[Bug tree-optimization/100126] New: missing -Wuninitialized using a trivial member of class with another nontrivial member

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100126

            Bug ID: 100126
           Summary: missing -Wuninitialized using a trivial member of
                    class with another nontrivial member
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

Reading an uninitialized trivial member of a struct that contains a member of a
type with a user-defined (or similar) ctor is not diagnosed.  The reason is
that the call to the ctor is (exceedingly) conservatively assumed to store into
the other members of the enclosing struct.

$ gcc -O1 -S -Wall -fdump-tree-uninit=/dev/stdout t.C

struct A
{
  char *p;
  A ();
};

struct B
{
  int i;
  A a;
};


void f ()
{
  B b;
  __builtin_printf ("%d\n", b.i);   // missing -Wuninitialized
}

;; Function f (_Z1fv, funcdef_no=0, decl_uid=2365, cgraph_uid=4,
symbol_order=3)

void f ()
{
  struct B b;
  int _1;

  <bb 2> [local count: 1073741824]:
  A::A (&b.a);                          <<< assumed to set b.i
  _1 = b.i;
  __builtin_printf ("%d\n", _1);        <<< missing -Wuninitialized
  b ={v} {CLOBBER};
  return;

}


This isn't C++ specific.  The same limitation affects C:

struct A
{
  char *p;
};

void init (struct A *);

struct B
{
  int i;
  struct A a;
};


void f ()
{
  struct B b;
  init (&b.a);
  __builtin_printf ("%d\n", b.i);
}