From: "David Turner" <dtur...@twopensource.com>
In a repository with many refs, check_refname_component can be a major
contributor to the runtime of some git commands. One such command is

git rev-parse HEAD

Timings for one particular repo, with about 60k refs, almost all
packed, are:

Old: 35 ms
New: 29 ms

Many other commands which read refs are also sped up.

Signed-off-by: David Turner <dtur...@twitter.com>
---
refs.c | 68 ++++++++++++++++++++++++++++++++++++++++--------------------------
1 file changed, 41 insertions(+), 27 deletions(-)

diff --git a/refs.c b/refs.c
index 28d5eca..62e2301 100644
--- a/refs.c
+++ b/refs.c
@@ -5,9 +5,32 @@
#include "dir.h"
#include "string-list.h"

+/* How to handle various characters in refnames:
+ * 0: An acceptable character for refs
+ * 1: End-of-component
+ * 2: ., look for a following . to reject .. in refs
+ * 3: @, look for a following { to reject @{ in refs
+ * 9: A bad character, reject ref
+ *
+ * See below for the list of illegal characters, from which
+ * this table is derived.

The "see below" doesn't quite scan right.  Perhaps
   The character handling rules above are used to
   derive the table below.

+ */
+static unsigned char refname_disposition[] = {
+ 1, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9,
+ 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9, 9,
+ 9, 0, 0, 0, 0, 0, 0, 0, 0, 0, 9, 0, 0, 0, 2, 1,
+ 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 9, 0, 0, 0, 0, 9,
+ 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+ 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 9, 9, 0, 9, 0,
+ 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+ 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 3, 0, 0, 9, 9
+};
+
/*
- * Make sure "ref" is something reasonable to have under ".git/refs/";
- * We do not like it if:
+ * Try to read one refname component from the front of refname.
+ * Return the length of the component found, or -1 if the component is + * not legal. It is legal if it is something reasonable to have under
+ * ".git/refs/"; We do not like it if:
 *
 * - any path component of it begins with ".", or
 * - it has double dots "..", or
@@ -15,24 +38,7 @@
 * - it ends with a "/".
 * - it ends with ".lock"
 * - it contains a "\" (backslash)
- */

-/* Return true iff ch is not allowed in reference names. */
-static inline int bad_ref_char(int ch)
-{
- if (((unsigned) ch) <= ' ' || ch == 0x7f ||
-     ch == '~' || ch == '^' || ch == ':' || ch == '\\')
- return 1;
- /* 2.13 Pattern Matching Notation */
- if (ch == '*' || ch == '?' || ch == '[') /* Unsupported */
- return 1;
- return 0;
-}
-
-/*
- * Try to read one refname component from the front of refname. Return
- * the length of the component found, or -1 if the component is not
- * legal.
 */
static int check_refname_component(const char *refname, int flags)
{
@@ -40,17 +46,25 @@ static int check_refname_component(const char *refname, int flags)
 char last = '\0';

 for (cp = refname; ; cp++) {
- char ch = *cp;
- if (ch == '\0' || ch == '/')
+ unsigned char ch = (unsigned char) *cp;
+ char disp = refname_disposition[ch];
+ switch(disp) {
+ case 1:
+ goto out;
+ case 2:
+ if (last == '.')
+ return -1; /* Refname contains "..". */
 break;
- if (bad_ref_char(ch))
- return -1; /* Illegal character in refname. */
- if (last == '.' && ch == '.')
- return -1; /* Refname contains "..". */
- if (last == '@' && ch == '{')
- return -1; /* Refname contains "@{". */
+ case 3:
+ if (last == '@')
+ return -1; /* Refname contains "@{". */
+ break;
+ case 9:
+ return -1;
+ }
 last = ch;
 }
+out:
 if (cp == refname)
 return 0; /* Component has zero length. */
 if (refname[0] == '.') {
--
2.0.0.rc1.18.gf763c0f

--

Philip
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