[llvm] [DAG] Constant fold ISD::FSHL/FSHR nodes (PR #154480)

[Simon Pilgrim via llvm-commits]

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@@ -7054,6 +7066,30 @@ SDValue SelectionDAG::FoldConstantArithmetic(unsigned Opcode, const SDLoc &DL,
     }
   }
 
+  // Handle trinary-op special cases.
+  if (NumOps == 3) {
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RKSimon wrote:

we're unlikely to have any other integer ternary cases to handle tbh - better to just check `Opcode == ISD::FSHL || Opcode == ISD::FSHR` and remove the FoldValue helper for now.

https://github.com/llvm/llvm-project/pull/154480