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Re: [Patch, Fortran] PR 85599: warn about short-circuiting of logical expressions for non-pure functions
- From: Janus Weil <janus at gcc dot gnu dot org>
- To: Dominique d'Humières <dominiq at lps dot ens dot fr>
- Cc: gfortran <fortran at gcc dot gnu dot org>, gcc-patches <gcc-patches at gcc dot gnu dot org>
- Date: Thu, 12 Jul 2018 16:55:53 +0200
- Subject: Re: [Patch, Fortran] PR 85599: warn about short-circuiting of logical expressions for non-pure functions
- References: <3D4B88C8-39B3-4E6C-9FA0-4EB6F80788B5@lps.ens.fr> <CAKwh3qhFufJt6+AnFhTK7ZUhBQZqcjPSM5oS-BALfPrh7p=wPQ@mail.gmail.com> <F3EA47AF-66D3-4D3F-820E-F4920E8DF621@lps.ens.fr>
2018-07-12 16:35 GMT+02:00 Dominique d'Humières <email@example.com>:
>>>> after the dust of the heated discussion around this PR has settled a
>>>> bit, here is another attempt to implement at least some basic warnings
>>>> about compiler-dependent behavior concerning the short-circuiting of
>>>> logical expressions. …
>>> IMO your patch is missing the only point I agree with you on this issue, i.e.,
>> So you don't agree that it's a good idea to warn about non-portable code?
> Wel, as you might guess I don’t like warnings in general: too much false positives
> and missed targets (see recent bootstrap breakage).
So then: Do you have a better solution for the problem at hand?
The cleaner approach would certainly be to avoid short-circuiting of
impure functions altogether. If we can all agree that this is a good
idea, I'll be happy to submit a patch in that direction. But recent
discussions have shown that there is a broad opposition of hardliners
who favor optimization to portability. I don't really have the nerves
and the time to continue this endless fight, so for now I would be
satisfied if we at least had the warnings.
>>> What to do with
>>> if(x>0 .and. sort(x)<10.0*log(x)) …
>>> which is not portable to compilers computing the two expressions?
>> That's one of the typical cases that the patch should be able to
>> handle. If 'sort' is impure, this should trigger a warning with the
>> patch. If it is pure, it doesn't matter whether the rhs of the .and.
>> operator is executed (since there are no side effects). Or am I
>> missing something?
> I meant ‘sqrt’ (changed behind my back by the stupid spell-checker).
sqrt and log are both elemental, thus pure, so there will be no warning.