<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 15 (filtered medium)">
<style><!--
/* Font Definitions */
@font-face
{font-family:"Cambria Math";
panose-1:2 4 5 3 5 4 6 3 2 4;}
@font-face
{font-family:Calibri;
panose-1:2 15 5 2 2 2 4 3 2 4;}
/* Style Definitions */
p.MsoNormal, li.MsoNormal, div.MsoNormal
{margin:0cm;
margin-bottom:.0001pt;
font-size:11.0pt;
font-family:"Calibri",sans-serif;
mso-fareast-language:EN-US;}
a:link, span.MsoHyperlink
{mso-style-priority:99;
color:#0563C1;
text-decoration:underline;}
a:visited, span.MsoHyperlinkFollowed
{mso-style-priority:99;
color:#954F72;
text-decoration:underline;}
span.EmailStyle17
{mso-style-type:personal-compose;
font-family:"Calibri",sans-serif;
color:windowtext;}
span.k
{mso-style-name:k;}
span.err
{mso-style-name:err;}
span.n
{mso-style-name:n;}
span.bright
{mso-style-name:bright;}
.MsoChpDefault
{mso-style-type:export-only;
mso-fareast-language:EN-US;}
@page WordSection1
{size:612.0pt 792.0pt;
margin:72.0pt 90.0pt 72.0pt 90.0pt;}
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="FR-CA" link="#0563C1" vlink="#954F72">
<div class="WordSection1">
<p class="MsoNormal"><span lang="EN-CA">Hi folks,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA">With the recent support for Clang in Visual Studio 2019 [1], I thought we should revive and eventually land the /MP proposal I made a while ago [2].<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA">As it stands now, building with clang-cl is much slower than MSVC when used along MSBuild, see timings in summary [2].<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA">With this patch, the situation is greatly improved, clang-cl /MP performance is better in all cases [5].<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA">----<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA">When the MP option is active (default behavior) in a Visual Studio Project [3], MSBuild issues commands such as:<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA"><o:p> </o:p></span></p>
<p class="MsoNormal" style="text-indent:35.4pt"><span lang="EN-CA">clang-cl /MP file-A.cpp file-B.cpp file-C.cpp<o:p></o:p></span></p>
<p class="MsoNormal" style="text-indent:35.4pt"><span lang="EN-CA"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA">This creates a root clang-cl process, and then *<b>sequentially</b>* a cc1 sub-process for each CPP file in the list. Evidently this is slow and doesn’t use the full potential on modern multi-cores machines.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA">A workaround is to increase the number of parallel builds [4], but that is sub-optimal and is still slower than MSVC.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA">The /MP proposal I made in [2] simply maintains a list of cc1 sub-processes for each hardware core (or for the number of cores provided as an option to /MP).<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA">Waiting for termination in the root clang-cl process is done through a single OS primitive (see sys::WaitMany), which temporarily suspends the root process until at least one cc1 sub-process finishes. This ensure no cycles
are wasted waiting, and avoids useless context-switches to the root process.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA">----<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA">Most of the changes in the demo patch in [2] are: modernization of the process launching API (ProcessInfo), and support for sys::WaitMany, which I’d like to commit separately and incrementally.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA">Support for /MP is very small and mostly located in clang/trunk/lib/Driver/Compilation.cpp in the demo patch.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA">We could subsequently add a new flag –j to the regular clang driver, which mimics /MP. Further down the road, I’d like to discuss optional support for concrt multithreading which, from some preliminary testing, would
be much faster than the current cc1 sub-process invocation, at least on Windows 10 1703+.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA">Feel free to comment here or in the demo patch.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA">Thanks,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA">Alex.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-CA">[1] </span><a href="https://devblogs.microsoft.com/cppblog/clang-llvm-support-in-visual-studio/"><span lang="EN-CA">https://devblogs.microsoft.com/cppblog/clang-llvm-support-in-visual-studio/</span></a><o:p></o:p></p>
<p class="MsoNormal">[2] <a href="https://reviews.llvm.org/D52193">https://reviews.llvm.org/D52193</a><o:p></o:p></p>
<p class="MsoNormal">[3] <a href="https://docs.microsoft.com/en-us/previous-versions/bb385193(v=vs.140)">
https://docs.microsoft.com/en-us/previous-versions/bb385193(v=vs.140)</a><o:p></o:p></p>
<p class="MsoNormal">[4] <a href="https://docs.microsoft.com/en-us/visualstudio/ide/reference/options-dialog-box-projects-and-solutions-build-and-run?view=vs-2019">
https://docs.microsoft.com/en-us/visualstudio/ide/reference/options-dialog-box-projects-and-solutions-build-and-run?view=vs-2019</a><o:p></o:p></p>
<p class="MsoNormal">[5] <a href="https://reviews.llvm.org/D52193#1241023">https://reviews.llvm.org/D52193#1241023</a><o:p></o:p></p>
</div>
</body>
</html>