Re: Different behaviour for --find-renames between git diff and git merge?

From

Elijah Newren <newren@gmail.com>

Date

Dec 16, 2025, 19:44 UTC

Message-ID

<CABPp-BHTnP-3erFTJ23goreg=UJGWPwCwdN9LNKsVbB3Omjt9w@mail.gmail.com>

In-Reply-To

<61700785-5421-4fa8-8277-c0837b09a737@gmail.com>

Hi,

>

[...]

> In the following example, I start from an empty file and I modify it on
> one side of the history and move (rename) it on the other side. The
> rename between `branch` and the merge base is detected. So, can you tell
> me why in the following case the rename is not detected during the merge?
>
>     git switch -c master root
>
>     touch aaa
>     git add aaa
>     git commit -m 'aaa'
>
>     git switch -c branch
>     echo -ne 'A\nB\nC\n' > aaa
>     git add aaa
>     git commit -m 'A\nB\nC\n > aaa'
>
>     git switch master
>     mkdir dir
>     mv aaa dir/
>     git add aaa dir/
>     git commit -m 'aaa -> dir/'
>
>     git merge --no-edit branch

$ git diff master~1 master

diff --git a/aaa b/dir/aaa
similarity index 100%
rename from aaa
rename to dir/aaa

$ git diff --no-rename-empty master~1 master
diff --git a/aaa b/aaa
deleted file mode 100644
index e69de29..0000000
diff --git a/dir/aaa b/dir/aaa
new file mode 100644
index 0000000..e69de29

The merge machinery runs with the equivalent of --no-rename-empty:

$ git -C ~/floss/git grep rename_empty merge-ort.c
merge-ort.c:    diff_opts.flags.rename_empty = 0;

This comes from commit 4f7cb99ada26 (merge-recursive: don't detect
renames of empty files, 2012-03-22), and the commit message there
explains the rationale.  (The name of the option and how it is set has
changed since 2012, due to commit 0d1e0e7801bb (diff: make struct
diff_flags members lowercase, 2017-10-31)).  merge-ort copied that
behavior from merge-recursive.

So, although the merge machinery calls the same diff machinery that
`git diff` uses, it does pass slightly different defaults.  (There's a
couple others too; I believe the differences include rename_empty,
rename_limit, histogram vs myers, basename-guided similarity, and the
possibility of cached renames in a sequence of commits being
reapplied.  Users are unlikely to see any of these typically, though
you certainly did here.)