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

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

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

...

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.

to o->result and add_index_entry() is the way to do that (granted if
we feel confident we could add ADD_CACHE_JUST_APPEND which makes it
super cheap). This is the outcome of n-way merge,

all_trees_same_as_cache_tree() only gurantees the input condition (all
trees the same, index also the same) but it can't affect what fn does.
I don't think we can just simply skip and not update anything (like
o->diff_index_cached case) because o->result would be empty in the
end. And we need to create (temporary) o->result before we can swap it
to o->dst_index as the result of a merge operation.

...

I have a feeling that you're thinking we can swap o->src_index to
o->dst_index at the end? That might explain your confusion about
o->result (or I misread your replies horribly) and the original
index...

Yeah, thanks for figuring out my confusion and jogging my memory.