[PATCH 5/9] strbuf_addch: avoid calling strbuf_grow

We mark strbuf_addch as inline, because we expect it may be
called from a tight loop. However, the first thing it does
is call the non-inline strbuf_grow(), which can handle
arbitrary-sized growth. Since we know that we only need a
single character, we can use the inline strbuf_avail() to
quickly check whether we need to grow at all.

Our check is redundant when we do call strbuf_grow(), but
that's OK. The common case is that we avoid calling it at
all, and we have made that case faster.

On a silly pathological case:

  perl -le '
    print "[core]";
    print "key$_ = value$_" for (1..1000000)
  ' >input
  git config -f input core.key1

this dropped the time to run git-config from:

  real    0m0.159s
  user    0m0.152s
  sys     0m0.004s

to:

  real    0m0.140s
  user    0m0.136s
  sys     0m0.004s

for a savings of 12%.

Signed-off-by: Jeff King <redacted>
---
I doubt anybody will really notice this in practice with config files,
and for the most part we do not have tight loops of strbuf_addch
elsewhere. But it is such an easy optimization, I'd rather do it now
while we're thinking about it.

 strbuf.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/strbuf.h b/strbuf.h
index 1883494..01c5c63 100644
--- a/strbuf.h
+++ b/strbuf.h

@@ -205,7 +205,8 @@ extern int strbuf_cmp(const struct strbuf *, const struct strbuf *);
  */
 static inline void strbuf_addch(struct strbuf *sb, int c)
 {
-	strbuf_grow(sb, 1);
+	if (!strbuf_avail(sb))
+		strbuf_grow(sb, 1);
 	sb->buf[sb->len++] = c;
 	sb->buf[sb->len] = '\0';
 }

-- 
2.4.0.rc2.384.g7297a4a