[clang] [llvm] Add support for flag output operand "=@cc" for SystemZ. (PR #125970)

[Ulrich Weigand via llvm-commits]

================
@@ -8701,95 +8734,341 @@ SDValue SystemZTargetLowering::combineSETCC(
   return SDValue();
 }
 
-static bool combineCCMask(SDValue &CCReg, int &CCValid, int &CCMask) {
+static std::pair<SDValue, int> findCCUse(const SDValue &Val) {
+  auto *N = Val.getNode();
+  if (!N)
+    return std::make_pair(SDValue(), SystemZ::CCMASK_NONE);
+  switch (N->getOpcode()) {
+  default:
+    return std::make_pair(SDValue(), SystemZ::CCMASK_NONE);
+  case SystemZISD::IPM:
+    if (N->getOperand(0).getOpcode() == SystemZISD::CLC ||
+        N->getOperand(0).getOpcode() == SystemZ::CLST ||
+        N->getOperand(0).getOpcode() == SystemZISD::STRCMP)
+      return std::make_pair(N->getOperand(0), SystemZ::CCMASK_ICMP);
+    return std::make_pair(N->getOperand(0), SystemZ::CCMASK_ANY);
+  case ISD::SHL:
+  case ISD::SRA:
+  case ISD::SRL:
+    return findCCUse(N->getOperand(0));
+  case SystemZISD::SELECT_CCMASK: {
+    SDValue Op4CCReg = N->getOperand(4);
+    auto *Op4CCNode = Op4CCReg.getNode();
+    auto *CCValid = dyn_cast<ConstantSDNode>(N->getOperand(2));
+    if (!CCValid || !Op4CCNode)
+      return std::make_pair(SDValue(), SystemZ::CCMASK_NONE);
+    int CCValidVal = CCValid->getZExtValue();
+    if (Op4CCNode->getOpcode() == SystemZISD::ICMP ||
+        Op4CCNode->getOpcode() == SystemZISD::TM) {
+      auto [OpCC, OpCCValid] = findCCUse(Op4CCNode->getOperand(0));
+      if (OpCC != SDValue())
+        return std::make_pair(OpCC, OpCCValid);
+    }
+    auto [OpCC, OpCCValid] = findCCUse(Op4CCReg);
+    return OpCC != SDValue() ? std::make_pair(OpCC, OpCCValid)
+                             : std::make_pair(Op4CCReg, CCValidVal);
+  }
+  case ISD::ADD:
+  case ISD::AND:
+  case ISD::OR:
+  case ISD::XOR:
+    auto [Op0CC, Op0CCValid] = findCCUse(N->getOperand(0));
+    if (Op0CC != SDValue())
+      return std::make_pair(Op0CC, Op0CCValid);
+    return findCCUse(N->getOperand(1));
+  }
+}
+
+SmallVector<SDValue, 4>
+SystemZTargetLowering::simplifyAssumingCCVal(SDValue &Val, SDValue &CC,
+                                             DAGCombinerInfo &DCI) const {
+  const auto isValidBinaryOperation = [](const SDValue &Op, SDValue &Op0,
+                                         SDValue &Op1, unsigned &Opcode) {
+    auto *N = Op.getNode();
+    if (!N)
+      return false;
+    Opcode = N->getOpcode();
+    if (Opcode != ISD::ADD && Opcode != ISD::AND && Opcode != ISD::OR &&
+        Opcode != ISD::XOR)
+      return false;
+    Op0 = N->getOperand(0);
+    Op1 = N->getOperand(1);
+    return true;
+  };
+  if (isa<ConstantSDNode>(Val)) {
+    return {Val, Val, Val, Val};
+  }
+  auto *N = Val.getNode(), *CCNode = CC.getNode();
+  if (!N || !CCNode)
+    return {};
+  SelectionDAG &DAG = DCI.DAG;
+  SDLoc DL(N);
+  if (N->getOpcode() == SystemZISD::IPM) {
+    SmallVector<SDValue, 4> ShiftedCCVals;
----------------
uweigand wrote:

However, here it is critical that we verify the operand of `IPM` is actually `CCNode`, or else we mistakenly accept wrong code.

https://github.com/llvm/llvm-project/pull/125970