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