ref-filter: reuse output buffer

When we use `git for-each-ref`, every ref will allocate
its own output strbuf and error strbuf. But we can reuse
the final strbuf for each step ref's output. The error
buffer will also be reused, despite the fact that the git
will exit when `format_ref_array_item()` return a non-zero
value and output the contents of the error buffer.

The performance for `git for-each-ref` on the Git repository
itself with performance testing tool `hyperfine` changes from
23.7 ms ± 0.9 ms to 22.2 ms ± 1.0 ms. Optimization is relatively
minor.

At the same time, we apply this optimization to `git tag -l`
and `git branch -l`.

This approach is similar to the one used by 79ed0a5
(cat-file: use a single strbuf for all output, 2018-08-14)
to speed up the cat-file builtin.

Helped-by: Junio C Hamano <gitster@pobox.com>
Helped-by: Jeff King <peff@peff.net>
Helped-by: René Scharfe <l.s.r@web.de>
Signed-off-by: ZheNing Hu <adlternative@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 7 insertions, 6 deletions

diff --git a/builtin/for-each-ref.c b/builtin/for-each-ref.c
index 8520008604..b529228c62 100644
--- a/builtin/for-each-ref.c
+++ b/builtin/for-each-ref.c
@@ -22,6 +22,8 @@ int cmd_for_each_ref(int argc, const char **argv, const char *prefix)
 	struct ref_array array;
 	struct ref_filter filter;
 	struct ref_format format = REF_FORMAT_INIT;
+	struct strbuf output = STRBUF_INIT;
+	struct strbuf err = STRBUF_INIT;
 
 	struct option opts[] = {
 		OPT_BIT('s', "shell", &format.quote_style,
@@ -81,17 +83,16 @@ int cmd_for_each_ref(int argc, const char **argv, const char *prefix)
 	if (!maxcount || array.nr < maxcount)
 		maxcount = array.nr;
 	for (i = 0; i < maxcount; i++) {
-		struct strbuf output = STRBUF_INIT;
-		struct strbuf err = STRBUF_INIT;
-
+		strbuf_reset(&err);
+		strbuf_reset(&output);
 		if (format_ref_array_item(array.items[i], &format, &output, &err))
 			die("%s", err.buf);
 		fwrite(output.buf, 1, output.len, stdout);
 		putchar('\n');
-
-		strbuf_release(&err);
-		strbuf_release(&output);
 	}
+
+	strbuf_release(&err);
+	strbuf_release(&output);
 	ref_array_clear(&array);
 	return 0;
 }