[Discuss] Test for positive gives wacko results for underflowed values for the gcc compiler suite (including the gcc compiler, and g77 and gfortran compilers)

[pw]

Alan W. Irwin wrote:
>
> Anybody have a clue about what is going on here between the bad results for
> fortran and C on the one hand and good results for python on the other?
> 
> Is this a gcc (and g77 and gfortran) issue?  I would appreciate it if
> somebody would run the above C test codes with different C
> compilers/platforms.
 >

Setting the precision back to IEEE 754 single precision gives a
different answer.


       DOUBLE PRECISION x, xscale
       x = 1.d-126
       xscale = 1.d+127
       write(*,'(1p3d15.5)') x, xscale, x/xscale
       write(*,*) x/xscale.gt.0.d0
       write(*,*) x/xscale.gt.1.d-305
       write(*,*) log(x/xscale)
       end


test.exe

     1.00000-126    1.00000+127    1.00000-253
  T
  T
   -582.554028527494



Looks like somebody has mixed up 'float' and 'double' definitions
somewhere for this build.

What happens if gcc gets rolled back to a previous version?

Peter