[llvm] [SDAG][NVPTX] Add TLI check for preferring custom FP_TO_SINT operations to FP_TO_UINT (PR #132470)

[Kevin McAfee via llvm-commits]

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@@ -6179,6 +6179,33 @@ Instruction *NVPTXTargetLowering::emitTrailingFence(IRBuilderBase &Builder,
   return nullptr;
 }
 
+// Rather than default to SINT when both UINT and SINT are custom, we only
+// change the opcode when UINT is not legal and SINT is. UINT is preferred when
+// both are custom since unsigned CVT instructions can lead to slightly better
+// SASS code with fewer instructions.
+unsigned NVPTXTargetLowering::getFPToXIntOpcode(unsigned Op, EVT FromVT,
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kalxr wrote:

I think that because the i8 needs to be promoted, using custom requires that we support these operations in ReplaceNodeResults. Doing that seems like re-implementing PromoteIntRes_FP_TO_XINT, so I'm not sure it's worth it if we can use the hook instead. Thoughts?

Otherwise, the flow looks like:
PromoteIntegerResult->CustomLowerNode->ReplaceNodeResults->"LLVM ERROR: Unhandled custom legalization"

https://github.com/llvm/llvm-project/pull/132470