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ComputeMaskedBits: Make knownzero computation more aggressive for ctlz with undef zero.

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unsigned foo(unsigned x) { return 31 - __builtin_clz(x); }
now compiles into a single "bsrl" instruction on x86.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@147255 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Benjamin Kramer
2011-12-24 17:31:46 +00:00
parent 1fdfae05b0
commit 009da05e75
3 changed files with 24 additions and 4 deletions
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lib/Analysis/ValueTracking.cpp
+8 -1
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@@ -714,9 +714,16 @@ void llvm::ComputeMaskedBits(Value *V, const APInt &Mask,
if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
switch (II->getIntrinsicID()) {
default: break;
case Intrinsic::ctpop:
case Intrinsic::ctlz:
case Intrinsic::cttz: {
unsigned LowBits = Log2_32(BitWidth)+1;
// If this call is undefined for 0, the result will be less than 2^n.
if (II->getArgOperand(1) == ConstantInt::getTrue(II->getContext()))
LowBits -= 1;
KnownZero = APInt::getHighBitsSet(BitWidth, BitWidth - LowBits);
break;
}
case Intrinsic::ctpop: {
unsigned LowBits = Log2_32(BitWidth)+1;
KnownZero = APInt::getHighBitsSet(BitWidth, BitWidth - LowBits);
break;