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<p class="MsoNormal" style="margin-left:36.0pt">The inliner is a finicky beast. In response to your insults, I will say that although GHC has beautiful ideas in it, a lot of the details of the optimization passes and how they fit together *are* a bit of a crapshoot,
chosen by benchmarks rather than theory.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";mso-fareast-language:EN-US">It’s true that, particularly for fusion, inlining can make a huge difference. And GHC really does need help… it’s extremely hard for it to make the “right” choice
all the time.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";mso-fareast-language:EN-US">I strongly agree with Kim-Ee that we should play the game of “optimise by randomly mutating the program and pick the version that (today) happens to run faster”.
But I don’t think David is doing that. There is, at least a </span>Note: [List comprehensions and inlining].<span style="font-family:"Calibri","sans-serif";mso-fareast-language:EN-US"><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";mso-fareast-language:EN-US">What we should do is to
<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt;mso-list:l0 level1 lfo1"><![if !supportLists]><span style="font-family:Symbol;mso-fareast-language:EN-US"><span style="mso-list:Ignore">·<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><b><span style="font-family:"Calibri","sans-serif";mso-fareast-language:EN-US">understand</span></b><span style="font-family:"Calibri","sans-serif";mso-fareast-language:EN-US"> which the crucial inlining decisions are, and why<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt;mso-list:l0 level1 lfo1"><![if !supportLists]><span style="font-family:Symbol;mso-fareast-language:EN-US"><span style="mso-list:Ignore">·<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><b><span style="font-family:"Calibri","sans-serif";mso-fareast-language:EN-US">express</span></b><span style="font-family:"Calibri","sans-serif";mso-fareast-language:EN-US"> that understanding in a Note, so that subsequent people
looking at the code also understand<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt;mso-list:l0 level1 lfo1"><![if !supportLists]><span style="font-family:Symbol;mso-fareast-language:EN-US"><span style="mso-list:Ignore">·<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-family:"Calibri","sans-serif";mso-fareast-language:EN-US">remove any un-necessary code cruft, which sometimes accumulates during a period of experimentation, leaving the minimum necessary to achieve the effect<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";mso-fareast-language:EN-US">I’m not quite certain that all these steps have been done here, and they can be tiresome to do. But worth it!<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";mso-fareast-language:EN-US">Simon<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";mso-fareast-language:EN-US"><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""> Libraries [mailto:libraries-bounces@haskell.org]
<b>On Behalf Of </b>David Feuer<br>
<b>Sent:</b> 11 November 2014 09:12<br>
<b>To:</b> Kim-Ee Yeoh<br>
<b>Cc:</b> Haskell Libraries; ghc-devs<br>
<b>Subject:</b> Re: [GHC] #9781: Make list monad operations fuse<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
<p><br>
On Nov 11, 2014 3:56 AM, "Kim-Ee Yeoh" <<a href="mailto:ky3@atamo.com">ky3@atamo.com</a>> wrote:<br>
><br>
> From the patch fragment at<br>
><br>
> <a href="https://phabricator.haskell.org/D455?id=1311#inline-3123">https://phabricator.haskell.org/D455?id=1311#inline-3123</a><br>
><br>
> What's the justification for expanding out the definition of mapM from "sequence . map f" into do-notation and duplicated code?<br>
><br>
> Observe how mapM now duplicates code from sequence.<o:p></o:p></p>
<p>One good option might be to redefine sequence in terms of mapM.<o:p></o:p></p>
<p>> The absence of benchmarks is bad enough. What's worse is that the given excuse boils down to "Pity the poor compiler! Let's take over its work instead. Like, just in case, you know."<o:p></o:p></p>
<p>The excuse is that it actually makes a big difference for nofib. Why? I would have to guess it relates to what inlines when. The inliner is a finicky beast. In response to your insults, I will say that although GHC has beautiful ideas in it, a lot of the
details of the optimization passes and how they fit together *are* a bit of a crapshoot, chosen by benchmarks rather than theory. Theory sometimes comes up behind and explains why the benchmarks do what they do, but you can't expect every little change to
be backed up by some deep theoretical reason.<o:p></o:p></p>
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