[LLVMdev] RFC: PerfGuide for frontend authors (original) (raw)

Rodney M. Bates rodney_bates at lcwb.coop
Mon Feb 23 18:15:17 PST 2015

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 > _________________________ > LLVM Developers mailing list > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev >