[llvm] [NVPTX] Fix lowering of i1 SETCC (PR #115035)

[Artem Belevich via llvm-commits]

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@@ -18754,8 +18754,9 @@ SDValue DAGCombiner::rebuildSetCC(SDValue N) {
       if (LegalTypes)
         SetCCVT = getSetCCResultType(SetCCVT);
       // Replace the uses of XOR with SETCC
-      return DAG.getSetCC(SDLoc(N), SetCCVT, Op0, Op1,
-                          Equal ? ISD::SETEQ : ISD::SETNE);
+      const ISD::CondCode CC = Equal ? ISD::SETEQ : ISD::SETNE;
+      if (!LegalOperations || TLI.isCondCodeLegal(CC, Op0.getSimpleValueType()))
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Artem-B wrote:

This could definitely use a comment. AFAICT what we're doing here is "combine into setcc(NE/EQ) only if we know they are legal, which leaves xor(i1) untouched and allows us to use eventually use logic ops on predicates" .

Do we actually need to add this special case here?
It appears that we do manage to lower `setcc x, y, eq` as `xor.pred` already : https://godbolt.org/z/cMx9d8P33 

What happens if we remove the change here?

https://github.com/llvm/llvm-project/pull/115035