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While SimplifyDemandedBits constant folds this, we can't rely on it here. 

It's possible to craft an input that hits the recursion limits in a way
that SimplifyDemandedBits doesn't simplify the icmp but ComputeMaskedBits
can infer which bits are zero.

No test case as it depends on too many other things. Fixes PR9609.

llvm-svn: 128777
This commit is contained in:
Benjamin Kramer 2011-04-02 18:50:58 +00:00
parent 1473c9a7c4
commit 50a281a871
1 changed files with 7 additions and 2 deletions
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9
llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
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@ -913,8 +913,13 @@ Instruction *InstCombiner::transformSExtICmp(ICmpInst *ICI, Instruction &CI) {
if (KnownZeroMask.isPowerOf2()) {
Value *In = ICI->getOperand(0);
assert((Op1C->isZero() || Op1C->getValue() == KnownZeroMask) &&
"Constant icmp not folded?");
// If the icmp tests for a known zero bit we can constant fold it.
if (!Op1C->isZero() && Op1C->getValue() != KnownZeroMask) {
Value *V = Pred == ICmpInst::ICMP_NE ?
ConstantInt::getAllOnesValue(CI.getType()) :
ConstantInt::getNullValue(CI.getType());
return ReplaceInstUsesWith(CI, V);
}
if (!Op1C->isZero() == (Pred == ICmpInst::ICMP_NE)) {
// sext ((x & 2^n) == 0) -> (x >> n) - 1