[llvm] [AArch64][CostModel] Consider i32 --> i64 partial reduce cost as Invalid for FixedLength vectors (PR #165226)

[Sushant Gokhale via llvm-commits]

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@@ -5757,8 +5757,15 @@ InstructionCost AArch64TTIImpl::getPartialReductionCost(
       return Cost;
   }
 
+  if (!ST->useSVEForFixedLengthVectors() &&
+      (AccumLT.second.isFixedLengthVector() && ST->isNeonAvailable() &&
+       ST->hasDotProd()) &&
+      (AccumLT.second.getScalarType() == MVT::i64 &&
+       InputLT.second.getScalarType() == MVT::i32))
+    return Invalid;
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sushgokh wrote:

As explained in the writeup, this gets lowered to SMLAL[BT] when SVE2 is available as described by state T3 

https://github.com/llvm/llvm-project/pull/165226