[BUG] `git diff` treats an unchanged line as modified

Here are two files `old` and `new`.

The content of `old` file:

```

kasan_poison_slab(page);//

shuffle = shuffle_freelist(s, page);//

The content of `new` file:

```

kasan_poison_slab(page);

When using git to compare these two files, the result is:

```

diff --git a/old b/new
index 474ab07..d432a67 100644
--- a/old
+++ b/new
@@ -1,11 +1,6 @@



-       kasan_poison_slab(page);//
-
-       shuffle = shuffle_freelist(s, page);//
-
-       if (!shuffle) {
-               for_each_object_idx(p, idx, s, start, page->objects) {
-                       setup_object(s, page, p);
-                       if (likely(idx < page->objects))
+       kasan_poison_slab(page);
+
+       shuffle = shuffle_freelist(s, page);
```

However, the Linux diff command gives the following result:

```
--- old 2019-12-25 00:56:40.000000000 +0800
+++ new 2019-12-25 01:12:48.000000000 +0800
@@ -1,11 +1,6 @@



- kasan_poison_slab(page);//
+ kasan_poison_slab(page);

- shuffle = shuffle_freelist(s, page);//
-
- if (!shuffle) {
- for_each_object_idx(p, idx, s, start, page->objects) {
- setup_object(s, page, p);
- if (likely(idx < page->objects))
+ shuffle = shuffle_freelist(s, page);
```

I think the latter is correct because the fifth line is not changed
and it's still a line which only contains a whitespace.

Steps to Reproduce:

curl -L https://git.io/JeFNy | base64 -d > file.tgz
tar -xf file.tgz
git diff --no-index old new

Junio C Hamano· Dec 24, 2019, 19:34 UTC · re: Hao Lee · lore

Re: [BUG] `git diff` treats an unchanged line as modified

Hao Lee <haolee.swjtu@gmail.com> writes:

> I think the latter is correct because the fifth line is not changed
> and it's still a line which only contains a whitespace.

IOW, there is no *single* diff output that is correct.

Re: [BUG] `git diff` treats an unchanged line as modified

On Wed, 25 Dec 2019 at 03:34, Junio C Hamano <gitster@pobox.com> wrote:

> IOW, there is no *single* diff output that is correct.
>
> Among many "correct" diff output, there are ones that readers find
> them easier to understand and those that look suboptimal.  The
> differences are often personal tastes.

Thanks, this explanation is clear for me.

> "git diff" has options to tweak heuristics it uses to choose among
> many "correct" diff outputs, like --patience, --histogram, etc.
> Perhaps using one of them would produce one that match your taste
> better?
>