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

[Matt Arsenault via llvm-commits]

================
@@ -1162,6 +1202,165 @@ bool SeparateConstOffsetFromGEP::splitGEP(GetElementPtrInst *GEP) {
   return true;
 }
 
+// Helper function to check if an instruction has at least one GEP user
+bool XorToOrDisjointTransformer::hasGEPUser(const Value *V) {
+  for (const User *U : V->users()) {
+    if (isa<GetElementPtrInst>(U)) {
+      return true;
+    }
+  }
+  return false;
+}
+
+bool XorToOrDisjointTransformer::dominatesAllXors(
+    BinaryOperator *BaseXor, const XorOpList &XorsInGroup) {
+  for (const auto &XorEntry : XorsInGroup) {
+    BinaryOperator *XorInst = XorEntry.first;
+    if (XorInst != BaseXor && !DT.dominates(BaseXor, XorInst)) {
+      return false;
+    }
+  }
+  return true;
+}
+
+bool XorToOrDisjointTransformer::processXorGroup(Instruction *OriginalBaseInst,
+                                                 XorOpList &XorsInGroup) {
+  bool Changed = false;
+  if (XorsInGroup.size() <= 1)
+    return false;
+
+  // Sort XorsInGroup by the constant offset value in increasing order.
+  llvm::sort(
+      XorsInGroup.begin(), XorsInGroup.end(),
+      [](const auto &A, const auto &B) { return A.second.ult(B.second); });
+
+  // Dominance check
+  // The "base" XOR for dominance purposes is the one with the smallest
+  // constant.
+  BinaryOperator *XorWithSmallConst = XorsInGroup[0].first;
+
+  if (!dominatesAllXors(XorWithSmallConst, XorsInGroup)) {
+    LLVM_DEBUG(dbgs() << DEBUG_TYPE
+                      << ": Cloning and inserting XOR with smallest constant ("
+                      << *XorWithSmallConst
+                      << ") as it does not dominate all other XORs"
+                      << " in function " << F.getName() << "\n");
+
+    BinaryOperator *ClonedXor =
+        cast<BinaryOperator>(XorWithSmallConst->clone());
+    ClonedXor->setName(XorWithSmallConst->getName() + ".dom_clone");
+    ClonedXor->insertAfter(OriginalBaseInst);
+    LLVM_DEBUG(dbgs() << "  Cloned Inst: " << *ClonedXor << "\n");
+    Changed = true;
+    XorWithSmallConst = ClonedXor;
+  }
+
+  SmallVector<Instruction *, 8> InstructionsToErase;
+  const APInt SmallestConst =
+      cast<ConstantInt>(XorWithSmallConst->getOperand(1))->getValue();
+
+  // Main transformation loop: Iterate over the original XORs in the sorted
+  // group.
+  for (const auto &XorEntry : XorsInGroup) {
+    BinaryOperator *XorInst = XorEntry.first; // Original XOR instruction
+    const APInt ConstOffsetVal = XorEntry.second;
+
+    // Do not process the one with smallest constant as it is the base.
+    if (XorInst == XorWithSmallConst)
+      continue;
+
+    // Disjointness Check 1
+    APInt NewConstVal = ConstOffsetVal - SmallestConst;
+    if ((NewConstVal & SmallestConst) != 0) {
+      LLVM_DEBUG(dbgs() << DEBUG_TYPE << ": Cannot transform XOR in function "
+                        << F.getName() << ":\n"
+                        << "  New Const: " << NewConstVal << "\n"
+                        << "  Smallest Const: " << SmallestConst << "\n"
+                        << "  are not disjoint \n");
----------------
arsenm wrote:

```suggestion
                        << F.getName() << ":\n"
                        << "  New Const: " << NewConstVal 
                        << "  Smallest Const: " << SmallestConst << " are not disjoint\n");
```

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