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I'm moving forward with this now given no one has raised objections.<br>
<br>
Based on Sean's comments, the naming I'm going to use is:
FrontendInfo/PerfTips<br>
<br>
I plan on committing an initial version based on what was discussed
here without further review. I'm going to keep the first version
short, and then open it for others to contribute.<br>
<br>
Philip<br>
<br>
<div class="moz-cite-prefix">On 02/24/2015 02:13 PM, Sean Silva
wrote:<br>
</div>
<blockquote
cite="mid:CAHnXoanNNq4sNthTr2-8CoB8Py1yZS7j0AGaAG0dBWwY09vHQg@mail.gmail.com"
type="cite">
<div dir="ltr">SGTM.
<div><br>
</div>
<div>I like your idea of starting "perf tips" as sort of
isolated guidelines for better IRGen. Should allow some nice
incremental growth of the documentation. I expect some stuff
will need a bit more discussion, so I would like this document
to be in a directory docs/FrontendInfo/ or something like that
(bikeshed) so that we can easily split out into new docs as
needed for some breathing room, or to give some structure for
readers to follow.</div>
<div><br>
</div>
<div>Our optimizer and backends are our lifeblood, but frontends
are our reason for existence. This kind of frontend-oriented
documentation has been needed for a long time. Thanks for
kicking it off!<br>
</div>
<div>
<div>
<div><br>
</div>
<div>-- Sean Silva</div>
</div>
</div>
</div>
<div class="gmail_extra"><br>
<div class="gmail_quote">On Mon, Feb 23, 2015 at 4:46 PM, Philip
Reames <span dir="ltr"><<a moz-do-not-send="true"
href="mailto:listmail@philipreames.com" target="_blank">listmail@philipreames.com</a>></span>
wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">I'd like
to propose that we create a new Performance Guide document.
The target of this document will be frontend authors, not
necessarily LLVM contributors. The content will be a
collection of items a frontend author might want to know
about how to generate LLVM IR which will optimize well.<br>
<br>
Some ideas on topics that might be worthwhile:<br>
- Prefer sext over zext when value is known to be positive
in the language (e.g. range checked index on a GEP)<br>
- Avoid loading and storing first class aggregates (i.e.
they're not well supported in the optimizer)<br>
- Mark invariant locations - i.e. link to !invariant.load
and TBAA constant flags<br>
- Use globals not inttoptr for runtime structures - this
gives you dereferenceability information<br>
- Use function attributes where possible (nonnull, deref,
etc..)<br>
- Be ware of ordered and atomic memory operations (not well
optimized), depending on source language, might be faster to
use fences.<br>
- Range checks - make sure you test with the IRCE pass<br>
<br>
If folks are happy with the idea of having such a document,
I volunteer to create version 0.1 with one or two items.
After that, we can add to it as folks encounter ideas. The
initial content will be fairly minimal, I just want a link I
can send to folks in reviews to record comments made. :)<br>
<br>
Philip<br>
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</blockquote>
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