State of AutoFDO in GCC

[Xinliang David Li]

On Fri, Apr 23, 2021 at 9:54 AM Jan Hubicka <hubicka@ucw.cz> wrote:

> > On Fri, Apr 23, 2021 at 12:18 AM Martin Liška <mliska@suse.cz> wrote:
> >
> > > On 4/23/21 9:00 AM, Richard Biener via Gcc wrote:
> > > > On Fri, Apr 23, 2021 at 7:28 AM Xinliang David Li via Gcc
> > > > <gcc@gcc.gnu.org> wrote:
> > > >>
> > > >> Hi, the create_gcov tool was probably removed with the assumption
> that
> > > it
> > > >> was only used with Google GCC branch, but it is actually used with
> GCC
> > > >> trunk as well.
> > > >>
> > > >> Given that, the tool will be restored in the github repo. It seems
> to
> > > build
> > > >> and work fine with the regression test.
> > > >>
> > > >> The tool may ust work as it is right now, but there is no guarantee
> it
> > > >> won't break in the future unless someone in the GCC community tries
> to
> > > >> maintain it.
> > >
> > > Hi.
> > >
> > > The current situation is that AutoFDO doesn't work with pretty simple
> > > test-cases
> > > we have in testsuite:
> > >
> > > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71672
> > > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81379
> > >
> > > These are ~5 years old and nothing has happened.
>
> Basic functionality is to identify hot parts of programs and give basic
> idea of branch probabilities.  Bugs above are about other optimizations
> (indirect inlining, hot-cold partitioning and peeling). First one ought
> to be working but still indirect call profiling accounts for relatively
> minor portion of FDO benefits, so auto-FDO should be usefable even w/o
> these working if tooling was fixed.  Expecting hot-cold partitioning and
> peeling to work reliably for testcases designed for normalFDO is IMO bit
> unrealistic.
>
> Perf is not limited to x86, so we should get basic usability for all
> major architectures.  David, what CPUs are supported with auto-FDO on
> LLVM side?
> >
> > This is a major feature in Clang. Deprecating it in GCC will be
> unfortunate.
>
> I also like overall idea of auto-FDO just never had much time to get it
> into shape.  Every stage1 I am trying to fix something that was broken
> for a while (like ICF last stage1) and fix it, so option would be to
> focus on autoFDO this stage1 even though I have quite few other things
> in TODO list.  I never looked into the tools translating perf data to
> gcc compatible format and how easy would be to keep this alive.
>
> David, how does perf->LLVM path work these days?
>

It uses create_llvm_prof tool which is in the same git repo. The data
parsing part is shared with create_gcov, but the writer is obviously
different for the two tools.

David


> Honza
> >
> > David
> >
> > >
> > > Thoughts?
> > > Martin
> > >
> > > >
> > > > Having the tool third-party makes keeping the whole chain working
> more
> > > > difficult.
> > > >
> > > > Richard.
> > > >
> > > >> Thanks,
> > > >>
> > > >> David
> > > >>
> > > >> On Thu, Apr 22, 2021 at 3:29 PM Jan Hubicka <hubicka@ucw.cz> wrote:
> > > >>
> > > >>>> On 4/22/21 9:58 PM, Eugene Rozenfeld via Gcc wrote:
> > > >>>>> GCC documentation for AutoFDO points to create_gcov tool that
> > > converts
> > > >>> perf.data file into gcov format that can be consumed by gcc with
> > > >>> -fauto-profile (
> > > https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html,
> > > >>> https://gcc.gnu.org/wiki/AutoFDO/Tutorial).
> > > >>>>>
> > > >>>>> I noticed that the source code for create_gcov has been deleted
> from
> > > >>> https://github.com/google/autofdo on April 7. I asked about that
> > > change
> > > >>> in that repo and got the following reply:
> > > >>>>>
> > > >>>>>
> https://github.com/google/autofdo/pull/107#issuecomment-819108738
> > > >>>>>
> > > >>>>> "Actually we didn't use create_gcov and havn't updated
> create_gcov
> > > for
> > > >>> years, and we also didn't have enough tests to guarantee it works
> (It
> > > was
> > > >>> gcc-4.8 when we used and verified create_gcov). If you need it, it
> is
> > > >>> welcomed to update create_gcov and add it to the respository."
> > > >>>>>
> > > >>>>> Does this mean that AutoFDO is currently dead in gcc?
> > > >>>>
> > > >>>> Hello.
> > > >>>>
> > > >>>> Yes. I know that even basic test cases have been broken for years
> in
> > > the
> > > >>> GCC.
> > > >>>> It's new to me that create_gcov was removed.
> > > >>>>
> > > >>>> I tend to send patch to GCC that will remove AutoFDO from GCC.
> > > >>>> I known Bin spent some time working on AutoFDO, has he came up to
> > > >>> something?
> > > >>>
> > > >>> The GCC side of auto-FDO is not that hard.  We have most of
> > > >>> infrastructure in place, but stopping point for me was always
> > > difficulty
> > > >>> to get gcov-tool working.  If some maintainer steps up, I think I
> can
> > > >>> fix GCC side.
> > > >>>
> > > >>> I am bit unsure how important feature it is - we have FDO that
> works
> > > >>> quite well for most users but I know there are some users of the
> LLVM
> > > >>> implementation and there is potential to tie this with other
> hardware
> > > >>> events to asist i.e. if conversion (where one wants to know how
> well
> > > CPU
> > > >>> predicts the jump rather than just the jump probability) which I
> always
> > > >>> found potentially interesting.
> > > >>>
> > > >>> Honza
> > > >>>>
> > > >>>> Martin
> > > >>>>
> > > >>>>>
> > > >>>>> Thanks,
> > > >>>>>
> > > >>>>> Eugene
> > > >>>>>
> > > >>>>
> > > >>>
> > >
> > >
>