[InstCombine] prefer xor with -1 because 'not' is easier to understand (PR32706)

This matches the demanded bits behavior in the DAG and should fix:
https://bugs.llvm.org/show_bug.cgi?id=32706

Differential Revision: https://reviews.llvm.org/D32255

llvm-svn: 300977

1 files changed, 14 insertions, 4 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
index 2f6e411345f..d7e709c9cca 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
@@ -277,10 +277,20 @@ Value *InstCombiner::SimplifyDemandedUseBits(Value *V, APInt DemandedMask,
       return InsertNewInstWith(And, *I);
     }
 
-    // If the RHS is a constant, see if we can simplify it.
-    // FIXME: for XOR, we prefer to force bits to 1 if they will make a -1.
-    if (ShrinkDemandedConstant(I, 1, DemandedMask))
-      return I;
+    // If the RHS is a constant, see if we can change it. Don't alter a -1
+    // constant because that's a canonical 'not' op, and that is better for
+    // combining, SCEV, and codegen.
+    const APInt *C;
+    if (match(I->getOperand(1), m_APInt(C)) && !C->isAllOnesValue()) {
+      if ((*C | ~DemandedMask).isAllOnesValue()) {
+        // Force bits to 1 to create a 'not' op.
+        I->setOperand(1, ConstantInt::getAllOnesValue(VTy));
+        return I;
+      }
+      // If we can't turn this into a 'not', try to shrink the constant.
+      if (ShrinkDemandedConstant(I, 1, DemandedMask))
+        return I;
+    }
 
     // If our LHS is an 'and' and if it has one use, and if any of the bits we
     // are flipping are known to be set, then the xor is just resetting those