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

Don’t forget ghc 9 is already out! :) On Mon, Feb 15, 2021 at 2:10 PM Troels Henriksen 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 writes:

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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@sigkill.dk>:

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Carter Schonwald writes:

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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/d0839412bdd11884d75a1494dd5de5191833...

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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.

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