[Mlir-commits] [mlir] [mlir][async]: Make async.execute operation with RecursiveMemoryEffects trait (PR #116544)

[Aviad Cohen]

AviadCo wrote:

> > Can you elaborate on the motivation and the correctness in the description of this PR please? This is surprising to me and I don't quite understand why this op can be Pure at the moment. Thanks.
> 
> Sure, the case I have is `scf.if` with result which means that I must have an `else` region. For example:
> 
> ```
>      %res = scf.if cond -> (!async.token) {
>       ...
>     } else {
>       %token = async.execute {
>         async.yield
>       }
>       scf.yield %token : !async.token
>     }
> ```
> 
> In some phase, I reach the point where the `async` is being inside `if` that has no longer have result, for example:
> 
> ```
>     scf.if cond {
>       ...
>       %token = async.execute {
>           .....
>       }
>       // Token is being used
>     } else {
>       async.execute {
>         async.yield
>       }
>     }
> ```
> 
> In this case, I would expect that the whole `else` will be eliminated (folder) as the execute has no memory effect inside it and has no users. With `Pure` this will be optimized to:
> 
> ```
>     scf.if cond {
>         %token = async.execute {
>             .....
>         }
>         // Token is being used
>     }
> ```
> 
> Keeping my specific flow apart, it seems legic to remove the execute region if it has no usages and have no memory effects like `scf.for` does.

@joker-eph I confused between `Pure` and `RecursiveMemoryEffects` - please see 

https://github.com/llvm/llvm-project/pull/116544