On Sat, Nov 03, 2001 at 02:03:12AM +0000, [EMAIL PROTECTED] wrote:
> Method2 is broken because it gives erroneous results.
> In my original post I presented a small program that
> called three different versions of a RDTSC functions
> (1,2,2A) and only method 1 gave results that were in any
> posssible way correct. Since these are three different
> functions, there can't be any optimization issues
> between the source code in the three functions and it's
> not at all obvious -- at least to me -- where
> a "volatile" declaration could be put. If someone could
> post a modified version of my program in method 2 or
> method 2a worked, I'd be very grateful.
> > > ---method1---
> > > __inline__ unsigned long long int rdtsc(void)
> > > {
> > > unsigned long long int x;
> > > __asm__ volatile (".byte 0x0f, 0x31" : "=A" (x));
> > > return x;
> > > }
> > >
> > >
> > > ---method2A---
> > > __inline__ unsigned long long int hwtime(void)
> > > {
> > > unsigned long long int x;
> > > __asm__("rdtsc\n\t"
> > > "mov %%edx, %%ecx\n\t"
> > > :"=A" (x));
> > > return x;
> > > }
Things, severally:
1) Use __volatile__, not volatile. Just an ANISism for clarity.
2) The mov is, in practice, as much use as a chocolate teapot. You're
achieving absolutely nada.
3) In the case you're using 2A, [It's been a long time since I did any
"serious" assembly], don't you need to give some clobber registers?
Specifically, something like:
---method2A-clobber---
__inline__ unsigned long long int hwtime(void)
{
unsigned long long int x;
__asm__("rdtsc\n\t"
"mov %%edx, %%ecx\n\t"
:"=A" (x)
:
: "ecx");
return x;
}
Or %ecx, or whatever it is. Can't remember, as I'm not using it.
In the end, here's some actual code I'm now using:
/* Ripped straight out of "vanilla" kernel */
#define rdtscll(x) \
__asm__ __volatile__ ("rdtsc" : "=A" (x))
<snip>
long long int time_blocked,counter0,counter1;
do_some_funky_init_stuff();
while (1) {
rdtscll(counter0);
pthread_wait_np ();
rdtscll(counter1);
time_blocked = counter1-counter0;
rtf_put(fifo_out, (char *) &time_blocked,sizeof(time_blocked));
}
</snip>
Obviously, that achieves very little except to tell you what _other_
processes in your system are doing to aid in the jitter of your piece
of code.
But still, it gets the point across, yes?
Gary (-;
PS Norm, you've been instrumental in getting me this far, and thanks
(=
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