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

[Ulrich Weigand via llvm-commits]

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@@ -8702,61 +8738,264 @@ SDValue SystemZTargetLowering::combineSETCC(
   return SDValue();
 }
 
+static SmallSet<int, 4> convertCCMaskToCCValsSet(int Mask) {
+  SmallSet<int, 4> CCVals;
+  size_t Pos = 0;
+  while (Mask) {
+    if (Mask & 0x1)
+      CCVals.insert(3 - Pos);
+    Mask >>= 1;
+    ++Pos;
+  }
+  return CCVals;
+}
+
+static std::pair<SDValue, int> findCCUse(const SDValue &Val) {
+  auto *N = Val.getNode();
+  if (!N)
+    return {Val, SystemZ::CCMASK_NONE};
+  if (isa<ConstantSDNode>(Val))
+    return std::make_pair(SDValue(), SystemZ::CCMASK_NONE);
+  else if (N->getOpcode() == ISD::CopyFromReg && N->getNumOperands() > 1) {
+    if (auto *RN = cast<RegisterSDNode>(N->getOperand(1))) {
+      if (RN->getReg() == SystemZ::CC)
+        return {Val, SystemZ::CCMASK_ANY};
+    }
+  } else if (N->getOpcode() == SystemZISD::IPM)
+    return std::make_pair(N->getOperand(0), SystemZ::CCMASK_ANY);
+  else if (N->getOpcode() == ISD::SRL)
+    return findCCUse(N->getOperand(0));
+  else if (N->getOpcode() == SystemZISD::ICMP)
+    return findCCUse(N->getOperand(0));
+  else if (N->getOpcode() == SystemZISD::TM)
+    return findCCUse(N->getOperand(0));
+  else if (N->getOpcode() == SystemZISD::SELECT_CCMASK) {
+    SDValue SelectCCReg = N->getOperand(4);
+    auto [OpCC, OpCCValid] = findCCUse(SelectCCReg);
+    auto *OpCCNode = OpCC.getNode();
+    if (OpCCNode && OpCCNode != SelectCCReg.getNode())
+      return std::make_pair(OpCC, OpCCValid);
+    auto *CCValid = dyn_cast<ConstantSDNode>(N->getOperand(2));
+    if (CCValid)
+      return std::make_pair(SelectCCReg, CCValid->getZExtValue());
+  } else if (N->getOpcode() == ISD::ADD || N->getOpcode() == ISD::AND ||
+             N->getOpcode() == ISD::OR || N->getOpcode() == ISD::XOR ||
+             N->getOpcode() == ISD::AND) {
+    auto [Op0CC, Op0CCValid] = findCCUse(N->getOperand(0));
+    if (isa<ConstantSDNode>(N->getOperand(1)))
+      return std::make_pair(Op0CC, Op0CCValid);
+    auto [Op1CC, Op1CCValid] = findCCUse(N->getOperand(1));
+    auto *N0 = Op0CC.getNode(), *N1 = Op1CC.getNode();
+    if (N0 && N1 && N0 == N1 && Op0CCValid == Op1CCValid)
+      return std::make_pair(Op0CC, Op0CCValid);
----------------
uweigand wrote:

I'm not sure it is necessary to always recurse into both sides.  I think we should just return the first CC we find.

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