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Date: Tue, 12 Jan 2021 09:45:17 -0800 From: Fāng-ruì Sòng <maskray@...gle.com> To: Nick Desaulniers <ndesaulniers@...gle.com> Cc: Bill Wendling <morbo@...gle.com>, Jonathan Corbet <corbet@....net>, Masahiro Yamada <masahiroy@...nel.org>, Linux Doc Mailing List <linux-doc@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>, Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>, clang-built-linux <clang-built-linux@...glegroups.com>, Andrew Morton <akpm@...ux-foundation.org>, Nathan Chancellor <natechancellor@...il.com>, Sami Tolvanen <samitolvanen@...gle.com>, Alistair Delva <adelva@...gle.com> Subject: Re: [PATCH v2] pgo: add clang's Profile Guided Optimization infrastructure On Tue, Jan 12, 2021 at 9:37 AM 'Nick Desaulniers' via Clang Built Linux <clang-built-linux@...glegroups.com> wrote: > > On Mon, Jan 11, 2021 at 9:14 PM Bill Wendling <morbo@...gle.com> wrote: > > > > From: Sami Tolvanen <samitolvanen@...gle.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 now be used by the compiler: > > > > $ make LLVM=1 KCFLAGS=-fprofile-use=vmlinux.profdata ... > > > > This initial submission is restricted to x86, as that's the platform we > > Please drop all changes to arch/* that are not to arch/x86/ then; we > can cross that bridge when we get to each arch. For example, there's > no point disabling PGO for architectures LLVM doesn't even have a > backend for. > > > know works. This restriction can be lifted once other platforms have > > been verified to work with PGO. > > > > Note that this method of profiling the kernel is clang-native and isn't > > compatible with clang's gcov support in kernel/gcov. > > Then the Kconfig option should depend on !GCOV so that they are > mutually exclusive and can't be selected together accidentally; such > as by bots doing randconfig tests. The profile formats (Clang PGO, Clang gcov, GCC gcov/PGO) are different but Clang PGO can be used with Clang's gcov implementation: clang -fprofile-generate --coverage a.cc; ./a.out => default*.profraw + a.gcda > <large snip> > > > +static inline int inst_prof_popcount(unsigned long long value) > > +{ > > + value = value - ((value >> 1) & 0x5555555555555555ULL); > > + value = (value & 0x3333333333333333ULL) + > > + ((value >> 2) & 0x3333333333333333ULL); > > + value = (value + (value >> 4)) & 0x0F0F0F0F0F0F0F0FULL; > > + > > + return (int)((unsigned long long)(value * 0x0101010101010101ULL) >> 56); > > +} > > The kernel has a portable popcnt implementation called hweight64 if > you #include <asm-generic/bitops/hweight.h>; does that work here? > https://en.wikipedia.org/wiki/Hamming_weight > -- > Thanks, > ~Nick Desaulniers > > -- > You received this message because you are subscribed to the Google Groups "Clang Built Linux" group. > To unsubscribe from this group and stop receiving emails from it, send an email to clang-built-linux+unsubscribe@...glegroups.com. > To view this discussion on the web visit https://groups.google.com/d/msgid/clang-built-linux/CAKwvOdk%2BNqhzC_4wFbQMJmLMQWoDSjQiRJyCGe5dsWkqK_NJJQ%40mail.gmail.com.
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