[llvm] [LV] Add initial support for vectorizing literal struct return values (PR #109833)

[Benjamin Maxwell via llvm-commits]

================
@@ -1506,6 +1514,8 @@ InstructionCost VPWidenRecipe::computeCost(ElementCount VF,
     return Ctx.TTI.getArithmeticInstrCost(Instruction::Mul, VectorTy,
                                           Ctx.CostKind);
   }
+  case Instruction::ExtractValue:
+    return 0;
----------------
MacDue wrote:

If by the "right thing" you mean "Am I costing the return values based on the target-specific call ABI?" then the answer is no. I think the answer to that is _also_ "no" for non-struct-return functions too (I don't think there's any costing based on how the target passes arguments/returns values). 

I don't really think that needs doing in this patch (which just adds basic support). Since nothing is using struct-return vectorization right now. Currently, it needs things to be manually annotated with `vector-function-abi-variant`, and I don't really think your example is a very realistic use case. 


https://github.com/llvm/llvm-project/pull/109833