Before people start replying pointing out that my program has a bug (I
should have told GAS that cpuid clobbers eax, ebx, ecx and edx) - it still
appears that __volatile__ is needed.
Regards,
Tigran.
On Sat, 27 Mar 1999, Tigran Aivazian wrote:
> Hi guys,
>
> I think __volatile__ is needed when we calculate rdtsc in get_cycles().
> because this simple program will print the wrong value otherwise.
>
> Regards,
> Tigran.
>
> /*
> Compile with: CFLAGS = -Wall -Wstrict-prototypes -O2 -g -fomit-frame-pointer
> */
>
> #include <stdio.h>
> #include <unistd.h>
> #include <sys/types.h>
>
> typedef unsigned long cycles_t;
>
> static inline cycles_t get_cycles(void)
> {
> unsigned long eax, edx;
> __asm__ __volatile__ ("rdtsc":"=a" (eax), "=d" (edx));
> return eax;
> }
>
> int main(int argc, char *argv[])
> {
> cycles_t x, y;
>
> x = get_cycles();
> __asm__("cpuid");
> y = get_cycles();
> fprintf(stderr, "%lu\n", y - x);
> return 0;
> }
>
> Hence this simple patch (for include/asm-i386/timex.h):
>
> --- timex.h.0 Sat Mar 27 12:41:54 1999
> +++ timex.h Sat Mar 27 12:42:07 1999
> @@ -39,7 +39,7 @@
> #else
> unsigned long eax, edx;
>
> - __asm__("rdtsc":"=a" (eax), "=d" (edx));
> + __asm__ __volatile__ ("rdtsc":"=a" (eax), "=d" (edx));
> return eax;
> #endif
> }
>
>
>
-
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