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    SubjectRe: [PATCH] x86: Turn off GCC branch probability heuristics

    * Linus Torvalds <torvalds@linux-foundation.org> wrote:

    > On Sat, Apr 11, 2015 at 11:57 AM, Thomas Gleixner <tglx@linutronix.de> wrote:
    > >
    > > I thinks its just the no-guess one:
    > >
    > > text data dec patch reduction
    > > 7563475 1781048 10302987
    > > 7192973 1780024 9931461 no-guess -4.8%
    > > 7354819 1781048 958464 align-1 -2.7%
    > > 7192973 1780024 9931461 no-guess + align-1 -4.8%
    >
    > Yeah, a 5% code expansion is a big deal. Sadly, it looks like
    > 'no-guess' also disables our explicit likely/unlikely handling.

    So I spent some time trying to get as much code size reduction as
    possible via GCC optimization options, and the total savings possible
    are 10.1%:

    text data bss dec filename
    12566391 1617840 1089536 15273767 vmlinux.vanilla
    11416805 1617840 1089536 14124181 vmlinux.combo
    10532552 1596080 1089536 13218168 vmlinux.Os

    (combo patch attached below.)

    The -Os savings are 19% total - but as you mentioned before, it's
    sometimes achieved through unacceptable techniques.

    Unfortunately I found no other GCC options to achieve what -Os does -
    the missing 9% can purely be achieved via -Os, with no cherry-picking
    possible.

    The other, smaller savings are:

    + # Reduces vmlinux size by 0.25%:
    + KBUILD_CFLAGS += -fno-caller-saves
    +
    + # Reduces vmlinux size by 1.10%:
    + KBUILD_CFLAGS += -fno-inline-small-functions
    +
    + # Reduces vmlinux size by about 0.95%:
    + KBUILD_CFLAGS += -fno-tree-ch

    (each of them has to be double checked to make sure it leads to
    nothing silly and unacceptable - I just blindly tried to find GCC
    options that impacted kernel code size.)

    Thanks,

    Ingo


    ---
    arch/x86/Makefile | 24 ++++++++++++++++++++++++
    1 file changed, 24 insertions(+)

    diff --git a/arch/x86/Makefile b/arch/x86/Makefile
    index 5ba2d9ce82dc..999e94685d12 100644
    --- a/arch/x86/Makefile
    +++ b/arch/x86/Makefile
    @@ -77,10 +77,34 @@ else
    KBUILD_AFLAGS += -m64
    KBUILD_CFLAGS += -m64

    + # Pack jump targets tightly, don't align them to the default 16 bytes:
    + KBUILD_CFLAGS += -falign-jumps=1
    +
    + # Pack functions tightly as well:
    + KBUILD_CFLAGS += -falign-functions=1
    +
    + # Pack loops tightly as well:
    + KBUILD_CFLAGS += -falign-loops=1
    +
    # Don't autogenerate traditional x87 instructions
    KBUILD_CFLAGS += $(call cc-option,-mno-80387)
    KBUILD_CFLAGS += $(call cc-option,-mno-fp-ret-in-387)

    + #
    + # Don't guess branch probabilities, follow the code and unlikely()/likely() hints,
    + # which reduces vmlinux size by about 5.4%:
    + #
    + KBUILD_CFLAGS += -fno-guess-branch-probability
    +
    + # Reduces vmlinux size by 0.25%:
    + KBUILD_CFLAGS += -fno-caller-saves
    +
    + # Reduces vmlinux size by 1.10%:
    + KBUILD_CFLAGS += -fno-inline-small-functions
    +
    + # Reduces vmlinux size by about 0.95%:
    + KBUILD_CFLAGS += -fno-tree-ch
    +
    # Use -mpreferred-stack-boundary=3 if supported.
    KBUILD_CFLAGS += $(call cc-option,-mpreferred-stack-boundary=3)




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