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<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">Thanks Gael.
<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">As you may know, we’ve focused a lot on compile time in the last year or so, and 8.4 is a lot faster. I think there are still plenty of bad cases, but things are better.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">Our compile-time perf page is here:
<a href="https://ghc.haskell.org/trac/ghc/wiki/Performance/Compiler">https://ghc.haskell.org/trac/ghc/wiki/Performance/Compiler</a><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">It links to a list of open perf tickets:
<a href="https://ghc.haskell.org/trac/ghc/query?status=!closed&failure=Compile-time+performance+bug">
https://ghc.haskell.org/trac/ghc/query?status=!closed&failure=Compile-time+performance+bug</a><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">What helps above all are reproducible test cases. Package X compiles much more slowly with 8.2 than with 7.10, say. You did exactly that, thank you – can you open a ticket explaining exactly
how to reproduce it, and what measurements you took. (Matthew’s numbers seemed two orders of magnitude different; I wonder why?)<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">Another thing that is really helpful is people who characterise what the problem is:<o:p></o:p></span></p>
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<li class="MsoListParagraph" style="margin-left:0cm;mso-list:l0 level1 lfo1"><span style="font-family:"Calibri",sans-serif">Is GHC generating a lot more code? -dshow-passes can reveal this.<o:p></o:p></span></li><li class="MsoListParagraph" style="margin-left:0cm;mso-list:l0 level1 lfo1"><span style="font-family:"Calibri",sans-serif">Is it GHC at all, or is it link-times, or some other part of the tool chain?<o:p></o:p></span></li><li class="MsoListParagraph" style="margin-left:0cm;mso-list:l0 level1 lfo1"><span style="font-family:"Calibri",sans-serif">If it is GHC, and the output of compilation is no bigger than before, where is the time going? -dshow-passes gives a rough idea of that
too.<o:p></o:p></span></li><li class="MsoListParagraph" style="margin-left:0cm;mso-list:l0 level1 lfo1"><span style="font-family:"Calibri",sans-serif">Is there a particular characteristic of the program that seems to trigger the poor behaviour? Can you accentuate that characteristic
to make it behave wildly badly?<o:p></o:p></span></li></ul>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">Everyone: GHC is your compiler! We need your help.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">Simon<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif">From:</span></b><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif"> ghc-devs [mailto:ghc-devs-bounces@haskell.org]
<b>On Behalf Of </b>Deest, Gaël<br>
<b>Sent:</b> 04 July 2017 08:17<br>
<b>To:</b> ghc-devs@haskell.org<br>
<b>Cc:</b> Manuel Chakravarty <manuel.chakravarty@tweag.io><br>
<b>Subject:</b> Compilation time<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<span style="font-size:9.5pt">Hi all,<o:p></o:p></span></p>
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<span style="font-size:9.5pt">As you are probably well aware, GHC performance has been a growing concern over the last few years. Many Haskell programmers complain that build time has significantly increased over the last few releases. However, to our knowledge,
there isn't much data available to substantiate this claim and the severity of these problems is not well known.<o:p></o:p></span></p>
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<span style="font-size:9.5pt">That's why we would like to bring some anecdotal evidence to your attention that seems to indicate really major performance regressions. We stumbled upon the CI of the data-reify package, which is built against all GHC releases
since 2010 :<br>
<br>
<a href="https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Ftravis-ci.org%2Fku-fpg%2Fdata-reify&data=02%7C01%7Csimonpj%40microsoft.com%7C1bb3bed0e1b040d80b4508d4c2acb803%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636347494457518816&sdata=Gd9NDAt7jus3ewaGX0GgWwB0%2FjkjX3ZW0cB%2B5LEWE2E%3D&reserved=0" target="_blank">https://travis-ci.org/ku-fpg/data-reify</a><o:p></o:p></span></p>
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<span style="font-size:9.5pt">tl;dr: Build time has gone from 1 min 32s for GHC 7.0 to 4 min 35s for GHC 8.2. The 8.2 release alone seems to have increased compilation time by almost 2 minutes, with the current development branch bringing only minor performance
improvements.<o:p></o:p></span></p>
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<span style="font-size:9.5pt">Of course, this single data point is not sufficient to establish how severe and widespread these problems are. More data could probably be gathered from other packages. However, it certainly matches our (subjective) experience
and we felt important to report it to you.<o:p></o:p></span></p>
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<span style="font-size:9.5pt">Regards,<br clear="all">
</span><o:p></o:p></p>
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-- <o:p></o:p></p>
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Gaël Deest<o:p></o:p></p>
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Tweag I/O<o:p></o:p></p>
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