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    SubjectRe: [patch] __volatile__ needed in get_cycles()?
    On Wed, 31 Mar 1999, Andrea Arcangeli wrote:

    > On Tue, 30 Mar 1999, Andrea Arcangeli wrote:
    >
    > >>Yes, I like the above, I did not know you can do
    > >>
    > >>__asm__ __volatile__("": :);
    > >>
    > >>to stop compiler from re-ordering things (because I never looked at wmb()
    > >>macro). (and I like the "pseudo-smiley" at the end :).
    > >
    > >Infact you can't do that (thanks to MikeG for make me noticing this) ;).
    > >It's been my fault. While I thought that the C compiler shouldn't look
    > >throught the contents of the asm (""), it seems he is optimizing it away
    > >(too much smart ;) even if it's an "nop" and not a "".
    >
    > Well as Horst pointed out to me the C compiler shouldn't really optimize
    > it away! It looks like a C compiler bug. (I know it's offtopic but I post
    > it here too because maybe somebody read my previous email).

    It's not off-topic, compiler technicalities are very important in the
    kernel. As to it being optimized away.. it didn't. I think that your
    initial look was dead center.. the second rdtsc was combined with the
    first, which is a lot different from throwing it away. (I would find
    it easy to view combination of two rdtsc insns as a bug tho, given the
    purpose of that insn :)

    > If somebody need a testcase ask to MikeG ;).

    In case anyone is interested, here's the code. Is it a bug? I wonder..
    all gcc/egcs versions here do basically the same thing. (combine)

    typedef unsigned long cycles_t;

    static inline cycles_t get_cycles (void)
    {
    unsigned long eax, edx;

    __asm__ ("rdtsc":"=a" (eax), "=d" (edx));
    return eax;
    }

    static inline cycles_t get_cycles_ordered(void)
    {
    /* I know I should not use `register' but I can't resist ;). -Andrea */
    register cycles_t foo;

    __asm__ __volatile__ ("lock; addl $0,0(%%esp)": :);
    foo = get_cycles();
    __asm__ __volatile__("": :);

    return foo;
    }

    extern cycles_t tsca, tscb, diff;

    void darn()
    {
    tsca = tscb = get_cycles_ordered(); /*just wanted to see:)*/
    tscb = get_cycles_ordered();
    diff = tscb - tsca;
    }

    -Mike


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