[Bug c++/58116] New: missed-optimization: const temporaries could be promoted to static

[Bug c++/58116] New: missed-optimization: const temporaries could be promoted to static

[luto at mit dot edu]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58116

            Bug ID: 58116
           Summary: missed-optimization: const temporaries could be
                    promoted to static
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: luto at mit dot edu

This code:

struct S
{
  int a, b, c;
};

extern void callee(const S &s);

void test()
{
  const S s{1,2,3};
  callee(s);

  callee((const S){1,2,3});
}

would be shorter and faster if both S instances were promoted to statics.  This
would be correct because it's undefined behavior to modify a non-mutable member
of an object declared const in C++.  (I'm not sure about C.)

[Bug c++/58116] missed-optimization: const temporaries could be promoted to static

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58116

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
I think there is a duplicated bug of this already.

[Bug c++/58116] missed-optimization: const temporaries could be promoted to static

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58116

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2014-01-17
     Ever confirmed|0                           |1

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed.

[Bug c++/58116] missed-optimization: const temporaries could be promoted to static

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58116

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |INVALID

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Actually this is an invalid optimization.

Take:
struct S
{
  int a, b, c;
};

extern void callee(const S &s);

void test()
{
  const S s{1,2,3};
  callee(s);

  callee((const S){1,2,3});
}

void test1()
{
  callee((const S){1,2,3});
}

void callee(const S &s)
{
  static const S *a;
  if (!a)
    {
      a = &s;
      test();
    }
  if (a == &s)
    __builtin_abort();
  return;
}

>From http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59863#c1:
If the object address can escape and the function can be called 
recursively, this would violate the requirement for distinct objects to 
have distinct addresses.  (See discussion on comp.std.c, "uniqueness of 
automatic objects", Nov 2008; I'm not sure if there was a corresponding 
GCC bug report / fix.)

[Bug c++/58116] missed-optimization: const temporaries could be promoted to static

[luto at mit dot edu]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58116

--- Comment #4 from Andy Lutomirski <luto at mit dot edu> ---
Sorry -- I forgot about the recursive / threaded case.

(I keep meaning to propose something like [[non_unique_address]] to enable
optimizations like this and things zero-byte struct members.)