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
