[Bug middle-end/77294] New: __builtin_object_size inconsistent for member arrays

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77294

            Bug ID: 77294
           Summary: __builtin_object_size inconsistent for member arrays
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

Bbesides bug 77293, further testing of my patch for bug 71831 also revealed
that __builtin_object_size yields inconsistent results for member arrays
depending on how an element of the array is referenced (using slightly
different but equivalent expressions) and on whether or not an offset into the
array is an integer constant.  The following test case shows the inconsistency
both between the iterations of the first and within the first one.  The output
is expected to be consistent both between the two iterations but also within
each one of them (i.e., I would expect each line of output to show the same two
numbers).  When (type & 1) is set, I would also expect to see a larger result
than when the bit is clear based on the manual saying "if [the least
significant bit ] is set, a closest surrounding subobject is considered the
object a pointer points to."

$ (set -x && cat xyz.c && for N in 1 i; do /build/gcc-trunk-svn/gcc/xgcc -B
/build/gcc-trunk-svn/gcc -DN=$N -O2 xyz.c && ./a.out; done)
+ cat xyz.c
struct __attribute__ ((packed)) A { char a [3]; char b [5]; };

struct A a;

int main (void)
{
  int i = 1;
  __builtin_printf ("type 0: %zu %zu\n"
                    "type 1: %zu %zu\n"
                    "type 2: %zu %zu\n"
                    "type 3: %zu %zu\n",
                    __builtin_object_size (&a.a[0] + N, 0),
                    __builtin_object_size (&a.a[N] + 0, 0),
                    __builtin_object_size (&a.a[0] + N, 1),
                    __builtin_object_size (&a.a[N] + 0, 1),
                    __builtin_object_size (&a.a[0] + N, 2),
                    __builtin_object_size (&a.a[N] + 0, 2),
                    __builtin_object_size (&a.a[0] + N, 3),
                    __builtin_object_size (&a.a[N] + 0, 3));
}


+ for N in 1 i
+ /build/gcc-trunk-svn/gcc/xgcc -B /build/gcc-trunk-svn/gcc -DN=1 -O2 xyz.c
+ ./a.out
type 0: 7 7
type 1: 2 2
type 2: 7 7
type 3: 7 2
+ for N in 1 i
+ /build/gcc-trunk-svn/gcc/xgcc -B /build/gcc-trunk-svn/gcc -DN=i -O2 xyz.c
+ ./a.out
type 0: 7 7
type 1: 7 7
type 2: 7 7
type 3: 7 7