[InstCombine] enhance fold for subtract-from-constant -> xor

A low-bit mask is not required:
https://alive2.llvm.org/ce/z/yPShss

This matches the SDAG implementation that was updated at:
8b756713140f

1 files changed, 5 insertions, 7 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
index f53cce1..01821a7 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -1965,14 +1965,12 @@ Instruction *InstCombinerImpl::visitSub(BinaryOperator &I) {
       return BinaryOperator::CreateAdd(X, ConstantExpr::getSub(C, C2));
   }
 
+  // If there's no chance any bit will need to borrow from an adjacent bit:
+  // sub C, X --> xor X, C
   const APInt *Op0C;
-  if (match(Op0, m_APInt(Op0C)) && Op0C->isMask()) {
-    // Turn this into a xor if LHS is 2^n-1 and the remaining bits are known
-    // zero.
-    KnownBits RHSKnown = computeKnownBits(Op1, 0, &I);
-    if ((*Op0C | RHSKnown.Zero).isAllOnes())
-      return BinaryOperator::CreateXor(Op1, Op0);
-  }
+  if (match(Op0, m_APInt(Op0C)) &&
+      (~computeKnownBits(Op1, 0, &I).Zero).isSubsetOf(*Op0C))
+    return BinaryOperator::CreateXor(Op1, Op0);
 
   {
     Value *Y;