Re: [PATCH][C++] Save memory and reallocations in name-lookup

[Dimitrios Apostolou <jimis at gmx dot net>]

Hi,

On Fri, 17 Aug 2012, Jakub Jelinek wrote:
> On Fri, Aug 17, 2012 at 06:41:37AM -0500, Gabriel Dos Reis wrote:
> > I am however concerned with:
> >
> > >   static void
> > >   store_bindings (tree names, VEC(cxx_saved_binding,gc) **old_bindings)
> > >   {
> > > !   static VEC(tree,heap) *bindings_need_stored = NULL;
> >
> > I would be more comfortable to see the cache be on per-scope
> > (e.g. namespace scope) basis as opposed
> > a blanket global cache stored in a global variable.
>
> It is not any kind of cache.  It could be in theory an automatic variable
> vector pointer, it is only used during that function.  The reason why it is
> static variable instead is just to avoid constant allocation/deallocation
> of the vector, this way after the first call it will be already allocated
> (but, upon entry to store_bindings will always be empty).

Why not use stack vector of sufficient size for most cases then? I usually 
do something like:

      VEC (tree, stack) *bindings_need_stored = VEC_alloc (tree, stack, 32);
      ...
      VEC_free (tree, stack, bindings_need_stored);

if I've measured that 32 (or whatever) is enough for most cases. I 
don't have a clue about this case though.


Dimitris