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

    * Ingo Molnar <mingo@kernel.org> wrote:

    > Btw., totally off topic, the following NOP caught my attention:
    >
    > > 5a: 66 0f 1f 44 00 00 nopw 0x0(%rax,%rax,1)
    >
    > That's a dead NOP that boats the function a bit, added for the 16 byte
    > alignment of one of the jump targets.

    Another thing caught my attention (and I'm hijacking the RCU thread
    again): GCC's notion of how to place branches seems somewhat random,
    and rather bloaty.

    So I tried the experiment below on an x86 defconfig, turning off GCC's
    branch heuristics, and it's rather surprising:

    text data bss dec filename
    12566447 1617840 1089536 15273823 vmlinux.fguess-branch-probability
    11923593 1617840 1089536 14630969 vmlinux.fno-guess-branch-probability

    That's an 5.4% code size improvement!

    So maybe we should try this, as it results in much more predictable
    (and more compact!) code by default - and allows us to shape loops and
    branches in a natural fashion: by their placement, and also via
    likely()/unlikely() hints when absolutely necessary.

    Thoughts?

    Thanks,

    Ingo


    =================>
    From: Ingo Molnar <mingo@kernel.org>
    Date: Sat, 11 Apr 2015 11:16:30 +0200
    Subject: [PATCH] x86: Turn off GCC branch probability heuristics

    Not-Signed-off-by: Ingo Molnar <mingo@kernel.org>
    ---
    arch/x86/Makefile | 3 +++
    1 file changed, 3 insertions(+)

    diff --git a/arch/x86/Makefile b/arch/x86/Makefile
    index 5ba2d9ce82dc..7c12b3f56915 100644
    --- a/arch/x86/Makefile
    +++ b/arch/x86/Makefile
    @@ -81,6 +81,9 @@ else
    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:
    + KBUILD_CFLAGS += -fno-guess-branch-probability
    +
    # Use -mpreferred-stack-boundary=3 if supported.
    KBUILD_CFLAGS += $(call cc-option,-mpreferred-stack-boundary=3)


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