compiler/optimizing/constant_folding.cc

/*
 * 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 instructions that can be evaluated
// as constants.
class HConstantFoldingVisitor : public HGraphDelegateVisitor {
 public:
  explicit HConstantFoldingVisitor(HGraph* graph)
      : HGraphDelegateVisitor(graph) {}

 private:
  void VisitBasicBlock(HBasicBlock* block) OVERRIDE;

  void VisitUnaryOperation(HUnaryOperation* inst) OVERRIDE;
  void VisitBinaryOperation(HBinaryOperation* inst) OVERRIDE;

  void VisitTypeConversion(HTypeConversion* inst) OVERRIDE;
  void VisitDivZeroCheck(HDivZeroCheck* inst) OVERRIDE;

  DISALLOW_COPY_AND_ASSIGN(HConstantFoldingVisitor);
};

// This visitor tries to simplify operations with an absorbing input,
// yielding 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 VisitAbove(HAbove* instruction) OVERRIDE;
  void VisitAboveOrEqual(HAboveOrEqual* instruction) OVERRIDE;
  void VisitBelow(HBelow* instruction) OVERRIDE;
  void VisitBelowOrEqual(HBelowOrEqual* instruction) OVERRIDE;

  void VisitAnd(HAnd* instruction) OVERRIDE;
  void VisitCompare(HCompare* 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() {
  HConstantFoldingVisitor visitor(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.
  visitor.VisitReversePostOrder();
}


void HConstantFoldingVisitor::VisitBasicBlock(HBasicBlock* block) {
  // Traverse this block's instructions (phis don't need to be
  // processed) in (forward) order and replace the ones that can be
  // statically evaluated by a compile-time counterpart.
  for (HInstructionIterator it(block->GetInstructions()); !it.Done(); it.Advance()) {
    it.Current()->Accept(this);
  }
}

void HConstantFoldingVisitor::VisitUnaryOperation(HUnaryOperation* inst) {
  // Constant folding: replace `op(a)' with a constant at compile
  // time if `a' is a constant.
  HConstant* constant = inst->TryStaticEvaluation();
  if (constant != nullptr) {
    inst->ReplaceWith(constant);
    inst->GetBlock()->RemoveInstruction(inst);
  }
}

void HConstantFoldingVisitor::VisitBinaryOperation(HBinaryOperation* inst) {
  // Constant folding: replace `op(a, b)' with a constant at
  // compile time if `a' and `b' are both constants.
  HConstant* constant = inst->TryStaticEvaluation();
  if (constant != nullptr) {
    inst->ReplaceWith(constant);
    inst->GetBlock()->RemoveInstruction(inst);
  } else {
    InstructionWithAbsorbingInputSimplifier simplifier(GetGraph());
    inst->Accept(&simplifier);
  }
}

void HConstantFoldingVisitor::VisitTypeConversion(HTypeConversion* inst) {
  // Constant folding: replace `TypeConversion(a)' with a constant at
  // compile time if `a' is a constant.
  HConstant* constant = inst->AsTypeConversion()->TryStaticEvaluation();
  if (constant != nullptr) {
    inst->ReplaceWith(constant);
    inst->GetBlock()->RemoveInstruction(inst);
  }
}

void HConstantFoldingVisitor::VisitDivZeroCheck(HDivZeroCheck* inst) {
  // We can safely remove the check if the input is a non-null constant.
  HInstruction* check_input = inst->InputAt(0);
  if (check_input->IsConstant() && !check_input->AsConstant()->IsArithmeticZero()) {
    inst->ReplaceWith(check_input);
    inst->GetBlock()->RemoveInstruction(inst);
  }
}


void InstructionWithAbsorbingInputSimplifier::VisitShift(HBinaryOperation* instruction) {
  DCHECK(instruction->IsShl() || instruction->IsShr() || instruction->IsUShr());
  HInstruction* left = instruction->GetLeft();
  if (left->IsConstant() && left->AsConstant()->IsArithmeticZero()) {
    // Replace code looking like
    //    SHL dst, 0, shift_amount
    // with
    //    CONSTANT 0
    instruction->ReplaceWith(left);
    instruction->GetBlock()->RemoveInstruction(instruction);
  }
}

void InstructionWithAbsorbingInputSimplifier::VisitAbove(HAbove* instruction) {
  if (instruction->GetLeft()->IsConstant() &&
      instruction->GetLeft()->AsConstant()->IsArithmeticZero()) {
    // Replace code looking like
    //    ABOVE dst, 0, src  // unsigned 0 > src is always false
    // with
    //    CONSTANT false
    instruction->ReplaceWith(GetGraph()->GetConstant(Primitive::kPrimBoolean, 0));
    instruction->GetBlock()->RemoveInstruction(instruction);
  }
}

void InstructionWithAbsorbingInputSimplifier::VisitAboveOrEqual(HAboveOrEqual* instruction) {
  if (instruction->GetRight()->IsConstant() &&
      instruction->GetRight()->AsConstant()->IsArithmeticZero()) {
    // Replace code looking like
    //    ABOVE_OR_EQUAL dst, src, 0  // unsigned src >= 0 is always true
    // with
    //    CONSTANT true
    instruction->ReplaceWith(GetGraph()->GetConstant(Primitive::kPrimBoolean, 1));
    instruction->GetBlock()->RemoveInstruction(instruction);
  }
}

void InstructionWithAbsorbingInputSimplifier::VisitBelow(HBelow* instruction) {
  if (instruction->GetRight()->IsConstant() &&
      instruction->GetRight()->AsConstant()->IsArithmeticZero()) {
    // Replace code looking like
    //    BELOW dst, src, 0  // unsigned src < 0 is always false
    // with
    //    CONSTANT false
    instruction->ReplaceWith(GetGraph()->GetConstant(Primitive::kPrimBoolean, 0));
    instruction->GetBlock()->RemoveInstruction(instruction);
  }
}

void InstructionWithAbsorbingInputSimplifier::VisitBelowOrEqual(HBelowOrEqual* instruction) {
  if (instruction->GetLeft()->IsConstant() &&
      instruction->GetLeft()->AsConstant()->IsArithmeticZero()) {
    // Replace code looking like
    //    BELOW_OR_EQUAL dst, 0, src  // unsigned 0 <= src is always true
    // with
    //    CONSTANT true
    instruction->ReplaceWith(GetGraph()->GetConstant(Primitive::kPrimBoolean, 1));
    instruction->GetBlock()->RemoveInstruction(instruction);
  }
}

void InstructionWithAbsorbingInputSimplifier::VisitAnd(HAnd* instruction) {
  HConstant* input_cst = instruction->GetConstantRight();
  if ((input_cst != nullptr) && input_cst->IsZeroBitPattern()) {
    // Replace code looking like
    //    AND dst, src, 0
    // with
    //    CONSTANT 0
    instruction->ReplaceWith(input_cst);
    instruction->GetBlock()->RemoveInstruction(instruction);
  }
}

void InstructionWithAbsorbingInputSimplifier::VisitCompare(HCompare* instruction) {
  HConstant* input_cst = instruction->GetConstantRight();
  if (input_cst != nullptr) {
    HInstruction* input_value = instruction->GetLeastConstantLeft();
    if (Primitive::IsFloatingPointType(input_value->GetType()) &&
        ((input_cst->IsFloatConstant() && input_cst->AsFloatConstant()->IsNaN()) ||
         (input_cst->IsDoubleConstant() && input_cst->AsDoubleConstant()->IsNaN()))) {
      // Replace code looking like
      //    CMP{G,L}-{FLOAT,DOUBLE} dst, src, NaN
      // with
      //    CONSTANT +1 (gt bias)
      // or
      //    CONSTANT -1 (lt bias)
      instruction->ReplaceWith(GetGraph()->GetConstant(Primitive::kPrimInt,
                                                       (instruction->IsGtBias() ? 1 : -1)));
      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->IsArithmeticZero()) {
    // 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()->IsArithmeticZero()) {
    // 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 optimization
  // opportunity.
  if (instruction->GetLeft() == instruction->GetRight()) {
    // Replace code looking like
    //    SUB dst, src, src
    // with
    //    CONSTANT 0
    // Note that we cannot optimize `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