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<p class="MsoNormal">Oh, now I get it. I misunderstood you.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal" style="margin-left:36.0pt"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">But the point is, I do *<b>not</b>* want to set optLevel. I want to turn on individual optimizations. But as it stands today, if I turn on any optimizations,
that doesn’t do anything unless I *<b>also</b>* set optLevel to >= 1.<o:p></o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Yes I agree. Nothing should consult `optLevel`; indeed maybe we shouldn’t even record it permanently.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">The place in Pipeline that Sylvain identified is most important.
<o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal" style="margin-left:36.0pt"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">So when we compute the CoreTodos, instead of looking at the optLevel, we’d consult the set of optimizations turned on, and go “yup, at least one of
these needs core-to-core simpl passes, so we better get on it”, going to the same branch that current is gated by optLevel >= 1.</span><o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Yes I agree with this. Another alternative might be to kill off the -O0 pipeline entirely (in GHC.Core.Opt.Pipeline.getCoreToDo) and just have one pipeline that does not-very-much when no optimisations are enabled. This would be more
robust (no need to have a list of flags to consult to decide which path to follow), but could have unforeseen consequences (what happens in the -0 pipeline if all optimisations are off? Same as the -O0 pipeline?).<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I’d be happy with either of these changes, if someone wants to offer a patch. Certainly open a ticket with this discussion so it doesn’t get lost.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Simon<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-size:8.0pt;mso-fareast-language:EN-GB">PS: I am leaving Microsoft at the end of November 2021, at which point
<a href="mailto:simonpj@microsoft.com"><span style="color:#0563C1">simonpj@microsoft.com</span></a> will cease to work. Use
<a href="mailto:simon.peytonjones@gmail.com"><span style="color:#0563C1">simon.peytonjones@gmail.com</span></a> instead. (For now, it just forwards to simonpj@microsoft.com.)<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><b><span lang="EN-US" style="mso-fareast-language:EN-GB">From:</span></b><span lang="EN-US" style="mso-fareast-language:EN-GB"> Erdi, Gergo <Gergo.Erdi@sc.com>
<br>
<b>Sent:</b> 11 October 2021 09:21<br>
<b>To:</b> Simon Peyton Jones <simonpj@microsoft.com><br>
<b>Cc:</b> Montelatici, Raphael Laurent <Raphael.Montelatici@sc.com>; 'GHC' <ghc-devs@haskell.org><br>
<b>Subject:</b> RE: -O* does more than what's in optLevelFlags?<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
<p style="margin:15.0pt"><span style="font-size:9.0pt;font-family:"Arial",sans-serif;color:#317100">PUBLIC<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-GB"><o:p> </o:p></span></p>
<div>
<p class="msipheader7557e74b" style="margin:0cm"><span style="font-size:9.0pt;font-family:"Arial",sans-serif;color:#317100">PUBLIC</span><o:p></o:p></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-GB"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">But the point is, I do *<b>not</b>* want to set optLevel. I want to turn on individual optimizations. But as it stands today, if I turn on any optimizations, that doesn’t do
anything unless I *<b>also</b>* set optLevel to >= 1.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">A nicer design in my mind would be if the semantics of optLevel is fully defined in terms of optimization flags. I want “-O1 -fno-strictness” to be exactly equivalent to “-fpolynomial-complexity
-fremove-space-leaks -fstrictness -fno-strictness” which of course is equivalent to “-fpolynomial-complexity -fremove-space-leaks -fno-strictness” (assuming that the O1 flag is defined to turn on three flags that ensure polynomial complexity, remove space
leaks, and turn on strictness analysis, just as a way of example). So when we compute the CoreTodos, instead of looking at the optLevel, we’d consult the set of optimizations turned on, and go “yup, at least one of these needs core-to-core simpl passes, so
we better get on it”, going to the same branch that current is gated by optLevel >= 1.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">Would that make sense?<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">Also, this is NOT just an API documentation issue! The GHC user’s guide itself wrongly claims:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal" style="margin-bottom:18.0pt;line-height:18.0pt;background:#FCFCFC">
<span style="font-size:12.0pt;font-family:"Arial",sans-serif;color:#404040;mso-fareast-language:EN-GB">The </span><span style="font-size:9.0pt;font-family:Consolas;color:#111153;mso-fareast-language:EN-GB">-O*</span><span style="font-size:12.0pt;font-family:"Arial",sans-serif;color:#404040;mso-fareast-language:EN-GB"> options
specify convenient “packages” of optimisation flags; the </span><span style="font-size:9.0pt;font-family:Consolas;color:#111153;mso-fareast-language:EN-GB">-f*</span><span style="font-size:12.0pt;font-family:"Arial",sans-serif;color:#404040;mso-fareast-language:EN-GB"> options
described later on specify <i>individual</i> optimisations to be turned on/off; the </span><span style="font-size:9.0pt;font-family:Consolas;color:#111153;mso-fareast-language:EN-GB">-m*</span><span style="font-size:12.0pt;font-family:"Arial",sans-serif;color:#404040;mso-fareast-language:EN-GB"> options
specify <i>machine-specific</i> optimisations to be turned on/off.<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-bottom:18.0pt;line-height:18.0pt;background:#FCFCFC">
<span style="font-size:12.0pt;font-family:"Arial",sans-serif;color:#404040;mso-fareast-language:EN-GB">[…]<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-bottom:18.0pt;line-height:18.0pt;background:#FCFCFC">
<span style="font-size:12.0pt;font-family:"Arial",sans-serif;color:#404040;mso-fareast-language:EN-GB">There are <i>many</i> options that affect the quality of code produced by GHC. Most people only have a general goal, something like “Compile quickly” or “Make
my program run like greased lightning.” The following “packages” of optimisations (or lack thereof) should suffice.<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-bottom:18.0pt;line-height:18.0pt;background:#FCFCFC">
<span style="font-size:12.0pt;font-family:"Arial",sans-serif;color:#404040;mso-fareast-language:EN-GB"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif"><a href="https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fdownloads.haskell.org%2F~ghc%2Flatest%2Fdocs%2Fhtml%2Fusers_guide%2Fusing-optimisation.html%23o-convenient-packages-of-optimisation-flags&data=04%7C01%7Csimonpj%40microsoft.com%7C5afa7f7726d04c4e8e7008d98c903bd6%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637695374886882049%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=EE6ARdxQR07lCZCjIPLJKxjIBcvCufBj6yWk8VpFPvE%3D&reserved=0">https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/using-optimisation.html#o-convenient-packages-of-optimisation-flags</a><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">I’m happy to submit a documentation patch if there is no way to actually support this model, but I would prefer much more if we can implement what is claimed in the documentation
here </span><span style="font-size:10.0pt;font-family:"Segoe UI Emoji",sans-serif">😊</span><span style="font-size:10.0pt;font-family:"Arial",sans-serif"><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US" style="mso-fareast-language:EN-GB">From:</span></b><span lang="EN-US" style="mso-fareast-language:EN-GB"> Simon Peyton Jones <<a href="mailto:simonpj@microsoft.com">simonpj@microsoft.com</a>>
<br>
<b>Sent:</b> Monday, October 11, 2021 4:06 PM<br>
<b>To:</b> Erdi, Gergo <<a href="mailto:Gergo.Erdi@sc.com">Gergo.Erdi@sc.com</a>><br>
<b>Cc:</b> Montelatici, Raphael Laurent <<a href="mailto:Raphael.Montelatici@sc.com">Raphael.Montelatici@sc.com</a>>; 'GHC' <<a href="mailto:ghc-devs@haskell.org">ghc-devs@haskell.org</a>><br>
<b>Subject:</b> [External] RE: -O* does more than what's in optLevelFlags?<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="mso-fareast-language:EN-GB"><o:p> </o:p></span></p>
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<p class="MsoNormal" style="margin-left:36.0pt"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">Is this a documentation bug, an implementation bug (as in, if any of the relevant opts are set, then the CoreToDos should always include the optimization
steps selected), or a design bug (there is no way to support this meaningfully)?<o:p></o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Maybe a documentation bug? You should update the `optLevel` field of `DynFlags` only via calling `setOptLevel`, not by setting it directly. <o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">What other design would make sense? We want to support ghc -O -fno-strictness, where the -O switches on a bunch of flags, and -fno-strictness turns off strictness. The order matters.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">One difficulty is that I’m not even sure where one would <i>
look</i> for that documentation. We don’t really have a comprehensive GHC User Manual description of the GHC API: Section 7.2 “Using GHC as a library” is vestigial. I would be Absolutely Fantastic, if someone (Gergo, even) felt able to flesh it out.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Simon<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-size:8.0pt;mso-fareast-language:EN-GB">PS: I am leaving Microsoft at the end of November 2021, at which point
<a href="mailto:simonpj@microsoft.com"><span style="color:#0563C1">simonpj@microsoft.com</span></a> will cease to work. Use
<a href="mailto:simon.peytonjones@gmail.com"><span style="color:#0563C1">simon.peytonjones@gmail.com</span></a> instead. (For now, it just forwards to
<a href="mailto:simonpj@microsoft.com">simonpj@microsoft.com</a>.)<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><b><span lang="EN-US" style="mso-fareast-language:EN-GB">From:</span></b><span lang="EN-US" style="mso-fareast-language:EN-GB"> ghc-devs <<a href="mailto:ghc-devs-bounces@haskell.org">ghc-devs-bounces@haskell.org</a>>
<b>On Behalf Of </b>Erdi, Gergo via ghc-devs<br>
<b>Sent:</b> 11 October 2021 08:54<br>
<b>To:</b> 'GHC' <<a href="mailto:ghc-devs@haskell.org">ghc-devs@haskell.org</a>><br>
<b>Cc:</b> Montelatici, Raphael Laurent <<a href="mailto:Raphael.Montelatici@sc.com">Raphael.Montelatici@sc.com</a>><br>
<b>Subject:</b> RE: -O* does more than what's in optLevelFlags?<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
<p class="msipheader7557e74b" style="margin:0cm"><span style="font-size:9.0pt;font-family:"Arial",sans-serif;color:#317100">PUBLIC</span><o:p></o:p></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-GB"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">I’ve done some digging into this, and it turns out the DynFlag’s `optLevel` itself is used at some places, most notably when creating the main [CoreToDo]. So turning on all these
flags on their own doesn’t equal setting -On for the right “n”; in fact, currently setting most of these flags does NOTHING on its own unless -On with n>=1 is *<b>also</b>* passed on the command line, and there is no command line flag to *<b>only</b>* turn
on Core optimizations in the abstract, without actually turning any specific ones on.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">Is this a documentation bug, an implementation bug (as in, if any of the relevant opts are set, then the CoreToDos should always include the optimization steps selected), or
a design bug (there is no way to support this meaningfully)?<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US" style="mso-fareast-language:EN-GB">From:</span></b><span lang="EN-US" style="mso-fareast-language:EN-GB"> Erdi, Gergo
<br>
<b>Sent:</b> Monday, October 11, 2021 12:09 PM<br>
<b>To:</b> 'GHC' <<a href="mailto:ghc-devs@haskell.org">ghc-devs@haskell.org</a>><br>
<b>Cc:</b> Montelatici, Raphael Laurent <<a href="mailto:Raphael.Montelatici@sc.com">Raphael.Montelatici@sc.com</a>><br>
<b>Subject:</b> -O* does more than what's in optLevelFlags?<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
<p class="msipheader7557e74b" style="margin:0cm"><span style="font-size:9.0pt;font-family:"Arial",sans-serif;color:#317100">PUBLIC</span><o:p></o:p></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-GB"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">What is set by -O* that is not included in optLevelFlags?<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">I would have thought that setting all the flags implied by, e.g., -O1, would be the same as setting -O1 itself. But this is not the case! Here are all the flags for O1 from optLevelFlags:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">Opt_DoLambdaEtaExpansion<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">Opt_DoEtaReduction<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">Opt_LlvmTBAA<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">Opt_CallArity<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">Opt_Exitification<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">Opt_CaseMerge<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">Opt_CaseFolding<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">Opt_CmmElimCommonBlocks<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">Opt_CmmSink<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">Opt_CmmStaticPred<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">Opt_CSE<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">Opt_StgCSE<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">Opt_EnableRewriteRules<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">Opt_FloatIn<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">Opt_FullLaziness<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">Opt_IgnoreAsserts<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">Opt_Loopification<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">Opt_CfgBlocklayout<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">Opt_Specialise<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">Opt_CrossModuleSpecialise<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">Opt_InlineGenerics<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">Opt_Strictness<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">Opt_UnboxSmallStrictFields<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">Opt_CprAnal<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">Opt_WorkerWrapper<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">Opt_SolveConstantDicts<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">Opt_NumConstantFolding<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">And here are the ones that are set by O0 (the default) but not by O1:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">Opt_IgnoreInterfacePragmas<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">Opt_OmitInterfacePragmas<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">So I expected that the following two invocations of GHC would be equivalent:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<ol style="margin-top:0cm" start="1" type="1">
<li class="MsoListParagraphCxSpFirst" style="margin-left:0cm;mso-add-space:auto;mso-list:l1 level1 lfo3">
<span style="font-family:"Lucida Console"">ghc -O1</span><span style="font-size:10.0pt;font-family:"Arial",sans-serif"><o:p></o:p></span></li><li class="MsoListParagraphCxSpLast" style="margin-left:0cm;mso-add-space:auto;mso-list:l1 level1 lfo3">
<span style="font-family:"Lucida Console"">ghc -fdo-lambda-eta-expansion -fdo-eta-reduction -fllvm-tbaa -fcall-arity -fexitification -fcase-merge -fcase-folding -fcmm-elim-common-blocks -fcmm-sink -fcmm-static-pred -fcse -fstg-cse -fenable-rewrite-rules -ffloat-in
-ffull-laziness -fignore-asserts -floopification -fblock-layout-cfg -fspecialise -fcross-module-specialise -finline-generics -fstrictness -funbox-small-strict-fields -fcpr-anal -fworker-wrapper -fsolve-constant-dicts -fnum-constant-folding -fno-ignore-interface-pragmas
-fno-omit-interface-pragmas</span><span style="font-size:10.0pt;font-family:"Arial",sans-serif"><o:p></o:p></span></li></ol>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">However, just by observing the output of -dshow-passes, I can see that while -O1 applies all these optimizations, the second version does NOT, even though I have turned on each
and every one of them one by one. <o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">Looking at compiler/GHC/Driver/Session.hs, it is not at all clear that -O* should do more than just setting the flags from optLevelFlags. What other flags are implied by -O*?<o:p></o:p></span></p>
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