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

//===- InstCombineCasts.cpp -----------------------------------------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements the visit functions for cast operations.
//
//===----------------------------------------------------------------------===//

#include "InstCombine.h"
#include "llvm/Target/TargetData.h"
#include "llvm/Support/PatternMatch.h"
using namespace llvm;
using namespace PatternMatch;

/// CanEvaluateInDifferentType - Return true if we can take the specified value
/// and return it as type Ty without inserting any new casts and without
/// changing the computed value.  This is used by code that tries to decide
/// whether promoting or shrinking integer operations to wider or smaller types
/// will allow us to eliminate a truncate or extend.
///
/// This is a truncation operation if Ty is smaller than V->getType(), or an
/// extension operation if Ty is larger.
///
/// If CastOpc is a truncation, then Ty will be a type smaller than V.  We
/// should return true if trunc(V) can be computed by computing V in the smaller
/// type.  If V is an instruction, then trunc(inst(x,y)) can be computed as
/// inst(trunc(x),trunc(y)), which only makes sense if x and y can be
/// efficiently truncated.
///
/// If CastOpc is a sext or zext, we are asking if the low bits of the value can
/// bit computed in a larger type, which is then and'd or sext_in_reg'd to get
/// the final result.
bool InstCombiner::CanEvaluateInDifferentType(Value *V, const Type *Ty,
                                              unsigned CastOpc,
                                              int &NumCastsRemoved){
  // We can always evaluate constants in another type.
  if (isa<Constant>(V))
    return true;
  
  Instruction *I = dyn_cast<Instruction>(V);
  if (!I) return false;
  
  const Type *OrigTy = V->getType();
  
  // If this is an extension or truncate, we can often eliminate it.
  if (isa<TruncInst>(I) || isa<ZExtInst>(I) || isa<SExtInst>(I)) {
    // If this is a cast from the destination type, we can trivially eliminate
    // it, and this will remove a cast overall.
    if (I->getOperand(0)->getType() == Ty) {
      // If the first operand is itself a cast, and is eliminable, do not count
      // this as an eliminable cast.  We would prefer to eliminate those two
      // casts first.
      if (!isa<CastInst>(I->getOperand(0)) && I->hasOneUse())
        ++NumCastsRemoved;
      return true;
    }
  }

  // We can't extend or shrink something that has multiple uses: doing so would
  // require duplicating the instruction in general, which isn't profitable.
  if (!I->hasOneUse()) return false;

  unsigned Opc = I->getOpcode();
  switch (Opc) {
  case Instruction::Add:
  case Instruction::Sub:
  case Instruction::Mul:
  case Instruction::And:
  case Instruction::Or:
  case Instruction::Xor:
    // These operators can all arbitrarily be extended or truncated.
    return CanEvaluateInDifferentType(I->getOperand(0), Ty, CastOpc,
                                      NumCastsRemoved) &&
           CanEvaluateInDifferentType(I->getOperand(1), Ty, CastOpc,
                                      NumCastsRemoved);

  case Instruction::UDiv:
  case Instruction::URem: {
    // UDiv and URem can be truncated if all the truncated bits are zero.
    uint32_t OrigBitWidth = OrigTy->getScalarSizeInBits();
    uint32_t BitWidth = Ty->getScalarSizeInBits();
    if (BitWidth < OrigBitWidth) {
      APInt Mask = APInt::getHighBitsSet(OrigBitWidth, OrigBitWidth-BitWidth);
      if (MaskedValueIsZero(I->getOperand(0), Mask) &&
          MaskedValueIsZero(I->getOperand(1), Mask)) {
        return CanEvaluateInDifferentType(I->getOperand(0), Ty, CastOpc,
                                          NumCastsRemoved) &&
               CanEvaluateInDifferentType(I->getOperand(1), Ty, CastOpc,