[PATCH 2/2] [GSOC] ref-filter: reuse output buffer

["ZheNing Hu via GitGitGadget" <gitgitgadget@gmail.com>]

From: ZheNing Hu <adlternative@gmail.com>

When we use `git for-each-ref`, every ref will allocate
its own output strbuf. But we can reuse the final strbuf
for each step ref's output.

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: Jeff King <peff@peff.net>
Helped-by: René Scharfe <l.s.r@web.de>
Signed-off-by: ZheNing Hu <adlternative@gmail.com>
---
 builtin/branch.c       |  9 +++++----
 builtin/for-each-ref.c | 12 ++++++------
 builtin/tag.c          | 12 ++++++------
 3 files changed, 17 insertions(+), 16 deletions(-)

diff --git a/builtin/branch.c b/builtin/branch.c
index bcc00bcf182d..00081de1aed8 100644
--- a/builtin/branch.c
+++ b/builtin/branch.c
@@ -411,6 +411,8 @@ static void print_ref_list(struct ref_filter *filter, struct ref_sorting *sortin
 {
 	int i;
 	struct ref_array array;
+	struct strbuf out = STRBUF_INIT;
+	struct strbuf err = STRBUF_INIT;
 	int maxwidth = 0;
 	const char *remote_prefix = "";
 	char *to_free = NULL;
@@ -440,8 +442,7 @@ static void print_ref_list(struct ref_filter *filter, struct ref_sorting *sortin
 	ref_array_sort(sorting, &array);
 
 	for (i = 0; i < array.nr; i++) {
-		struct strbuf out = STRBUF_INIT;
-		struct strbuf err = STRBUF_INIT;
+		strbuf_reset(&out);
 		if (format_ref_array_item(array.items[i], format, &out, &err))
 			die("%s", err.buf);
 		if (column_active(colopts)) {
@@ -452,10 +453,10 @@ static void print_ref_list(struct ref_filter *filter, struct ref_sorting *sortin
 			fwrite(out.buf, 1, out.len, stdout);
 			putchar('\n');
 		}
-		strbuf_release(&err);
-		strbuf_release(&out);
 	}
 
+	strbuf_release(&err);
+	strbuf_release(&out);
 	ref_array_clear(&array);
 	free(to_free);
 }
diff --git a/builtin/for-each-ref.c b/builtin/for-each-ref.c
index 8520008604e3..bf24c595c526 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 +83,15 @@ 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(&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;
 }
diff --git a/builtin/tag.c b/builtin/tag.c
index d92d8e110b4d..592af1d154ea 100644
--- a/builtin/tag.c
+++ b/builtin/tag.c
@@ -39,6 +39,8 @@ static int list_tags(struct ref_filter *filter, struct ref_sorting *sorting,
 		     struct ref_format *format)
 {
 	struct ref_array array;
+	struct strbuf output = STRBUF_INIT;
+	struct strbuf err = STRBUF_INIT;
 	char *to_free = NULL;
 	int i;
 
@@ -64,17 +66,15 @@ static int list_tags(struct ref_filter *filter, struct ref_sorting *sorting,
 	ref_array_sort(sorting, &array);
 
 	for (i = 0; i < array.nr; i++) {
-		struct strbuf output = STRBUF_INIT;
-		struct strbuf err = STRBUF_INIT;
-
+		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);
 	free(to_free);
 
-- 
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