[Bug tree-optimization/71831] New: __builtin_object_size poor results with no optimization

["msebor at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71831

            Bug ID: 71831
           Summary: __builtin_object_size poor results with no
                    optimization
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

The __builtin_object_size intrinsic returns the size of an object pointed to by
its first argument.  The function relies on optimization to determine the sizes
of objects referenced by complex expressions such as arguments of inlined
functions.  However, when optimization isn't enabled the function returns
either zero or -1 even in straightforward cases to indicate that it doesn't
know.  Enhancing the function to work just a little harder in some of these
basic cases would make it much more useful, both to users and also within GCC
itself.  For example, the function could easily handle all of the expressions
in the example below (it only handles the first one).

In addition, treating the function as a constant expression in these limited
cases (i.e., where the size of the object can unambiguously be determined by
folding) would also make it usable in more contexts (both in C and especially
in C++ constexpr functions).  In C++, this functionality already exists within
the front end (a constexpr function must determine whether or not an indirect
access to an object via a pointer is within the bounds of the object and reject
access that are not), but it is not conveniently exposed to users.

$ cat xyz.c && /home/msebor/build/gcc-trunk-svn/gcc/xgcc -B
/home/msebor/build/gcc-trunk-svn/gcc -S -Wall -Wextra -Wpedantic
-fdump-tree-optimized=/dev/stdout xyz.c | grep -e ^f -e __builtin_object_size
char a [7];

int f0 (void)
{
  return __builtin_object_size (a, 0);
}

int f1 (void)
{
  return __builtin_object_size (a + 3, 0);
}

int f2 (void)
{
  void *p = a;
  return __builtin_object_size (p, 0);
}
f0 ()
f1 ()
  _2 = __builtin_object_size (_1, 0);
f2 ()
  _1 = __builtin_object_size (p_2, 0);