[PATCH] checkout: eliminate unnecessary merge for trivial checkout

Teach git to avoid unnecessary merge during trivial checkout.  When
running 'git checkout -b foo' git follows a common code path through
the expensive merge_working_tree even when it is unnecessary.  As a
result, 95% of the time is spent in merge_working_tree doing the 2-way
merge between the new and old commit trees that is unneeded.

The time breakdown is as follows:

    merge_working_tree <-- 95%
        unpack_trees <-- 80%
            traverse_trees <-- 50%
            cache_tree_update <-- 17%
            mark_new_skip_worktree <-- 10%

With a large repo, this cost is pronounced.  Using "git checkout -b r"
to create and switch to a new branch costs 166 seconds (all times worst
case with a cold file system cache).

git.c:406               trace: built-in: git 'checkout' '-b' 'r'
read-cache.c:1667       performance: 17.442926555 s: read_index_from
name-hash.c:128         performance: 2.912145231 s: lazy_init_name_hash
read-cache.c:2208       performance: 4.387713335 s: write_locked_index
trace.c:420             performance: 166.458921289 s: git command:
                                        'c:\Users\benpeart\bin\git.exe' 'checkout' '-b' 'r'
Switched to a new branch 'r'

By adding a test to skip the unnecessary call to merge_working_tree in
this case reduces the cost to 16 seconds.

git.c:406               trace: built-in: git 'checkout' '-b' 's'
read-cache.c:1667       performance: 16.100742476 s: read_index_from
trace.c:420             performance: 16.461547867 s: git command: 'c:\Users\benpeart\bin\git.exe' 'checkout' '-b' 's'
Switched to a new branch 's'

Signed-off-by: Ben Peart <redacted>
---
 builtin/checkout.c | 23 +++++++++++++++++++----
 1 file changed, 19 insertions(+), 4 deletions(-)

diff --git a/builtin/checkout.c b/builtin/checkout.c
index 8672d07..595d64b 100644
--- a/builtin/checkout.c
+++ b/builtin/checkout.c

@@ -827,10 +827,25 @@ static int switch_branches(const struct checkout_opts *opts,
 		parse_commit_or_die(new->commit);
 	}
 
-	ret = merge_working_tree(opts, &old, new, &writeout_error);
-	if (ret) {
-		free(path_to_free);
-		return ret;
+	/*
+	 * Optimize the performance of checkout when the current and
+	 * new branch have the same OID and avoid the trivial merge.
+	 * For example, a "git checkout -b foo" just needs to create
+	 * the new ref and report the stats.
+	 */
+	if (!old.commit || !new->commit
+		|| oidcmp(&old.commit->object.oid, &new->commit->object.oid)
+		|| !opts->new_branch || opts->new_branch_force || opts->new_orphan_branch
+		|| opts->patch_mode || opts->merge || opts->force || opts->force_detach
+		|| opts->writeout_stage || !opts->overwrite_ignore
+		|| opts->ignore_skipworktree || opts->ignore_other_worktrees
+		|| opts->new_branch_log || opts->branch_exists || opts->prefix
+		|| opts->source_tree) {
+		ret = merge_working_tree(opts, &old, new, &writeout_error);
+		if (ret) {
+			free(path_to_free);
+			return ret;
+		}
 	}
 
 	if (!opts->quiet && !old.path && old.commit && new->commit != old.commit)

-- 
2.10.0.windows.1