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find_applet_by_name: add an example of faster linear search code

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Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
This commit is contained in:
Denys Vlasenko
2016-04-02 22:54:23 +02:00
parent 8b0f459af7
commit a93e4fd376
1 changed files with 77 additions and 3 deletions
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80
libbb/appletlib.c
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@@ -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
}