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

[msebor at gcc dot gnu.org via Gcc-bugs]

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;

}