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From:   Bill Wendling <morbo@google.com>
Date:   Mon, 11 Jan 2021 12:23:08 -0800
Message-ID: <CAGG=3QWo5_wwTMHtif4BzFssByaW1ScvpaEH1p1nZ6ymVggLjA@mail.gmail.com>
Subject: Re: [PATCH] pgo: add clang's Profile Guided Optimization infrastructure
To:     Fangrui Song <maskray@google.com>
Cc:     Jonathan Corbet <corbet@lwn.net>,
        Masahiro Yamada <masahiroy@kernel.org>,
        linux-doc@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>,
        Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Nathan Chancellor <natechancellor@gmail.com>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Sami Tolvanen <samitolvanen@google.com>
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On Mon, Jan 11, 2021 at 12:12 PM Fangrui Song <maskray@google.com> wrote:
>
> On 2021-01-11, 'Bill Wendling' via Clang Built Linux wrote:
> >From: Sami Tolvanen <samitolvanen@google.com>
> >
> >Enable the use of clang's Profile-Guided Optimization[1]. To generate a
> >profile, the kernel is instrumented with PGO counters, a representative
> >workload is run, and the raw profile data is collected from
> >/sys/kernel/debug/pgo/profraw.
> >
> >The raw profile data must be processed by clang's "llvm-profdata" tool before
> >it can be used during recompilation:
> >
> >  $ cp /sys/kernel/debug/pgo/profraw vmlinux.profraw
> >  $ llvm-profdata merge --output=vmlinux.profdata vmlinux.profraw
> >
> >Multiple raw profiles may be merged during this step.
> >
> >The data can be used either by the compiler if LTO isn't enabled:
> >
> >    ... -fprofile-use=vmlinux.profdata ...
> >
> >or by LLD if LTO is enabled:
> >
> >    ... -lto-cs-profile-file=vmlinux.profdata ...
>
> This LLD option does not exist.
> LLD does have some `--lto-*` options but the `-lto-*` form is not supported
> (it clashes with -l) https://reviews.llvm.org/D79371
>
That's strange. I've been using that option for years now. :-) Is this
a recent change?

> (There is an earlier -fprofile-instr-generate which does
> instrumentation in Clang, but the option does not have broad usage.
> It is used more for code coverage, not for optimization.
> Noticeably, it does not even implement the Kirchhoff's current law
> optimization)
>
Right. I've been told outside of this email that -fprofile-generate is
the prefered flag to use.

> -fprofile-use= is used by both regular PGO and context-sensitive PGO (CSPGO).
>
> clang -flto=thin -fprofile-use= passes -plugin-opt=cs-profile-path= to the linker.
> For regular PGO, this option is effectively a no-op (confirmed with CSPGO main developer).
>
> So I think the "or by LLD if LTO is enabled:" part should be removed.

But what if you specify the linking step explicitly? Linux doesn't
call "clang" when linking, but "ld.lld".

-bw