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    Subject[RFC patch 10/41] lttng-optimize-write-to-page-function-remove-some-memcpy-calls
    Zhaolei :
    > Hello, Mathieu
    >
    > Why not use instructions generated by gcc instead of memcpy on arch without
    > 64bit write as:
    > case 4: *(u32 *)dest = *(const u32 *)src;
    > break;
    > case 8: *(u64 *)dest = *(const u64 *)src;
    > break;
    >
    > IMHO, even on arch without 64bit write, memcpy is more complex.

    #include <inttypes.h>

    char dest[100];
    char src[100];

    typedef uint64_t u64;
    typedef uint32_t u32;

    void gcc_u64(void)
    {
    asm("/* begin */");
    *(u64 *)dest = *(const u64 *)src;
    asm("/* end */");
    }


    movl src, %eax
    movl src+4, %edx
    movl %eax, dest
    movl %edx, dest+4


    void twice_u32(void)
    {
    asm("/* begin */");
    ((u32 *)dest)[0] = ((const u32 *)src)[0];
    ((u32 *)dest)[1] = ((const u32 *)src)[1];
    asm("/* end */");
    }
    movl src, %eax
    movl %eax, dest
    movl src+4, %eax
    movl %eax, dest+4

    gcc seems to do a better register scheduler than my code, so I think
    it's not so bad. I will take your proposal.

    Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
    CC: Zhaolei <zhaolei@cn.fujitsu.com>
    ---
    include/linux/ltt-relay.h | 21 ++++++++++++---------
    1 file changed, 12 insertions(+), 9 deletions(-)

    Index: linux-2.6-lttng/include/linux/ltt-relay.h
    ===================================================================
    --- linux-2.6-lttng.orig/include/linux/ltt-relay.h 2009-03-05 15:40:02.000000000 -0500
    +++ linux-2.6-lttng/include/linux/ltt-relay.h 2009-03-05 15:40:42.000000000 -0500
    @@ -215,13 +215,16 @@ static inline void ltt_relay_do_copy(voi
    case 4:
    *(u32 *)dest = *(const u32 *)src;
    break;
    -#if (BITS_PER_LONG == 64)
    case 8:
    *(u64 *)dest = *(const u64 *)src;
    break;
    -#endif
    default:
    - memcpy(dest, src, len);
    + /*
    + * What we really want here is an inline memcpy, but we don't
    + * have constants, so gcc generally uses a function call.
    + */
    + for (; len > 0; len--)
    + *(u8 *)dest++ = *(const u8 *)src++;
    }
    }
    #else
    @@ -256,19 +259,19 @@ static inline void ltt_relay_do_copy(voi
    goto memcpy_fallback;
    *(u32 *)dest = *(const u32 *)src;
    break;
    -#if (BITS_PER_LONG == 64)
    case 8:
    if (unlikely(!addr_aligned(dest, src, 8)))
    goto memcpy_fallback;
    *(u64 *)dest = *(const u64 *)src;
    break;
    -#endif
    default:
    - goto memcpy_fallback;
    + /*
    + * What we really want here is an inline memcpy, but we don't
    + * have constants, so gcc generally uses a function call.
    + */
    + for (; len > 0; len--)
    + *(u8 *)dest++ = *(const u8 *)src++;
    }
    - return;
    -memcpy_fallback:
    - memcpy(dest, src, len);
    }
    #endif

    --
    Mathieu Desnoyers
    OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68


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