[Patch, Fortran] PR 85599: warn about short-circuiting of logical expressions for non-pure functions

[Janus Weil]

Hi all,

here is another update of the patch. It cures the previously-mentioned
problems with generic interfaces, adds some documentation (as
suggested by Dominique) and slightly enhances the error message
(mentioning the impurity of the function we're warning about).

I tested it on a fairly large code base and found no further false
positives. Also it still regtests cleanly. Ok for trunk?

Cheers,
Janus


2018-07-15  Thomas Koenig  <tkoenig@gcc.gnu.org>
        Janus Weil  <janus@gcc.gnu.org>

        PR fortran/85599
        * dump-parse-tree.c (show_attr): Add handling of implicit_pure.
        * gfortran.texi: Add chapter on evaluation of logical expressions.
        * resolve.c (implicit_pure_function): New function.
        (check_pure_function): Use it here.
        (impure_function_callback): New function.
        (resolve_operator): Call it via gfc_expr_walker.


2018-07-15  Janus Weil  <janus@gcc.gnu.org>

        PR fortran/85599
        * gfortran.dg/short_circuiting.f90: New test.



2018-07-13 10:02 GMT+02:00 Janus Weil <janus@gcc.gnu.org>:
> Just noticed another problematic case: Calls to generic interfaces are
> not detected as implicitly pure, see enhanced test case in attachment.
> Will look into this problem on the weekend ...
>
> Cheers,
> Janus
>
> 2018-07-12 21:43 GMT+02:00 Janus Weil <janus@gcc.gnu.org>:
>> Hi all,
>>
>> here is a minor update of the patch, which cures some problems with
>> implicitly pure functions in the previous version.
>>
>> Most implicitly pure functions were actually detected correctly, but
>> implicitly pure functions that called other implicitly pure functions
>> were not detected properly, and therefore could trigger a warning.
>> This is fixed in the current version, which still regtests cleanly
>> (note that alloc-comp-3.f90 currently fails due to PR 86417). The test
>> case is also enhanced to include the problematic case.
>>
>> Ok for trunk?
>>
>> Cheers,
>> Janus
>>
>>
>>
>> 2018-07-11 23:06 GMT+02:00 Janus Weil <janus@gcc.gnu.org>:
>>> Hi all,
>>>
>>> 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.
>>>
>>> As a reminder (and recap of the previous discussion), the Fortran
>>> standard unfortunately is a bit sloppy in this area: It allows
>>> compilers to short-circuit the second operand of .AND. / .OR.
>>> operators, but does not require this. As a result, compilers can do
>>> what they want without conflicting with the standard, and they do:
>>> gfortran does short-circuiting (via TRUTH_ANDIF_EXPR/TRUTH_ORIF_EXPR),
>>> ifort does not.
>>>
>>> I'm continuing here the least-invasive approach of keeping gfortran's
>>> current behavior, but warning about cases where compilers may produce
>>> different results.
>>>
>>> The attached patch is very close to the version I posted previously
>>> (which was already approved by Janne), with the difference that the
>>> warnings are now triggered by -Wextra and not -Wsurprising (which is
>>> included in -Wall), as suggested by Nick Maclaren. I think this is
>>> more reasonable, since not everyone may want to see these warnings.
>>>
>>> Note that I don't want to warn about all possible optimizations that
>>> might be allowed by the standard, but only about those that are
>>> actually problematic in practice and result in compiler-dependent
>>> behavior.
>>>
>>> The patch regtests cleanly on x86_64-linux-gnu. Ok for trunk?
>>>
>>> Cheers,
>>> Janus
>>>
>>>
>>> 2018-07-11  Thomas Koenig  <tkoenig@gcc.gnu.org>
>>>         Janus Weil  <janus@gcc.gnu.org>
>>>
>>>         PR fortran/85599
>>>         * dump-parse-tree (show_attr): Add handling of implicit_pure.
>>>         * resolve.c (impure_function_callback): New function.
>>>         (resolve_operator): Call it vial gfc_expr_walker.
>>>
>>>
>>> 2018-07-11  Janus Weil  <janus@gcc.gnu.org>
>>>
>>>         PR fortran/85599
>>>         * gfortran.dg/short_circuiting.f90: New test.
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