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ash: fix quadratic matching slowdown is ${v/*foo*/repl} (really bad one)

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It is especially bad with patterns starting with "*".

With ASH_OPTIMIZE_FOR_SIZE=y, only those are optimized, +few bytes:
   text    data     bss     dec     hex filename
 836337     441    7564  844342   ce236 busybox_old
 836341     441    7564  844346   ce23a busybox_unstripped

With ASH_OPTIMIZE_FOR_SIZE off, we also optimize patterns _ending_ with "*",
which costs about 80 bytes:
   text    data     bss     dec     hex filename
 836656     441    7564  844661   ce375 busybox_old
 836732     441    7564  844737   ce3c1 busybox_unstripped

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
This commit is contained in:
Denys Vlasenko 2010-03-13 16:19:04 +01:00
parent 6eaeb7737d
commit b76356b28e
1 changed files with 59 additions and 12 deletions
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71
shell/ash.c
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@ -6040,25 +6040,61 @@ scanleft(char *startp, char *rmesc, char *rmescend UNUSED_PARAM, char *str, int
}
static char *
scanright(char *startp, char *rmesc, char *rmescend, char *str, int quotes,
int zero)
scanright(char *startp, char *rmesc, char *rmescend, char *pattern, int quotes, int match_at_start)
{
#if !ENABLE_ASH_OPTIMIZE_FOR_SIZE
int try2optimize = match_at_start;
#endif
int esc = 0;
char *loc;
char *loc2;
for (loc = str - 1, loc2 = rmescend; loc >= startp; loc2--) {
/* If we called by "${v/pattern/repl}" or "${v//pattern/repl}":
* startp="escaped_value_of_v" rmesc="raw_value_of_v"
* rmescend=""(ptr to NUL in rmesc) pattern="pattern" quotes=match_at_start=1
* Logic:
* loc starts at NUL at the end of startp, loc2 starts at the end of rmesc,
* and on each iteration they go back two/one char until they reach the beginning.
* We try to find a match in "raw_value_of_v", "raw_value_of_", "raw_value_of" etc.
*/
/* TODO: document in what other circumstances we are called. */
for (loc = pattern - 1, loc2 = rmescend; loc >= startp; loc2--) {
int match;
char c = *loc2;
const char *s = loc2;
if (zero) {
if (match_at_start) {
*loc2 = '\0';
s = rmesc;
}
match = pmatch(str, s);
match = pmatch(pattern, s);
//bb_error_msg("pmatch(pattern:'%s',s:'%s'):%d", pattern, s, match);
*loc2 = c;
if (match)
return loc;
#if !ENABLE_ASH_OPTIMIZE_FOR_SIZE
if (try2optimize) {
/* Maybe we can optimize this:
* if pattern ends with unescaped *, we can avoid checking
* shorter strings: if "foo*" doesnt match "raw_value_of_v",
* it wont match truncated "raw_value_of_" strings too.
*/
unsigned plen = strlen(pattern);
/* Does it end with "*"? */
if (plen != 0 && pattern[--plen] == '*') {
/* "xxxx*" is not escaped */
/* "xxx\*" is escaped */
/* "xx\\*" is not escaped */
/* "x\\\*" is escaped */
int slashes = 0;
while (plen != 0 && pattern[--plen] == '\\')
slashes++;
if (!(slashes & 1))
break; /* ends with unescaped "*" */
}
try2optimize = 0;
}
#endif
loc--;
if (quotes) {
if (--esc < 0) {
@ -6248,7 +6284,7 @@ subevalvar(char *p, char *str, int strloc, int subtype,
#if ENABLE_ASH_BASH_COMPAT
if (subtype == VSREPLACE || subtype == VSREPLACEALL) {
char *idx, *end, *restart_detect;
char *idx, *end;
if (!repl) {
repl = parse_sub_pattern(str, varflags & VSQUOTE);
@ -6257,17 +6293,19 @@ subevalvar(char *p, char *str, int strloc, int subtype,
}
/* If there's no pattern to match, return the expansion unmolested */
if (*str == '\0')
if (str[0] == '\0')
return 0;
len = 0;
idx = startp;
end = str - 1;
while (idx < end) {
try_to_match:
loc = scanright(idx, rmesc, rmescend, str, quotes, 1);
if (!loc) {
/* No match, advance */
restart_detect = stackblock();
char *restart_detect = stackblock();
skip_matching:
STPUTC(*idx, expdest);
if (quotes && (unsigned char)*idx == CTLESC) {
idx++;
@ -6279,7 +6317,16 @@ subevalvar(char *p, char *str, int strloc, int subtype,
idx++;
len++;
rmesc++;
continue;
/* continue; - prone to quadratic behavior, smarter code: */
if (idx >= end)
break;
if (str[0] == '*') {
/* Pattern is "*foo". If "*foo" does not match "long_string",
* it would never match "ong_string" etc, no point in trying.
*/
goto skip_matching;
}
goto try_to_match;
}
if (subtype == VSREPLACEALL) {
@ -6294,7 +6341,7 @@ subevalvar(char *p, char *str, int strloc, int subtype,
}
for (loc = repl; *loc; loc++) {
restart_detect = stackblock();
char *restart_detect = stackblock();
STPUTC(*loc, expdest);
if (stackblock() != restart_detect)
goto restart;
@ -6303,7 +6350,7 @@ subevalvar(char *p, char *str, int strloc, int subtype,
if (subtype == VSREPLACE) {
while (*idx) {
restart_detect = stackblock();
char *restart_detect = stackblock();
STPUTC(*idx, expdest);
if (stackblock() != restart_detect)
goto restart;
@ -6332,7 +6379,7 @@ subevalvar(char *p, char *str, int strloc, int subtype,
if (subtype < 0 || subtype > 7)
abort();
#endif
/* zero = subtype == VSTRIMLEFT || subtype == VSTRIMLEFTMAX */
/* zero = (subtype == VSTRIMLEFT || subtype == VSTRIMLEFTMAX) */
zero = subtype >> 1;
/* VSTRIMLEFT/VSTRIMRIGHTMAX -> scanleft */
scan = (subtype & 1) ^ zero ? scanleft : scanright;