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

[Graham Hunter via llvm-commits]

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@@ -3144,6 +3144,21 @@ InstructionCost AArch64TTIImpl::getCastInstrCost(unsigned Opcode, Type *Dst,
       {ISD::SIGN_EXTEND, MVT::nxv8i32, MVT::nxv8i16, 2},
       {ISD::SIGN_EXTEND, MVT::nxv8i64, MVT::nxv8i16, 6},
       {ISD::SIGN_EXTEND, MVT::nxv4i64, MVT::nxv4i32, 2},
+
+      // Add cost for extending and converting to illegal -too wide- scalable
+      // Extending one size (e.g. i32 -> f64) takes 2 unpacks and 2 fcvts, while
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huntergr-arm wrote:

The fcvt instructions seem to have 1/2 to 1/8 the throughput (depending on type) compared to simple arithmetic instructions, e.g. `add`, so I bumped the cost of those. The numbers may not be the best overall, but don't seem to lead to regressions at present. We may want to try a range of values at some point to see if there's a better estimate.

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