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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 "constant_folding.h"
namespace art {
// This visitor tries to simplify operations that yield a constant. For example
// `input * 0` is replaced by a null constant.
class InstructionWithAbsorbingInputSimplifier : public HGraphVisitor {
public:
explicit InstructionWithAbsorbingInputSimplifier(HGraph* graph) : HGraphVisitor(graph) {}
private:
void VisitShift(HBinaryOperation* shift);
void VisitAnd(HAnd* instruction) OVERRIDE;
void VisitMul(HMul* instruction) OVERRIDE;
void VisitOr(HOr* instruction) OVERRIDE;
void VisitRem(HRem* instruction) OVERRIDE;
void VisitShl(HShl* instruction) OVERRIDE;
void VisitShr(HShr* instruction) OVERRIDE;
void VisitSub(HSub* instruction) OVERRIDE;
void VisitUShr(HUShr* instruction) OVERRIDE;
void VisitXor(HXor* instruction) OVERRIDE;
};
void HConstantFolding::Run() {
InstructionWithAbsorbingInputSimplifier simplifier(graph_);
// Process basic blocks in reverse post-order in the dominator tree,
// so that an instruction turned into a constant, used as input of
// another instruction, may possibly be used to turn that second
// instruction into a constant as well.
for (HReversePostOrderIterator it(*graph_); !it.Done(); it.Advance()) {
HBasicBlock* block = it.Current();
// Traverse this block's instructions in (forward) order and
// replace the ones that can be statically evaluated by a
// compile-time counterpart.
for (HInstructionIterator inst_it(block->GetInstructions());
!inst_it.Done(); inst_it.Advance()) {
HInstruction* inst = inst_it.Current();
if (inst->IsBinaryOperation()) {
// Constant folding: replace `op(a, b)' with a constant at
// compile time if `a' and `b' are both constants.
HConstant* constant = inst->AsBinaryOperation()->TryStaticEvaluation();
if (constant != nullptr) {
inst->ReplaceWith(constant);
inst->GetBlock()->RemoveInstruction(inst);
} else {
inst->Accept(&simplifier);
}
} else if (inst->IsUnaryOperation()) {
// Constant folding: replace `op(a)' with a constant at compile
// time if `a' is a constant.
HConstant* constant = inst->AsUnaryOperation()->TryStaticEvaluation();
if (constant != nullptr) {
inst->ReplaceWith(constant);
inst->GetBlock()->RemoveInstruction(inst);
}
} else if (inst->IsDivZeroCheck()) {
// We can safely remove the check if the input is a non-null constant.
HDivZeroCheck* check = inst->AsDivZeroCheck();
HInstruction* check_input = check->InputAt(0);
if (check_input->IsConstant() && !check_input->AsConstant()->IsZero()) {
check->ReplaceWith(check_input);
check->GetBlock()->RemoveInstruction(check);
}
}
}
}
}
void InstructionWithAbsorbingInputSimplifier::VisitShift(HBinaryOperation* instruction) {
DCHECK(instruction->IsShl() || instruction->IsShr() || instruction->IsUShr());
HInstruction* left = instruction->GetLeft();
if (left->IsConstant() && left->AsConstant()->IsZero()) {
// Replace code looking like
// SHL dst, 0, shift_amount
// with
// CONSTANT 0
instruction->ReplaceWith(left);
instruction->GetBlock()->RemoveInstruction(instruction);
}
}
void InstructionWithAbsorbingInputSimplifier::VisitAnd(HAnd* instruction) {
HConstant* input_cst = instruction->GetConstantRight();
if ((input_cst != nullptr) && input_cst->IsZero()) {
// Replace code looking like
// AND dst, src, 0
// with
// CONSTANT 0
instruction->ReplaceWith(input_cst);
instruction->GetBlock()->RemoveInstruction(instruction);
}
}
void InstructionWithAbsorbingInputSimplifier::VisitMul(HMul* instruction) {
HConstant* input_cst = instruction->GetConstantRight();
Primitive::Type type = instruction->GetType();
if (Primitive::IsIntOrLongType(type) &&
(input_cst != nullptr) && input_cst->IsZero()) {
// Replace code looking like
// MUL dst, src, 0
// with
// CONSTANT 0
// Integral multiplication by zero always yields zero, but floating-point
// multiplication by zero does not always do. For example `Infinity * 0.0`
// should yield a NaN.
instruction->ReplaceWith(input_cst);
instruction->GetBlock()->RemoveInstruction(instruction);
}
}
void InstructionWithAbsorbingInputSimplifier::VisitOr(HOr* instruction) {
HConstant* input_cst = instruction->GetConstantRight();
if (input_cst == nullptr) {
return;
}
if (Int64FromConstant(input_cst) == -1) {
// Replace code looking like
// OR dst, src, 0xFFF...FF
// with
// CONSTANT 0xFFF...FF
instruction->ReplaceWith(input_cst);
instruction->GetBlock()->RemoveInstruction(instruction);
}
}
void InstructionWithAbsorbingInputSimplifier::VisitRem(HRem* instruction) {
Primitive::Type type = instruction->GetType();
if (!Primitive::IsIntegralType(type)) {
return;
}
HBasicBlock* block = instruction->GetBlock();
if (instruction->GetLeft()->IsConstant() &&
instruction->GetLeft()->AsConstant()->IsZero()) {
// Replace code looking like
// REM dst, 0, src
// with
// CONSTANT 0
instruction->ReplaceWith(instruction->GetLeft());
block->RemoveInstruction(instruction);
}
HConstant* cst_right = instruction->GetRight()->AsConstant();
if (((cst_right != nullptr) &&
(cst_right->IsOne() || cst_right->IsMinusOne())) ||
(instruction->GetLeft() == instruction->GetRight())) {
// Replace code looking like
// REM dst, src, 1
// or
// REM dst, src, -1
// or
// REM dst, src, src
// with
// CONSTANT 0
instruction->ReplaceWith(GetGraph()->GetConstant(type, 0));
block->RemoveInstruction(instruction);
}
}
void InstructionWithAbsorbingInputSimplifier::VisitShl(HShl* instruction) {
VisitShift(instruction);
}
void InstructionWithAbsorbingInputSimplifier::VisitShr(HShr* instruction) {
VisitShift(instruction);
}
void InstructionWithAbsorbingInputSimplifier::VisitSub(HSub* instruction) {
Primitive::Type type = instruction->GetType();
if (!Primitive::IsIntegralType(type)) {
return;
}
HBasicBlock* block = instruction->GetBlock();
// We assume that GVN has run before, so we only perform a pointer
// comparison. If for some reason the values are equal but the pointers are
// different, we are still correct and only miss an optimisation
// opportunity.
if (instruction->GetLeft() == instruction->GetRight()) {
// Replace code looking like
// SUB dst, src, src
// with
// CONSTANT 0
// Note that we cannot optimise `x - x` to `0` for floating-point. It does
// not work when `x` is an infinity.
instruction->ReplaceWith(GetGraph()->GetConstant(type, 0));
block->RemoveInstruction(instruction);
}
}
void InstructionWithAbsorbingInputSimplifier::VisitUShr(HUShr* instruction) {
VisitShift(instruction);
}
void InstructionWithAbsorbingInputSimplifier::VisitXor(HXor* instruction) {
if (instruction->GetLeft() == instruction->GetRight()) {
// Replace code looking like
// XOR dst, src, src
// with
// CONSTANT 0
Primitive::Type type = instruction->GetType();
HBasicBlock* block = instruction->GetBlock();
instruction->ReplaceWith(GetGraph()->GetConstant(type, 0));
block->RemoveInstruction(instruction);
}
}
} // namespace art