Re: [patch, fortran] Handling of .and. and .or. expressions

[Janus Weil <janus at gcc dot gnu dot org>]

2018-06-26 23:16 GMT+02:00 Jakub Jelinek <jakub@redhat.com>:
> On Tue, Jun 26, 2018 at 11:12:40PM +0200, Thomas Koenig wrote:
>> Hi Janus,
>>
>> with your patch, we would only warn about
>>
>> var .and. func()
>>
>> and not about
>>
>> func() .and. var.
>>
>> AFAIK, TRUTH_AND_EXPR does not guarantee that func() will
>> always be exectued, or if it does, I have not seen the
>> documentation; it just happens to match what we currently
>> see (which may be due to missed optimizatins in the back end,
>> or the lack of test cases exposing this).
>
> If you are talking about what the middle-end does, TRUTH_AND_EXPR
> always evaluates side-effects in both operands, while for
> TRUTH_ANDIF_EXPR it evaluates the second operand only if the first operand
> is not false.

thanks for the comments. Looking into gfc_conv_expr_op, I see:

    case INTRINSIC_AND:
      code = TRUTH_ANDIF_EXPR;
      lop = 1;
      break;

    case INTRINSIC_OR:
      code = TRUTH_ORIF_EXPR;
      lop = 1;
      break;

That seems to indicate that Fortran's logical .AND. operator is
translated into TRUTH_ANDIF_EXPR, right? Also the empirically observed
behavior matches Jakub's description of TRUTH_ANDIF_EXPR, where the
first operator is always evaluated and the second one only on demand.
Therefore I think my patch is correct in warning about an impure
function only if it occurs in the second operand of an and/or
operator.

Cheers,
Janus