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Re: Strange SIGFPE (underflow?) with gfortran [ -mieee ?]


Rob wrote:
Rob <spamrefuse at yahoo dot com> wrote:
I'm using GCC 4.1.2, from SVN as of about two weeks
ago. It's on an AlphaServer system with Linux
installed.


I have this piece of Fortran code:


	      R1=-708.397
	      R2=EXP(R1)
	      R3=1-R2

Compiling with gfortran is OK. However, when running
the executable, I get an SIGFPE Arithmetic exception
at the line with the R3 assignment. Using gdb, I get
these values:

  R1 = -708.39697265625
  R2 = 2.2238412332569656e-308
  R3 = 0

The value of R3 should be 1, as R2 is close to zero.
I suppose the FPE is an underflow exception, but
why is the underflow not squashed to zero?


However, when I change the first line to R1=708.396

(change last digit from 7 to 6) then the SIGFPE
disappears when running the executable. In that
case I have (using gdb):

  R1 = -708.39599609375
  R2 = 2.2260140139667223e-308
  R3 = 1


Is this a problem with the gfortran compiler?


Is there a flag I can pass during compilation
and/or linking, to avoid such problems?

I kind of solved this problem with the "-mieee" flag, a specific flag for the Alpha system. I have two concerns here:

  1) Do I still get the correct arithmetic with
     this flag? I mean, I don't want to suppress
     the SIGFPE and continue the program with
     random values in my variables.

  2) The manpage says that the -mieee flag will
     produce less efficient code. Is there another
     flag that also solves my problem, but without
     compromising on the code's efficiency?

Thank you!
Rob.

Rob,
what I don't understand in your posting is that you not only "change last digit from 7 to 6" but you also change the sign of R1 (from - to +)?





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