Re: # Re: [RFD] -ffinite-math-only.

[Toon Moene <toon at moene dot indiv dot nluug dot nl>]

Brad Lucier wrote:

> > Toon Moene wrote:
> > No, your mixing "undefined behaviour" with "yields a processor-dependent 
> > approximation to".  Undefined behaviour in Fortran speak means: The 
> > `processor' (compiler+assembler+linker+OS+hardware) can do what it likes 
> > with your program, because it just broke the rules.
>
> Actually, I don't think I'm mixing those two things.  I just looked through
> the f77 standard and I can't find where dividing by zero, for example,
> leads to "undefined behavior".  It may be there, but I can't find it.
http://www.fortran.com/f77_std/rjcnf-6.html#sh-6.6:

Any arithmetic operation whose result is not mathematically defined is 
prohibited in the execution of an executable program.  Examples are 
dividing by zero and raising a zero-valued primary to a zero-valued or 
negative valued exponent.  Raising a negative-valued primary to a real 
or double precision power is also prohibited.

You're right.  It doesn't say "undefined behaviour", it says: 
"prohibited".  I think that's just as good :-)

> > Well, I'd say that if people use Fortran 77, they should either (1) know 
> > the model of arithmetic it supplies and adhere to that 
> I don't think that f77 *has* a model of arithmetic precise enough to
> "adhere to". 
It *is* quite precise in what is prohibited.  The operations that 
produce NaNs or Infinities are prohibited.  That means that someone who 
relies on a Fortran 77 program to do this correctly is relying on an 
extension of Fortran 77.

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