Re: [patch, fortran, doc] Explicitly mention undefined overflow

[Thomas Koenig <tkoenig@netcologne.de>]

I wrote:

> 
>> Yes, that's fine for trunk. I wonder if it is worth being explicit that
>> linear congruential pseudo-random number generators can and do fail at 
>> -O3?
> 
> I don't think we should put this into the docs, because that can change
> at any time.  Maybe into porting_to.html, though (where I have only
> mentioned this as a general issue with linear congruential generators,
> without mentioning specific options. Current text can be seen at
> https://gcc.gnu.org/gcc-13/porting_to.html ).
> 
> Hm....

Breaking things actually goes back further than I thought.
Taking the random number generator from rnseed and running
it 10 times with my system gfortran 9.4.0 gets me, at
different levels of optimization:

$ for a in -O0 -O1 -O2; do echo $a; gfortran $a genuni.f90 && ./a.out; done
-O0
   0.269411892
   0.891386986
   0.444042951
   0.779210865
   0.500058949
   0.666437685
   0.666963458
   0.462416053
   0.376364112
    2.90278494E-02
-O1
   0.269411892
   0.891386986
   0.444042951
   0.779210865
   0.500058949
   0.666437685
   0.666963458
   0.462416053
   0.376364112
    2.90278494E-02
-O2
  -0.730588138
   0.891386986
  -0.555957019
  -0.220789105
  -0.499941051
   0.666437685
  -0.333036542
   0.462416053
   0.376364112
    2.90278494E-02

and for current trunk it is

$ for a in -O0 -O1 -O2; do echo $a; gfortran $a genuni.f90 && ./a.out; done
-O0
   0.269411892
   0.891386986
   0.444042951
   0.779210865
   0.500058949
   0.666437685
   0.666963458
   0.462416053
   0.376364112
    2.90278494E-02
-O1
   0.269411892
   0.891386986
   0.444042951
   0.779210865
   0.500058949
   0.666437685
   0.666963458
   0.462416053
   0.376364112
    2.90278494E-02
-O2
   0.211324871
   0.211324871
   0.211324871
   0.211324871
   0.211324871
   0.211324871
   0.211324871
   0.211324871
   0.211324871
   0.211324871

so it the general problem is not restricted to -O3 and not
to current trunk, it depends on the details.

I doubt that the result from 9.4.0 was expected, but rather
nobody noticed.  Or, bringing out the pseudo-RNG into a
different setting changed things.

So... any suggestions on how to improve the current wording?

Best regards

	Thomas