Why do the reverse binder swap transformation?

[Jaro Reinders]

This might be an example: 
https://gitlab.haskell.org/ghc/ghc/-/issues/22902#note_479305

Cheers,
Jaro

On 14-07-2023 17:30, Rodrigo Mesquita wrote:
> Dear GHC devs,
> 
> I’m wondering about the reverse binder swap transformation, the one in which we substitute occurrences of the case binder by occurrences of the scrutinee (when the scrut. is a variable):
> 
> 	case x of z { r -> e }
>          ===>
> 	case x of z { r -> e[x/z] }
> 
> My question is: why do we do this transformation? An example in which this transformation is beneficial would be great too.
> 
> The Note I’ve found about it, Note [Binder-swap during float-out], wasn’t entirely clear to me:
> 
> 	4. Note [Binder-swap during float-out]
> 	   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 	   In the expression
> 	        case x of wild { p -> ...wild... }
> 	   we substitute x for wild in the RHS of the case alternatives:
> 	        case x of wild { p -> ...x... }
> 	   This means that a sub-expression involving x is not "trapped" inside the RHS.
> 	   And it's not inconvenient because we already have a substitution.
> 
> 	  Note that this is EXACTLY BACKWARDS from the what the simplifier does.
> 	  The simplifier tries to get rid of occurrences of x, in favour of wild,
> 	  in the hope that there will only be one remaining occurrence of x, namely
> 	  the scrutinee of the case, and we can inline it.
> 
> Many thanks,
> Rodrigo
> 
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