[llvm-dev] [RFC] Profile guided section layout

[Michael Spencer via llvm-dev]

On Mon, Jul 31, 2017 at 5:26 PM, Rafael Espíndola <
rafael.espindola at gmail.com> wrote:

> I updated the patch to read a call graph from a text file.
>
> I tested it with the attached call.txt from lld linking chromium.
> Unfortunately the resulting lld doesn't seem any faster. One thing I
> noticed is that the most used symbols seem to be at the end of the
> file.
>
> In any case, can you add tests and send the lld patch for review?
>
> Thanks,
> Rafael
>
>
Patch up on phab: https://reviews.llvm.org/D36351


- Michael Spencer


>
>
> On 31 July 2017 at 15:19, Davide Italiano <davide at freebsd.org> wrote:
> > I'm definitely interested in seeing this moving forward, (phabricator
> > will make the review easy), quick question, why the option is phrased
> > as negative? (i.e. --disable instead of --enable?)
> >
> > Thanks,
> >
> > --
> > Davide
> >
> > On Mon, Jul 31, 2017 at 3:12 PM, Rafael Espíndola via llvm-dev
> > <llvm-dev at lists.llvm.org> wrote:
> >> A rebased version of the lld patch is attached.
> >>
> >> Cheers,
> >> Rafael
> >>
> >>
> >> On 31 July 2017 at 15:11, Rafael Avila de Espindola
> >> <rafael.espindola at gmail.com> wrote:
> >>> Tobias Edler von Koch <tobias at codeaurora.org> writes:
> >>>
> >>>> Hi Rafael,
> >>>>
> >>>> On 07/31/2017 04:20 PM, Rafael Avila de Espindola via llvm-dev wrote:
> >>>>> However, do we need to start with instrumentation? The original paper
> >>>>> uses sampling with good results and current intel cpus can record
> every
> >>>>> branch in a program.
> >>>>>
> >>>>> I would propose starting with just an lld patch that reads the call
> >>>>> graph from a file. The format would be very similar to what you
> propose,
> >>>>> just weight,caller,callee.
> >>>>
> >>>> The advantage of the proposed approach (weighted callgraph section) is
> >>>> that it's completely transparent: it works regardless of the
> particular
> >>>> profiling methodology (as long as there's !perf metadata when the pass
> >>>> runs). For this reason, it fits neatly into an *existing* PGO-based
> >>>> build flow. I only need to add 1 compiler flag to enable it. That's a
> >>>> big plus.
> >>>>
> >>>> On the other hand, I could see how your idea (callgraph input file for
> >>>> linker) would be useful in situations where I just want to do section
> >>>> layout but no PGO in the compiler... and of course for testing of the
> >>>> linker's sorting algorithm.
> >>>>
> >>>> So there's merits in both, but for my use cases Michael's original
> >>>> approach is the most practical.
> >>>
> >>> Yes, I must stress that I am not proposing having the option of reading
> >>> the callgraph from another file *instead* of reading it from .o
> >>> files. Just that doing it first decouples most of the lld patch form
> the
> >>> llvm changes and can be useful in cases where only samples are
> available.
> >>>
> >>> Cheers,
> >>> Rafael
> >>
> >> _______________________________________________
> >> LLVM Developers mailing list
> >> llvm-dev at lists.llvm.org
> >> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >>
> >
> >
> >
> > --
> > Davide
> >
> > "There are no solved problems; there are only problems that are more
> > or less solved" -- Henri Poincare
>
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