Re: RFD - cleaning up C identifier lookup

[Geoff Keating <geoffk at geoffk dot org>]

Zack Weinberg <zack at codesourcery dot com> writes:

> Geoff Keating <geoffk at geoffk dot org> writes:
> 
> > Zack Weinberg <zack at codesourcery dot com> writes:
> >> Thus, we can save two more pointers per identifier by removing both
> >> fields and maintaining a global list of such declarations instead;
> >> under normal circumstances this list will be empty.
> >
> > I'm not quite sure how this would work.  Would the list only be
> > searched when a declaration is seen that is known to be block scope
> > external?  Or would it need to be searched on *every* decl lookup?
> > The first is almost certainly a win, the second maybe not.
> 
> It would need to be searched for every declaration with external
> linkage.  Since the operation is just a walk down a tree_list
> comparing pointers, and the list should be short, I don't think it'll
> be worse.

One example I saw was that in a header, the programmer had written
something like:

static inline int get_foo1 (void) 
{
  extern int internal_foo1;
  return internal_foo1 + FOO1_BIAS;
}
...  (for foo2, foo3, etc., up to maybe foo20, with get & set operations)
presumably so that 'internal_foo1' would be hidden from the user of the
header but still getting the benefits of inlining.

In this case, would the list have 20 elements, or just one?  I think
one would still be OK, but 20 might get bad.  I don't think more than
one should be needed, since the elements can be (should be) kept out
of the list when they're no longer in scope.

-- 
- Geoffrey Keating <geoffk at geoffk dot org>