[Bug lto/114713] New: incorrect TBAA for struct with flexible array member or GNU zero size

[muecker at gwdg dot de via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114713

            Bug ID: 114713
           Summary: incorrect TBAA for struct with flexible array member
                    or GNU zero size
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: lto
          Assignee: unassigned at gcc dot gnu.org
          Reporter: muecker at gwdg dot de
  Target Milestone: ---

If one mixes a struct with FAM and one with zero size at the end, then TBAA
considers them incompatible for aliasing purposes when using LTO. One could
decide that those are incompatible types, but this would break code that mixes
both, e.g. during a transition time to standard FAMs. Also affects old versions
of GCC.

https://godbolt.org/z/xodMK9sqE


struct foo { int x; char a[]; };

void test_bar(void* b);

__attribute__((noinline))
int test_foo(struct foo* a, void* b)
{
        a->x = 1;
        test_bar(b);
        return a->x;
}

int main()
{
        struct foo y;

        if (2 != test_foo(&y, &y))
                __builtin_abort();

        return 0;
}

// TU2
struct foo { int x; char a[0]; };

void test_bar(void* b)
{
        struct foo *p = b;
        p->x = 2;
}