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

[Juan Manuel Martinez Caamaño via llvm-commits]

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@@ -1162,6 +1167,179 @@ 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)) {
----------------
jmmartinez wrote:

I don't understand what does "Closest" mean. It's the `xor` that is the nearest to `A` (and that is not `&I`) ? Or the nearest to `&I` ? Should it dominate `&I` or not ?

I think that what you're really looking for is a `xor` that has `A` and a constant as operands and that dominate `&I`.

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