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    SubjectRe: [PATCH] x86: Use __builtin_memset and __builtin_memcpy for memset/memcpy
    On Mon, 28 Sep 2009 11:34:33 +0200

    turns out doing this unconditional is not a good idea due to gcc 3.x;
    the updated patch below only does the change for gcc 4.x

    From e86cf2618a2e489cafcafb4361569c4d0853156e Mon Sep 17 00:00:00 2001
    From: Arjan van de Ven <>
    Date: Mon, 28 Sep 2009 11:21:32 +0200
    Subject: [PATCH] x86: Use __builtin_memset and __builtin_memcpy for memset/memcpy

    GCC provides reasonable memset/memcpy functions itself, with __builtin_memset
    and __builtin_memcpy. For the "unknown" cases, it'll fall back to our
    current existing functions, but for fixed size versions it'll inline
    something smart. Quite often that will be the same as we have now,
    but sometimes it can do something smarter (for example, if the code
    then sets the first member of a struct, it can do a shorter memset).

    In addition, and this is more important, gcc knows which registers and
    such are not clobbered (while for our asm version it pretty much
    acts like a compiler barrier), so for various cases it can avoid reloading

    The effect on codesize is shown below on my typical laptop .config:

    text data bss dec hex filename
    5605675 2041100 6525148 14171923 d83f13 vmlinux.before
    5595849 2041668 6525148 14162665 d81ae9 vmlinux.after

    Due to some not-so-good behavior in the gcc 3.x series, this change
    is only done for GCC 4.x and above.

    Signed-off-by: Arjan van de Ven <>
    arch/x86/include/asm/string_32.h | 9 +++++++++
    1 files changed, 9 insertions(+), 0 deletions(-)

    diff --git a/arch/x86/include/asm/string_32.h b/arch/x86/include/asm/string_32.h
    index ae907e6..3d3e835 100644
    --- a/arch/x86/include/asm/string_32.h
    +++ b/arch/x86/include/asm/string_32.h
    @@ -177,10 +177,15 @@ static inline void *__memcpy3d(void *to, const void *from, size_t len)

    +#if (__GNUC__ >= 4)
    +#define memcpy(t, f, n) __builtin_memcpy(t, f, n)
    #define memcpy(t, f, n) \
    (__builtin_constant_p((n)) \
    ? __constant_memcpy((t), (f), (n)) \
    : __memcpy((t), (f), (n)))
    * kmemcheck becomes very happy if we use the REP instructions unconditionally,
    @@ -316,11 +321,15 @@ void *__constant_c_and_count_memset(void *s, unsigned long pattern,
    : __memset_generic((s), (c), (count)))

    #define __HAVE_ARCH_MEMSET
    +#if (__GNUC__ >= 4)
    +#define memset(s, c, count) __builtin_memset(s, c, count)
    #define memset(s, c, count) \
    (__builtin_constant_p(c) \
    ? __constant_c_x_memset((s), (0x01010101UL * (unsigned char)(c)), \
    (count)) \
    : __memset((s), (c), (count)))

    * find the first occurrence of byte 'c', or 1 past the area if none

    Arjan van de Ven Intel Open Source Technology Centre
    For development, discussion and tips for power savings,

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      Last update: 2009-09-28 14:23    [W:0.022 / U:57.156 seconds]
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