[PATCH v1] add: speed up cmd_add() by utilizing read_cache_preload()

[Ben Peart <peartben@gmail.com>]

From: Ben Peart <benpeart@microsoft.com>

During an "add", a call is made to run_diff_files() which calls
check_remove() for each index-entry.  The preload_index() code distributes
some of the costs across multiple threads.

Because the files checked are restricted to pathspec, adding individual
files makes no measurable impact but on a Windows repo with ~200K files,
'git add .' drops from 6.3 seconds to 3.3 seconds for a 47% savings.

Signed-off-by: Ben Peart <benpeart@microsoft.com>
---

Notes:
    Base Ref: master
    Web-Diff: https://github.com/benpeart/git/commit/fc4830b545
    Checkout: git fetch https://github.com/benpeart/git add-preload-index-v1 && git checkout fc4830b545

 builtin/add.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/builtin/add.c b/builtin/add.c
index ad49806ebf..f65c172299 100644
--- a/builtin/add.c
+++ b/builtin/add.c
@@ -445,11 +445,6 @@ int cmd_add(int argc, const char **argv, const char *prefix)
 		return 0;
 	}
 
-	if (read_cache() < 0)
-		die(_("index file corrupt"));
-
-	die_in_unpopulated_submodule(&the_index, prefix);
-
 	/*
 	 * Check the "pathspec '%s' did not match any files" block
 	 * below before enabling new magic.
@@ -459,6 +454,10 @@ int cmd_add(int argc, const char **argv, const char *prefix)
 		       PATHSPEC_SYMLINK_LEADING_PATH,
 		       prefix, argv);
 
+	if (read_cache_preload(&pathspec) < 0)
+		die(_("index file corrupt"));
+
+	die_in_unpopulated_submodule(&the_index, prefix);
 	die_path_inside_submodule(&the_index, &pathspec);
 
 	if (add_new_files) {

base-commit: 4ede3d42dfb57f9a41ac96a1f216c62eb7566cc2
-- 
2.18.0.windows.1