[Bug tree-optimization/77706] New: Optimize away some local static constructors

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77706

            Bug ID: 77706
           Summary: Optimize away some local static constructors
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jakub at gcc dot gnu.org
  Target Milestone: ---

Similar to PR77705, but even harder:

struct S { int a; };
void baz (S *);
#if __cpp_constexpr >= 200704
constexpr
#endif
inline S foo () { return (S) { 2 }; }

void
bar ()
{
  static S t = foo ();
  baz (&t);
}

inline void
bar2 ()
{
  static S t = foo ();
  baz (&t);
}

void
bar3 ()
{
  bar2 ();
}

Here we have:
  static struct S t;
  unsigned char _1;
  int _2;

  <bb 2>:
  _1 = __atomic_load_1 (&_ZGVZ3barvE1t, 2);
  if (_1 == 0)
    goto <bb 4>;
  else
    goto <bb 3>;

  <bb 3>:
  goto <bb 6>;

  <bb 4>:
  _2 = __cxa_guard_acquire (&_ZGVZ3barvE1t);
  if (_2 != 0)
    goto <bb 5>;
  else
    goto <bb 3>;

  <bb 5>:
  MEM[(struct S *)&t] = 2;
  __cxa_guard_release (&_ZGVZ3barvE1t);

  <bb 6>:

it would be nice to turn that into static struct S t = { 2 }; and get rid of
the guard var and atomics/__cxa_guard*, but we need to consider ABI issues if
the guard var is visible outside of the TU.