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

[Alex MacLean 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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AlexMaclean wrote:

Yep, we get stuck in an infinite loop: legalization converts the setcc into an xor and then dag combiner converts it back to a setcc over and over again. I've added a comment explaining. 

> It appears that we do manage to lower setcc x, y, eq as xor.pred already : https://godbolt.org/z/cMx9d8P33

Good catch, this is the result of some old table-gen patterns which handled `eq` and `ne`. I've removed these as the legalization approach handles these cases and is more powerful since it allows DAGCombiner to manipulate the logical ops created. 


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