On 02/ 2/12 04:45 AM, Dima Pasechnik wrote:
Here is an illustration of the same phenomenon on x86_64. There, of course,
8-byte floats are double, so the code to demonstrate
the problem is as follows:
#include<stdio.h>
#include<math.h>
int main ()
{
double x = 6.0;
printf("sizof(double)=%d\n",sizeof(double));
printf("lgamma (%.20f)=%.20f\n", x, lgamma(x));
printf("tgamma (%.20f)=%.20f\n", x, tgamma(x));
printf("exp(lgamma) (%.20f)=%.20f\n", x, exp(lgamma(x)));
return 0;
}
On x86_64 Linux, it outputs
sizof(double)=8
lgamma (6.00000000000000000000)=4.78749174278204581157
tgamma (6.00000000000000000000)=119.99999999999997157829
exp(lgamma) (6.00000000000000000000)=119.99999999999997157829
On MacOSX, libc has an honest gamma(), which does the job properly:
sizof(double)=8
lgamma (6.00000000000000000000)=4.78749174278204492339
tgamma (6.00000000000000000000)=120.00000000000000000000
exp(lgamma) (6.00000000000000000000)=119.99999999999987210231
So eglibc does a bad job computing gamma() on double (i.e. on 8-byte
floats)...
Report upstream?
Dima
Here's two from OpenSolaris on x86 - one with gcc, the other with the Sun
compiler 'cc'
drkirkby@hawk:~$ gcc test.c -lm
drkirkby@hawk:~$ ./a.out
sizof(double)=8
lgamma (6.00000000000000000000)=4.78749174278204581157
tgamma (6.00000000000000000000)=120.00000000000000000000
exp(lgamma) (6.00000000000000000000)=119.99999999999997157829
drkirkby@hawk:~$ cc test.c -lm
drkirkby@hawk:~$ ./a.out
sizof(double)=8
lgamma (6.00000000000000000000)=4.78749174278204581157
tgamma (6.00000000000000000000)=120.00000000000000000000
exp(lgamma) (6.00000000000000000000)=119.99999999999997157829
Same result on each I think (someone can check, as I really must rush and cant
check every digit).
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