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string.c: compare the first byte before memcmp() searching backward #7186

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Merged
matz merged 1 commit into from
Aug 15, 2026
Merged

string.c: compare the first byte before memcmp() searching backward #7186

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12 changes: 10 additions & 2 deletions src/string.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1127,10 +1127,15 @@ str_byterindex(mrb_value str, mrb_value sub, mrb_int pos)

sbeg = RSTR_PTR(ps);
t = RSTRING_PTR(sub);
/* The first byte has to match wherever the rest does, and comparing it here
settles all but the positions that carry it. Handing every position to
memcmp() instead pays for a call at each one, which is what the search
spends nearly all of its time on where the needle is not there to find. */
const char head = t[0];
/* count down an index rather than a pointer: stepping a pointer past the
first byte to end the search would leave the buffer */
for (mrb_int i = pos; 0 <= i; i--) {
if (memcmp(sbeg+i, t, len) == 0) {
if (sbeg[i] == head && memcmp(sbeg+i, t, len) == 0) {
return i;
}
}
Expand Down Expand Up @@ -1162,8 +1167,11 @@ str_char_rindex(mrb_value str, mrb_value sub, mrb_int pos)
/* a match may start only at a character boundary, and `pos` need not be
one: the clamp above answers the last byte `sub` fits at */
s = mrb_utf8_char_head(sbeg, s, send);
/* see str_byterindex(): the first byte settles all but the positions
carrying it, and it is read here anyway to step back from */
const char head = t[0];
for (;;) {
if ((mrb_int)(send - s) >= len && memcmp(s, t, len) == 0) {
if (*s == head && (mrb_int)(send - s) >= len && memcmp(s, t, len) == 0) {
return (mrb_int)(s - sbeg);
}
/* the character before `s`, which there is none of once the search has
Expand Down
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