Pattern matching desugaring regression? Re: [Haskell-cafe] Why does my module take so long to compile?

[Carter Schonwald]

Don’t forget ghc 9 is already out! :)

On Mon, Feb 15, 2021 at 2:10 PM Troels Henriksen <athas at sigkill.dk> wrote:

> It is very likely that issue 17386 is the issue.  With
>
> {-# OPTIONS_GHC -Wno-overlapping-patterns -Wno-incomplete-patterns
> -Wno-incomplete-uni-patterns -Wno-incomplete-record-updates #-}
>
> my module(s) compile very quickly.  I'll wait and see if GHC 9 does
> better before I try to create a smaller case (and now I at least have a
> workaround).
>
> Sebastian Graf <sgraf1337 at gmail.com> writes:
>
> > Hi,
> >
> > I'm not sure I see all the context of the conversation, but it is
> entirely
> > possible that code with many local constraints regresses the
> pattern-match
> > checker (which is accounted to Desugaring in the profile emitted by -v2),
> > I'm afraid. That simply has to do with the fact that we now actually care
> > about them, previously they were mostly discarded.
> >
> > I'd be glad if you submitted a relatively isolated reproducer of what is
> > fast with 8.8 and slow with 8.10 (even better 9.0).
> > I hope that things have improved since we fixed
> > https://gitlab.haskell.org/ghc/ghc/-/issues/17836, which is part of 9.0
> but
> > not of 8.10.
> >
> > Cheers,
> > Sebastian
> >
> > Am Mo., 15. Feb. 2021 um 19:04 Uhr schrieb Troels Henriksen <
> > athas at sigkill.dk>:
> >
> >> Carter Schonwald <carter.schonwald at gmail.com> writes:
> >>
> >> > Ccing ghc devs since that’s a better forum perhaps
> >> > Crazy theory:
> >> >
> >> > this is a regression due the the partial changes to pattern matching
> >> > coverage checking in 8.10 that finished / landed in ghc 9
> >> >
> >> > Why:
> >> > Desugaring is when pattern/ case statement translation happens I
> think?
> >> > And the only obvious “big thing” is that you have some huge , albeit
> sane
> >> > for a compiler, pattern matching
> >> >
> >> > I’d first check if the new ghc 9 release doesn’t have that regression
> in
> >> > build time that you experienced.  And if it does file a ticket.
> >> >
> >> > I may be totally wrong, but that seems like a decent likelihood !
> >>
> >> You may be right!  Another module that regressed is also mainly
> >> characterised by large-but-not-insane case expressions:
> >>
> >>
> https://github.com/diku-dk/futhark/blob/d0839412bdd11884d75a1494dd5de5191833f39e/src/Futhark/Optimise/Simplify/Rules.hs
> >>
> >> I'll try to split these modules up a little bit (I should have done so a
> >> while ago anyway) and maybe that will make the picture even clearer.
> >>
> >> --
> >> \  Troels
> >> /\ Henriksen
> >> _______________________________________________
> >> ghc-devs mailing list
> >> ghc-devs at haskell.org
> >> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
> >>
>
> --
> \  Troels
> /\ Henriksen
>
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