path: root/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp

//===- InstCombineSimplifyDemanded.cpp ------------------------------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file contains logic for simplifying instructions based on information
// about how they are used.
//
//===----------------------------------------------------------------------===//


#include "InstCombine.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/Support/PatternMatch.h"

using namespace llvm;
using namespace llvm::PatternMatch;

/// ShrinkDemandedConstant - Check to see if the specified operand of the
/// specified instruction is a constant integer.  If so, check to see if there
/// are any bits set in the constant that are not demanded.  If so, shrink the
/// constant and return true.
static bool ShrinkDemandedConstant(Instruction *I, unsigned OpNo,
                                   APInt Demanded) {
  assert(I && "No instruction?");
  assert(OpNo < I->getNumOperands() && "Operand index too large");

  // If the operand is not a constant integer, nothing to do.
  ConstantInt *OpC = dyn_cast<ConstantInt>(I->getOperand(OpNo));
  if (!OpC) return false;

  // If there are no bits set that aren't demanded, nothing to do.
  Demanded = Demanded.zextOrTrunc(OpC->getValue().getBitWidth());
  if ((~Demanded & OpC->getValue()) == 0)
    return false;

  // This instruction is producing bits that are not demanded. Shrink the RHS.
  Demanded &= OpC->getValue();
  I->setOperand(OpNo, ConstantInt::get(OpC->getType(), Demanded));
  return true;
}



/// SimplifyDemandedInstructionBits - Inst is an integer instruction that
/// SimplifyDemandedBits knows about.  See if the instruction has any
/// properties that allow us to simplify its operands.
bool InstCombiner::SimplifyDemandedInstructionBits(Instruction &Inst) {
  unsigned BitWidth = Inst.getType()->getScalarSizeInBits();
  APInt KnownZero(BitWidth, 0), KnownOne(BitWidth, 0);
  APInt DemandedMask(APInt::getAllOnesValue(BitWidth));

  Value *V = SimplifyDemandedUseBits(&Inst, DemandedMask,
                                     KnownZero, KnownOne, 0);
  if (V == 0) return false;
  if (V == &Inst) return true;
  ReplaceInstUsesWith(Inst, V);
  return true;
}

/// SimplifyDemandedBits - This form of SimplifyDemandedBits simplifies the
/// specified instruction operand if possible, updating it in place.  It returns
/// true if it made any change and false otherwise.
bool InstCombiner::SimplifyDemandedBits(Use &U, APInt DemandedMask,
                                        APInt &KnownZero, APInt &KnownOne,
                                        unsigned Depth) {
  Value *NewVal = SimplifyDemandedUseBits(U.get(), DemandedMask,
                                          KnownZero, KnownOne, Depth);