Re: [PATCH v9] pgo: add clang's Profile Guided Optimization infrastructure

Peter Zijlstra
2021-06-14T09:01:55Z

On Sat, Jun 12, 2021 at 01:56:41PM -0700, Bill Wendling wrote:
> On Sat, Jun 12, 2021 at 1:25 PM Peter Zijlstra <peterz@infradead.org> wrote:
> > On Sat, Jun 12, 2021 at 12:10:03PM -0700, Bill Wendling wrote:
> > Yes it is, but is that sufficient in this case? It very much isn't for
> > KASAN, UBSAN, and a whole host of other instrumentation crud. They all
> > needed their own 'bugger-off' attributes.
> >
> > > > We've got KCOV and GCOV support already. Coverage is also not an
> > > > argument mentioned anywhere else. Coverage can go pound sand, we really
> > > > don't need a third means of getting that.
> > > >
> > > Those aren't useful for clang-based implementations. And I like to
> > > look forward to potential improvements.
> >
> > I look forward to less things doing the same over and over. The obvious
> > solution if of course to make clang use what we have, not the other way
> > around.
> >
> That is not the obvious "solution".

Because having GCOV, KCOV and PGO all do essentially the same thing
differently, makes heaps of sense?

I understand that the compilers actually generates radically different
instrumentation for the various cases, but essentially they're all
collecting (function/branch) arcs.

I'm thinking it might be about time to build _one_ infrastructure for
that and define a kernel arc format and call it a day.

Note that if your compiler does arcs with functions (like gcc, unlike
clang) we can also trivially augment the arcs with PMU counter data. I
once did that for userspace.