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/*
* Copyright (C) 2015 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 "boolean_simplifier.h"
namespace art {
// Returns true if 'block1' and 'block2' are empty, merge into the same single
// successor and the successor can only be reached from them.
static bool BlocksDoMergeTogether(HBasicBlock* block1, HBasicBlock* block2) {
if (!block1->IsSingleGoto() || !block2->IsSingleGoto()) return false;
HBasicBlock* succ1 = block1->GetSuccessors().Get(0);
HBasicBlock* succ2 = block2->GetSuccessors().Get(0);
return succ1 == succ2 && succ1->GetPredecessors().Size() == 2u;
}
// Returns true if the outcome of the branching matches the boolean value of
// the branching condition.
static bool PreservesCondition(HInstruction* input_true, HInstruction* input_false) {
return input_true->IsIntConstant() && input_true->AsIntConstant()->IsOne()
&& input_false->IsIntConstant() && input_false->AsIntConstant()->IsZero();
}
// Returns true if the outcome of the branching is exactly opposite of the
// boolean value of the branching condition.
static bool NegatesCondition(HInstruction* input_true, HInstruction* input_false) {
return input_true->IsIntConstant() && input_true->AsIntConstant()->IsZero()
&& input_false->IsIntConstant() && input_false->AsIntConstant()->IsOne();
}
// Returns an instruction with the opposite boolean value from 'cond'.
static HInstruction* GetOppositeCondition(HInstruction* cond) {
HGraph* graph = cond->GetBlock()->GetGraph();
ArenaAllocator* allocator = graph->GetArena();
if (cond->IsCondition()) {
HInstruction* lhs = cond->InputAt(0);
HInstruction* rhs = cond->InputAt(1);
if (cond->IsEqual()) {
return new (allocator) HNotEqual(lhs, rhs);
} else if (cond->IsNotEqual()) {
return new (allocator) HEqual(lhs, rhs);
} else if (cond->IsLessThan()) {
return new (allocator) HGreaterThanOrEqual(lhs, rhs);
} else if (cond->IsLessThanOrEqual()) {
return new (allocator) HGreaterThan(lhs, rhs);
} else if (cond->IsGreaterThan()) {
return new (allocator) HLessThanOrEqual(lhs, rhs);
} else if (cond->IsGreaterThanOrEqual()) {
return new (allocator) HLessThan(lhs, rhs);
}
} else if (cond->IsIntConstant()) {
HIntConstant* int_const = cond->AsIntConstant();
if (int_const->IsZero()) {
return graph->GetIntConstant(1);
} else {
DCHECK(int_const->IsOne());
return graph->GetIntConstant(0);
}
}
LOG(FATAL) << "Instruction " << cond->DebugName() << " used as a condition";
UNREACHABLE();
}
void HBooleanSimplifier::Run() {
// Iterate in post order in the unlikely case that removing one occurrence of
// the pattern empties a branch block of another occurrence. Otherwise the
// order does not matter.
for (HPostOrderIterator it(*graph_); !it.Done(); it.Advance()) {
HBasicBlock* block = it.Current();
if (!block->EndsWithIf()) continue;
// Find elements of the pattern.
HIf* if_instruction = block->GetLastInstruction()->AsIf();
HBasicBlock* true_block = if_instruction->IfTrueSuccessor();
HBasicBlock* false_block = if_instruction->IfFalseSuccessor();
if (!BlocksDoMergeTogether(true_block, false_block)) {
continue;
}
HBasicBlock* merge_block = true_block->GetSuccessors().Get(0);
if (!merge_block->HasSinglePhi()) {
continue;
}
HPhi* phi = merge_block->GetFirstPhi()->AsPhi();
HInstruction* true_value = phi->InputAt(merge_block->GetPredecessorIndexOf(true_block));
HInstruction* false_value = phi->InputAt(merge_block->GetPredecessorIndexOf(false_block));
// Check if the selection negates/preserves the value of the condition and
// if so, generate a suitable replacement instruction.
HInstruction* if_condition = if_instruction->InputAt(0);
HInstruction* replacement;
if (NegatesCondition(true_value, false_value)) {
replacement = GetOppositeCondition(if_condition);
if (replacement->GetBlock() == nullptr) {
block->InsertInstructionBefore(replacement, if_instruction);
}
} else if (PreservesCondition(true_value, false_value)) {
replacement = if_condition;
} else {
continue;
}
// Replace the selection outcome with the new instruction.
phi->ReplaceWith(replacement);
merge_block->RemovePhi(phi);
// Link the start/end blocks and remove empty branches.
graph_->MergeEmptyBranches(block, merge_block);
// Remove the original condition if it is now unused.
if (!if_condition->HasUses()) {
if_condition->GetBlock()->RemoveInstruction(if_condition);
}
}
}
} // namespace art
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