[Bug tree-optimization/120947] New: __builtin_object_size should understand allocations pointed by pointers within a struct

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120947

            Bug ID: 120947
           Summary: __builtin_object_size should understand allocations
                    pointed by pointers within a struct
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: siddhesh at gcc dot gnu.org
  Target Milestone: ---

Considering this program:

typedef __SIZE_TYPE__ size_t;

extern void pin_pointer(void *);
extern int some_value(void);

struct magic_ {
        char unused[9]; // at least 9
};

struct magic_map {
        struct magic_ *magic;
};

static int
coalesce_entries(struct magic_ **ma)
{
  size_t slen;

  slen = sizeof (**ma);
  *ma = __builtin_malloc (slen);

  for (unsigned i = 0; i < 1; i++)
    {
      char b[1024] = {};
      struct magic_ *ptr = *ma;
      (void) __builtin___memcpy_chk (ptr, b, sizeof (*ptr),
                                     __builtin_dynamic_object_size (ptr, 0));
    }
  return 0;
}

struct magic_map *
apprentice_load(void)
{
  char buf[128]; // did not shrink, but needs to be more than 100
  struct magic_map *map2;

  map2 = __builtin_malloc (sizeof (*map2));

  pin_pointer(&buf);
  coalesce_entries(&map2->magic);
  pin_pointer(map2);
}

$ gcc/cc1 -quiet -fdump-tree-objsz-details -O1 ../countedby.c -o /dev/null

The objsz pass is unable to see the allocation assigned to *ma since the
gimple_assign analysis is unable to determine if there's a usable estimate for
object size and as a result, it bails out.  This should be supportable to some
extent by walking vdefs of *ma to see if we can find a definite allocation,
like in this case.