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    SubjectRe: [PATCH v9] pgo: add clang's Profile Guided Optimization infrastructure
    On Wed, Apr 07, 2021 at 02:17:04PM -0700, '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 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
    > 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, unlike
    > the clang support in kernel/gcov.
    >
    > [1] https://clang.llvm.org/docs/UsersManual.html#profile-guided-optimization
    >
    > Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
    > Co-developed-by: Bill Wendling <morbo@google.com>
    > Signed-off-by: Bill Wendling <morbo@google.com>
    > Tested-by: Nick Desaulniers <ndesaulniers@google.com>
    > Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
    > Reviewed-by: Fangrui Song <maskray@google.com>

    Thanks for sending this again! I'm looking forward to using it.

    Masahiro and Andrew, unless one of you would prefer to take this in your
    tree, I figure I can snag it to send to Linus.

    Anyone else have feedback?

    Thanks!

    -Kees

    --
    Kees Cook

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