lkml.org 
[lkml]   [2007]   [Nov]   [24]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [RFC] Documentation about unaligned memory access
    On Sat, 24 Nov 2007 15:50:52 +0000
    Luciano Rocha <strange@nsk.no-ip.org> wrote:

    >
    > Dumb memcpy (while (len--) { *d++ = *s++ }) will have alignment problems
    > in any case. Intelligent ones, like the one provided in glibc, first copy
    > bytes till output is aligned (C file) *or* size is a multiple (i686 asm file)
    > of word size, and then it copies word-by-word.
    >
    > Linux's x86_64 memcpy does the opposite, copies 64bit words, and then
    > copies the last bytes.
    >
    > So, in effect, as long as no packed structures are used, memcpy should
    > be safer on *int, etc., than *char, as the compiler ensures
    > word-alignment.
    >

    It most certainly does not. gcc will assume that an int* has int alignment. memcpy() is a builtin, which gcc can translate to pretty much anything. And C specifies that a pointer to foo, will point to a real object of type foo, so gcc can't be blamed for the unsafe typecasts. I have tested this the hard way, so this is not just speculation.

    E.g., we have the following struct:

    struct foo
    {
    u8 a[4];
    u32 b;
    };

    This struct will have a size of 8 bytes and an alignment of 4 bytes (caused by the member b). Now take the following code:

    void copy_foo(struct foo *dst, struct foo *src)
    {
    *dst = *src;
    }

    On a platform that supports 64-bit loads and stores (e.g. AVR32, where I got hit by this), this will generate:

    LD r1, (src)
    ST r1, (dst)

    Now if I replace that with:

    void copy_foo(struct foo *dst, struct foo *src)
    {
    memcpy(dst, src, sizeof(struct foo));
    }

    then it will generate the same code. So I cannot use copy_foo() to transfer a struct foo either out of, or into a packet buffer.

    In other words, memcpy() does _not_ save you from alignment issues. If you cast from char* or void* to something else, you better be damn sure the alignment is correct because gcc will assume it is.

    Rgds
    --
    -- Pierre Ossman

    Linux kernel, MMC maintainer http://www.kernel.org
    PulseAudio, core developer http://pulseaudio.org
    rdesktop, core developer http://www.rdesktop.org
    -
    To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
    the body of a message to majordomo@vger.kernel.org
    More majordomo info at http://vger.kernel.org/majordomo-info.html
    Please read the FAQ at http://www.tux.org/lkml/

    \
     
     \ /
      Last update: 2007-11-24 17:21    [W:2.448 / U:0.048 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site