[PATCH] Simplify _Hashtable::_M_merge_multi

[François Dumont]

Hi

A minor simplification.
libstdc++: Simplify _Hashtable::_M_merge_multi

When merging two hashtable instances of the same type we do not need
to go through _M_src_hash_code that also check for identical Hash functor
type.

libstdc++-v3/ChangeLog:

        * include/bits/hashtable.h 
(_Hashtable<>::_M_merge_multi(_Hashtable&)):
        Remove usage of _M_src_hash_code and check directly if Hash is 
stateless
        in the method.

Tested under Linux x64.

Ok to commit ?

François

diff --git a/libstdc++-v3/include/bits/hashtable.h 
b/libstdc++-v3/include/bits/hashtable.h
index d6d76a743bb..3bcff67a377 100644
--- a/libstdc++-v3/include/bits/hashtable.h
+++ b/libstdc++-v3/include/bits/hashtable.h
@@ -1328,8 +1328,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
          {
            const auto& __node = static_cast<__node_type&>(*__prev->_M_nxt);
            const key_type& __k = _ExtractKey{}(__node._M_v());
-           // Hash code from this->hash_function():
-           auto __code = _M_src_hash_code(__src.hash_function(), __k, __node);
+           __hash_code __code;
+           if constexpr (std::is_empty_v<_Hash>)
+             // If the node has a cached hash code, it's OK to use it.
+             __code = this->_M_hash_code(__node);
+           else
+             __code = this->_M_hash_code(__k);
+
            // Bucket index in __src, using code from __src.hash_function():
            size_type __src_bkt = __src._M_bucket_index(__node);
            auto __nh = __src._M_extract_node(__src_bkt, __prev);