[ConstantRange] Improve ConstantRange::binaryXor (#80146)

`ConstantRange::binaryXor` gives poor results as it currently depends on
`KnownBits::operator^`.
Since `sub A, B` is canonicalized into `xor A, B` if `B` is the subset
of `A`, this patch reverts the transform in `ConstantRange::binaryXor`,
which will give better results.

Alive2: https://alive2.llvm.org/ce/z/bmTMV9
Fixes #79696.

1 files changed, 16 insertions, 1 deletions

diff --git a/llvm/lib/IR/ConstantRange.cpp b/llvm/lib/IR/ConstantRange.cpp
index cbb64b2..3394a1e 100644
--- a/llvm/lib/IR/ConstantRange.cpp
+++ b/llvm/lib/IR/ConstantRange.cpp
@@ -1467,7 +1467,22 @@ ConstantRange ConstantRange::binaryXor(const ConstantRange &Other) const {
   if (isSingleElement() && getSingleElement()->isAllOnes())
     return Other.binaryNot();
 
-  return fromKnownBits(toKnownBits() ^ Other.toKnownBits(), /*IsSigned*/false);
+  KnownBits LHSKnown = toKnownBits();
+  KnownBits RHSKnown = Other.toKnownBits();
+  KnownBits Known = LHSKnown ^ RHSKnown;
+  ConstantRange CR = fromKnownBits(Known, /*IsSigned*/ false);
+  // Typically the following code doesn't improve the result if BW = 1.
+  if (getBitWidth() == 1)
+    return CR;
+
+  // If LHS is known to be the subset of RHS, treat LHS ^ RHS as RHS -nuw/nsw
+  // LHS. If RHS is known to be the subset of LHS, treat LHS ^ RHS as LHS
+  // -nuw/nsw RHS.
+  if ((~LHSKnown.Zero).isSubsetOf(RHSKnown.One))
+    CR = CR.intersectWith(Other.sub(*this), PreferredRangeType::Unsigned);
+  else if ((~RHSKnown.Zero).isSubsetOf(LHSKnown.One))
+    CR = CR.intersectWith(this->sub(Other), PreferredRangeType::Unsigned);
+  return CR;
 }
 
 ConstantRange