On Mon, May 20, 2013 at 04:42:38PM +0200, Michael Haggerty wrote:
> >> * Many callers store the empty string ("") as the name; for example,
> >> most of the entries created during a run of rev-list have "" as
> >> their name. This means that lots of needless copies of "" are being
> >> made. I think that the best solution to this problem would be to
> >> store NULL rather than "" for such entries, but I haven't figured
> >> out all of the places where the name is used.
> >
> > Use strbufs?
> >
> > No allocation (except for the strbuf object itself) is needed for
> > empty strings, and string ownership and be transferred to and from it
> > to prevent extra copies.
>
> That would cost two extra size_t per object_array_entry. I have the
> feeling that this structure is used often enough that the extra overhead
> would be a disadvantage, but I'm not sure.
>
> The obvious alternative would be to teach users to deal with NULL and
> either add another constructor alternative that transfers string
> ownership or *always* transfer string ownership and change the callers
> to call xstrdup() if they don't already own the name string. I think I
> will try that approach first.
You could use the same trick that strbuf does: instead of NULL, point to
a well-known empty string literal. Readers do not have to care about
this optimization at all; only writers need to recognize the well-known
pointer value. And since we do not update in place but only eventually
free, it really is just that anyone calling free() would do "if (name !=
well_known_empty_string)".
-Peff
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