Fix regexp performance regression for patterns starting with s/k

Commit 981ee02c7c ("Fix performance problem with /k/i and /s/i") was
merged for Ruby 4.0 to enable partial Boyer-Moore optimization for
patterns containing 's' or 'k' by using the prefix before those
characters.

However, when 's' or 'k' appears at the start of a pattern (no usable
prefix), set_bm_skip() returns 0 and the code returned early without
setting any optimization mode, leaving reg->optimize at
ONIG_OPTIMIZE_NONE. This caused up to 30x slowdown for patterns like
/slackware/i when matched against strings with non-ASCII characters.

This patch keeps the improvement from 981ee02c7c for patterns with
3+ char prefix, while fixing the regression by falling back to
ONIG_OPTIMIZE_EXACT_IC with the full pattern when the usable prefix
is less than 3 characters.

Before: /\bslackware\b/i with non-ASCII string: 2.24 us/op
After:  /\bslackware\b/i with non-ASCII string: 0.70 us/op (3.2x faster)

[Bug #21824]

0 files changed, 0 insertions, 0 deletions