On Tue, Jan 12, 2021 at 9:37 AM 'Nick Desaulniers' via Clang Built
Linux [off-list ref] wrote:

On Mon, Jan 11, 2021 at 9:14 PM Bill Wendling [off-list ref] wrote:

quoted

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 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.

quoted

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>

quoted

+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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