[llvm] [SeparateConstOffsetFromGEP] Decompose constant xor operand if possible (PR #150438)

[Matt Arsenault via llvm-commits]

================
@@ -743,6 +759,67 @@ Value *ConstantOffsetExtractor::removeConstOffset(unsigned ChainIndex) {
   return NewBO;
 }
 
+/// Analyze XOR instruction to extract disjoint constant bits for address
+/// folding
+///
+/// This function identifies bits in an XOR constant operand that are disjoint
+/// from the base operand's known set bits. For these disjoint bits, XOR behaves
+/// identically to addition, allowing us to extract them as constant offsets
+/// that can be folded into addressing modes.
+///
+/// Transformation: `Base ^ Const` becomes `(Base ^ NonDisjointBits) +
+/// DisjointBits` where DisjointBits = Const & KnownZeros(Base)
+///
+/// Example with ptr having known-zero low bit:
+///   Original: `xor %ptr, 3`    ; 3 = 0b11
+///   Analysis: DisjointBits = 3 & KnownZeros(%ptr) = 0b11 & 0b01 = 0b01
+///   Result:   `(xor %ptr, 2) + 1` where 1 can be folded into address mode
+///
+/// \param XorInst The XOR binary operator to analyze
+/// \return APInt containing the disjoint bits that can be extracted as offset,
+///         or zero if no disjoint bits exist
+APInt ConstantOffsetExtractor::extractDisjointBitsFromXor(
+    BinaryOperator *XorInst) {
+  assert(XorInst && XorInst->getOpcode() == Instruction::Xor &&
+         "Expected XOR instruction");
+
+  const unsigned BitWidth = XorInst->getType()->getScalarSizeInBits();
+  Value *BaseOperand;
+  ConstantInt *XorConstant;
+
+  // Match pattern: xor BaseOperand, Constant.
+  if (!match(XorInst, m_Xor(m_Value(BaseOperand), m_ConstantInt(XorConstant))))
+    return APInt::getZero(BitWidth);
+
+  // Compute known bits for the base operand.
+  const SimplifyQuery SQ(DL);
+  const KnownBits BaseKnownBits = computeKnownBits(BaseOperand, SQ);
+  const APInt &ConstantValue = XorConstant->getValue();
+
+  // Identify disjoint bits: constant bits that are known zero in base.
+  const APInt DisjointBits = ConstantValue & BaseKnownBits.Zero;
+
+  // Early exit if no disjoint bits found.
+  if (DisjointBits.isZero())
+    return APInt::getZero(BitWidth);
+
+  // Compute the remaining non-disjoint bits that stay in the XOR.
+  const APInt NonDisjointBits = ConstantValue & ~DisjointBits;
+
+  // FIXME: Enhance XOR constant extraction to handle nested binary operations.
+  // Currently we only extract disjoint bits from the immediate XOR constant,
+  // but we could recursively process cases like:
+  //   xor (add %base, C1), C2  ->  add %base, (C1 ^ disjoint_bits(C2))
+  // This requires careful analysis to ensure the transformation preserves
+  // semantics, particularly around sign extension and overflow behavior.
+
+  // Add the non-disjoint constant to the user chain for later transformation
+  // This will replace the original constant in the XOR with the new
+  // constant.
+  UserChain.push_back(ConstantInt::get(XorInst->getType(), NonDisjointBits));
----------------
arsenm wrote:

The caller seems like it's already trying to add this to UserChain, based on the constant being 0 already. Can this whole thing just fold to return DisjointBits above? 

https://github.com/llvm/llvm-project/pull/150438