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Unexpected FP div-by-zero with /64
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mecej4



Joined: 31 Oct 2006
Posts: 1484

PostPosted: Mon Mar 08, 2021 1:15 pm    Post subject: Reply with quote

OK, thanks! It must be the math library used by Gfortran/GCC, then.
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JohnCampbell



Joined: 16 Feb 2006
Posts: 2268
Location: Sydney

PostPosted: Tue Mar 09, 2021 12:22 am    Post subject: Reply with quote

The hex value for TANH would clarify any problem.
Code:
program mterr
use iso_fortran_env
implicit none
integer i,ix,it
real x,tnhx,f
!
write (*,*) compiler_version()
print *
print *,'            x     x(IEEE hex) tanh(hex)      tanh(x)     1/x - 1/tanh(x)'
print *,'    -------------   --------  --------   -------------   ---------------'     
ix = Z'33DBE6F8'
do i=1,16
   x = transfer(ix,x)
   tnhx = tanh(x)
   it = transfer(tnhx,it)
   f = 1.0/x - 1.0/tnhx
   print 10,i,x,ix,it,tnhx,f
   ix = ix+1
end do
10 format(i2,ES16.7,1x,2Z10,ES16.7,ES15.5)
end program
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mecej4



Joined: 31 Oct 2006
Posts: 1484

PostPosted: Tue Mar 09, 2021 12:07 pm    Post subject: Reply with quote

John, I raised this issue on CLF.

A few of the responders who had older versions of Gfortran on Linux did report seeing the 1.0s in the output.

Most of them had a recent version of Gfortran on Linux, MacOS, or even Android (on a Nokia phone!), and did not see the error.
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