Re: [PATCH] read-cache.c: optimize reading index format v4

> > PS. I notice that v4 does not pad to align entries at 4 byte boundary
> > like v2/v3. This could cause a slight slow down on x86 and segfault on
> > some other platforms.
>
> Care to elaborate?
>
> Long time ago, we used to mmap and read directly from the index file
> contents, requiring either an unaligned read or padded entries.  But
> that was eons ago and we first read and convert from on-disk using
> get_be32() etc. to in-core structure, so I am not sure what you mean
> by "segfault" here.
>

> > @@ -1898,7 +1884,8 @@ int do_read_index(struct index_state *istate, const char *path, int must_exist)
> >       struct cache_header *hdr;
> >       void *mmap;
> >       size_t mmap_size;
> > -     struct strbuf previous_name_buf = STRBUF_INIT, *previous_name;
> > +     const struct cache_entry *previous_ce = NULL;
> > +     struct cache_entry *dummy_entry = NULL;
> >
> >       if (istate->initialized)
> >               return istate->cache_nr;
> > @@ -1936,11 +1923,10 @@ int do_read_index(struct index_state *istate, const char *path, int must_exist)
> >       istate->initialized = 1;
> >
> >       if (istate->version == 4) {
> > -             previous_name = &previous_name_buf;
> > +             previous_ce = dummy_entry = make_empty_transient_cache_entry(0);
>
> I do like the idea of passing the previous ce around to tell the
> next one what the previous name was, but I would have preferred to
> see this done a bit more cleanly without requiring us to support "a
> dummy entry with name whose length is 0"; a real cache entry never
> has zero-length name, and our code may want to enforce it as a
> sanity check.
>
> I think we can just call create_from_disk() with NULL set to
> previous_ce in the first round; of course, the logic to assign the
> one we just created to previous_ce must check istate->version,
> instead of "is previous_ce NULL?" (which is an indirect way to check
> the same thing used in this patch).