Re: [PATCH] branch: let --delete-merged find squash merged branches

[Phillip Wood <phillip.wood123@gmail.com>]

On 29/09/2026 12:26, D. Ben Knoble wrote:
> On Tue, Sep 29, 2026 at 3:33 AM Harald Nordgren via GitGitGadget
> <gitgitgadget@gmail.com> wrote:
> 
> …in the rebase case, I would expect something like git-log's
> --cherry-mark option (or really the algorithm behind it, git-cherry,
> and git-range-diff) 

That's what I was expecting as well. It would be worth carefully 
studying the implementation of git-cherry. "git cherry A...B" 
precalculates the patch-ids from the side of the merge base that has the 
fewest commits and then walks the other side to compare them. While it 
is walking the other side I think it also looks at which paths were 
changed to avoid calculating the patch-id for commits that cannot match. 
It also batches fetches the blobs it needs in partial clones.

As far as I can see the implementation here makes a separate upstream 
revision walk for each branch, and recalculates the upstream diffs each 
time which seems less efficient than it could be.
> to be useful for identifying rebased branches. But
> of course even rebase-merged branches can end up with minor
> differences (say, a commit was made upstream before that branch was
> rebased with an identical change; no conflict occurs, but the new
> commit differs from the old by not having that change).

Yes if a branch has been rebased before it is merged it may be altered 
such that we cannot detect it.
> In the squash case, I suppose the best we can do is check that all our
> changes were applied at some point between the merge-base and the tip.
> There probably won't be any tree-same commits, though maybe a
> (premature?) optimization can return early if the trees match exactly.

If we have

(topic)  D - C - B - A
                       \
  (main)    M - Q - P - O -
             \          /
               - - S - -

where M is a squashed merge of topic I think we have

     M^2^{tree} == topic^{tree}
     Merge-base(M^1, M^2) == Merge-base(topic, topic@{upstream})
     $(git rev-list --count --right-only M^1...M^2) == 1

If you know your repository only has squash merges that were not rebased 
it would be a lot more efficient to just look at the trees and 
merge-bases, especially in a blobless clone. Having an option to turn 
off the patch-id based detection would probably be useful in that case.

I think detecting branches that have been squashed and/or rebased is a 
useful improvement, but it needs careful implementation to be efficient 
enough that it is practical in large repositories and I'm unlikely to 
have time to closely review it.

Thanks

Phillip

> It looked like you don't distinguish the 2 cases in the code, and I
> think that's reasonable: we wouldn't know a priori whether to check
> for a rebased series or a squashed commit, so we'd have to run both
> checks, and the latter presumably subsumes the former.
> 
> Anyway, I can see how this would all be fairly expensive---on one repo
> I work in, git-range-diff can be somewhat slow depending on how many
> commits are in the range, I think. I don't know if it's worth trying
> to state that for folks, though? If we ever make improvements to
> performance, we'd have to remember to remove the "this may be slow"
> text.
> 
>> After the release of 2.56, I saw people liking the --delete-merged
>>     feature, but asking for this. A lot of people, me included prefer
>>     squash-merge and it currently doesn't work with --delete-merged.
> 
> Btw, I wonder if you can share where you saw this? 2.56 was released
> so recently I'm (pleasantly) surprised there's already feedback on
> this!
>