Floating-point exceptions are signalling in new gfortran version

Question

I am currently working to debug a subroutine of some software that my boss has written back in the 90s. There seems to be a floating-point exception that occurs in a do loop of a particular subroutine:

16  irad=1,incmax
    rr1=rr2
    rr2=rr2+rdiv
      if(rr1.gt.rlimit) goto 16
      if(pts(irad).gt.0.0) then
         discrm=(rmsden(mt,irad)/pts(irad))
     1          -((average(mt,irad)**2)/(pts(irad)**2))
      else
         discrm=0.0
      endif
      if(discrm.ge.0.0) then
         rmsden(mt,irad)=sqrt(discrm)
      else
         rmsden(mt,irad)=0.0
      endif

      average(mt,irad)=average(mt,irad)/pts(irad)
      average(mt,irad)=(average(mt,irad)*100.0)/bigmost(mt)
      rmsden(mt,irad)=(rmsden(mt,irad)*100.0)/bigmost(mt)
      denbot(mt,irad)=(denbot(mt,irad)*100.0)/bigmost(mt)
      dentop(mt,irad)=(dentop(mt,irad)*100.0)/bigmost(mt)
      iradmax(mt)=irad


 17   if(iverbose0.ge.1) then
         ipts=pts(irad)
         iptszero=ptszero(irad)
         if(ipts.eq.0) then
            average(mt,irad)=0.0
            rmsden(mt,irad)=0.0
            denbot(mt,irad)=0.0
            dentop(mt,irad)=0.0
         endif
         write(6,99) rr1,rr2,ipts,iptszero,average(mt,irad),
     1               rmsden(mt,irad),denbot(mt,irad),dentop(mt,irad)
 99      format(1x,2f9.2,2i8,2f8.1,2f8.1)
      endif
 16   continue
      stop 'PIPPA'

If I put the stop 'PIPPA' statement before "16  continue", the there are no errors. However, if the stop statement goes after the "16  continue", I get: 
Note: The following floating-point exceptions are signalling: IEEE_INVALID_FLAG
STOP PIPPA

This didn't happen before upgrading the GCC compilers/libraries. I admit that I have found a lot of resources through Googling, but I am still unable to debug this. I have also tried the --fpe-trap flags upon compilation, however they do not output anything.

Why does gfortran now complain?

Answer 1

There seems to be two questions here. Lets try to answer each one.

1. Why does the new GFortran version print such a message

Since the Fortran 2008 standard requires that execution of the STOP and ERROR STOP statements outputs a list of currently signaling FP exceptions (if such a thing is supported by the system), GFortran follows this as of GFortran version 4.9. See https://gcc.gnu.org/ml/fortran/2013-06/msg00072.html .

2. Why does my code trigger this exception, and why does it only happen with the new GFortran version

Very likely, the exception was signalling before as well, but since it wasn't printed at the STOP statement you weren't aware of it. Since the example you have showed isn't self-contained (I can't compile and test it), I can only recommend you try the usual debugging command line options such as "-fcheck=all, -ffpe-trap=invalid,zero,overflow -g -Wall -Wextra -Werror -pedantic", check running your program under valgrind etc.