[LLVMdev] RFC: PerfGuide for frontend authors

[Philip Reames]

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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