[LLVMdev] RFC: PerfGuide for frontend authors

[Sean Silva]

On Fri, Feb 27, 2015 at 9:25 PM, Philip Reames <listmail at philipreames.com>
wrote:

>
> > On Feb 27, 2015, at 3:58 PM, Hal Finkel <hfinkel at anl.gov> wrote:
> >
> > ----- Original Message -----
> >> From: "Philip Reames" <listmail at philipreames.com>
> >> To: "LLVM Developers Mailing List" <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:
> I'll add these Monday, but am not going to take the time to write much.
> Any expansion you (or anyone else) want to do would be welcome
>
> >
> > 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
> I was thinking of a more general: use metadata and function attributes.
>
> I don't want to end up duplicating content from the Lang ref here.  I was
> thinking that this page should cover the things you can't learn by just
> reading langref.
>

You can use links to langref if necessary. The audience of this document is
people looking for how to generate code that will optimize well. We should
bring to their attention any relevant features.

LangRef has a very low density for people looking for how to generate code
that will optimize well, and I consider it unreasonable to expect them to
trawl through it just to pick out a couple small morsels that you are
already aware of and can direct them to.

Regardless, I think there is more to say about these attributes/metadata
than LangRef provides. E.g., LangRef doesn't provide a clear picture of
what transformations these things enable, how profitable they can be, and
what sort of code they trigger on. For example, a frontend author may be
able to get information from their frontend to add noalias, but doing so
will require some nontrivial changes to their frontend (and/or slow it
down): is it worth it for them to take the time to change their frontend?
In order to make this decision, we need to provide them with the
information they need to estimate the benefit noalias will bring them. Of
course, it's impossible to predict the exact benefit, but we should provide
enough information to allow the frontend author to at least prioritize the
order in which they investigate using each of these features.

-- Sean Silva


>
> >
> > 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
> Do you find these help in practice?  The few experiments I ran were
> neutral at best and harmful in one or two cases.  Do you have suggestions
> on how and when to use them?
>
> I am using invariant.load, tbaas is constant flag, and a custom hook for
> zero initialized memory from my allocation routines.
> >
> > 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
> >>> _______________________________________________
> >>> LLVM Developers mailing list
> >>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> >>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> >>
> >> _______________________________________________
> >> LLVM Developers mailing list
> >> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> >> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> >
> > --
> > Hal Finkel
> > Assistant Computational Scientist
> > Leadership Computing Facility
> > Argonne National Laboratory
>
> _______________________________________________
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