Re: State of AutoFDO in GCC

[Xinliang David Li <davidxl@google.com>]

On Fri, Apr 23, 2021 at 12:00 AM Richard Biener <richard.guenther@gmail.com>
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.
>
> I think if we want to keep the feature it makes sense to provide
> create_gcov
> functionality either directly from perf (input data producer) or from gcc
> (data consumer).  Of course I have no idea about its complexity, license
> or implementation language ...
>
>
Right. What it takes is a perf data (can be text format) parser to produce
the format GCC needs, but someone in the community needs to take the lead.
It should not involve too much effort.

David

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
> > > > >
> > > >
> > >
>