[llvm] [SeparateConstOffsetFromGEP] Decompose constant xor operand if possible (PR #135788)

[Sumanth Gundapaneni via llvm-commits]

================
@@ -1162,6 +1168,162 @@ bool SeparateConstOffsetFromGEP::splitGEP(GetElementPtrInst *GEP) {
   return true;
 }
 
+bool SeparateConstOffsetFromGEP::decomposeXor(Function &F) {
+  bool FunctionChanged = false;
+  SmallVector<std::pair<Instruction *, Value *>, 16> ReplacementsToMake;
+
+  for (BasicBlock &BB : F) {
+    for (Instruction &I : BB) {
+      if (I.getOpcode() == Instruction::Xor) {
+        if (Value *Replacement = tryFoldXorToOrDisjoint(I)) {
+          ReplacementsToMake.push_back({&I, Replacement});
+          FunctionChanged = true;
+        }
+      }
+    }
+  }
+
+  if (!ReplacementsToMake.empty()) {
+    LLVM_DEBUG(dbgs() << "Applying " << ReplacementsToMake.size()
+                      << " XOR->OR Disjoint replacements in " << F.getName()
+                      << "\n");
+    for (auto &Pair : ReplacementsToMake) {
+      Pair.first->replaceAllUsesWith(Pair.second);
+    }
+    for (auto &Pair : ReplacementsToMake) {
+      Pair.first->eraseFromParent();
+    }
+  }
+
+  return FunctionChanged;
+}
+
+static llvm::Instruction *findClosestSequentialXor(Value *A, Instruction &I) {
+  llvm::Instruction *ClosestUser = nullptr;
+  for (llvm::User *User : A->users()) {
+    if (auto *UserInst = llvm::dyn_cast<llvm::Instruction>(User)) {
+      if (UserInst->getOpcode() != Instruction::Xor || UserInst == &I)
+        continue;
+      if (!ClosestUser) {
+        ClosestUser = UserInst;
+      } else {
+        // Compare instruction positions.
+        if (UserInst->comesBefore(ClosestUser)) {
+          ClosestUser = UserInst;
+        }
+      }
+    }
+  }
+  return ClosestUser;
+}
+
+/// Try to transform I = xor(A, C1) into or disjoint(Y, C2)
+/// where Y = xor(A, C0) is another existing instruction dominating I,
+/// C2 = C0 ^ C1, and A is known to be disjoint with C2.
+///
+/// @param I  The XOR instruction being visited.
+/// @return The replacement Value* if successful, nullptr otherwise.
+Value *SeparateConstOffsetFromGEP::tryFoldXorToOrDisjoint(Instruction &I) {
+  assert(I.getOpcode() == Instruction::Xor && "Instruction must be XOR");
+
+  // Check if I has at least one GEP user.
+  bool HasGepUser = false;
+  for (User *U : I.users()) {
+    if (isa<GetElementPtrInst>(U)) {
+      HasGepUser = true;
+      break;
+    }
+  }
+  // If no user is a GEP instruction, abort the transformation.
+  if (!HasGepUser) {
+    LLVM_DEBUG(
+        dbgs() << "SeparateConstOffsetFromGEP: Skipping XOR->OR DISJOINT for "
+               << I << " because it has no GEP users.\n");
+    return nullptr;
+  }
+
+  Value *Op0 = I.getOperand(0);
+  Value *Op1 = I.getOperand(1);
+  ConstantInt *C1 = dyn_cast<ConstantInt>(Op1);
+  Value *A = Op0;
+
+  // Bail out of there is not constant operand.
+  if (!C1) {
+    C1 = dyn_cast<ConstantInt>(Op0);
+    if (!C1)
+      return nullptr;
+    A = Op1;
+  }
+
+  if (isa<UndefValue>(A))
+    return nullptr;
+
+  APInt C1_APInt = C1->getValue();
+  unsigned BitWidth = C1_APInt.getBitWidth();
+  Type *Ty = I.getType();
+
+  // --- Step 2: Find Dominating Y = xor A, C0 ---
+  Instruction *FoundUserInst = nullptr; // Instruction Y
+  APInt C0_APInt;
+
+  auto UserInst = findClosestSequentialXor(A, I);
+
+  BinaryOperator *UserBO = cast<BinaryOperator>(UserInst);
----------------
sgundapa wrote:

I will add a nullptr check here before I use this instruction.

https://github.com/llvm/llvm-project/pull/135788