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

[Fangrui Song]

On 2021-01-11, Bill Wendling wrote:
> On Mon, Jan 11, 2021 at 12:12 PM Fangrui Song <maskray@xxxxxxxxxx> wrote:
> > On 2021-01-11, 'Bill Wendling' via Clang Built Linux wrote:
> > >From: Sami Tolvanen <samitolvanen@xxxxxxxxxx>
> > >
> > >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?

The more frequently used options (specifyed by the clang driver) are
-plugin-opt=... (options implemented by LLVMgold.so).
`-lto-*` is rare.

> > (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".

Regular PGO+LTO does not need -plugin-opt=cs-profile-path=
CSPGO+LTO needs it.
Because -fprofile-use= may be used by both, Clang driver adds it.
CSPGO is relevant in this this patch, so the linker option does not need to be mentioned.