Re: [PATCH] Optimize prefixcmp()

Johannes Schindelin schrieb:

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.

 static inline int prefixcmp(const char *str, const char *prefix)
 {
-	return strncmp(str, prefix, strlen(prefix));
+	for (; ; str++, prefix++)
+		if (!*prefix)
+			return 0;
+		else if (*str != *prefix)
+			return (unsigned char)*prefix - (unsigned char)*str;
 }
 
 static inline int strtoul_ui(char const *s, int base, unsigned int *result)

prefixcmp() was already optimized before -- only for a different use
case.  At a number of callsites the prefix is a string literal, which
allowed the compiler to perform the strlen() call at compile time.

The patch increases the text size considerably: the file "git" is
2,620,938 without and 2,640,450 with the patch in my build (there are
136 callsites in builtin*.c).  The new version of prefixcmp() shouldn't
be inlined any more, as the benefit of doing so is gone.

Is there a portable way to let the preprocessor decide if
prefixcmp_literal() or prefixcmp_generic() is to be used, depending on
the prefix being a string literal or not?

René