While simplify opt

[Pangoraw]

@wsmoses you mentioned at some point that unlike scf.for, stablehlo.while cannot reference values which are not passed as arguments. Should I update this optimization to only apply to stablehlo.constant ?

[wsmoses]

Yeah I think you should update this as a special case of constants.

Also clearly we want a follow up pass here [which may be https://github.com//pull/173] to remove unused iter args (or those which only copmute things for themselves. e.g. if you have the equivalent of

%res = while iter = %0 {

} {
nextiter = iter * 10
yield nextiter
}

and res is unused. We don't need the iter arrgument, nor nextiter [since it is only used to compute iter]

[wsmoses]

@GleasonK I remember somewhere seeing that the spec for whileop [perhaps in practice] required all variables used within the whileop region to be passed in as iterargs. However, I don't see that in the spec: https://github.com/openxla/stablehlo/blob/main/docs/spec.md#while

Can you remind me what the requirements are ?

[GleasonK]

Hm, I'll look more closely next week but that's an HLO requirement, not sure if it's a stablehlo req, we have an MLIR pass to fix up any while ops that violate the HLO requirement

[wsmoses]

Interesting, if you have a link to that that would be good to look at. There are some potential questions / perf optimizations there (for example one could choose to either pass both X and Y as tier args or potentially if both are functions of Z one could just pass in Z and compute X and Y from it)