[Bug fortran/24518] Intrinsic MOD incorrect for large arg1/arg2 and slow.

["fxcoudert at gcc dot gnu dot org" <gcc-bugzilla@gcc.gnu.org>]

------- Comment #2 from fxcoudert at gcc dot gnu dot org  2006-06-20 12:22 -------
Why exactly aren't we using BUILT_IN_FMOD{F,,L}?

$ cat a.f90   
  real*8 :: x = 10.0e9
  do i = 10, 22
    x = 10d0 * x
    print '(a,i2,a,g14.8," = ",g14.8)', "mod (10**",i,", 1.7_8) = ", &
                                        fmod (x, 1.7_8), mod (x, 1.7_8);
  end do
end
$ cat a.c
#include <math.h>
double fmod_ (double *x, double *y)
{ return fmod(*x,*y); }
$ gfortran a.f90 a.c && ./a.out
mod (10**10, 1.7_8) =  1.3000026     =  1.3000026    
mod (10**11, 1.7_8) =  1.1000261     =  1.1000261    
mod (10**12, 1.7_8) = 0.80026120     = 0.80026150    
mod (10**13, 1.7_8) =  1.2026123     =  1.2026138    
mod (10**14, 1.7_8) = 0.12612289     = 0.12609863    
mod (10**15, 1.7_8) =  1.2612289     =  1.2607422    
mod (10**16, 1.7_8) = 0.71228945     =  5.8125000    
mod (10**17, 1.7_8) = 0.32289469     = -50.687500    
mod (10**18, 1.7_8) =  1.5289470     =  364.00000    
mod (10**19, 1.7_8) =  1.6894697     = -.70000000E+20
mod (10**20, 1.7_8) =  1.5946971     = -.70000000E+21
mod (10**21, 1.7_8) = 0.64697063     = -.70000000E+22
mod (10**22, 1.7_8) = 0.86970627     = -.70000000E+23

It's actually slower: 55% slower at -O0 and 230% slower at -O2, on a loop with
real(kind=8) variables modulo. But then, we're already testing whether the
division can be represented by an integer, we could call fmod only in the case
where it's not possible. Opinions?


-- 

fxcoudert at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |fxcoudert at gcc dot gnu dot
                   |                            |org
   Last reconfirmed|2006-01-24 04:26:11         |2006-06-20 12:22:57
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24518