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Teach instcombine to turn trunc(srl x, c) -> srl (trunc(x), c) when safe.

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This implements InstCombine/cast.ll:test34.  It fires hundreds of times on
176.gcc.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@32009 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner 2006-11-29 07:04:07 +00:00
parent 41e2103133
commit 6aa5eb19d5
1 changed files with 33 additions and 1 deletions
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34
lib/Transforms/Scalar/InstructionCombining.cpp
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@ -5986,7 +5986,39 @@ Instruction *InstCombiner::commonIntCastTransforms(CastInst &CI) {
}
Instruction *InstCombiner::visitTrunc(CastInst &CI) {
return commonIntCastTransforms(CI);
if (Instruction *Result = commonIntCastTransforms(CI))
return Result;
Value *Src = CI.getOperand(0);
const Type *Ty = CI.getType();
unsigned DestBitWidth = Ty->getPrimitiveSizeInBits();
if (Instruction *SrcI = dyn_cast<Instruction>(Src)) {
switch (SrcI->getOpcode()) {
default: break;
case Instruction::LShr:
// We can shrink lshr to something smaller if we know the bits shifted in
// are already zeros.
if (ConstantInt *ShAmtV = dyn_cast<ConstantInt>(SrcI->getOperand(1))) {
unsigned ShAmt = ShAmtV->getZExtValue();
// Get a mask for the bits shifting in.
uint64_t Mask = (~0ULL >> (64-ShAmt)) << DestBitWidth;
if (SrcI->hasOneUse() && MaskedValueIsZero(SrcI->getOperand(0), Mask)) {
if (ShAmt >= DestBitWidth) // All zeros.
return ReplaceInstUsesWith(CI, Constant::getNullValue(Ty));
// Okay, we can shrink this. Truncate the input, then return a new
// shift.
Value *V = InsertCastBefore(SrcI->getOperand(0), Ty, CI);
return new ShiftInst(Instruction::LShr, V, SrcI->getOperand(1));
}
}
break;
}
}
return 0;
}
Instruction *InstCombiner::visitZExt(CastInst &CI) {