Re: Detecting memfun staticness

=?ISO-8859-1?Q?Joaqu=EDn_M_L=F3pez_Mu=F1oz?= <>
Mon, 1 Sep 2008 16:04:22 CST
On 1 sep, 21:39, Greg Herlihy <> wrote:

On Aug 28, 1:34 pm, Joaqu?n M L?pez Mu?oz <> wrote:

Say I've got the following template function

   template<typename T>
   void call_f(int x)
     T t;


And now I'd like to optimize call_f's implementation by avoiding the
creation of the T object when it is not necessary (as in bar, whose
memfun f is static).

To avoid the overhead of initializing a "T" object each time
call_f<T>() is invoked, simply have the T object declared inside
call_f() - be static as well:

    template <class T>
    void call_f(int x)
         static T t;


Well, my probem statement is a simplification of the real scenario;
actually, I'm not creating t, but rather retrieving a reference to
it through a Meyers singleton, something like this:

   template<class T>
   struct singleton
     static T& get()
       static T t;
       return t;

   template <class T>
   void call_f(int x)

The problem is that the compiler does not optimize
the singleton<T>::get() call away when T::f is static because
obtaining the singleton reference has side effects, namely the
creation of t the first time get() is called. The translation
of your proposal to this scenario:

   template <class T>
   void call_f(int x)
     T& t=singleton<T>::get();

is also suboptimal because the *presence* of a static local incurs
an execution overhead every time call_f is invoked, precisely
for the same reason as above, first-time-creation code must be
executed on every call_f invocation.

Joaqu?n M L?pez Mu?oz
Telef?nica, Investigaci?n y Desarrollo

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