<html 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=utf-8">
<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:0in;
font-size:11.0pt;
font-family:"Calibri",sans-serif;}
span.EmailStyle20
{mso-style-type:personal-reply;
font-family:"Calibri",sans-serif;
color:windowtext;}
.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;}
/* List Definitions */
@list l0
{mso-list-id:2077505821;
mso-list-template-ids:2064289668;}
ol
{margin-bottom:0in;}
ul
{margin-bottom:0in;}
--></style>
</head>
<body lang="EN-US" link="#0563C1" vlink="#954F72" style="word-wrap:break-word">
<div class="WordSection1">
<p class="MsoNormal">To second what Jez said here: the new Mach-O backend is still pretty young (e.g. it wasn’t even the default Mach-O backend in LLVM 12). It’s being actively developed, and there’s still the possibility of fairly invasive changes being required
to support new features or as we gain more implementation experience.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">One of our main motivations for LLD for Mach-O was to improve incremental build speeds for our developers. We’ve found that link speed is a major component of incremental build times; LLD’s speed is a huge win there, and we care a lot about
maintaining that speed. I’m CCing Nico, since he’s also been actively benchmarking LLD for Mach-O against Chromium builds (and reporting and fixing speed regressions).<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Reid’s proposals (better error handling and eliminating globals) are completely reasonable. In general though, I really appreciate the LLD codebase’s simplicity and think it’s very valuable for understanding and maintaining the code. I
haven’t worked too much with the rest of LLVM or Clang, so I’m not at all trying to compare LLD with them or cast aspersions on those codebases; I’m just speaking from my personal perspective. For example, having (mostly) separate codebases for each LLD port
set off my “code duplication” spidey senses when I first started working with LLD, but while it does lead to some amount of duplication, there’s also a lot of subtle behavior differences between platforms, and having some amount of duplication is IMO a better
tradeoff than e.g. having common functions that have a bunch of conditionals for each platform, or trying to come up with common abstract interfaces that are specialized for each platform, or so on. I really hope we can maintain that simplicity to whatever
extent possible.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Thanks,<o:p></o:p></p>
<p class="MsoNormal">Shoaib<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<div style="border:none;border-top:solid #B5C4DF 1.0pt;padding:3.0pt 0in 0in 0in">
<p class="MsoNormal"><b><span style="font-size:12.0pt;color:black">From: </span></b><span style="font-size:12.0pt;color:black">llvm-dev <llvm-dev-bounces@lists.llvm.org> on behalf of Jez Ng via llvm-dev <llvm-dev@lists.llvm.org><br>
<b>Reply-To: </b>Jez Ng <jezng@fb.com><br>
<b>Date: </b>Friday, June 11, 2021 at 2:14 PM<br>
<b>To: </b>"llvm-dev@lists.llvm.org" <llvm-dev@lists.llvm.org><br>
<b>Subject: </b>Re: [llvm-dev] RFC: Revisiting LLD-as-a-library design<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal" style="background:white"><span style="font-size:12.0pt;color:black">As one of the people working on the new Mach-O backend, my main concerns are:<o:p></o:p></span></p>
</div>
<div>
<ol start="1" type="1">
<li class="MsoNormal" style="color:black;mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l0 level1 lfo1;background:white">
<span style="font-size:12.0pt">The Mach-O backend is still very much in flux, and will likely remain so until the end of this year. Whoever undertakes this library-fication should sync up with us to avoid e.g. awkward merge conflicts.<o:p></o:p></span></li><li class="MsoNormal" style="color:black;mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l0 level1 lfo1;background:white">
<span style="font-size:12.0pt">Performance is very important to us, and this library-fication should not regress it. Right now, we don't have a good benchmarking service set up (the LNT server only benchmarks LLD-ELF); we've been mostly just profiling things
locally. I can send these benchmarking instructions to whomever takes on this effort.<o:p></o:p></span></li></ol>
<div>
<p class="MsoNormal" style="background:white"><span style="font-size:12.0pt;color:black">Jez<o:p></o:p></span></p>
</div>
</div>
</div>
</body>
</html>