[Bug middle-end/95818] wrong "used uninitialized" warning

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95818

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|c                           |middle-end
         Resolution|---                         |INVALID
                 CC|                            |msebor at gcc dot gnu.org
             Status|WAITING                     |RESOLVED

--- Comment #6 from Martin Sebor <msebor at gcc dot gnu.org> ---
This instance of the warning looks familiar but I can't reproduce it with my
build of the kernel.  The code in the test case isn't valid (it triggers both
-Wstrict-aliasing and -Wuninitialized), and I don't think it's safe either. 
Only the leading 48 bits of the struct are initialized (by GCC; the program
initializes just 35 bits) but the access is to the full 64 bits.  Clearing the
whole struct by calling memset avoids the -Wuninitialized.

A small test case that reproduces both warnings is below.

$ cat z.c && gcc -O2 -S -Wall -fdump-tree-uninit=/dev/stdout z.c
struct S
{
  int a;
  short i: 1;
};

void f (long);

void g (void)
{
  struct S s;
  s.a = 0;
  s.i = 1;
  f (*(long*)&s);
}
z.c: In function ‘g’:
z.c:14:7: warning: dereferencing type-punned pointer will break strict-aliasing
rules [-Wstrict-aliasing]
   14 |   f (*(long*)&s);
      |       ^~~~~~~~~
z.c:14:3: warning: ‘s’ is used uninitialized [-Wuninitialized]
   14 |   f (*(long*)&s);
      |   ^~~~~~~~~~~~~~
z.c:11:12: note: ‘s’ declared here
   11 |   struct S s;
      |            ^

;; Function g (g, funcdef_no=0, decl_uid=1937, cgraph_uid=1, symbol_order=0)

g ()
{
  struct S s;
  long int _1;

  <bb 2> [local count: 1073741824]:
  s.a = 0;
  s.i = -1;
  _1 = MEM[(long int *)&s];
  f (_1); [tail call]
  s ={v} {CLOBBER};
  return;

}