[llvm] [AArch64][CostModel] Increase the cost of illegal SVE int-to-fp converts (PR #130756)

[Benjamin Maxwell via llvm-commits]

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@@ -2811,6 +2811,19 @@ InstructionCost AArch64TTIImpl::getCastInstrCost(unsigned Opcode, Type *Dst,
             BF16Tbl, ISD, DstTy.getSimpleVT(), SrcTy.getSimpleVT()))
       return AdjustCost(Entry->Cost);
 
+  // Symbolic constants for the SVE sitofp/uitofp entries in the table below
+  // The cost of unpacking twice is artificially increased for now in order
+  // to avoid regressions against NEON, which will use tbl instructions directly
+  // instead of multiple layers of [s|u]unpk[lo|hi].
+  // We use the unpacks in cases where the destination type is illegal and
+  // requires splitting of the input, even if the input type itself is legal.
+  // FIXME: Use tbl instructions for SVE as well, at least in cases where the
+  //        conversion is done in a loop.
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MacDue wrote:

```suggestion
```

https://github.com/llvm/llvm-project/pull/130756