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libbb: code shrink and speed up find_applet_by_name()

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find_applet_by_name() determines the appropriate range of applet
indices to check for the given name and performs a linear search in
applet_names[].

Revise the code so the index of the upper bound of the range, 'max',
isn't calculated.  Instead check the value of the first non-matching
character to see if we've reached the end of the range.

This new code speeds up the time to find a valid applet name by 6%
and halves the time to detect that a given name is invalid.  The
average time to detect an invalid name is now the same as for a valid
one.

function                                             old     new   delta
find_applet_by_name                                  155     133     -22
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-22)             Total: -22 bytes

Signed-off-by: Ron Yorston <rmy@pobox.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
This commit is contained in:
Ron Yorston 2021-01-29 13:22:48 +00:00 committed by Denys Vlasenko
parent e8fe9f9635
commit 59120c3303
1 changed files with 17 additions and 79 deletions
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96
libbb/appletlib.c
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@ -176,7 +176,7 @@ void FAST_FUNC bb_show_usage(void)
int FAST_FUNC find_applet_by_name(const char *name)
{
unsigned i, max;
unsigned i;
int j;
const char *p;
@ -200,105 +200,43 @@ int FAST_FUNC find_applet_by_name(const char *name)
#endif
p = applet_names;
i = 0;
#if KNOWN_APPNAME_OFFSETS <= 0
max = NUM_APPLETS;
i = 0;
#else
max = NUM_APPLETS * KNOWN_APPNAME_OFFSETS;
i = NUM_APPLETS * (KNOWN_APPNAME_OFFSETS - 1);
for (j = ARRAY_SIZE(applet_nameofs)-1; j >= 0; j--) {
const char *pp = applet_names + applet_nameofs[j];
if (strcmp(name, pp) >= 0) {
//bb_error_msg("name:'%s' >= pp:'%s'", name, pp);
p = pp;
i = max - NUM_APPLETS;
break;
}
max -= NUM_APPLETS;
i -= NUM_APPLETS;
}
max /= (unsigned)KNOWN_APPNAME_OFFSETS;
i /= (unsigned)KNOWN_APPNAME_OFFSETS;
//bb_error_msg("name:'%s' starting from:'%s' i:%u max:%u", name, p, i, max);
//bb_error_msg("name:'%s' starting from:'%s' i:%u", name, p, i);
#endif
/* Open-coded linear search 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;
/* Do we see "name\0" in applet_names[p] position? */
while ((ch = *p) == name[j]) {
if (ch == '\0') {
while (*p) {
/* Do we see "name\0" at current position in applet_names? */
for (j = 0; *p == name[j]; ++j) {
if (*p++ == '\0') {
//bb_error_msg("found:'%s' i:%u", name, i);
return i; /* yes */
}
p++;
j++;
}
/* No.
* p => 1st non-matching char in applet_names[],
* skip to and including NUL.
*/
while (ch != '\0')
ch = *++p;
p++;
/* No. Have we gone too far, alphabetically? */
if (*p > name[j]) {
//bb_error_msg("break:'%s' i:%u", name, i);
break;
}
/* No. Move to the start of the next applet name. */
while (*p++ != '\0')
continue;
i++;
}
return -1;
#endif
}