El dom 06 ene 2013 22:31:56 CLST, Theodore Ts'o escribió:
On Sun, Jan 06, 2013 at 09:53:47PM -0300, Cristian Rodríguez wrote:
Yeah, I asked GCC developers exactly this, was told to fill a
enhancement request.
If you could also sned them a bug/enhancement request to use a more
optimized version of __popcountdi2, that would be great. I'm not sure
it helps e2fsprogs much, since it's too hard for us to tell whether we
are using a version of the gcc runtime that has a optimized or
unuptomized version of builtin_popcount().
But since it doesn't make that much difference, my preference is to
just ignore builtin_popcount() for now. If someone is really using
128TB ext4 file systems, and cares about that extra 6 seconds of CPU,
it's probably going to require the ugly approach of using x86 asm
statements to determine whether or not we're running on a CPU that
supports the popcount instruction or not....
with a recent compiler it goes something like this..
unsigned int popcnt(unsigned int w) __attribute__ ((ifunc
("resolve_popcnt")));
__attribute__ ((__target__ ("popcnt")))
static unsigned int hw_popcnt(unsigned int w)
{
return __builtin_popcount(w);
}
static unsigned int soft_popcnt(unsigned int w)
{
return __builtin_popcount(w);
}
static void (*resolve_popcnt (void)) (void)
{
#if (__GNUC__ > 4) || (__GNUC__ == 4 && __GNUC_MINOR__ >= 8)
__builtin_cpu_init();
if (__builtin_cpu_supports("popcnt"))
return hw_popcnt;
#else
unsigned int eax, ebx, ecx, edx;
if (__get_cpuid (1, &eax, &ebx, &ecx, &edx))
if (ecx & bit_POPCNT)
return hw_popcnt;
#endif
/* If magic does not work, or running old cpu.. */
return soft_popcnt;
}
then call "popcnt" function in the code, this flies in x86 && ELF &&
GCC >= 4.6 only though.
The CPU detection code only runs once at load time btw.
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