HHVM PCRE cache needs rewriting.
* m_overflow is just for deallocation -- once the shared cache is full, every call to preg_match() causes another compilation
* StaticString leakage, even when the shared cache is full. This is apparently a workaround for AtomicHashArray's key limitations "You must use atomically compare-and-swappable integer keys, or a different container class."
* AtomicHashArray is complex but very limited: no LRU, size limit baked in, "Can grow to a maximum of about 18 times the initial capacity, but performance degrades linearly with growth."
* Only item count is accounted for, size not accounted for in capacity limit
There's absolutely no chance of rewriting MediaWiki such that it doesn't eventually exhaust the shared regex cache.
Existing HHVM bug report: https://github.com/facebook/hhvm/issues/1182
Done, review pending: https://reviews.facebook.net/D25515