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

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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>
This commit is contained in:
René Scharfe 2020-02-04 22:24:24 +01:00 committed by Junio C Hamano
parent ddc42ec786
commit 1c56fc2084
1 changed files with 14 additions and 6 deletions
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20
builtin/name-rev.c
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@ -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,