[llvm] [AArch64][CostModel] Alter sdiv/srem cost where the divisor is constant (PR #123552)

[David Green via llvm-commits]

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@@ -5,29 +5,29 @@ target datalayout = "e-m:e-i8:8:32-i16:16:32-i64:64-i128:128-n32:64-S128"
 
 define i32 @srem() {
 ; CHECK-LABEL: 'srem'
-; CHECK-NEXT:  Cost Model: Found an estimated cost of 14 for instruction: %I128 = srem i128 undef, undef
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davemgreen wrote:

Anything with undef should be treated like a variable, not a constant. It is a way of making the tests simpler to write, so think of them as srem by an unknown value (`srem i64 %a, %b`).

The `ISD == ISD::SDIV || ISD == ISD::UDIV` check was added as #122236 (and this patch) was only handling divide by a constant, so the srem/urem by an unknown is not expected to produce the right costs yet. I would expect it the srems to be something like cost(sdiv) + mul + sub, but was hoping to go over them properly in a later patch. The base cost of a divide should be changed too. Unfortunately these are all taking longer than I would have liked, if we can get #122469 in then that unblocks the sve costs for this one.

https://github.com/llvm/llvm-project/pull/123552