stack variables and loops

[Wesley Smith]

Hey wow!
That's very informative.
thanks,
wes

On Wed, Mar 19, 2008 at 6:17 AM, Diego Novillo <dnovillo@google.com> wrote:
> 2008/3/19 Wesley Smith <wesley.hoke@gmail.com>:
>
>
>  >  Vec3 *v = vectors;
>  >  for(int i=0; i < 100; i++) {
>  >        Vec3 v_rand;
>  >         v_rand.random();   //make a random vector;
>  >
>  >           *v++ += v_rand;
>  >  }
>  >
>  >
>  >  /////example 2
>  >
>  >  Vec3 *v = vectors;
>  >   Vec3 v_rand;
>  >  for(int i=0; i < 100; i++) {
>  >         v_rand.random();   //make a random vector;
>  >
>  >           *v++ += v_rand;
>  >  }
>  >
>  >
>  >  Clearly there are scope differences between the 2 examples, but in
>  >  terms of performance, how do compilers handle these situations.
>
>  The first version needs to have a ctor and dtor call invoked on every
>  iteration of the loop.  Depending on what they do that will slow the
>  code down.  If you insert a printf() call in Vec3's constructor and
>  destructor you'll see the difference.  With GCC 4+ you can also use
>  -fdump-tree-gimple to get a C-like representation of your program.
>
>
>  Diego.
>