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    Subject[tip:x86/build] x86-64, gcc: Use -mpreferred-stack-boundary= 3 if supported
    Commit-ID:  d9b0cde91c60da0ed5f92cdc3ac878142e6b5f27
    Gitweb: http://git.kernel.org/tip/d9b0cde91c60da0ed5f92cdc3ac878142e6b5f27
    Author: H.J. Lu <hjl.tools@gmail.com>
    AuthorDate: Tue, 29 May 2012 14:31:23 -0700
    Committer: H. Peter Anvin <hpa@zytor.com>
    CommitDate: Sat, 23 Jun 2012 19:25:22 -0700

    x86-64, gcc: Use -mpreferred-stack-boundary=3 if supported

    On x86-64, the standard ABI requires alignment to 16 bytes. However,
    this is not actually necessary in the kernel (we don't do SSE except
    in very controlled ways); and furthermore, the standard kernel entry
    on x86-64 actually leaves the stack on an odd 8-byte boundary, which
    means that gcc will generate extra instructions to keep the stack
    *mis*aligned!

    gcc 4.8 adds an -mpreferred-stack-boundary=3 option to override this
    and lets us save some stack space and a handful of instructions.

    Note that this causes us to pass -mno-sse twice; this is redundant,
    but necessary since the cc-option test will fail unless -mno-sse is
    passed on the same command line.

    [ hpa: rewrote the patch description ]

    Signed-off-by: H.J. Lu <hjl.tools@gmail.com>
    Link: http://lkml.kernel.org/r/CAMe9rOqPfy3JcZRLaUeCjBe9BVY-P6e0uaSbMi5hvS-6WwQueg@mail.gmail.com
    Signed-off-by: H. Peter Anvin <hpa@zytor.com>
    ---
    arch/x86/Makefile | 3 +++
    1 files changed, 3 insertions(+), 0 deletions(-)

    diff --git a/arch/x86/Makefile b/arch/x86/Makefile
    index 1f25214..b0c5276 100644
    --- a/arch/x86/Makefile
    +++ b/arch/x86/Makefile
    @@ -49,6 +49,9 @@ else
    KBUILD_AFLAGS += -m64
    KBUILD_CFLAGS += -m64

    + # Use -mpreferred-stack-boundary=3 if supported.
    + KBUILD_CFLAGS += $(call cc-option,-mno-sse -mpreferred-stack-boundary=3)
    +
    # FIXME - should be integrated in Makefile.cpu (Makefile_32.cpu)
    cflags-$(CONFIG_MK8) += $(call cc-option,-march=k8)
    cflags-$(CONFIG_MPSC) += $(call cc-option,-march=nocona)

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