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

Fangrui Song
2021-01-11T20:32:06Z

On 2021-01-11, Bill Wendling wrote:
>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?

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.