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

[Bill Wendling]

On Mon, Jan 11, 2021 at 12:31 PM Fangrui Song <maskray@xxxxxxxxxx> wrote:
> 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.

I'm still a bit confused. Are you saying that when clang uses
`-flto=thin -fprofile-use=foo` that the profile file "foo" is embedded
into the bitcode file so that when the linker's run it'll be used?

This is the workflow:

clang ... -fprofile-use=vmlinux.profdata ... -c -o foo.o foo.c
clang ... -fprofile-use=vmlinux.profdata ... -c -o bar.o bar.c
ld.lld ... <output file> foo.o bar.o

Are you saying that we don't need to have
"-plugin-opt=cs-profile-path=vmlinux.profdata" on the "ld.lld ..."
line?

-bw