Precise timing

SDL Development
1

Hello,
i was trying to do precise timing and i found something useful in sdl
sources. It uses rdtsc on intel but i need it for PPC.
So i added
#ifdef PPC
#define rdtsc(t)
{ uint32 hilo = (uint32)(&t);
asm volatile (
“1: \n”
“mftbu 4 \n”
“mftb %1 \n”
“mftbu %0 \n”
“sub. 4, 4, %0 \n”
“bne 1b \n”
:: “r” (hilo), “r”((hilo+1)) : “4”, “cc” ); }
#else
…
it seems like it is working but doesn’t someone know about some
easyier way to do this?

And is it possible to make this part of SDL?

Thanks
Jan Kratochvil–

2

Btw, I would make a couple changes to the assembly itself:

  • use a temp variable instead of always overwriting r4. This way the
    compiler is free to give you anything.
  • those should be output variables, not input variables. That’s
    serious, and requires some slightly different syntax (loosely tested):

#include <stdint.h>
void mftb(uint64_t *tb)
{
uint32_t *tbwords = (uint32_t *)tb;
uint32_t tbu, tbl, temp;

 /* from section 2.2.1 of the 32-bit PowerPC PEM */
 __asm__ __volatile__(
     "1:\n"
     "mftbu  %2\n"
     "mftb   %0\n"
     "mftbu  %1\n"
     "cmpw   %2,%1\n"
     "bne    1b\n"
 : "=r"(tbl), "=r"(tbu), "=r"(temp)
 :
 : "cc");

 tbwords[0] = tbu;
 tbwords[1] = tbl;

}

-HollisOn Apr 11, 2004, at 1:24 PM, Jan Kratochvil wrote:

#define rdtsc(t)
{ uint32 hilo = (uint32)(&t);
asm volatile (
“1: \n”
“mftbu 4 \n”
“mftb %1 \n”
“mftbu %0 \n”
“sub. 4, 4, %0 \n”
“bne 1b \n”
:: “r” (hilo), “r”((hilo+1)) : “4”, “cc” ); }

3

Please don’t call it “rdtsc”. You wouldn’t call it “mftb” on an x86
would you…?

Looking at the ifdefs in SDL_systimer.c, I’d say SDL_StartTicks and
SDL_GetTicks should be broken into their own conditionally-compiled
files, e.g. ticks.c and ticks_x86_tsc.c. Then you can write
ticks_ppc_tb.c and everyone’s happy.

Of course, notice that USE_RDTSC is commented out on x86. The same
issue exists on SMP PPC: the timebases are close, but not perfectly
synced. Consecutive calls to mftb could appear to go backwards if your
process is switched to another CPU in between. I see “Use RDTSC” is
still on the TODO list for x86…

-HollisOn Apr 11, 2004, at 1:24 PM, Jan Kratochvil wrote:

i was trying to do precise timing and i found something useful in sdl
sources. It uses rdtsc on intel but i need it for PPC.
So i added
#ifdef PPC
#define rdtsc(t)
{ uint32 hilo = (uint32)(&t);
asm volatile (
“1: \n”
“mftbu 4 \n”
“mftb %1 \n”
“mftbu %0 \n”
“sub. 4, 4, %0 \n”
“bne 1b \n”
:: “r” (hilo), “r”((hilo+1)) : “4”, “cc” ); }
#else
…
it seems like it is working but doesn’t someone know about some
easyier way to do this?