On Sat, Mar 12, 2011 at 03:27:31PM +0100, Julien PUYDT wrote:
> Le 12/03/2011 12:11, Aurelien Jarno a écrit :
> >On Sat, Mar 12, 2011 at 09:48:53AM +0100, Julien PUYDT wrote:
> >>Package: libc6
> >>Version: 2.11.2-13
> >>
> >>The following piece of code :
> >>
> >>#include<stdio.h>
> >>#include<math.h>
> >>
> >>int
> >>main (int argc,
> >> char* argv[])
> >>{
> >> long double x = 6.0;
> >> printf ("tgammal (%20Lf)=%20Lf\n", x, tgammal (x));
> >> return 0;
> >>}
> >>
> >>Prints, on an x86 debian unstable (eglibc 2.11.2-11) :
> >>tgammal ( 6.000000)= 120.000000
> >>And on an ARMel debian unstable (eglibc 2.11.2-13) :
> >>tgammal (6.00000000000000000000)=119.99999999999997157829
> >
> >On armel, long double is same the type as a double, and thus tgamma()
> >and tgammal() are the same functions. On x86 long double and double are
> >different types, and thus tgamma() and tgammal() are different
> >functions.
> >
> >Your test code with long double and tgammal() on armel gives as expected
> >exactly the same result as double and tgamma() on x86. I don't see any
> >problem here, this function works as expected.
>
> Let's see :
> 119.99999999999997157829
> 0123456789ABCD <- digit counting
> That makes 12 good digits -- and 119 needs 7 more (it's between 2**6
> and 2**7), so that makes 19 good digits.
>
> Isn't that a little short? If I read
> http://en.wikipedia.org/wiki/IEEE_754-2008#Basic_formats well, even
> simple precision boasts 23 digits.
23 binary digits, which means 7.22 decimal digit. For double, it lists
15.95 digit, so your example is in the expected precision range.
> The above computation may be a little naive. Can you point me to the
> more precise place where the precision of those special functions is
> normalized?
>
I don't thing the standard normalizes the precision of these functions,
at least I haven't found anything. OTOH, it normalizes the precision of
the different types.
--
Aurelien Jarno GPG: 1024D/F1BCDB73
aurel...@aurel32.net http://www.aurel32.net
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