hashtable.h (_Hashtable<>::rehash): Review comment.

hashtable.h (_Hashtable<>::rehash): Review comment.

2019-05-04  François Dumont  <fdumont@gcc.gnu.org>

	* include/bits/hashtable.h (_Hashtable<>::rehash): Review comment.
	* include/bits/hashtable_policy.h
	(_Prime_rehash_policy::_M_bkt_for_elements): Use __builtin_ceill.
	(_Power2_rehash_policy::_M_bkt_for_elements): Likewise.
	(_Power2_rehash_policy::_M_next_bkt): Enforce returning a result not
	smaller than input value rather than always greater. Preserve
	_M_next_resize if called with 0 input. Use __builtin_floorl.
	(_Power2_rehash_policy::_M_need_rehash): Rehash only if number of
	elements + number of insertions is greater than _M_next_resize. Start
	with 11 buckets if not told otherwise. Use __builtin_floorl.
	(_Rehash_base<>::reserve): Use rehash policy _M_bkt_for_elements.
	* src/c++11/hashtable_c++0x.cc (_Prime_rehash_policy::_M_next_bkt):
	Preserve _M_next_resize if called with 0 input. Use __builtin_floorl.
	(_Prime_rehash_policy::_M_need_rehash): Start with 11 buckets if not
	told otherwise. Use __builtin_floorl.
	* testsuite/23_containers/unordered_set/hash_policy/71181.cc: Adapt test
	to also validate _Power2_rehash_policy.
	* testsuite/23_containers/unordered_set/hash_policy/power2_rehash.cc:
	Adapt.

From-SVN: r270868