[llvm] [AMDGPU] SelDAG: fix lowering of undefined workitem intrinsics (PR #126058)

[Matt Arsenault via llvm-commits]

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@@ -8790,11 +8790,28 @@ SDValue SITargetLowering::LowerINTRINSIC_WO_CHAIN(SDValue Op,
                              AMDGPUFunctionArgInfo::LDS_KERNEL_ID);
   }
   case Intrinsic::amdgcn_workitem_id_x:
-    return lowerWorkitemID(DAG, Op, 0, MFI->getArgInfo().WorkItemIDX);
+    if (!MFI->getArgInfo().WorkItemIDX) {
+      // It's undefined behavior if a function marked with the amdgpu-no-*
+      // attributes uses the corresponding intrinsic.
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arsenm wrote:

I thought it was poison? This should return undef, not 0?

https://github.com/llvm/llvm-project/pull/126058