[llvm] [ARM]Adjust cost of muls in SMLAL patterns (PR #122713)

[David Green via llvm-commits]

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@@ -1458,16 +1458,62 @@ InstructionCost ARMTTIImpl::getArithmeticInstrCost(
   if (LooksLikeAFreeShift())
     return 0;
 
+  // When targets have both DSP and MVE we find that the
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davemgreen wrote:

Is it worth reducing the cost when we don't have MVE (but do have DSP) too? LLVM isn't great at trying to cost-model multiple instructions that can combine into one.  I think I would expect the mul to still have a cost, but the sext's to be free - I'm not sure if that would do better or worse in practice.

https://github.com/llvm/llvm-project/pull/122713