[Bug c++/99975] New: wrong variable alignment on a locally redeclared overaligned extern variable

[Bug c++/99975] New: wrong variable alignment on a locally redeclared overaligned extern variable

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99975

            Bug ID: 99975
           Summary: wrong variable alignment on a locally redeclared
                    overaligned extern variable
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

Perhaps due to the same underlying bug as pr99974, the C++ front end determines
the wrong alignment from an overaligned extern variable redeclared locally
without the alignment attribute.

Both the C front end as well as other compilers (Clang and ICC) behave
correctly and determine the same alignment in both functions.

$ cat z.C && /build/gcc-master/gcc/xgcc -B /build/gcc-master/gcc -O -S -Wall
-fdump-tree-optimized=/dev/stdout z.C
extern __attribute__ ((aligned (32))) int i;

int fa32 ()
{
  return __alignof__ (i);   // folded to 32 (good)
}

int ga32 ()
{
  extern int i;
  return __alignof__ (i);   // folded to 4 (bug)
}

;; Function fa32 (_Z4fa32v, funcdef_no=0, decl_uid=2347, cgraph_uid=1,
symbol_order=0)

int fa32 ()
{
  <bb 2> [local count: 1073741824]:
  return 32;

}



;; Function ga32 (_Z4ga32v, funcdef_no=1, decl_uid=2349, cgraph_uid=2,
symbol_order=1)

int ga32 ()
{
  <bb 2> [local count: 1073741824]:
  return 4;

}

[Bug c++/99975] wrong variable alignment on a locally redeclared overaligned extern variable

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99975

--- Comment #1 from Martin Sebor <msebor at gcc dot gnu.org> ---
Surprisingly, other variable attributes such as deprecated and alloc_size (the
latter applied to a pointer to a function) do work as expected:

$ cat z.C && gcc -S -Wall z.C
int f ()
{
  extern __attribute__ ((deprecated ("I was deprecated in f()"))) int i;

  return i;
}

int gv()
{
  extern int i;

  return i;
}
z.C: In function ‘int f()’:
z.C:5:10: warning: ‘i’ is deprecated: I was deprecated in f()
[-Wdeprecated-declarations]
    5 |   return i;
      |          ^
z.C:3:71: note: declared here
    3 |  extern __attribute__ ((deprecated ("I was deprecated in f()"))) int i;
      |                                                                      ^

z.C: In function ‘int gv()’:
z.C:12:10: warning: ‘i’ is deprecated: I was deprecated in f()
[-Wdeprecated-declarations]
   12 |   return i;
      |          ^
z.C:3:71: note: declared here
    3 |  extern __attribute__ ((deprecated ("I was deprecated in f()"))) int i;
      |                                                                      ^

[Bug c++/99975] wrong variable alignment on a locally redeclared overaligned extern variable

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99975

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2024-09-19

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed.