find_applet_by_name: add an example of faster linear search code

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

diff --git a/libbb/appletlib.c b/libbb/appletlib.c
index aeaf238f1aa20151b946b44e8ed860369dfd1160..18583f91a53935f91a22e248e9623171a6ed536d 100644 (file)
--- a/libbb/appletlib.c
+++ b/libbb/appletlib.c
@@ -141,10 +141,28 @@ void FAST_FUNC bb_show_usage(void)
 
 int FAST_FUNC find_applet_by_name(const char *name)
 {
-       unsigned i, max;
-       int j;
+       unsigned i, j, max;
        const char *p;
 
+/* The commented-out word-at-a-time code is ~40% faster, but +160 bytes.
+ * "Faster" here saves ~0.5 microsecond of real time - not worth it.
+ */
+#if 0 /*BB_UNALIGNED_MEMACCESS_OK && BB_LITTLE_ENDIAN*/
+       uint32_t n32;
+
+       /* Handle all names < 2 chars long early */
+       if (name[0] == '\0')
+               return -1; /* "" is not a valid applet name */
+       if (name[1] == '\0') {
+               if (!ENABLE_TEST)
+                       return -1; /* 1-char name is not valid */
+               if (name[0] != ']')
+                       return -1; /* 1-char name which isn't "[" is not valid */
+               /* applet "[" is always applet #0: */
+               return 0;
+       }
+#endif
+
        p = applet_names;
        i = 0;
 #if KNOWN_APPNAME_OFFSETS <= 0
@@ -166,7 +184,62 @@ int FAST_FUNC find_applet_by_name(const char *name)
        //bb_error_msg("name:'%s' starting from:'%s' i:%u max:%u", name, p, i, max);
 #endif
 
-       /* Open-coding without strcmp/strlen calls for speed */
+       /* Open-coded linear seatch without strcmp/strlen calls for speed */
+
+#if 0 /*BB_UNALIGNED_MEMACCESS_OK && BB_LITTLE_ENDIAN*/
+       /* skip "[\0" name, it's surely not it */
+       if (ENABLE_TEST && LONE_CHAR(p, '['))
+               i++, p += 2;
+       /* All remaining applet names in p[] are at least 2 chars long */
+       /* name[] is also at least 2 chars long */
+
+       n32 = (name[0] << 0) | (name[1] << 8) | (name[2] << 16);
+       while (i < max) {
+               uint32_t p32;
+               char ch;
+
+               /* Quickly check match of the first 3 bytes */
+               move_from_unaligned32(p32, p);
+               p += 3;
+               if ((p32 & 0x00ffffff) != n32) {
+                       /* Most likely case: 3 first bytes do not match */
+                       i++;
+                       if ((p32 & 0x00ff0000) == '\0')
+                               continue; // p[2] was NUL
+                       p++;
+                       if ((p32 & 0xff000000) == '\0')
+                               continue; // p[3] was NUL
+                       /* p[0..3] aren't matching and none is NUL, check the rest */
+                       while (*p++ != '\0')
+                               continue;
+                       continue;
+               }
+
+               /* Unlikely branch: first 3 bytes ([0..2]) match */
+               if ((p32 & 0x00ff0000) == '\0') {
+                       /* name is 2-byte long, it is full match */
+                       //bb_error_msg("found:'%s' i:%u", name, i);
+                       return i;
+               }
+               /* Check remaining bytes [3..NUL] */
+               ch = (p32 >> 24);
+               j = 3;
+               while (ch == name[j]) {
+                       if (ch == '\0') {
+                               //bb_error_msg("found:'%s' i:%u", name, i);
+                               return i;
+                       }
+                       ch = *++p;
+                       j++;
+               }
+               /* Not a match. Skip it, including NUL */
+               while (ch != '\0')
+                       ch = *++p;
+               p++;
+               i++;
+       }
+       return -1;
+#else
        while (i < max) {
                char ch;
                j = 0;
@@ -189,6 +262,7 @@ int FAST_FUNC find_applet_by_name(const char *name)
                i++;
        }
        return -1;
+#endif
 }