libbb: much faster concat_path_file()

function                                             old     new   delta
concat_path_file                                      68     127     +59

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 75 insertions, 0 deletions

diff --git a/libbb/concat_path_file.c b/libbb/concat_path_file.c
index 5b4b7f113..dec9588ea 100644
--- a/libbb/concat_path_file.c
+++ b/libbb/concat_path_file.c
@@ -17,6 +17,7 @@
 
 char* FAST_FUNC concat_path_file(const char *path, const char *filename)
 {
+#if 0
         char *lc;
 
         if (!path)
@@ -25,4 +26,78 @@ char* FAST_FUNC concat_path_file(const char *path, const char *filename)
         while (*filename == '/')
                 filename++;
         return xasprintf("%s%s%s", path, (lc==NULL ? "/" : ""), filename);
+#else
+/* ^^^^^^^^^^^ timing of xasprintf-based code above:
+ * real 7.074s
+ * user 0.156s <<<
+ * sys  6.394s
+ *      "rm -rf" of a Linux kernel tree from tmpfs (run time still dominated by in-kernel work, though)
+ * real 6.989s
+ * user 0.055s <<< 3 times less CPU used
+ * sys  6.450s
+ * vvvvvvvvvvv timing of open-coded malloc+memcpy code below (+59 bytes):
+ */
+        char *buf, *p;
+        size_t n1, n2, n3;
+
+        while (*filename == '/')
+                filename++;
+
+        if (!path || !path[0])
+                return xstrdup(filename);
+
+        n1 = strlen(path);
+        n2 = (path[n1 - 1] != '/'); /* 1: "path has no trailing slash" */
+        n3 = strlen(filename) + 1;
+
+        buf = xmalloc(n1 + n2 + n3);
+        p = mempcpy(buf, path, n1);
+        if (n2)
+                *p++ = '/';
+        p = mempcpy(p, filename, n3);
+        return buf;
+#endif
+}
+
+/* If second component comes from struct dirent,
+ * it's possible to eliminate one strlen() by using name length
+ * provided by kernel in struct dirent. See below.
+ * However, the win seems to be insignificant.
+ */
+
+#if 0
+
+/* Extract d_namlen from struct dirent */
+static size_t get_d_namlen(const struct dirent *de)
+{
+#if defined(_DIRENT_HAVE_D_NAMLEN)
+        return de->d_namlen;
+#elif defined(_DIRENT_HAVE_D_RECLEN)
+        const size_t prefix_sz = offsetof(struct dirent, d_name);
+        return de->d_reclen - prefix_sz;
+#else
+        return strlen(de->d_name);
+#endif
+}
+
+char* FAST_FUNC concat_path_dirent(const char *path, const struct dirent *de)
+{
+        char *buf, *p;
+        size_t n1, n2, n3;
+
+        if (!path || !path[0])
+                return xstrdup(de->d_name);
+
+        n1 = strlen(path);
+        n2 = (path[n1 - 1] != '/');
+        n3 = get_d_namlen(de) + 1;
+
+        buf = xmalloc(n1 + n2 + n3);
+        p = mempcpy(buf, path, n1);
+        if (n2)
+                *p++ = '/';
+        p = mempcpy(p, de->d_name, n3);
+        return buf;
 }
+
+#endif