[PATCH v2 3/3] read-cache: micro-optimize expand_name_field() to speed up V4 index parsing.

 - strbuf_remove() in expand_name_field() is not exactly a good fit
   for stripping a part at the end, _setlen() would do the same job
   and is much cheaper.

 - the open-coded loop to find the end of the string in
   expand_name_field() can't beat an optimized strlen()

I used p0002-read-cache.sh to generate some performance data on the
cumulative impact:

100,000 files

Test                                HEAD~3           HEAD
---------------------------------------------------------------------------
read_cache/discard_cache 1000 times 14.08(0.03+0.09) 8.71(0.01+0.09) -38.1%

1,000,000 files

Test                                HEAD~3            HEAD
------------------------------------------------------------------------------
read_cache/discard_cache 1000 times 201.77(0.03+0.07) 149.68(0.04+0.07) -25.8%

Suggested by: Nguyễn Thái Ngọc Duy [off-list ref]
Signed-off-by: Ben Peart <redacted>
---
 read-cache.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/read-cache.c b/read-cache.c
index f768004617..f5e7c86c42 100644
--- a/read-cache.c
+++ b/read-cache.c

@@ -1754,9 +1754,8 @@ static unsigned long expand_name_field(struct strbuf *name, const char *cp_)
 
 	if (name->len < len)
 		die("malformed name field in the index");
-	strbuf_remove(name, name->len - len, len);
-	for (ep = cp; *ep; ep++)
-		; /* find the end */
+	strbuf_setlen(name, name->len - len);
+	ep = cp + strlen((const char *)cp);
 	strbuf_add(name, cp, ep - cp);
 	return (const char *)ep + 1 - cp_;
 }

-- 
2.18.0.windows.1