[LLVMdev] RFC: PerfGuide for frontend authors

Mon Mar 2 11:15:25 PST 2015

On 02/27/2015 05:06 PM, David Jones wrote:
> I will second point #1 above. It bit me.
>
> May I suggest that for each of these, something is said (ideally an 
> example) of how to do these things using the API? It's pretty 
> straightforward when writing IR assembly, but not so obvious when 
> you're building up the IR using the various calls to methods defined 
> in include/llvm/IR.
As always, patches welcome.  :)

Though, honestly, I think this is somewhat beyond the scope of the 
document.  I'm expecting the audience for this to be people who already 
have familiar with the various APIs.  This definitely isn't geared 
towards folks just starting out.
>
> It might also be worthwhile to add information on how to actually 
> invoke the optimizer and code generator programmatically. My code was 
> based on the sources for llc in LLVM 3.2, but things have changed 
> since then. If something as innocuous as forgetting to tweak something 
> in a PassManager will result in correct, but suboptimal code 
> generation then that's worth knowing.  (Point #1 is a good example. I 
> had provided a DataLayout to the TargetMachine at code generation 
> time, but I did not add it to the Module. Everything ran fine, nothing 
> asserted, but certain obvious optimizations just did not happen.)
While I agree that information on how to invoke the optimizer 
programmatic belongs somewhere, I'm not sure it needs to be documented, 
particularly here.  This is well covered by the tutorial.  Is there 
something specific you think is missing from there?

FYI, I will actively reject patches for things that only effect previous 
versions.  As we move forward, we will branch this doc as part of each 
release, but that's going to be it.  Trying to document every missed 
optimization that ever existed in LLVM is a inherently futile effort I 
have no interest in getting into.  This is already documented in the 
commit logs and (possibly) release notes.
>
>
> On Fri, Feb 27, 2015 at 6:58 PM, Hal Finkel <hfinkel at anl.gov 
> <mailto:hfinkel at anl.gov>> wrote:
>
>     ----- Original Message -----
>     > From: "Philip Reames" <listmail at philipreames.com
>     <mailto:listmail at philipreames.com>>
>     > To: "LLVM Developers Mailing List" <llvmdev at cs.uiuc.edu
>     <mailto:llvmdev at cs.uiuc.edu>>
>     > Sent: Friday, February 27, 2015 5:34:36 PM
>     > Subject: Re: [LLVMdev] RFC: PerfGuide for frontend authors
>     >
>     > 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.
>
>     First, thanks for working on this! Some things (perhaps) worth
>     mentioning:
>
>      1. Make sure that a DataLayout is provided (this will likely
>     become required in the near future, but is certainly important for
>     optimization).
>
>      2. Add nsw/nuw/fast-math flags as appropriate
>
>      3. Add noalias/align/dereferenceable/nonnull to function
>     arguments and return values as appropriate
>
>      4. Mark functions as readnone/readonly/nounwind when known
>     (especially for external functions)
>
>      5. Use ptrtoint/inttoptr sparingly (they interfere with pointer
>     aliasing analysis), prefer GEPs
>
>      6. Use the lifetime.start/lifetime.end and
>     invariant.start/invariant.end intrinsics where possible
>
>      7. Use pointer aliasing metadata, especially tbaa metadata, to
>     communicate otherwise-non-deducible pointer aliasing facts
>
>      8. Use the "most-private" possible linkage types for the
>     functions being defined (private, internal or linkonce_odr preferably)
>
>      -Hal
>
>     >
>     > 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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>     >
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>     >
>
>     --
>     Hal Finkel
>     Assistant Computational Scientist
>     Leadership Computing Facility
>     Argonne National Laboratory
>     _______________________________________________
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>

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