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<p class="msipheader7557e74b" style="margin:0in"><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>
<div>
<div style="border:none;border-top:solid #E1E1E1 1.0pt;padding:3.0pt 0in 0in 0in">
<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' <ghc-devs@haskell.org><br>
<b>Cc:</b> Montelatici, Raphael Laurent <Raphael.Montelatici@sc.com><br>
<b>Subject:</b> -O* does more than what's in optLevelFlags?<o:p></o:p></span></p>
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</div>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="msipheader7557e74b" style="margin:0in"><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:0in" start="1" type="1">
<li class="MsoListParagraphCxSpFirst" style="margin-left:0in;mso-add-space:auto;mso-list:l2 level1 lfo6">
<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:0in;mso-add-space:auto;mso-list:l2 level1 lfo6">
<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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