<html xmlns:v="urn:schemas-microsoft-com:vml" xmlns:o="urn:schemas-microsoft-com:office:office" xmlns:w="urn:schemas-microsoft-com:office:word" xmlns:m="http://schemas.microsoft.com/office/2004/12/omml" xmlns="http://www.w3.org/TR/REC-html40">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=us-ascii">
<meta name="Generator" content="Microsoft Word 14 (filtered medium)">
<style><!--
/* Font Definitions */
@font-face
{font-family:Calibri;
panose-1:2 15 5 2 2 2 4 3 2 4;}
@font-face
{font-family:Tahoma;
panose-1:2 11 6 4 3 5 4 4 2 4;}
/* Style Definitions */
p.MsoNormal, li.MsoNormal, div.MsoNormal
{margin:0in;
margin-bottom:.0001pt;
font-size:12.0pt;
font-family:"Times New Roman","serif";}
a:link, span.MsoHyperlink
{mso-style-priority:99;
color:blue;
text-decoration:underline;}
a:visited, span.MsoHyperlinkFollowed
{mso-style-priority:99;
color:purple;
text-decoration:underline;}
p.MsoAcetate, li.MsoAcetate, div.MsoAcetate
{mso-style-priority:99;
mso-style-link:"Balloon Text Char";
margin:0in;
margin-bottom:.0001pt;
font-size:8.0pt;
font-family:"Tahoma","sans-serif";}
span.BalloonTextChar
{mso-style-name:"Balloon Text Char";
mso-style-priority:99;
mso-style-link:"Balloon Text";
font-family:"Tahoma","sans-serif";}
span.EmailStyle19
{mso-style-type:personal-reply;
font-family:"Calibri","sans-serif";
color:#1F497D;}
.MsoChpDefault
{mso-style-type:export-only;
font-size:10.0pt;}
@page WordSection1
{size:8.5in 11.0in;
margin:1.0in 1.0in 1.0in 1.0in;}
div.WordSection1
{page:WordSection1;}
--></style><!--[if gte mso 9]><xml>
<o:shapedefaults v:ext="edit" spidmax="1026" />
</xml><![endif]--><!--[if gte mso 9]><xml>
<o:shapelayout v:ext="edit">
<o:idmap v:ext="edit" data="1" />
</o:shapelayout></xml><![endif]-->
</head>
<body lang="EN-US" link="blue" vlink="purple">
<div class="WordSection1">
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">If AutoFDO is using debug info to map instructions back to source locations, then the problem is that debug info (at least DWARF) cannot describe the situation
where one instruction in effect comes from two different places. This is a long-standing problem for debugging optimized code, and I guess would affect AutoFDO as well. We can either assign the instruction to one of the source locations (which is a lie part
of the time) or we can say we don't know where it comes from (which is kind of always true as the origin is ambiguous). I know that the debugging experience is not great the way things are now, but I don't know whether it would be better if we started saying
we don't know where the code comes from.<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">There are a variety of optimizations that would have to address this: branch folding, various combines, tail merging, probably more. Right now I think they
all just pick one somewhat arbitrarily.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">--paulr<o:p></o:p></span></p>
<p class="MsoNormal"><a name="_MailEndCompose"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></a></p>
<div style="border:none;border-left:solid blue 1.5pt;padding:0in 0in 0in 4.0pt">
<div>
<div style="border:none;border-top:solid #B5C4DF 1.0pt;padding:3.0pt 0in 0in 0in">
<p class="MsoNormal"><b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> llvm-dev [mailto:llvm-dev-bounces@lists.llvm.org]
<b>On Behalf Of </b>David Callahan via llvm-dev<br>
<b>Sent:</b> Wednesday, August 17, 2016 9:07 AM<br>
<b>To:</b> Sanjay Patel; Xinliang David Li<br>
<b>Cc:</b> LLVM Dev Mailing list<br>
<b>Subject:</b> Re: [llvm-dev] AutoFDO sample profiles v. SelectInst,<o:p></o:p></span></p>
</div>
</div>
<p class="MsoNormal"><o:p> </o:p></p>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Calibri","sans-serif";color:black">I agree, this a fundamental problem with how the AutoFDO maps addresses to statements.<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Calibri","sans-serif";color:black"><o:p> </o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Calibri","sans-serif";color:black">I have an experimental build where, rather than turning off certain optimizations, I change the DebugLoc information when we hoist instructions into a new execution
context. That avoids the problem of wrong branch weights but means some branch weights are inferred rather than measured. <o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Calibri","sans-serif";color:black"><o:p> </o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Calibri","sans-serif";color:black">Perhaps rather than limit optimizations we have a variant of ‘-g’ which tolerates this kind of change<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Calibri","sans-serif";color:black"><o:p> </o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Calibri","sans-serif";color:black"><o:p> </o:p></span></p>
</div>
<div style="border:none;border-top:solid #B5C4DF 1.0pt;padding:3.0pt 0in 0in 0in">
<p class="MsoNormal"><b><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:black">From:
</span></b><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:black">Sanjay Patel <<a href="mailto:spatel@rotateright.com">spatel@rotateright.com</a>><br>
<b>Date: </b>Wednesday, August 17, 2016 at 8:19 AM<br>
<b>To: </b>Xinliang David Li <<a href="mailto:xinliangli@gmail.com">xinliangli@gmail.com</a>><br>
<b>Cc: </b>David Callahan <<a href="mailto:dcallahan@fb.com">dcallahan@fb.com</a>>, LLVM Dev Mailing list <<a href="mailto:llvm-dev@lists.llvm.org">llvm-dev@lists.llvm.org</a>><br>
<b>Subject: </b>Re: [llvm-dev] AutoFDO sample profiles v. SelectInst,<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Calibri","sans-serif";color:black"><o:p> </o:p></span></p>
</div>
<div>
<div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Calibri","sans-serif";color:black"><o:p> </o:p></span></p>
<div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Calibri","sans-serif";color:black">On Fri, Aug 12, 2016 at 12:15 PM, Xinliang David Li via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a>> wrote:<o:p></o:p></span></p>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Calibri","sans-serif";color:black">+dehao.
<o:p></o:p></span></p>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Calibri","sans-serif";color:black"><o:p> </o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Calibri","sans-serif";color:black">There are two potential problems:<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Calibri","sans-serif";color:black"><o:p> </o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Calibri","sans-serif";color:black">1) the branch gets eliminated in the binary that is being profiled, so there is no profile data<o:p></o:p></span></p>
</div>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Calibri","sans-serif";color:black"><o:p> </o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Calibri","sans-serif";color:black">This seems like a fundamental problem for PGO. Maybe it is also responsible for this bug:
<a href="https://urldefense.proofpoint.com/v2/url?u=https-3A__llvm.org_bugs_show-5Fbug.cgi-3Fid-3D27359&d=DQMFaQ&c=5VD0RTtNlTh3ycd41b3MUw&r=lFyiPUrFdOHdaobP7i4hoA&m=ySKs86N9LiBaTDmiRjbJJIGlO-z7BHXRaRnz-hS1Gno&s=ZkL9ORQxX1oNnyUFSaW9NT6PcpKuF8Ow5gqA-aEp7YA&e=">
https://llvm.org/bugs/show_bug.cgi?id=27359</a> ?<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Calibri","sans-serif";color:black"> <o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Calibri","sans-serif";color:black">Should we limit select optimizations in IR for a PGO-training build? Or should there be a 'select smasher' pass later in the pipeline that turns selects into
branches for a PGO-training build? (I don't have a good understanding of PGO, so I'm just throwing out ideas...maybe a better question is: how do other compilers handle this?)<o:p></o:p></span></p>
</div>
</div>
<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Calibri","sans-serif";color:black"><o:p> </o:p></span></p>
</div>
</div>
</div>
</div>
</div>
</div>
</body>
</html>