[Bug c++/14769] New: Block extern incorrectly resolved to file-scope internal linkage object

[Bug c++/14769] New: Block extern incorrectly resolved to file-scope internal linkage object

[algrant at acm dot org]

In the following it is clear from the example in the C++ Standard 3.5.#6 that 
the block extern declaration refers to a separate object, i.e. that there is an 
internal-linkage i and an external-linkage i in the same TU.  As in C a block 
extern that does not 'see' an internal-linkage declaration (as in this case 
where hidden by the formal) gets external linkage; C 6.2.2#7 says that having 
internal and external linkage for the same name is UB but no such prohibition 
applies in C++.  G++ incorrectly resolves the block extern to the file-scope 
object (which in C++ is internal linkage) but should instead generate an 
external reference for the i object that is declared in the block extern.

#include <stdio.h>

const int i = 123;
int main(int i, char **) {
  {
    extern int i;
    printf("i = %u\n", i);
  }
  return 0;
}

-- 
           Summary: Block extern incorrectly resolved to file-scope internal
                    linkage object
           Product: gcc
           Version: 3.3.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: algrant at acm dot org
                CC: gcc-bugs at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14769

[Bug c++/14769] Block extern incorrectly resolved to file-scope internal linkage object

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2004-03-29 12:15 -------
It seems to work correctly for me (for -fPIC and without I will only show with):
(without the extern int i;):
main:
        pushl   %ebp
        movl    %esp, %ebp
        pushl   %ebx
        subl    $20, %esp
        call    .LPR3
        addl    $_GLOBAL_OFFSET_TABLE_, %ebx
        andl    $-16, %esp
        movl    $0, %eax
        subl    %eax, %esp
        movl    8(%ebp), %eax
        movl    %eax, 4(%esp)
        leal    .LC0@GOTOFF(%ebx), %eax
        movl    %eax, (%esp)
        call    printf@PLT
        movl    $0, %eax
        movl    -4(%ebp), %ebx
        leave
        ret
See how there is no reference to i, meaning that i is already "inlined".
Here are the asm for with the "extern int i;":
main:
        pushl   %ebp
        movl    %esp, %ebp
        pushl   %ebx
        subl    $20, %esp
        call    .LPR3
        addl    $_GLOBAL_OFFSET_TABLE_, %ebx
        andl    $-16, %esp
        movl    $0, %eax
        subl    %eax, %esp
        movl    i@GOT(%ebx), %eax
        movl    (%eax), %eax
        movl    %eax, 4(%esp)
        leal    .LC0@GOTOFF(%ebx), %eax
        movl    %eax, (%esp)
        call    printf@PLT
        movl    $0, %eax
        movl    -4(%ebp), %ebx
        leave
        ret
See how it goes through GOT,meaning it will pick up the extern one and not really always the local one.

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14769

[Bug c++/14769] Block extern incorrectly resolved to file-scope internal linkage object

[algrant at acm dot org]

------- Additional Comments From algrant at acm dot org  2004-03-29 14:31 -------
1) add a second translation unit
  extern int i = 5;

2) compile, link, and run the two translation units.  If it prints 5 it's ok, 
if it prints 123 it's broken.  For me it prints 123.



-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14769

[Bug c++/14769] [DR 426] Block extern incorrectly resolved to file-scope internal linkage object

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2004-03-30 18:07 -------
This looks related to DR 426: <http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/cwg_active.html#426>.
so confirming to ...

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|                            |1
   Last reconfirmed|0000-00-00 00:00:00         |2004-03-30 18:07:41
               date|                            |
            Summary|Block extern incorrectly    |[DR 426] Block extern
                   |resolved to file-scope      |incorrectly resolved to
                   |internal linkage object     |file-scope internal linkage
                   |                            |object


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14769

[Bug c++/14769] [DR 426] Block extern incorrectly resolved to file-scope internal linkage object

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2004-03-30 18:09 -------
Suspended as it is still active (because they are drafting the resulotion) but it looks like it is going to be  
undefined behavior so GCC is "right".

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |SUSPENDED


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14769

[Bug c++/14769] [DR 426] Block extern incorrectly resolved to file-scope internal linkage object

[pinskia at gcc dot gnu dot org]

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |wrong-code
   Last reconfirmed|2004-06-29 02:03:16         |2004-09-28 03:36:55
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14769

[Bug c++/14769] [DR 426] Block extern incorrectly resolved to file-scope internal linkage object

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=14769

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |tabloid.adroit at gmail dot com

--- Comment #7 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
*** Bug 94764 has been marked as a duplicate of this bug. ***

[Bug c++/14769] [DR 426] Block extern incorrectly resolved to file-scope internal linkage object

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=14769

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|wrong-code                  |accepts-invalid
             Blocks|                            |94404
              Alias|                            |cwg426
             Status|SUSPENDED                   |NEW

--- Comment #6 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
> [Adopted at the February/March, 2017 meeting.]

The defect report has now been adopted for C++20, and the code is illformed
(invalid).

I don't know if this is illformed with no diagnostic required or not but I
suspect it requires a diagnostic.

So now this is an accepts invalid.

Also note clang rejects the example program in comment #0 as illformed already
as the types are different const int vs int.


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94404
[Bug 94404] [meta-bug] C++ core issues

[Bug c++/14769] [DR 426] Block extern incorrectly resolved to file-scope internal linkage object

[pi3orama at gmail dot com]

------- Comment #5 from pi3orama at gmail dot com  2009-11-26 03:31 -------
How about the below C++ program? According to recent draft, the func() in //2:
is clearly "internal linkage", but g++ resolve it as external linkage.

static void func() { }

int main()
{
        extern void func();
        func();    // 1:
        {
                extern void func();
                func();    //2:
        }
        return 0;
}


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14769