Re: excess precision for 'float'

[Ondřej Bílka]

Precision of floats is implementation dependant and AFAIR compiler is
allowed do float arithmetic at compile time with higher precision than
at host machine. What is result when floats are read from standard
input?
On Sun, Oct 09, 2011 at 12:44:07PM +0200, Bastien ROUCARIES wrote:
> On Sun, Oct 9, 2011 at 12:31 AM, Bruno Haible <address@hidden> wrote:
> > It is well-known that 'double' computation results on x86 processors can
> > carry excess precision. Namely, if the value is stored in a floating-point
> > register (80 bits total, 64 bits mantissa) without passing through a
> > memory store, it can carry excess precision. The two ways to prevent
> > such excess precision (with GCC) are:
> >  - option -ffloat-store,
> >  - cast to 'volatile double'.
> >
> > It is not so well-known that this phenomenon can also happen with 'float'
> > computations.
> >
> > Here's a program:
> > ======================================================================
> > #include <stdio.h>
> > #include <math.h>
> > int main ()
> > {
> >  printf ("%.12g\n", (double) 0.9272951f);
> >  printf ("%.12g\n", (double) 0.92729513f);
> >  printf ("%.12g\n", (double) 0.92729514f);
> >  printf ("%.12g\n", (double) 0.92729515f);
> >  printf ("%.12g\n", (double) 0.92729516f);
> >  printf ("%.12g\n", (double) 0.92729517f);
> >  printf ("%.12g\n", (double) 0.92729518f);
> >  printf ("%.12g\n", (double) 0.92729519f);
> >  printf ("%.12g\n", (double) 0.9272952f);
> >  printf ("%.12g\n", (double) 0.9272953f);
> >  printf ("\n");
> >  printf ("%.12g\n", (double) atan2f (0.8f, 0.6f));
> >  printf ("%.12g\n", (double) (float) atan2f (0.8f, 0.6f));
> >  printf ("%.12g\n", (double) (volatile float) atan2f (0.8f, 0.6f));
> >  return 0;
> > }
> > ======================================================================
> >
> > Output on mingw:
> >
> > 0.927295088768
> > 0.927295148373
> > 0.927295148373
> > 0.927295148373
> > 0.927295148373
> > 0.927295148373
> > 0.927295207977
> > 0.927295207977
> > 0.927295207977
> > 0.927295327187
> >
> > 0.927295206081
> > 0.927295206081
> > 0.927295206081
> >
> > You can see that there are no 'float' values between
> > 0.927295148373 and 0.927295207977, yet the atan2f() function
> > - which is declared to return 'float' - has returned a value
> > between them.
> >
> > Does anyone know whether this can also happen with glibc functions?
> > Or are glibc functions that return 'float' really returning 'float'
> > values?
> 
> This can happen and it is clearly a bug for me (as a hpc coder) but
> not for gcc maintener.
> 
> Serge L of inria is the person to contact for this case. Will find his 
> contact.
> 
> Thanks
> 
> Bastien
> 
> 
> > Bruno
> > --
> > In memoriam Engin Ceber <http://en.wikipedia.org/wiki/Engin_Ceber>
> >
> >

-- 

user to computer ration too low.