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/Target/TargetData.h"
#include "llvm/IntrinsicInst.h"

using namespace llvm;


/// 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);
  if (NewVal == 0) return false;
  U = NewVal;
  return true;
}


/// SimplifyDemandedUseBits - This function attempts to replace V with a simpler
/// value based on the demanded bits.  When this function is called, it is known
/// that only the bits set in DemandedMask of the result of V are ever used
/// downstream. Consequently, depending on the mask and V, it may be possible
/// to replace V with a constant or one of its operands. In such cases, this
/// function does the replacement and returns true. In all other cases, it
/// returns false after analyzing the expression and setting KnownOne and known
/// to be one in the expression.  KnownZero contains all the bits that are known
/// to be zero in the expression. These are provided to potentially allow the
/// caller (which might recursively be SimplifyDemandedBits itself) to simplify
/// the expression. KnownOne and KnownZero always follow the invariant that 
/// KnownOne & KnownZero == 0. That is, a bit can't be both 1 and 0. Note that
/// the bits in KnownOne and KnownZero may only be accurate for those bits set
/// in DemandedMask. Note also that the bitwidth of V, DemandedMask, KnownZero
/// and KnownOne must all be the same.
///
/// This returns null if it did not change anything and it permits no
/// simplification.  This returns V itself if it did some simplification of V's
/// operands based on the information about what bits are demanded. This returns