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

[Jay Foad 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");
+      continue;
+    }
+
+    // Disjointness Check 2
+    KnownBits KnownBaseBits(
+        XorWithSmallConst->getType()->getScalarSizeInBits());
+    computeKnownBits(XorWithSmallConst, KnownBaseBits, DL, 0, nullptr,
+                     XorWithSmallConst, &DT);
+    if ((KnownBaseBits.Zero & NewConstVal) == NewConstVal) {
+      LLVM_DEBUG(dbgs() << DEBUG_TYPE
+                        << ": Transforming XOR to OR (disjoint) in function "
+                        << F.getName() << ":\n"
+                        << "  Xor: " << *XorInst << "\n"
+                        << "  Base Val: " << *XorWithSmallConst << "\n"
+                        << "  New Const: " << NewConstVal << "\n");
+
+      auto *NewOrInst = BinaryOperator::CreateDisjointOr(
+          XorWithSmallConst,
+          ConstantInt::get(OriginalBaseInst->getType(), NewConstVal),
+          XorInst->getName() + ".or_disjoint", XorInst->getIterator());
+
+      NewOrInst->copyMetadata(*XorInst);
+      XorInst->replaceAllUsesWith(NewOrInst);
+      LLVM_DEBUG(dbgs() << "  New Inst: " << *NewOrInst << "\n");
+      InstructionsToErase.push_back(XorInst); // Mark original XOR for deletion
+
+      Changed = true;
+    } else {
+      LLVM_DEBUG(
+          dbgs() << DEBUG_TYPE
+                 << ": Cannot transform XOR (not proven disjoint) in function "
+                 << F.getName() << ":\n"
+                 << "  Xor: " << *XorInst << "\n"
+                 << "  Base Val: " << *XorWithSmallConst << "\n"
+                 << "  New Const: " << NewConstVal << "\n");
+    }
+  }
+
+  for (Instruction *I : InstructionsToErase)
+    I->eraseFromParent();
+
+  return Changed;
+}
+
+// Try to transform XOR(A, B+C) in to XOR(A,C) + B where XOR(A,C) becomes
+// the base for memory operations. This transformation is true under the
+// following conditions
+// Check 1 -  B and C are disjoint.
+// Check 2 - XOR(A,C) and B are disjoint.
+//
+// This transformation is beneficial particularly for GEPs because:
+// 1. OR operations often map better to addressing modes than XOR
+// 2. Disjoint OR operations preserve the semantics of the original XOR
+// 3. This can enable further optimizations in the GEP offset folding pipeline
+bool XorToOrDisjointTransformer::run() {
+  bool Changed = false;
+
+  // Collect all candidate XORs
+  for (Instruction &I : instructions(F)) {
+    Instruction *Op0 = nullptr;
+    ConstantInt *C1 = nullptr;
+    BinaryOperator *MatchedXorOp = nullptr;
+
+    // Attempt to match the instruction 'I' as XOR operation.
+    if (match(&I, m_CombineAnd(m_Xor(m_Instruction(Op0), m_ConstantInt(C1)),
+                               m_BinOp(MatchedXorOp))) &&
+        hasGEPUser(MatchedXorOp))
+      XorGroups[Op0].push_back({MatchedXorOp, C1->getValue()});
+  }
+
+  if (XorGroups.empty())
+    return false;
+
+  // Process each group of XORs
+  for (auto &GroupPair : XorGroups) {
+    Instruction *OriginalBaseInst = GroupPair.first;
+    XorOpList &XorsInGroup = GroupPair.second;
----------------
jayfoad wrote:

Nit:
```suggestion
  for (auto &[OriginalBaseInst, XorsInGroup] : XorGroups) {
```

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