When determining if we can fold (x >> C1) << C2, the bits that we need to verify are zero

are not the low bits of x, but the bits that WILL be the low bits after the operation completes.

llvm-svn: 122529

1 files changed, 2 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
index e3232a491b7..a0c5ef5338f 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
@@ -168,8 +168,9 @@ static bool CanEvaluateShifted(Value *V, unsigned NumBits, bool isLeftShift,
     // We can always turn lshr(c1)+shl(c2) -> lshr(c3)+and(c4), but it isn't
     // profitable unless we know the and'd out bits are already zero.
     if (CI->getZExtValue() > NumBits) {
+      unsigned LowBits = CI->getZExtValue() - NumBits;
       if (MaskedValueIsZero(I->getOperand(0),
-                            APInt::getLowBitsSet(TypeWidth, NumBits)))
+                          APInt::getLowBitsSet(TypeWidth, NumBits) << LowBits))
         return true;
     }