[libcxx] Avoid hash key in __hash_table::find() if no buckets yet.

If the hash table has no buckets yet, it's empty and the find will do fast
return end().  Then compute hash key is useless and can be avoided, since
it could be expensive for some key types, such as long string.

This is a small optimization but useful in cases like a checklist (
implemented as unordered_set) that is mostly empty.

Author: arbiny

libcxx/include/__hash_table

@@ -1771,9 +1771,9 @@ template <class _Tp, class _Hash, class _Equal, class _Alloc>
 template <class _Key>
 typename __hash_table<_Tp, _Hash, _Equal, _Alloc>::iterator
 __hash_table<_Tp, _Hash, _Equal, _Alloc>::find(const _Key& __k) {
-  size_t __hash  = hash_function()(__k);
   size_type __bc = bucket_count();
   if (__bc != 0) {
+    size_t __hash       = hash_function()(__k);
     size_t __chash      = std::__constrain_hash(__hash, __bc);
     __next_pointer __nd = __bucket_list_[__chash];
     if (__nd != nullptr) {
@@ -1792,9 +1792,9 @@ template <class _Tp, class _Hash, class _Equal, class _Alloc>
 template <class _Key>
 typename __hash_table<_Tp, _Hash, _Equal, _Alloc>::const_iterator
 __hash_table<_Tp, _Hash, _Equal, _Alloc>::find(const _Key& __k) const {
-  size_t __hash  = hash_function()(__k);
   size_type __bc = bucket_count();
   if (__bc != 0) {
+    size_t __hash       = hash_function()(__k);
     size_t __chash      = std::__constrain_hash(__hash, __bc);
     __next_pointer __nd = __bucket_list_[__chash];
     if (__nd != nullptr) {