[llvm] [AArch64] Improve urem by constant costs (PR #122236)

[David Green via llvm-commits]

davemgreen wrote:

> @davemgreen I was just looking at the examples with 3 different scenarios for the constant in my mind:
> 
> 1. If the constant is pow-2
> 2. If the constant is uniform
> 3. If the constant is negative
> 
> So, essentially, its like having 8 possibilities for the constant with these 3 variables.
> 
> Now, things change pretty quickly if the constant is negative(I think I said that things dont change much if the constant is negative on the other patch but I was wrong). e.g.
> 
> ```
> for <4 x i32> :
> a) positive constant: ushr
> b) negative constant: ushr + umull2 + umull + uzp2 + ushr     (for certain cases)
> ```
> 
> The non-uniform case also brings in more complexity but not that much.
> 
> So, maybe, can be break this patch so that we conquer just few possibilities(from 8 possibilities) in every patch at a time ? This will ensure precision in cost estimation and easy verification I believe.

I believe for udiv/urem there are only really 2 cases - positive power-2 and everything else. The power-2 cases almost never come up as they are canonicalised early and are mostly added for completeness. Like you say I don't think there is a lot of difference between uniform and non-uniforms, it just takes the worst-case.

The exact cost will depend on the constant though, and we are currently keeping things simple and assuming no simplification happens. Some positive constants will simplify, some negative ones will, it is hard to tell without doing a lot of maths to see what the exact instructions will be. We could try to use DivisionByConstantInfo to get a more precise expansion and figure out what will simplify, but that feels a bit more complex than is helpful considering the other costs that could be improved. Maybe one for the future, if you don't want to look into it yourself.

https://github.com/llvm/llvm-project/pull/122236