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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Hi David!<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Thanks again for your help! I was wondering if you could clarify one thing for me?<o:p></o:p></span></p>
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I find mention of “hot arc” optimization (-fprofile-arcs) , but I’m unclear if this is the same thing. Does Clang PGO do block reordering?<o:p></o:p></p>
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It does reordering, but does not do splitting/partitioning.<o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">I take this to mean that PGO does block reordering within the function? I don’t see that the clang drive passes anything to the linker to drive function ordering
at the linker level as well. Is there something there that I missed, or are you aware of any readily available tools to do so? If not, we’ve done some work locally on enabling that which we will continue.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Thanks
</span><span style="font-size:11.0pt;font-family:Wingdings;color:#1F497D">J</span><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">--randy<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif">From:</span></b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif"> Xinliang David Li [<a href="mailto:xinliangli@gmail.com">mailto:xinliangli@gmail.com</a>]
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<b>Sent:</b> Wednesday, May 27, 2015 10:21 AM<br>
<b>To:</b> Lee Hunt<br>
<b>Cc:</b> <a href="mailto:llvmdev@cs.uiuc.edu">llvmdev@cs.uiuc.edu</a><br>
<b>Subject:</b> Re: [LLVMdev] Capabilities of Clang's PGO (e.g. improving code density)<o:p></o:p></span></p>
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<p class="MsoNormal">On Wed, May 27, 2015 at 10:11 AM, Lee Hunt <<a href="mailto:leehu@exchange.microsoft.com" target="_blank">leehu@exchange.microsoft.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Thanks! CIL [LeeHu] for a few comments…</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> </span><o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif">From:</span></b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif"> Xinliang David Li [mailto:<a href="mailto:xinliangli@gmail.com" target="_blank">xinliangli@gmail.com</a>]
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<b>Sent:</b> Wednesday, May 27, 2015 9:29 AM<br>
<b>To:</b> Lee Hunt<br>
<b>Cc:</b> <a href="mailto:llvmdev@cs.uiuc.edu" target="_blank">llvmdev@cs.uiuc.edu</a><br>
<b>Subject:</b> Re: [LLVMdev] Capabilities of Clang's PGO (e.g. improving code density)</span><o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">On Tue, May 26, 2015 at 8:47 PM, Lee Hunt <<a href="mailto:leehu@exchange.microsoft.com" target="_blank">leehu@exchange.microsoft.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Hello –<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">I’m an Engineer in Microsoft Office after looking into possible advantages of using PGO for our Android Applications.<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">We at Microsoft have deep experience with Visual C++’s
<a href="https://urldefense.proofpoint.com/v2/url?u=https-3A__msdn.microsoft.com_en-2Dus_library_e7k32f4k.aspx&d=AwMFAg&c=8hUWFZcy2Z-Za5rBPlktOQ&r=Mfk2qtn1LTDThVkh6-oGglNfMADXfJdty4_bhmuhMHA&m=CDx6fJHiO_U5ya1dHZhv-O5nAU_botD-I7BAyxPZXZE&s=L5s90Jkxqk45FMvD7qA0Visu71cC_bqMyLK3h0RSZtU&e=" target="_blank">
Profile Guided Optimization</a> and often see 10% or more reduction in the size of application code loaded after using PGO for key scenarios (e.g. application launch). <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">yes. This is true for the GCC too. Clang's PGO does not shrink code size yet.<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="color:#1F497D"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">[LeeHu] Note: I’m not talking about shrinking code size, but rather reordering it such that only ‘active’
branches within the profiled functions are grouped together in ‘hot’ code pages. This is a very big optimization for us in VC++ toolchain in PGO.</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">We also have the “/LTCG” flag – which is seemingly similar to the “-flto” Clang flag -- that *<b>does*</b>
shrink code by various means (dead code removal, common IL tree collapsing) because it can see all the object code for an entire produced target binary (e.g. .exe or .dll).</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Does -flto also shrink code?</span><o:p></o:p></p>
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<p class="MsoNormal">That depends on other options used (e.g, -Os). With LTO, compiler sees larger scope, performs cross module inlines and dead function eliminations. It does have more opportunities to shrink code.<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> Making application launch quickly is very important to us, and reducing the number of code pages loaded helps with this goal.<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Before we dig into turning it on, I’m wondering if there’s any pre-existing research / case studies about possible code page reduction seen from other Clang PGO-enabled applications?
It sounds like there is some possible instrumented run performance problems due to counter contention resulting in sluggish performance and perhaps skewed profile data:
<a href="https://urldefense.proofpoint.com/v2/url?u=https-3A__groups.google.com_forum_-23-21topic_llvm-2Ddev_cDqYgnxNEhY&d=AwMFAg&c=8hUWFZcy2Z-Za5rBPlktOQ&r=Mfk2qtn1LTDThVkh6-oGglNfMADXfJdty4_bhmuhMHA&m=CDx6fJHiO_U5ya1dHZhv-O5nAU_botD-I7BAyxPZXZE&s=YaUiiOgIrmA6Io5p4aWzmppYDAKyp8ddTwozd_l-Wjg&e=" target="_blank">
https://groups.google.com/forum/#!topic/llvm-dev/cDqYgnxNEhY</a>. <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Counter contention is one issue. Redundant counter updates is another major issue (due to the early instrumentation). We are working on the later and see great speed ups.<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">I’d like an overview of the optimizations that PGO does, but I don’t find much from looking at the Clang PGO section:
<a href="https://urldefense.proofpoint.com/v2/url?u=http-3A__clang.llvm.org_docs_UsersManual.html-23profile-2Dguided-2Doptimization&d=AwMFAg&c=8hUWFZcy2Z-Za5rBPlktOQ&r=Mfk2qtn1LTDThVkh6-oGglNfMADXfJdty4_bhmuhMHA&m=CDx6fJHiO_U5ya1dHZhv-O5nAU_botD-I7BAyxPZXZE&s=cKiMsZqz31mbPqwGaH_hX2B8sTtFSJ65A4_vbF-fkB4&e=" target="_blank">
http://clang.llvm.org/docs/UsersManual.html#profile-guided-optimization</a>.<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Profile data is not used in any IPA passes yet. It is used by any post inline optimizations though -- including block layout, register allocator etc.<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">[LeeHu]: sorry for naïve question, but what is IPA?
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<p class="MsoNormal">Inter-procedural analysis/optimizations.<o:p></o:p></p>
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<p class="MsoNormal"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">And what post-inline optimizations are currently being done? We’re currently using Clang 3.5 if that
matters.</span><o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">For example, from reading different pages on how Clang PGO, it’s unclear if it does “block reordering” (i.e. moving unexecuted code blocks to a distant code page, leaving only ‘hot’
executed code packed together for greater code density).<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">LLVM's block placement uses branch probability and frequency data, but there is no function splitting optimization yet.<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> I find mention of “hot arc” optimization (-fprofile-arcs) , but I’m unclear if this is the same thing. Does Clang PGO do block reordering?<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">It does reordering, but does not do splitting/partitioning.<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">David<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Thanks,<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">--Lee<o:p></o:p></p>
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_______________________________________________<br>
LLVM Developers mailing list<br>
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<a href="http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev" target="_blank">http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev</a><o:p></o:p></p>
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