize);
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visited->SetBit(graph->GetEntryBlock()->GetBlockId());
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worklist.push_back(graph->GetEntryBlock());
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while (!worklist.empty()) {
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HBasicBlock* block = worklist.back();
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worklist.pop_back();
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int block_id = block->GetBlockId();
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DCHECK(visited->IsBitSet(block_id));
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ArrayRef<HBasicBlock* const> live_successors(block->GetSuccessors());
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HInstruction* last_instruction = block->GetLastInstruction();
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if (last_instruction->IsIf()) {
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HIf* if_instruction = last_instruction->AsIf();
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HInstruction* condition = if_instruction->InputAt(0);
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if (condition->IsIntConstant()) {
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if (condition->AsIntConstant()->IsTrue()) {
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live_successors = live_successors.SubArray(0u, 1u);
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DCHECK_EQ(live_successors[0], if_instruction->IfTrueSuccessor());
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} else {
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DCHECK(condition->AsIntConstant()->IsFalse()) << condition->AsIntConstant()->GetValue();
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live_successors = live_successors.SubArray(1u, 1u);
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DCHECK_EQ(live_successors[0], if_instruction->IfFalseSuccessor());
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}
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}
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} else if (last_instruction->IsPackedSwitch()) {
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HPackedSwitch* switch_instruction = last_instruction->AsPackedSwitch();
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HInstruction* switch_input = switch_instruction->InputAt(0);
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if (switch_input->IsIntConstant()) {
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int32_t switch_value = switch_input->AsIntConstant()->GetValue();
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int32_t start_value = switch_instruction->GetStartValue();
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// Note: Though the spec forbids packed-switch values to wrap around, we leave
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// that task to the verifier and use unsigned arithmetic with it's "modulo 2^32"
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// semantics to check if the value is in range, wrapped or not.
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uint32_t switch_index =
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static_cast<uint32_t>(switch_value) - static_cast<uint32_t>(start_value);
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if (switch_index < switch_instruction->GetNumEntries()) {
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live_successors = live_successors.SubArray(switch_index, 1u);
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DCHECK_EQ(live_successors[0], block->GetSuccessors()[switch_index]);
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} else {
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live_successors = live_successors.SubArray(switch_instruction->GetNumEntries(), 1u);
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DCHECK_EQ(live_successors[0], switch_instruction->GetDefaultBlock());
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}
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}
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}
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for (HBasicBlock* successor : live_successors) {
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// Add only those successors that have not been visited yet.
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if (!visited->IsBitSet(successor->GetBlockId())) {
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visited->SetBit(successor->GetBlockId());
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worklist.push_back(successor);
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}
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}
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}
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}
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void HDeadCodeElimination::MaybeRecordDeadBlock(HBasicBlock* block) {
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if (stats_ != nullptr) {
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stats_->RecordStat(MethodCompilationStat::kRemovedDeadInstruction,
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block->GetPhis().CountSize() + block->GetInstructions().CountSize());
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}
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}
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void HDeadCodeElimination::MaybeRecordSimplifyIf() {
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if (stats_ != nullptr) {
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stats_->RecordStat(MethodCompilationStat::kSimplifyIf);
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}
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}
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static bool HasInput(HCondition* instruction, HInstruction* input) {
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return (instruction->InputAt(0) == input) ||
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(instruction->InputAt(1) == input);
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}
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static bool HasEquality(IfCondition condition) {
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switch (condition) {
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case kCondEQ:
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case kCondLE:
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case kCondGE:
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case kCondBE:
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case kCondAE:
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return true;
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case kCondNE:
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case kCondLT:
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case kCondGT:
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case kCondB:
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case kCondA:
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return false;
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}
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}
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static HConstant* Evaluate(HCondition* condition, HInstruction* left, HInstruction* right) {
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if (left == right && !DataType::IsFloatingPointType(left->GetType())) {
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return condition->GetBlock()->GetGraph()->GetIntConstant(
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HasEquality(condition->GetCondition()) ? 1 : 0);
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}
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if (!left->IsConstant() || !right->IsConstant()) {
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return nullptr;
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}
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if (left->IsIntConstant()) {
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return condition->Evaluate(left->AsIntConstant(), right->AsIntConstant());
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} else if (left->IsNullConstant()) {
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return condition->Evaluate(left->AsNullConstant(), right->AsNullConstant());
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} else if (left->IsLongConstant()) {
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return condition->Evaluate(left->AsLongConstant(), right->AsLongConstant());
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} else if (left->IsFloatConstant()) {
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return condition->Evaluate(left->AsFloatConstant(), right->AsFloatConstant());
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} else {
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DCHECK(left->IsDoubleConstant());
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return condition->Evaluate(left->AsDoubleConstant(), right->AsDoubleConstant());
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}
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}
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static bool RemoveNonNullControlDependences(HBasicBlock* block, HBasicBlock* throws) {
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// Test for an if as last statement.
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if (!block->EndsWithIf()) {
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return false;
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}
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HIf* ifs = block->GetLastInstruction()->AsIf();
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// Find either:
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// if obj == null
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// throws
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// else
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// not_throws
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// or:
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// if obj != null
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// not_throws
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// else
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// throws
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HInstruction* cond = ifs->InputAt(0);
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HBasicBlock* not_throws = nullptr;
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if (throws == ifs->IfTrueSuccessor() && cond->IsEqual()) {
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not_throws = ifs->IfFalseSuccessor();
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} else if (throws == ifs->IfFalseSuccessor() && cond->IsNotEqual()) {
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not_throws = ifs->IfTrueSuccessor();
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} else {
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return false;
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}
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DCHECK(cond->IsEqual() || cond->IsNotEqual());
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HInstruction* obj = cond->InputAt(1);
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if (obj->IsNullConstant()) {
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obj = cond->InputAt(0);
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} else if (!cond->InputAt(0)->IsNullConstant()) {
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return false;
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}
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// Scan all uses of obj and find null check under control dependence.
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HBoundType* bound = nullptr;
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const HUseList<HInstruction*>& uses = obj->GetUses();
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for (auto it = uses.begin(), end = uses.end(); it != end;) {
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HInstruction* user = it->GetUser();
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++it; // increment before possibly replacing
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if (user->IsNullCheck()) {
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HBasicBlock* user_block = user->GetBlock();
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if (user_block != block &&
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user_block != throws &&
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block->Dominates(user_block)) {
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if (bound == nullptr) {
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ReferenceTypeInfo ti = obj->GetReferenceTypeInfo();
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bound = new (obj->GetBlock()->GetGraph()->GetAllocator()) HBoundType(obj);
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bound->SetUpperBound(ti, /*can_be_null*/ false);
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bound->SetReferenceTypeInfo(ti);
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bound->SetCanBeNull(false);
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not_throws->InsertInstructionBefore(bound, not_throws->GetFirstInstruction());
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}
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user->ReplaceWith(bound);
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user_block->RemoveInstruction(user);
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}
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}
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}
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return bound != nullptr;
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}
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// Simplify the pattern:
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//
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// B1
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// / \
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// | foo() // always throws
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// \ goto B2
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// \ /
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// B2
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//
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// Into:
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//
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// B1
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// / \
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// | foo()
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// | goto Exit
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// | |
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// B2 Exit
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//
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// Rationale:
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// Removal of the never taken edge to B2 may expose
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// other optimization opportunities, such as code sinking.
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bool HDeadCodeElimination::SimplifyAlwaysThrows() {
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// Make sure exceptions go to exit.
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if (graph_->HasTryCatch()) {
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return false;
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}
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HBasicBlock* exit = graph_->GetExitBlock();
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if (exit == nullptr) {
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return false;
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}
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bool rerun_dominance_and_loop_analysis = false;
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// Order does not matter, just pick one.
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for (HBasicBlock* block : graph_->GetReversePostOrder()) {
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HInstruction* first = block->GetFirstInstruction();
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HInstruction* last = block->GetLastInstruction();
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// Ensure only one throwing instruction appears before goto.
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if (first->AlwaysThrows() &&
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first->GetNext() == last &&
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last->IsGoto() &&
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block->GetPhis().IsEmpty() &&
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block->GetPredecessors().size() == 1u) {
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DCHECK_EQ(block->GetSuccessors().size(), 1u);
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HBasicBlock* pred = block->GetSinglePredecessor();
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HBasicBlock* succ = block->GetSingleSuccessor();
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// Ensure no computations are merged through throwing block.
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// This does not prevent the optimization per se, but would
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// require an elaborate clean up of the SSA graph.
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if (succ != exit &&
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!block->Dominates(pred) &&
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pred->Dominates(succ) &&
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succ->GetPredecessors().size() > 1u &&
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succ->GetPhis().IsEmpty()) {
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block->ReplaceSuccessor(succ, exit);
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rerun_dominance_and_loop_analysis = true;
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MaybeRecordStat(stats_, MethodCompilationStat::kSimplifyThrowingInvoke);
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// Perform a quick follow up optimization on object != null control dependences
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// that is much cheaper to perform now than in a later phase.
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if (RemoveNonNullControlDependences(pred, block)) {
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MaybeRecordStat(stats_, MethodCompilationStat::kRemovedNullCheck);
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}
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}
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}
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}
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// We need to re-analyze the graph in order to run DCE afterwards.
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if (rerun_dominance_and_loop_analysis) {
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graph_->ClearLoopInformation();
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graph_->ClearDominanceInformation();
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graph_->BuildDominatorTree();
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return true;
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}
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return false;
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}
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// Simplify the pattern:
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//
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// B1 B2 ...
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// goto goto goto
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// \ | /
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// \ | /
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// B3
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// i1 = phi(input, input)
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// (i2 = condition on i1)
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// if i1 (or i2)
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// / \
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// / \
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// B4 B5
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//
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// Into:
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//
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// B1 B2 ...
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// | | |
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// B4 B5 B?
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//
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// Note that individual edges can be redirected (for example B2->B3
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// can be redirected as B2->B5) without applying this optimization
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// to other incoming edges.
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//
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// This simplification cannot be applied to catch blocks, because
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// exception handler edges do not represent normal control flow.
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// Though in theory this could still apply to normal control flow
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// going directly to a catch block, we cannot support it at the
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// moment because the catch Phi's inputs do not correspond to the
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// catch block's predecessors, so we cannot identify which
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// predecessor corresponds to a given statically evaluated input.
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//
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// We do not apply this optimization to loop headers as this could
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// create irreducible loops. We rely on the suspend check in the
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// loop header to prevent the pattern match.
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//
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// Note that we rely on the dead code elimination to get rid of B3.
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bool HDeadCodeElimination::SimplifyIfs() {
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bool simplified_one_or_more_ifs = false;
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bool rerun_dominance_and_loop_analysis = false;
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for (HBasicBlock* block : graph_->GetReversePostOrder()) {
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HInstruction* last = block->GetLastInstruction();
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HInstruction* first = block->GetFirstInstruction();
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if (!block->IsCatchBlock() &&
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last->IsIf() &&
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block->HasSinglePhi() &&
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block->GetFirstPhi()->HasOnlyOneNonEnvironmentUse()) {
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bool has_only_phi_and_if = (last == first) && (last->InputAt(0) == block->GetFirstPhi());
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bool has_only_phi_condition_and_if =
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!has_only_phi_and_if &&
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first->IsCondition() &&
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HasInput(first->AsCondition(), block->GetFirstPhi()) &&
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(first->GetNext() == last) &&
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(last->InputAt(0) == first) &&
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first->HasOnlyOneNonEnvironmentUse();
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if (has_only_phi_and_if || has_only_phi_condition_and_if) {
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DCHECK(!block->IsLoopHeader());
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HPhi* phi = block->GetFirstPhi()->AsPhi();
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bool phi_input_is_left = (first->InputAt(0) == phi);
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// Walk over all inputs of the phis and update the control flow of
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// predecessors feeding constants to the phi.
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// Note that phi->InputCount() may change inside the loop.
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for (size_t i = 0; i < phi->InputCount();) {
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HInstruction* input = phi->InputAt(i);
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HInstruction* value_to_check = nullptr;
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if (has_only_phi_and_if) {
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if (input->IsIntConstant()) {
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value_to_check = input;
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}
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} else {
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DCHECK(has_only_phi_condition_and_if);
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if (phi_input_is_left) {
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value_to_check = Evaluate(first->AsCondition(), input, first->InputAt(1));
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} else {
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value_to_check = Evaluate(first->AsCondition(), first->InputAt(0), input);
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}
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}
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if (value_to_check == nullptr) {
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// Could not evaluate to a constant, continue iterating over the inputs.
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++i;
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} else {
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HBasicBlock* predecessor_to_update = block->GetPredecessors()[i];
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HBasicBlock* successor_to_update = nullptr;
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if (value_to_check->AsIntConstant()->IsTrue()) {
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successor_to_update = last->AsIf()->IfTrueSuccessor();
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} else {
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DCHECK(value_to_check->AsIntConstant()->IsFalse())
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<< value_to_check->AsIntConstant()->GetValue();
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successor_to_update = last->AsIf()->IfFalseSuccessor();
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}
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predecessor_to_update->ReplaceSuccessor(block, successor_to_update);
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phi->RemoveInputAt(i);
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simplified_one_or_more_ifs = true;
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if (block->IsInLoop()) {
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rerun_dominance_and_loop_analysis = true;
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}
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// For simplicity, don't create a dead block, let the dead code elimination
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// pass deal with it.
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if (phi->InputCount() == 1) {
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break;
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}
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}
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}
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if (block->GetPredecessors().size() == 1) {
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phi->ReplaceWith(phi->InputAt(0));
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block->RemovePhi(phi);
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if (has_only_phi_condition_and_if) {
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// Evaluate here (and not wait for a constant folding pass) to open
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// more opportunities for DCE.
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HInstruction* result = first->AsCondition()->TryStaticEvaluation();
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if (result != nullptr) {
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first->ReplaceWith(result);
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block->RemoveInstruction(first);
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}
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}
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}
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if (simplified_one_or_more_ifs) {
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MaybeRecordSimplifyIf();
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}
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}
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}
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}
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// We need to re-analyze the graph in order to run DCE afterwards.
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if (simplified_one_or_more_ifs) {
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if (rerun_dominance_and_loop_analysis) {
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graph_->ClearLoopInformation();
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graph_->ClearDominanceInformation();
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graph_->BuildDominatorTree();
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} else {
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graph_->ClearDominanceInformation();
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// We have introduced critical edges, remove them.
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graph_->SimplifyCFG();
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graph_->ComputeDominanceInformation();
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graph_->ComputeTryBlockInformation();
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}
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}
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return simplified_one_or_more_ifs;
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}
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void HDeadCodeElimination::ConnectSuccessiveBlocks() {
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// Order does not matter. Skip the entry block by starting at index 1 in reverse post order.
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for (size_t i = 1u, size = graph_->GetReversePostOrder().size(); i != size; ++i) {
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HBasicBlock* block = graph_->GetReversePostOrder()[i];
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DCHECK(!block->IsEntryBlock());
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while (block->GetLastInstruction()->IsGoto()) {
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HBasicBlock* successor = block->GetSingleSuccessor();
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if (successor->IsExitBlock() || successor->GetPredecessors().size() != 1u) {
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break;
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}
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DCHECK_LT(i, IndexOfElement(graph_->GetReversePostOrder(), successor));
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block->MergeWith(successor);
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--size;
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DCHECK_EQ(size, graph_->GetReversePostOrder().size());
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DCHECK_EQ(block, graph_->GetReversePostOrder()[i]);
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// Reiterate on this block in case it can be merged with its new successor.
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}
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}
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}
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bool HDeadCodeElimination::RemoveDeadBlocks() {
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// Use local allocator for allocating memory.
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ScopedArenaAllocator allocator(graph_->GetArenaStack());
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// Classify blocks as reachable/unreachable.
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ArenaBitVector live_blocks(&allocator, graph_->GetBlocks().size(), false, kArenaAllocDCE);
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live_blocks.ClearAllBits();
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MarkReachableBlocks(graph_, &live_blocks);
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bool removed_one_or_more_blocks = false;
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bool rerun_dominance_and_loop_analysis = false;
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// Remove all dead blocks. Iterate in post order because removal needs the
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// block's chain of dominators and nested loops need to be updated from the
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// inside out.
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for (HBasicBlock* block : graph_->GetPostOrder()) {
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int id = block->GetBlockId();
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if (!live_blocks.IsBitSet(id)) {
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MaybeRecordDeadBlock(block);
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block->DisconnectAndDelete();
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removed_one_or_more_blocks = true;
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if (block->IsInLoop()) {
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rerun_dominance_and_loop_analysis = true;
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}
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}
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}
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// If we removed at least one block, we need to recompute the full
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// dominator tree and try block membership.
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if (removed_one_or_more_blocks) {
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if (rerun_dominance_and_loop_analysis) {
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graph_->ClearLoopInformation();
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graph_->ClearDominanceInformation();
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graph_->BuildDominatorTree();
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} else {
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graph_->ClearDominanceInformation();
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graph_->ComputeDominanceInformation();
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graph_->ComputeTryBlockInformation();
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}
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}
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return removed_one_or_more_blocks;
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}
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void HDeadCodeElimination::RemoveDeadInstructions() {
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// Process basic blocks in post-order in the dominator tree, so that
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// a dead instruction depending on another dead instruction is removed.
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for (HBasicBlock* block : graph_->GetPostOrder()) {
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// Traverse this block's instructions in backward order and remove
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// the unused ones.
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HBackwardInstructionIterator i(block->GetInstructions());
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// Skip the first iteration, as the last instruction of a block is
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// a branching instruction.
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DCHECK(i.Current()->IsControlFlow());
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for (i.Advance(); !i.Done(); i.Advance()) {
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HInstruction* inst = i.Current();
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DCHECK(!inst->IsControlFlow());
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if (inst->IsDeadAndRemovable()) {
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block->RemoveInstruction(inst);
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MaybeRecordStat(stats_, MethodCompilationStat::kRemovedDeadInstruction);
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}
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}
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}
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}
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bool HDeadCodeElimination::Run() {
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// Do not eliminate dead blocks if the graph has irreducible loops. We could
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// support it, but that would require changes in our loop representation to handle
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// multiple entry points. We decided it was not worth the complexity.
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if (!graph_->HasIrreducibleLoops()) {
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// Simplify graph to generate more dead block patterns.
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ConnectSuccessiveBlocks();
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bool did_any_simplification = false;
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did_any_simplification |= SimplifyAlwaysThrows();
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did_any_simplification |= SimplifyIfs();
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did_any_simplification |= RemoveDeadBlocks();
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if (did_any_simplification) {
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// Connect successive blocks created by dead branches.
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ConnectSuccessiveBlocks();
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}
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}
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SsaRedundantPhiElimination(graph_).Run();
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RemoveDeadInstructions();
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return true;
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}
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} // namespace art
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