[llvm] [RISCV] Combine vslide{up,down} x, poison -> x (PR #169013)

[Craig Topper via llvm-commits]

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@@ -21834,6 +21834,11 @@ SDValue RISCVTargetLowering::PerformDAGCombine(SDNode *N,
       return N->getOperand(0);
     break;
   }
+  case RISCVISD::VSLIDEDOWN_VL:
+  case RISCVISD::VSLIDEUP_VL:
+    if (N->getOperand(1)->isUndef())
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topperc wrote:

SelectionDAG support for poison is still in an early stage I think. It probably converts ISD::POISON to ISD::UNDEF in many cases still. I wouldn't be surprised if the vector was ISD::UNDEF by the time it gets here.

https://github.com/llvm/llvm-project/pull/169013