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

[Brandon Allbery]

They already said something about waiting on dependencies to catch up with
ghc9, IIRC.

On Mon, Feb 15, 2021 at 2:22 PM Carter Schonwald <carter.schonwald at gmail.com>
wrote:

> 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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-- 
brandon s allbery kf8nh
allbery.b at gmail.com
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