Re: State of AutoFDO in GCC

[Richard Biener <richard.guenther@gmail.com>]

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 ...

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
> > > >
> > >
> >