[Bug fortran/57360] New: Implement a warning for implied save

[Bug fortran/57360] New: Implement a warning for implied save

[Joost.VandeVondele at mat dot ethz.ch]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57360

            Bug ID: 57360
           Summary: Implement a warning for implied save
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: fortran
          Assignee: unassigned at gcc dot gnu.org
          Reporter: Joost.VandeVondele at mat dot ethz.ch

The following is a code snippet with an implicit save for the variable I.

 SUBROUTINE T()
 INTEGER :: I=1
 WRITE(6,*) I
 I=I+1
 END SUBROUTINE T

 CALL T()
 CALL T()
 END

Expecting this code to print twice 1 is one of the common mistakes for (C)
programmers starting with Fortran, see e.g.
http://www.cs.rpi.edu/~szymansk/OOF90/bugs.html

It would be nice if the compiler could optionally (-Wextra?) emit a warning for
implicit save attributes. Obviously the explicit code 'INTEGER, SAVE :: I=1'
should be fine.

[Bug fortran/57360] Implement a warning for implied save

[dominiq at lps dot ens.fr]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57360

Dominique d'Humieres <dominiq at lps dot ens.fr> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2015-10-20
     Ever confirmed|0                           |1

--- Comment #1 from Dominique d'Humieres <dominiq at lps dot ens.fr> ---
Usual answer!

[Bug fortran/57360] Implement a warning for implied save

[anlauf at gmx dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57360

Harald Anlauf <anlauf at gmx dot de> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |anlauf at gmx dot de

--- Comment #2 from Harald Anlauf <anlauf at gmx dot de> ---
I have a quite common "counter example", where a subroutine
is initialized on first invocation:

subroutine foo ()
  logical :: first = .true.
  write(6,*) first
  if (first) then
    first = .false.
    ! Do initialization
  end if
end subroutine foo

call foo ()
call foo ()
end

Should this one also get a warning?

[Bug fortran/57360] Implement a warning for implied save

[Joost.VandeVondele at mat dot ethz.ch]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57360

Joost VandeVondele <Joost.VandeVondele at mat dot ethz.ch> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |Joost.VandeVondele at mat dot ethz
                   |                            |.ch

--- Comment #3 from Joost VandeVondele <Joost.VandeVondele at mat dot ethz.ch> ---
(In reply to Harald Anlauf from comment #2)
> I have a quite common "counter example"
> Should this one also get a warning?

yes, while there are plenty of good uses of saved variables (really?), it
should be made more explicit i.e. just write 

logical, save :: first = .true.

as often with warnings, expert programmers won't need them, but novices might
(and they are useful to enforce good coding style with -Werror=foo). It is
certainly something I would add to the enforce list for our project.

[Bug fortran/57360] Implement a warning for implied save

[anlauf at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57360

anlauf at gcc dot gnu.org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |vivekrao4 at yahoo dot com

--- Comment #4 from anlauf at gcc dot gnu.org ---
*** Bug 102638 has been marked as a duplicate of this bug. ***

[Bug fortran/57360] Implement a warning for implied save

[w6ws at earthlink dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57360

--- Comment #5 from Walter Spector <w6ws at earthlink dot net> ---
IMHO this should be a "surprising" warning when -Wsurprising is specified.
  The message should suggest adding an explicit SAVE attribute to make the code
clear.

[Bug fortran/57360] Implement a warning for implied save

[kargl at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57360

kargl at gcc dot gnu.org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |kargl at gcc dot gnu.org

--- Comment #6 from kargl at gcc dot gnu.org ---
With this patch, 

diff --git a/gcc/fortran/decl.cc b/gcc/fortran/decl.cc
index 503ecb8d9b5..abb3579893f 100644
--- a/gcc/fortran/decl.cc
+++ b/gcc/fortran/decl.cc
@@ -2278,7 +2278,12 @@ add_init_expr_to_sym (const char *name, gfc_expr
**initp, locus *var_locus)

       sym->value = init;
       if (sym->attr.save == SAVE_NONE)
-       sym->attr.save = SAVE_IMPLICIT;
+       {
+         sym->attr.save = SAVE_IMPLICIT;
+         if (warn_surprising)
+           gfc_warning (OPT_Wsurprising, "Entity %qs at %L has an implicit "
+                     "SAVE attribute", sym->name, &sym->declared_at);
+       }
       *initp = NULL;
     }

I see

% gfcx -o z a.f90 -Wsurprising && ./z
a.f90:2:13:

    2 |  integer :: i=1
      |             1
Warning: Entity ‘i’ at (1) has an implicit SAVE attribute [-Wsurprising]
           1

[Bug fortran/57360] Implement a warning for implied save

[kargl at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57360

--- Comment #7 from kargl at gcc dot gnu.org ---
Upon some additional thinking, I wonder how useful this will be compared
to the possible volume of warning messages from modern Fortran.  Consider
this code:

module foo
   integer :: j = 2
   type a
      integer :: k = 3
   end type
   type(a) :: b = a(4)
   integer, target :: n
   integer, pointer :: m => n
end module foo

subroutine t()
 integer :: i=1
 write(6,*) i
 i=i+1
end subroutine t

 call t()
 call t()
end

with the patch at the end of this email, I see

% gfcx -c -Wsurprising -Wall a.f90
a.f90:2:14:

    2 |    integer :: j = 2
      |              1
Warning: Entity at (1) has an implicit SAVE attribute [-Wsurprising]
a.f90:6:14:

    6 |    type(a) :: b = a(4)
      |              1
Warning: Entity at (1) has an implicit SAVE attribute [-Wsurprising]
a.f90:7:22:

    7 |    integer, target :: n
      |                      1
Warning: Entity at (1) has an implicit SAVE attribute [-Wsurprising]
a.f90:8:23:

    8 |    integer, pointer :: m => n
      |                       1
Warning: Entity at (1) has an implicit SAVE attribute [-Wsurprising]
a.f90:12:13:

   12 |  integer :: i=1
      |             1
Warning: ‘i’ at (1) has an implicit SAVE attribute [-Wsurprising]


diff --git a/gcc/fortran/decl.cc b/gcc/fortran/decl.cc
index 503ecb8d9b5..d6ef37e51f2 100644
--- a/gcc/fortran/decl.cc
+++ b/gcc/fortran/decl.cc
@@ -2278,7 +2278,12 @@ add_init_expr_to_sym (const char *name, gfc_expr
**initp, locus *var_locus)

       sym->value = init;
       if (sym->attr.save == SAVE_NONE)
-       sym->attr.save = SAVE_IMPLICIT;
+       {
+         sym->attr.save = SAVE_IMPLICIT;
+         if (warn_surprising)
+           gfc_warning (OPT_Wsurprising, "%qs at %L has an implicit SAVE "
+                     "attribute", sym->name, &sym->declared_at);
+       }
       *initp = NULL;
     }

@@ -5868,7 +5873,12 @@ match_attr_spec (void)
        || gfc_current_state () == COMP_SUBMODULE)
       && !current_attr.save
       && (gfc_option.allow_std & GFC_STD_F2008) != 0)
-    current_attr.save = SAVE_IMPLICIT;
+    {
+      current_attr.save = SAVE_IMPLICIT;
+      if (warn_surprising)
+       gfc_warning (OPT_Wsurprising, "Entity at %C has an implicit SAVE "
+                    "attribute");
+    }

   colon_seen = 1;
   return MATCH_YES;

[Bug fortran/57360] Implement a warning for implied save

[w6ws at earthlink dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57360

--- Comment #8 from Walter Spector <w6ws at earthlink dot net> ---
Hi,

It is a good point.  The message is helpful when issued within a procedure.

At module scope, it doesn't mean much since everything at that level is SAVE
anyway.  This is similar to what happens in C:

int x = 3;  // statically allocated
void fn () {
  int i=3;  // stack allocated
}

Walter

-----Original Message-----
From: kargl at gcc dot gnu.org <gcc-bugzilla@gcc.gnu.org>
Sent: Jan 21, 2024 9:43 AM
To: <w6ws@earthlink.net>
Subject: [Bug fortran/57360] Implement a warning for implied save

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57360

--- Comment #7 from kargl at gcc dot gnu.org ---
Upon some additional thinking, I wonder how useful this will be compared
to the possible volume of warning messages from modern Fortran. Consider
this code:

module foo
integer :: j = 2
type a
integer :: k = 3
end type
type(a) :: b = a(4)
integer, target :: n
integer, pointer :: m => n
end module foo

subroutine t()
integer :: i=1
write(6,*) i
i=i+1
end subroutine t

call t()
call t()
end

with the patch at the end of this email, I see

% gfcx -c -Wsurprising -Wall a.f90
a.f90:2:14:

2 | integer :: j = 2
| 1
Warning: Entity at (1) has an implicit SAVE attribute [-Wsurprising]
a.f90:6:14:

6 | type(a) :: b = a(4)
| 1
Warning: Entity at (1) has an implicit SAVE attribute [-Wsurprising]
a.f90:7:22:

7 | integer, target :: n
| 1
Warning: Entity at (1) has an implicit SAVE attribute [-Wsurprising]
a.f90:8:23:

8 | integer, pointer :: m => n
| 1
Warning: Entity at (1) has an implicit SAVE attribute [-Wsurprising]
a.f90:12:13:

12 | integer :: i=1
| 1
Warning: &lsquo;i&rsquo; at (1) has an implicit SAVE attribute [-Wsurprising]


diff --git a/gcc/fortran/decl.cc b/gcc/fortran/decl.cc
index 503ecb8d9b5..d6ef37e51f2 100644
--- a/gcc/fortran/decl.cc
+++ b/gcc/fortran/decl.cc
@@ -2278,7 +2278,12 @@ add_init_expr_to_sym (const char *name, gfc_expr
**initp, locus *var_locus)

sym->value = init;
if (sym->attr.save == SAVE_NONE)
- sym->attr.save = SAVE_IMPLICIT;
+ {
+ sym->attr.save = SAVE_IMPLICIT;
+ if (warn_surprising)
+ gfc_warning (OPT_Wsurprising, "%qs at %L has an implicit SAVE "
+ "attribute", sym->name, &amp;sym->declared_at);
+ }
*initp = NULL;
}

@@ -5868,7 +5873,12 @@ match_attr_spec (void)
|| gfc_current_state () == COMP_SUBMODULE)
&amp;&amp; !current_attr.save
&amp;&amp; (gfc_option.allow_std &amp; GFC_STD_F2008) != 0)
- current_attr.save = SAVE_IMPLICIT;
+ {
+ current_attr.save = SAVE_IMPLICIT;
+ if (warn_surprising)
+ gfc_warning (OPT_Wsurprising, "Entity at %C has an implicit SAVE "
+ "attribute");
+ }

colon_seen = 1;
return MATCH_YES;

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[Bug fortran/57360] Implement a warning for implied save

[w6ws at earthlink dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57360

--- Comment #9 from Walter Spector <w6ws at earthlink dot net> ---
It appears that Lfortran issues a message for this case.  See:

https://github.com/j3-fortran/fortran_proposals/issues/83#issuecomment-1906266587