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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 "dead_code_elimination.h"
#include "utils/array_ref.h"
#include "base/bit_vector-inl.h"
#include "ssa_phi_elimination.h"
namespace art {
static void MarkReachableBlocks(HGraph* graph, ArenaBitVector* visited) {
ArenaVector<HBasicBlock*> worklist(graph->GetArena()->Adapter());
constexpr size_t kDefaultWorlistSize = 8;
worklist.reserve(kDefaultWorlistSize);
visited->SetBit(graph->GetEntryBlock()->GetBlockId());
worklist.push_back(graph->GetEntryBlock());
while (!worklist.empty()) {
HBasicBlock* block = worklist.back();
worklist.pop_back();
int block_id = block->GetBlockId();
DCHECK(visited->IsBitSet(block_id));
ArrayRef<HBasicBlock* const> live_successors(block->GetSuccessors());
HInstruction* last_instruction = block->GetLastInstruction();
if (last_instruction->IsIf()) {
HIf* if_instruction = last_instruction->AsIf();
HInstruction* condition = if_instruction->InputAt(0);
if (condition->IsIntConstant()) {
if (condition->AsIntConstant()->IsOne()) {
live_successors = live_successors.SubArray(0u, 1u);
DCHECK_EQ(live_successors[0], if_instruction->IfTrueSuccessor());
} else {
DCHECK(condition->AsIntConstant()->IsZero());
live_successors = live_successors.SubArray(1u, 1u);
DCHECK_EQ(live_successors[0], if_instruction->IfFalseSuccessor());
}
}
} else if (last_instruction->IsPackedSwitch()) {
HPackedSwitch* switch_instruction = last_instruction->AsPackedSwitch();
HInstruction* switch_input = switch_instruction->InputAt(0);
if (switch_input->IsIntConstant()) {
int32_t switch_value = switch_input->AsIntConstant()->GetValue();
int32_t start_value = switch_instruction->GetStartValue();
// Note: Though the spec forbids packed-switch values to wrap around, we leave
// that task to the verifier and use unsigned arithmetic with it's "modulo 2^32"
// semantics to check if the value is in range, wrapped or not.
uint32_t switch_index =
static_cast<uint32_t>(switch_value) - static_cast<uint32_t>(start_value);
if (switch_index < switch_instruction->GetNumEntries()) {
live_successors = live_successors.SubArray(switch_index, 1u);
DCHECK_EQ(live_successors[0], block->GetSuccessors()[switch_index]);
} else {
live_successors = live_successors.SubArray(switch_instruction->GetNumEntries(), 1u);
DCHECK_EQ(live_successors[0], switch_instruction->GetDefaultBlock());
}
}
}
for (HBasicBlock* successor : live_successors) {
// Add only those successors that have not been visited yet.
if (!visited->IsBitSet(successor->GetBlockId())) {
visited->SetBit(successor->GetBlockId());
worklist.push_back(successor);
}
}
}
}
void HDeadCodeElimination::MaybeRecordDeadBlock(HBasicBlock* block) {
if (stats_ != nullptr) {
stats_->RecordStat(MethodCompilationStat::kRemovedDeadInstruction,
block->GetPhis().CountSize() + block->GetInstructions().CountSize());
}
}
void HDeadCodeElimination::RemoveDeadBlocks() {
if (graph_->HasIrreducibleLoops()) {
// Do not eliminate dead blocks if the graph has irreducible loops. We could
// support it, but that would require changes in our loop representation to handle
// multiple entry points. We decided it was not worth the complexity.
return;
}
// Classify blocks as reachable/unreachable.
ArenaAllocator* allocator = graph_->GetArena();
ArenaBitVector live_blocks(allocator, graph_->GetBlocks().size(), false, kArenaAllocDCE);
MarkReachableBlocks(graph_, &live_blocks);
bool removed_one_or_more_blocks = false;
bool rerun_dominance_and_loop_analysis = false;
// Remove all dead blocks. Iterate in post order because removal needs the
// block's chain of dominators and nested loops need to be updated from the
// inside out.
for (HPostOrderIterator it(*graph_); !it.Done(); it.Advance()) {
HBasicBlock* block = it.Current();
int id = block->GetBlockId();
if (!live_blocks.IsBitSet(id)) {
MaybeRecordDeadBlock(block);
block->DisconnectAndDelete();
removed_one_or_more_blocks = true;
if (block->IsInLoop()) {
rerun_dominance_and_loop_analysis = true;
}
}
}
// If we removed at least one block, we need to recompute the full
// dominator tree and try block membership.
if (removed_one_or_more_blocks) {
if (rerun_dominance_and_loop_analysis) {
graph_->ClearLoopInformation();
graph_->ClearDominanceInformation();
graph_->BuildDominatorTree();
} else {
graph_->ClearDominanceInformation();
graph_->ComputeDominanceInformation();
graph_->ComputeTryBlockInformation();
}
}
// Connect successive blocks created by dead branches. Order does not matter.
for (HReversePostOrderIterator it(*graph_); !it.Done();) {
HBasicBlock* block = it.Current();
if (block->IsEntryBlock() || !block->GetLastInstruction()->IsGoto()) {
it.Advance();
continue;
}
HBasicBlock* successor = block->GetSingleSuccessor();
if (successor->IsExitBlock() || successor->GetPredecessors().size() != 1u) {
it.Advance();
continue;
}
block->MergeWith(successor);
// Reiterate on this block in case it can be merged with its new successor.
}
}
void HDeadCodeElimination::RemoveDeadInstructions() {
// Process basic blocks in post-order in the dominator tree, so that
// a dead instruction depending on another dead instruction is removed.
for (HPostOrderIterator b(*graph_); !b.Done(); b.Advance()) {
HBasicBlock* block = b.Current();
// Traverse this block's instructions in backward order and remove
// the unused ones.
HBackwardInstructionIterator i(block->GetInstructions());
// Skip the first iteration, as the last instruction of a block is
// a branching instruction.
DCHECK(i.Current()->IsControlFlow());
for (i.Advance(); !i.Done(); i.Advance()) {
HInstruction* inst = i.Current();
DCHECK(!inst->IsControlFlow());
if (!inst->HasSideEffects()
&& !inst->CanThrow()
&& !inst->IsSuspendCheck()
&& !inst->IsNativeDebugInfo()
// If we added an explicit barrier then we should keep it.
&& !inst->IsMemoryBarrier()
&& !inst->IsParameterValue()
&& !inst->HasUses()) {
block->RemoveInstruction(inst);
MaybeRecordStat(MethodCompilationStat::kRemovedDeadInstruction);
}
}
}
}
void HDeadCodeElimination::Run() {
RemoveDeadBlocks();
SsaRedundantPhiElimination(graph_).Run();
RemoveDeadInstructions();
}
} // namespace art
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