Always compute all the bits in ComputeMaskedBits.

This allows us to keep passing reduced masks to SimplifyDemandedBits, but
know about all the bits if SimplifyDemandedBits fails. This allows instcombine
to simplify cases like the one in the included testcase.

llvm-svn: 154011

1 files changed, 1 insertions, 2 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index 53089928c3f..ab2987ff24c 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -1028,9 +1028,8 @@ Instruction *InstCombiner::visitICmpInstWithInstAndIntCst(ICmpInst &ICI,
       // of the high bits truncated out of x are known.
       unsigned DstBits = LHSI->getType()->getPrimitiveSizeInBits(),
              SrcBits = LHSI->getOperand(0)->getType()->getPrimitiveSizeInBits();
-      APInt Mask(APInt::getHighBitsSet(SrcBits, SrcBits-DstBits));
       APInt KnownZero(SrcBits, 0), KnownOne(SrcBits, 0);
-      ComputeMaskedBits(LHSI->getOperand(0), Mask, KnownZero, KnownOne);
+      ComputeMaskedBits(LHSI->getOperand(0), KnownZero, KnownOne);
 
       // If all the high bits are known, we can do this xform.
       if ((KnownZero|KnownOne).countLeadingOnes() >= SrcBits-DstBits) {