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

Bill Wendling
2021-06-14T09:40:55Z

On Mon, Jun 14, 2021 at 2:01 AM Peter Zijlstra <peterz@infradead.org> wrote:
> 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?
>
It does when you're dealing with one toolchain without access to another.

> I understand that the compilers actually generates radically different
> instrumentation for the various cases, but essentially they're all
> collecting (function/branch) arcs.
>
That's true, but there's no one format for profiling data that's
usable between all compilers. I'm not even sure there's a good way to
translate between, say, gcov and llvm's format. To make matters more
complicated, each compiler's format is tightly coupled to a specific
version of that compiler. And depending on *how* the data is collected
(e.g. sampling or instrumentation), it may not give us the full
benefit of FDO/PGO.

> 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.
>
That may be nice, but it's a rather large request.

> 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.