Hello.
I have found that math sin function is unreasonable slow (400us on Athlon64 A2 4800+) for some values. It only happens on 64bit distribution.
Used versions:
Kubuntu 7.10 amd64 gutsy gutter
linux kernel 2.6.24
libc6 2.6.1-1ubuntu10
libc6-dev 2.6.1-1ubuntu10
Some of those values: -----------------------------------------------------------------------
0.93340582292648832662962377071381 00 b0 6b e3 75 de ed 3f 0x3fedde75e36bb000
2.3328432680770916363144351635128 d0 ad 38 bb a9 a9 02 40 0x4002a9a9bb38add0
3.7439477503636453548097051680088 00 f5 cd e0 9a f3 0d 40 0x400df39ae0cdf500
3.9225160069792437411706487182528 50 19 80 12 50 61 0f 40 0x400f615012801950
4.0711651639931289992091478779912 20 dc 4f 85 df 48 10 40 0x401048df854fdc20
4.7858438478542097982426639646292 c0 2f e9 3f b4 24 13 40 0x401324b43fe92fc0
5.9840767662578002727968851104379 a0 52 df d1 b1 ef 17 40 0x4017efb1d1df52a0
Short program for testing: ------------------------------------------------------------------
#include <stdlib.h>
#include <sstream>
#include <iostream>
#include <iomanip>
#include <math.h>
#include <sys/time.h>
using namespace std;
int main(int argc, char** argv) {
volatile union {
double dbl;
unsigned char hex[sizeof(double)];
} value;
if (argc == sizeof(double)+1) {
for (int i=0; i < sizeof(double); i++) {
istringstream s(argv[i+1]);
int tmp;
s >> hex >> tmp;
value.hex[i] = tmp;
}
} else if (argc == 2) {
istringstream s(argv[1]);
double tmp;
s >> tmp;
value.dbl = tmp;
} else {
cout << "usage: sintest 00 b0 6b e3 75 de ed 3f\n"
" sintest 0.93340582292648832662962377071381";
return (EXIT_FAILURE);
}
cout.precision(32);
cout << value.dbl << endl;
cout << "start\n";
struct timeval time1, time2;
gettimeofday(&time1, NULL);
for (int i=0; i < 10000; i++) {
volatile double out = sin(value.dbl);
}
gettimeofday(&time2, NULL);
long long diftime = 1000000ll * (time2.tv_sec - time1.tv_sec) +
(time2.tv_usec - time1.tv_usec);
cout << "end: " << diftime / 1000000ll << '.' << setw(6) << setfill('0') << diftime % 1000000ll << " s" << endl;
return (EXIT_SUCCESS);
}
Petr Cervenka
I would like to repost my previously deleted bug by (lazy IMHO)
<email address hidden>. The math sin function is at least 1000x slower on 64bit
distributions for special numbers (and carlos dosn't care about it).
I can't try it with CVS head, because I cannot connect to cvs through our firewall.
But even when I tried the latest snapshot, I couldn't build it (maybe another bug):
a - elf/dl-vdso.os
: /home/inova/
gcc -nostdlib -nostartfiles -r -o
/home/inova/
/home/inova/
/home/inova/
-Wl,-Map,
/home/inova/
definition of `__libc_
/home/inova/
first defined here
/home/inova/
`_dl_addr_
/home/inova/
definition of `_dl_addr_
/home/inova/
first defined here
collect2: ld returned 1 exit status
make[2]: *** [/home/
make[2]: Leaving directory `/home/
make[1]: *** [elf/subdir_lib] Error 2
make[1]: Leaving directory `/home/
make: *** [all] Error 2
Please, anyone with 64bit distribution and glibc CVS head, could you try the
attached example and post the time results of it? (or help me to build the
snapshot...)
Thank you
====== Original bug report =======
The math sin(double) function is in 64bit distribution (Kubuntu 7.10 AMD64 and
Fedora - unknown version) unreasonable slow (~400 microseconds on Atlon64 X2
4800+!!!) for some special values. In 32bit distribution is everything fine.
I captured some of those values:
0.9334058229264
2.3328432680770
3.7439477503636
3.9225160069792
4.0711651639931
4.7858438478542
5.9840767662578
Example:
#include <math.h>
int main(int argc, char** argv) {
volatile double value = 0.9334058229264
volatile double out;
int i;
for (i=0; i < 20000; i++)
out = sin(value);
return 0;
}