gcc-4.3 considers unaligned accesses on X86 as undefined - Kernel

This is a discussion on gcc-4.3 considers unaligned accesses on X86 as undefined - Kernel ; Hello x86 architecture maintainers, GCC-4.3 now considers that it is undefined behaviour to access memory through an int* that is not aligned to sizeof(int). At -O3 it generates vectorized code that _relies_ on the fact that pointers are always aligned ...

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Thread: gcc-4.3 considers unaligned accesses on X86 as undefined

  1. gcc-4.3 considers unaligned accesses on X86 as undefined

    Hello x86 architecture maintainers,

    GCC-4.3 now considers that it is undefined behaviour to access memory
    through an int* that is not aligned to sizeof(int).
    At -O3 it generates vectorized code that _relies_ on the fact that
    pointers are always aligned (unless you use packed attributes, etc.),
    and the resulting code crashes if the pointer is unaligned. (-O3 -msse
    on 32-bit, and simply -O3 on 64-bit since -msse is default)
    See this gcc bugreport: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35653
    [I do not really agree with this sudden change, because unaligned
    accesses have always been possible on x86, but the C99 standard does say
    it is undefined behaviour ...]

    I thought to inform you of this change in gcc's behaviour, because
    include/asm-x86/unaligned.h is no longer safe in the above context,
    especially that it is being used in a loop:
    http://lxr.linux.no/linux/net/blueto...ep/core.c#L153

    P.S.: I only compile my kernels with -O2, so I don't know if it actually
    crashes or not at -O3.

    Best regards,
    --Edwin

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  2. Re: gcc-4.3 considers unaligned accesses on X86 as undefined

    On Tue, 25 Mar 2008 22:51:09 +0200
    Török Edwin wrote:

    > Hello x86 architecture maintainers,
    >
    > GCC-4.3 now considers that it is undefined behaviour to access memory
    > through an int* that is not aligned to sizeof(int).
    > At -O3 it generates vectorized code

    FPU/MMX/SSE are not available or usable for the kernel anyway fortunately
    in this case.

    Alan
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  3. Re: gcc-4.3 considers unaligned accesses on X86 as undefined

    Török Edwin writes:

    > At -O3 it generates vectorized code that _relies_ on the fact that
    > pointers are always aligned (unless you use packed attributes, etc.),
    > and the resulting code crashes if the pointer is unaligned. (-O3 -msse
    > on 32-bit, and simply -O3 on 64-bit since -msse is default)

    The 2.6 kernel disables SSE and MMX code generation explicitely.
    2.4 should be also safe unless someone misguided recompiles it with -O3.

    -Andi
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