[PATCH][C++] Save memory/time when folding CONSTRUCTORs

[Jakub Jelinek]

On Wed, Feb 01, 2017 at 03:35:49PM +0100, Richard Biener wrote:
> > >  		    changed = false;
> > >  		    break;
> > >  		  }
> > > -		changed = true;
> > > +		if (! changed)
> > > +		  {
> > > +		    nelts = elts->copy ();
> > 
> > Isn't the above part unnecessarily expensive, e.g. for the case
> > where you have huge CONSTRUCTOR and recursive cp_fold changes already
> > the very first value?  Wouldn't it be better to just do:
> > 		    vec_safe_reserve (nelts, vec_safe_length (elts));
> > 		    vec_quick_grow (nelts, i);
> > 		    memcpy (nelts->address (), elts->address (),
> > 			    i * sizeof (constructor_elt));
> > and then:
> 
> It really depends on how many constructor elements usually fold.
> If every next element will fold then yes, otherwise a memcpy
> is going to be faster than individual vec_quick_push ()s with
> cache-trashing cp_fold calls inbetween.
> 
> But I didn't benchmark anything, I just looked at memory use
> (for the case where nothing folds).  And elts->copy () looks
> much "cleaner" than this reserve/grow/memcpy ;)

Maybe.  But then it would be better not to do:
+               constructor_elt e = { p->index, op };                                                                                              
+               (*nelts)[i] = e;                                                                                                                   
but just
		(*nelts)[i].value = op;
because (*nelts)[i].index has been already copied and is the same,
so no need to override it again.

	Jakub