[LLVMdev] RFC: PerfGuide for frontend authors

[Sean Silva]

SGTM.

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.

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!

-- Sean Silva

On Mon, Feb 23, 2015 at 4:46 PM, Philip Reames <listmail at philipreames.com>
wrote:

> 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.
>
> Some ideas on topics that might be worthwhile:
> - Prefer sext over zext when value is known to be positive in the language
> (e.g. range checked index on a GEP)
> - Avoid loading and storing first class aggregates (i.e. they're not well
> supported in the optimizer)
> - Mark invariant locations - i.e. link to !invariant.load and TBAA
> constant flags
> - Use globals not inttoptr for runtime structures - this gives you
> dereferenceability information
> - Use function attributes where possible (nonnull, deref, etc..)
> - Be ware of ordered and atomic memory operations (not well optimized),
> depending on source language, might be faster to use fences.
> - Range checks - make sure you test with the IRCE pass
>
> 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.  :)
>
> Philip
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