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

[Janus Weil]

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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