[LLVMdev] RFC: PerfGuide for frontend authors

[Hans Wennborg]

This is great. Can you add it to the release notes?

On Fri, Feb 27, 2015 at 3:34 PM, Philip Reames
<listmail at philipreames.com> wrote:
> The first version of this document is now live:
> http://llvm.org/docs/Frontend/PerformanceTips.html
>
> Please feel free to add to it directly.  Alternatively, feel free to reply
> to this thread with text describing an issue that should be documented.
> I'll make sure text gets turned into patches.
>
> Philip
>
>
> On 02/23/2015 04:46 PM, Philip Reames 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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