[LLVMdev] RFC: PerfGuide for frontend authors

[Rodney M. Bates]

This would be great for me.  I have had many questions about how optimizable different
llvm constructs are.

On 02/23/2015 06:46 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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-- 
Rodney Bates
rodney.m.bates at acm.org