[llvm] a0d457b - [X86] foldMaskAndShiftToScale - use MaskedValueIsZero to test for all-zero upper bits

[Simon Pilgrim via llvm-commits]

Author: Simon Pilgrim
Date: 2023-08-24T16:51:11+01:00
New Revision: a0d457bceca90122c7f9c1fef451f58208f92c6f

URL: https://github.com/llvm/llvm-project/commit/a0d457bceca90122c7f9c1fef451f58208f92c6f
DIFF: https://github.com/llvm/llvm-project/commit/a0d457bceca90122c7f9c1fef451f58208f92c6f.diff

LOG: [X86] foldMaskAndShiftToScale - use MaskedValueIsZero to test for all-zero upper bits

We were testing for an exact match of zero bits which isn't necessary (we don't care if lower bits are zero) - by inspection as its proven tricky to get a decent test case.

Noticed while clearing up D155472 regressions

Added: 
    

Modified: 
    llvm/lib/Target/X86/X86ISelDAGToDAG.cpp

Removed: 
    


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diff  --git a/llvm/lib/Target/X86/X86ISelDAGToDAG.cpp b/llvm/lib/Target/X86/X86ISelDAGToDAG.cpp
index c78d984a55363b..aeaa926e72fcbd 100644
--- a/llvm/lib/Target/X86/X86ISelDAGToDAG.cpp
+++ b/llvm/lib/Target/X86/X86ISelDAGToDAG.cpp
@@ -2108,8 +2108,8 @@ static bool foldMaskAndShiftToScale(SelectionDAG &DAG, SDValue N,
   }
   APInt MaskedHighBits =
     APInt::getHighBitsSet(X.getSimpleValueType().getSizeInBits(), MaskLZ);
-  KnownBits Known = DAG.computeKnownBits(X);
-  if (MaskedHighBits != Known.Zero) return true;
+  if (!DAG.MaskedValueIsZero(X, MaskedHighBits))
+    return true;
 
   // We've identified a pattern that can be transformed into a single shift
   // and an addressing mode. Make it so.