compiler/optimizing/constant_folding.cc - platform/art - Gitiles

 /*
  * 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 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() {
   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->IsTypeConversion()) {
         // 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);
         }
       } 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::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} 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->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
	/*
	* 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 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() {
	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->IsTypeConversion()) {
	// 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);
	}
	} 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::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} 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->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