[Bug c++/69697] New: incorrect initialization of static flexible array members

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69697

            Bug ID: 69697
           Summary: incorrect initialization of static flexible array
                    members
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

Similar to bug 69696, G++ accepts the questionable program below but emits
incorrect code overlaying the flexible array member over the variable j.  This
can be seen in the generated assembly file (below).  The underlying problem is
that G++ emits a .size directive that corresponds to the size of the type as
opposed to the ultimate size of the object (or its largest initializer).  This
was fixed in the C front end via bug 28865.

        .globl  i
        .bss
        .align 4
        .type   i, @object
        .size   i, 4
i:
        .zero   4
        .globl  a
        .align 4
        .type   a, @object
        .size   a, 4
a:
        .zero   4
        .globl  j
        .align 4
        .type   j, @object
        .size   j, 4
j:
        .zero   4

G++ should either reject such code with an error or it should lay down the
global variables consecutively.

$ cat x.c && /home/msebor/build/gcc-trunk-svn/gcc/xgcc
-B/home/msebor/build/gcc-trunk-svn/gcc -Wall -Wextra -Wpedantic -std=c++14
-xc++ x.c && ./a.out
int i;
struct A { int n, a[]; } a = i ? A({ 1, { 2 } }) : A({ 2, { 3, 4 } });
int j;

int main ()
{
    __builtin_printf ("i = %i, j = %i, a = { %i, { ", i, j, a.n);
    for (int i = 0; i != a.n; ++i)
        __builtin_printf ("%i, ", a.a[i]);
    __builtin_printf ("} }\n");
}
x.c:2:48: warning: initialization of a flexible array member [-Wpedantic]
 struct A { int n, a[]; } a = i ? A({ 1, { 2 } }) : A({ 2, { 3, 4 } });
                                                ^
x.c:2:69: warning: initialization of a flexible array member [-Wpedantic]
 struct A { int n, a[]; } a = i ? A({ 1, { 2 } }) : A({ 2, { 3, 4 } });
                                                                     ^
i = 0, j = 3, a = { 2, { 3, 4, } }