[llvm] [NVPTX] Lower bfloat16 add/mul/sub as fma on SM80 (PR #121065)

[Matt Arsenault via llvm-commits]

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@@ -853,6 +853,16 @@ NVPTXTargetLowering::NVPTXTargetLowering(const NVPTXTargetMachine &TM,
       AddPromotedToType(Op, MVT::bf16, MVT::f32);
   }
 
+  // Lower bf16 add/mul/sub as fma when it avoids promotion
+  for (const auto &Op : {ISD::FADD, ISD::FMUL, ISD::FSUB}) {
+    for (const auto &VT : {MVT::bf16, MVT::v2bf16}) {
+      if (getOperationAction(Op, VT) != Legal &&
+          getOperationAction(ISD::FMA, VT) == Legal) {
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arsenm wrote:

This is cumbersome, we usually don't write legalizer rules in terms of other legalizer rules.

I think you'd be best off just putting this logic into the default Expand action for add/fmul/fsub. If the FMA is legal, you emit the appropriate sequence before falling back to the default libcall expansion. Then you shouldn't need to touch the target rules here 

https://github.com/llvm/llvm-project/pull/121065