Re: [PATCH v2 4/4] unpack-trees: cheaper index update when walking by cache-tree

On Fri, Aug 10, 2018 at 9:40 PM Elijah Newren [off-list ref] wrote:

On Fri, Aug 10, 2018 at 12:30 PM Duy Nguyen [off-list ref] wrote:

quoted

On Fri, Aug 10, 2018 at 8:39 PM Elijah Newren [off-list ref] wrote:

...

quoted

quoted

Why do we still need to go through add_index_entry()?  I thought that
the whole point was that you already checked that at the current path,
the trees being unpacked were all equal and matched both the index and
the cache_tree.  If so, why is there any need for an update at all?
(Did I read your all_trees_same_as_cache_tree() function wrong, and
you don't actually know these all match in some important way?)

Unless fn is oneway_diff, we have to create a new index (in o->result)
based on o->src_index and some other trees. So we have to add entries

Oh, right, o->src_index may not equal o->dst_index (because of people
like me who call it that way from merge-recursive.c) and even if it
does, we still have the temporary o->result in the mean time.  I
should have remembered that; just didn't.

Your forgetting about this actually helps. I think the idea of
avoiding add_index_entry() may be worth considering.

We know that 90% of cases of unpack_trees() is from the_index to
the_index. So if instead of creating a full temporary index, where 90%
of it might be the same as source index, if we just mark in the source
index (e.g. in ce_flags) the entries that should be copied to
o->result and _not_ create them in o->result. When it's time to create
o->dst_index (which is the_index) from o->result, we could just do
little manipulation to delete stuff that the_index has but o->result
does not and add a bit more things. It is something that at least
sounds nice in my head, but I'm not sure if it works out...
-- 
Duy