[PATCH 6/9] strbuf_getwholeline: avoid calling strbuf_grow

As with the recent speedup to strbuf_addch, we can avoid calling strbuf_grow() in a tight loop of single-character adds by instead checking strbuf_avail.

Note that we would instead call strbuf_addch directly here, but it does more work than necessary: it will NUL-terminate the result for each character read. Instead, in this loop we read the characters one by one and then add the terminator manually at the end.

Running "git rev-parse refs/heads/does-not-exist" on a repo with an extremely large (1.6GB) packed-refs file went from (best-of-5):

  real    0m10.948s
  user    0m10.548s
  sys     0m0.412s

to:

  real    0m8.601s
  user    0m8.084s
  sys     0m0.524s

for a wall-clock speedup of 21%.

Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Jeff King <peff@peff.net>
---
Our "don't write a NUL for each character" optimization is only possible
because we're intimate with the strbuf details here. I thought about
making a strbuf_addch_unsafe interface to let other callers do this,
too. But the only other caller that would use it is the config reader,
and I measured only a 3% speedup there. Which I don't think is worth the
extra API complexity.

Whereas here it does make a big difference. Switching to strbuf_addch knocks us back up into the 9.5s range. I think the difference is that our lines are much longer than the tokens we're parsing in the config file. So the percentage of wasted NUL writes is much higher here.

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