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Date: Tue, 12 Jan 2021 09:37:01 -0800 From: Nick Desaulniers <ndesaulniers@...gle.com> To: Bill Wendling <morbo@...gle.com> Cc: 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 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. <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
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