name-rev: pre-size buffer in get_parent_name()

We can calculate the size of new name easily and precisely. Open-code
the xstrfmt() calls and grow the buffers as needed before filling them.
This provides a surprisingly large benefit when working with the
Chromium repository; here are the numbers measured using hyperfine
before:

Benchmark #1: ./git -C ../chromium/src name-rev --all
  Time (mean ± σ):      5.822 s ±  0.013 s    [User: 5.304 s, System: 0.516 s]
  Range (min … max):    5.803 s …  5.837 s    10 runs

... and with this patch:

Benchmark #1: ./git -C ../chromium/src name-rev --all
  Time (mean ± σ):      1.527 s ±  0.003 s    [User: 1.015 s, System: 0.511 s]
  Range (min … max):    1.524 s …  1.535 s    10 runs

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/builtin/name-rev.c b/builtin/name-rev.c
index 6701fb1569cf6d65b0fee3b8a96a30e7bc3f974c..793356edd164590fe5ea122304fb8f44e9d8b8e0 100644 (file)
--- a/builtin/name-rev.c
+++ b/builtin/name-rev.c
@@ -103,15 +103,23 @@ static struct rev_name *create_or_update_name(struct commit *commit,
 
 static char *get_parent_name(const struct rev_name *name, int parent_number)
 {
+       struct strbuf sb = STRBUF_INIT;
        size_t len;
 
        strip_suffix(name->tip_name, "^0", &len);
-       if (name->generation > 0)
-               return xstrfmt("%.*s~%d^%d", (int)len, name->tip_name,
-                              name->generation, parent_number);
-       else
-               return xstrfmt("%.*s^%d", (int)len, name->tip_name,
-                              parent_number);
+       if (name->generation > 0) {
+               strbuf_grow(&sb, len +
+                           1 + decimal_width(name->generation) +
+                           1 + decimal_width(parent_number));
+               strbuf_addf(&sb, "%.*s~%d^%d", (int)len, name->tip_name,
+                           name->generation, parent_number);
+       } else {
+               strbuf_grow(&sb, len +
+                           1 + decimal_width(parent_number));
+               strbuf_addf(&sb, "%.*s^%d", (int)len, name->tip_name,
+                           parent_number);
+       }
+       return strbuf_detach(&sb, NULL);
 }
 
 static void name_rev(struct commit *start_commit,