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

[Ulrich Weigand via llvm-commits]

================
@@ -8826,15 +9104,49 @@ SDValue SystemZTargetLowering::combineSELECT_CCMASK(
   int CCMaskVal = CCMask->getZExtValue();
   SDValue CCReg = N->getOperand(4);
 
-  if (combineCCMask(CCReg, CCValidVal, CCMaskVal))
-    return DAG.getNode(SystemZISD::SELECT_CCMASK, SDLoc(N), N->getValueType(0),
-                       N->getOperand(0), N->getOperand(1),
-                       DAG.getTargetConstant(CCValidVal, SDLoc(N), MVT::i32),
-                       DAG.getTargetConstant(CCMaskVal, SDLoc(N), MVT::i32),
-                       CCReg);
-  return SDValue();
-}
+  if (!combineCCMask(CCReg, CCValidVal, CCMaskVal, DCI))
+    return SDValue();
+
+  // Handle TrueVal and FalseVal in outermost select_ccmask.
+  SDValue TrueVal = N->getOperand(0);
+  SDValue FalseVal = N->getOperand(1);
+  const auto &&TrueSDVals = simplifyAssumingCCVal(TrueVal, CCReg, DCI);
+  const auto &&FalseSDVals = simplifyAssumingCCVal(FalseVal, CCReg, DCI);
+  // There might be cases where TrueSDVals and FalseSDVals are empty as
+  // TrueVal and FalseVal both are non-constant, and they have already been
+  // optimized by combineCCMask, we can not take early exit here, just bypass it
+  // and directly create a new SELECT_CCMASK.
+  if (!TrueSDVals.empty() && !FalseSDVals.empty()) {
+    SmallVector<SDValue, 4> MergedSDVals;
+    for (auto CC : {0, 1, 2, 3}) {
+      MergedSDVals.emplace_back(((CCMaskVal & (1 << (3 - CC))) != 0) &&
+                                        ((CCValidVal & (1 << (3 - CC))) != 0)
----------------
uweigand wrote:

CCValid check is redundant - CCMask is guaranteed to be a subset.

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