Re: Why doesn’t the simple optimizer fuse casts?

[Alexis King]

Hi Richard,

Nested casts can absolutely happen after the simple optimizer runs.
Suppose we produce a desugared expression like:

    let { y = x `cast` co1 } in y `cast` co2

The simple optimizer will inline y, since it only appears once, so now
we have a nested cast. Perhaps you mean you expect the casts to be
merged when the simple optimizer gets to the Cast expression itself,
but they aren’t, as it doesn’t use mkCast to reconstruct the result:

    -- from GHC.Core.SimpleOpt.simple_opt_expr
    go (Cast e co)      | isReflCo co' = go e
                        | otherwise    = Cast (go e) co'

I suppose a really simple fix would be to just use mkCast here instead
of Cast, but that wouldn’t be completely satisfying, since the merged
coercion wouldn’t be optimized. So you’d have to do something slightly
more complicated to detect if the result of `go e` was a cast
expression and combine the coercions before calling optCoercion.

Whether or not doing that would be a good idea is precisely what I’m
asking about. :)

Alexis

> On Apr 27, 2020, at 16:22, Richard Eisenberg <rae at richarde.dev> wrote:
> 
> Hi Alexis,
> 
> Nested casts shouldn't happen. The mkCast function gets rid of them.
> Someone somewhere is forgetting to call it. If you have a concrete
> program that leads to nested casts, post a bug report. :)
> 
> Thanks!
> Richard