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
