optimize cttz and ctlz when we can prove something about the
leading/trailing bits. Patch by Alastair Lynn!
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@92706 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Transforms/InstCombine/InstructionCombining.cpp b/lib/Transforms/InstCombine/InstructionCombining.cpp
index 1b08b0e..81640f3 100644
--- a/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -3144,7 +3144,40 @@
II->getOperand(1));
}
break;
-
+ case Intrinsic::cttz: {
+ // If all bits below the first known one are known zero,
+ // this value is constant.
+ const IntegerType *IT = cast<IntegerType>(II->getOperand(1)->getType());
+ uint32_t BitWidth = IT->getBitWidth();
+ APInt KnownZero(BitWidth, 0);
+ APInt KnownOne(BitWidth, 0);
+ ComputeMaskedBits(II->getOperand(1), APInt::getAllOnesValue(BitWidth),
+ KnownZero, KnownOne);
+ unsigned TrailingZeros = KnownOne.countTrailingZeros();
+ APInt Mask(APInt::getLowBitsSet(BitWidth, TrailingZeros));
+ if ((Mask & KnownZero) == Mask)
+ return ReplaceInstUsesWith(CI, ConstantInt::get(IT,
+ APInt(BitWidth, TrailingZeros)));
+
+ }
+ break;
+ case Intrinsic::ctlz: {
+ // If all bits above the first known one are known zero,
+ // this value is constant.
+ const IntegerType *IT = cast<IntegerType>(II->getOperand(1)->getType());
+ uint32_t BitWidth = IT->getBitWidth();
+ APInt KnownZero(BitWidth, 0);
+ APInt KnownOne(BitWidth, 0);
+ ComputeMaskedBits(II->getOperand(1), APInt::getAllOnesValue(BitWidth),
+ KnownZero, KnownOne);
+ unsigned LeadingZeros = KnownOne.countLeadingZeros();
+ APInt Mask(APInt::getHighBitsSet(BitWidth, LeadingZeros));
+ if ((Mask & KnownZero) == Mask)
+ return ReplaceInstUsesWith(CI, ConstantInt::get(IT,
+ APInt(BitWidth, LeadingZeros)));
+
+ }
+ break;
case Intrinsic::uadd_with_overflow: {
Value *LHS = II->getOperand(1), *RHS = II->getOperand(2);
const IntegerType *IT = cast<IntegerType>(II->getOperand(1)->getType());