[PATCH 06/10] dir.c: get rid of lazy initialization

From: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Date: Oct 4, 2021, 00:46 UTC
Message-ID: <patch-06.10-2b243d91696-20211004T002226Z-avarab@gmail.com>
In-Reply-To: <cover-00.10-00000000000-20211004T002226Z-avarab@gmail.com>

Remove the "Lazy initialization" in prep_exclude() left behind by aceb9429b37 (prep_exclude: remove the artificial PATH_MAX limit, 2014-07-14).

Now that every caller who sets up a "struct dir_struct" is using the DIR_INIT macro we can rely on it to have done the initialization. As noted in an analysis of the previous control flow[1] an earlier passing of of "dir->basebuf.buf" to strncmp() wasn't buggy, as we'd only reach that code on subsequent invocations of prep_exclude(), i.e. after this strbuf_init() had been run. But keeping track of that makes for hard-to-read code. Let's just rely on the initialization instead.

This does change the behavior of this code in that it won't be pre-growing the strbuf to a size of PATH_MAX. I think that's OK.

That we were using PATH_MAX at all is just a relic from this being a fixed buffer from way back in f87f9497486 (git-ls-files: --exclude mechanism updates., 2005-07-24).

Pre-allocating PATH_MAX was the opposite of an optimization in this case. I logged all "basebuf.buf" values when running the test suite, and by far the most common one (around 80%) is "", which we now won't allocate at all for, and just use the "strbuf_slopbuf".

The second most common one was "a/", followed by other common cases of short relative paths. So using the default "struct strbuf" growth pattern is a much better allocation optimization in this case.

1. https://lore.kernel.org/git/87sfxhohsj.fsf@evledraar.gmail.com/

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 dir.c | 8 --------
 dir.h | 4 +++-
 2 files changed, 3 insertions(+), 9 deletions(-)

diff --git a/dir.c b/dir.c
index 39fce3bcba7..efc87c2e405 100644
--- a/dir.c
+++ b/dir.c
@@ -1550,14 +1550,6 @@ static void prep_exclude(struct dir_struct *dir,
 	if (dir->pattern)
 		return;
 
-	/*
-	 * Lazy initialization. All call sites currently just
-	 * memset(dir, 0, sizeof(*dir)) before use. Changing all of
-	 * them seems lots of work for little benefit.
-	 */
-	if (!dir->basebuf.buf)
-		strbuf_init(&dir->basebuf, PATH_MAX);
-
 	/* Read from the parent directories and push them down. */
 	current = stk ? stk->baselen : -1;
 	strbuf_setlen(&dir->basebuf, current < 0 ? 0 : current);
diff --git a/dir.h b/dir.h
index ff3b4a7f602..e3757c6099e 100644
--- a/dir.h
+++ b/dir.h
@@ -342,7 +342,9 @@ struct dir_struct {
 	unsigned visited_directories;
 };
 
-#define DIR_INIT { 0 }
+#define DIR_INIT { \
+	.basebuf = STRBUF_INIT, \
+}
 
 struct dirent *readdir_skip_dot_and_dotdot(DIR *dirp);

-- 
2.33.0.1404.g83021034c5d