Re: [PATCH] Optimize prefixcmp()

On Sun, Dec 30, 2007 at 01:55:57PM +0000, Pierre Habouzit wrote:

On Sun, Dec 30, 2007 at 01:02:28PM +0000, Marco Costalba wrote:

quoted

Subject: [PATCH] Certain codepaths (notably "git log --pretty=format...") use

prefixcmp() extensively, with very short prefixes.  In those cases,
calling strlen() is a wasteful operation, so avoid it.

Initial patch by Johannes Schindelin.

Signed-off-by: Marco Costalba <redacted>
---
 git-compat-util.h |   11 ++++++++++-
 1 files changed, 10 insertions(+), 1 deletions(-)

diff --git a/git-compat-util.h b/git-compat-util.h
index 79eb10e..843a8f5 100644
--- a/git-compat-util.h
+++ b/git-compat-util.h

@@ -398,7 +398,16 @@ static inline int sane_case(int x, int high)

 static inline int prefixcmp(const char *str, const char *prefix)
 {
-	return strncmp(str, prefix, strlen(prefix));
+	do {
+		if (*str != *prefix)
+			return *(unsigned const char *)prefix - *(unsigned const char *)str;
+
+		if (!*(++prefix))
+			return 0;
+
+		str++;
+
+	} while (1);

  This code doesn't work if prefix is "". You want something like:

    for (; *prefix; prefix++, str++) {
        if (*str != *prefix)
            return *(unsigned const char *)prefix - *(unsigned const char *)str;
    }
    return 0;

  Which happens to be basically the same than what Dscho wrote, though I
suppose the compiler can compile that more efficiently than his code.


-- 
·O·  Pierre Habouzit
··O                                                madcoder@debian.org
OOO                                                http://www.madism.org