Alexis King writes: ...

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

Sounds like it would be worth a quick try. Given that this will get hit by the simplifier eventually anyways I suspect it won't make much of a difference one way or the other. That being said, I've been wrong before... Cheers, - Ben

Good spot. The very simple optimiser is, well, very simple. So let's not do anything complicated. But could you submit a ticket, and patch, to replace that Cast with mkCast? Thanks! Simon | -----Original Message----- | From: ghc-devs On Behalf Of Alexis King | Sent: 27 April 2020 22:58 | To: Richard Eisenberg | Cc: ghc-devs | Subject: Re: Why doesn’t the simple optimizer fuse casts? | | 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 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 | _______________________________________________ | ghc-devs mailing list | ghc-devs@haskell.org | https://nam06.safelinks.protection.outlook.com/?url=http%3A%2F%2Fmail.has | kell.org%2Fcgi-bin%2Fmailman%2Flistinfo%2Fghc- | devs&data=02%7C01%7Csimonpj%40microsoft.com%7C825fe41065dd450030e908d | 7eaf61d19%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637236215077850647 | &sdata=w%2Fk4JG%2FOmpwPoUAAGqdC62ssQSYcLQioY8T9%2BZ3XS8E%3D&reser | ved=0