[llvm] [AArch64] Add a basic i128 mul cost. (PR #203271)

[David Sherwood via llvm-commits]

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@@ -4409,6 +4409,15 @@ InstructionCost AArch64TTIImpl::getArithmeticInstrCost(
   case ISD::SUB:
     return LT.first; // Also works for i128
   case ISD::MUL:
+    // i128 multiply is umulh + 2*madd + mul and grows ~n^2.
+    if (Ty->getScalarSizeInBits() > 64) {
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david-arm wrote:

Should this explicitly check for 128-bit types? i256 is also a valid type and we do code generate muls for them: https://godbolt.org/z/jaE4T7eff

https://github.com/llvm/llvm-project/pull/203271