Re: [PATCH][Hashtable 4/6] Clean local_iterator implementation

[Jonathan Wakely via Gcc-patches]

On 17/11/19 22:03 +0100, François Dumont wrote:

Simplify local_iterator implementation. It makes local_iterator and iterator comparable which is used in debug containers.

    * include/bits/hashtable_policy.h (_Node_iterator_base()): New.
    (operator==(const _Node_iterator_base&, const _Node_iterator_base&)):
    Make hidden friend.
    (operator!=(const _Node_iterator_base&, const _Node_iterator_base&)):
    Make hidden friend.
    (_Local_iterator_base<>): Inherits _Node_iterator_base.
    (_Local_iterator_base<>::_M_cur): Remove.
    (_Local_iterator_base<>::_M_curr()): Remove.
    (operator==(const _Local_iterator_base&, const _Local_iterator_base&)):
    Remove.
    (operator!=(const _Local_iterator_base&, const _Local_iterator_base&)):
    Remove.
    * include/debug/unordered_map (unordered_map<>::_M_invalidate): Adapt.
    (unordered_multimap<>::_M_invalidate): Adapt.
    * include/debug/unordered_set (unordered_set<>::_M_invalidate): Adapt.
    (unordered_multiset<>::_M_invalidate): Adapt.

Tested under Linux x86_64.

OK for master, with one (non-essential) change mentioned below ...

diff --git a/libstdc++-v3/include/bits/hashtable_policy.h 
b/libstdc++-v3/include/bits/hashtable_policy.h
index f330f7f811b..5cc943b3d22 100644
--- a/libstdc++-v3/include/bits/hashtable_policy.h
+++ b/libstdc++-v3/include/bits/hashtable_policy.h
@@ -301,27 +301,24 @@ namespace __detail

      __node_type*  _M_cur;

+      _Node_iterator_base() = default;
      _Node_iterator_base(__node_type* __p) noexcept
      : _M_cur(__p) { }

      void
      _M_incr() noexcept
      { _M_cur = _M_cur->_M_next(); }
-    };

-  template<typename _Value, typename _Cache_hash_code>
-    inline bool
-    operator==(const _Node_iterator_base<_Value, _Cache_hash_code>& __x,
-              const _Node_iterator_base<_Value, _Cache_hash_code >& __y)
-    noexcept
-    { return __x._M_cur == __y._M_cur; }
+      friend bool
+      operator==(const _Node_iterator_base& __x, const _Node_iterator_base& 
__y)
+      noexcept
+      { return __x._M_cur == __y._M_cur; }

-  template<typename _Value, typename _Cache_hash_code>
-    inline bool
-    operator!=(const _Node_iterator_base<_Value, _Cache_hash_code>& __x,
-              const _Node_iterator_base<_Value, _Cache_hash_code>& __y)
-    noexcept
-    { return __x._M_cur != __y._M_cur; }
+      friend bool
+      operator!=(const _Node_iterator_base& __x, const _Node_iterator_base& 
__y)
+      noexcept
+      { return __x._M_cur != __y._M_cur; }

This operator!= can be surrounded by:

#if __cpp_impl_three_way_comparison < 201907L
...
#endif

because when three-way comparisons are supported (i.e. C++20 and
later) the operator!= can be generated by the compiler.

+    };

  /// Node iterators, used to iterate through all the hashtable.
  template<typename _Value, typename _Constant_iterators,
@@ -345,7 +342,7 @@ namespace __detail
                                                  const _Value&, _Value&>::type;

      _Node_iterator() noexcept
-      : __base_type(0) { }
+      : __base_type(nullptr) { }

      explicit
      _Node_iterator(__node_type* __p) noexcept
@@ -394,7 +391,7 @@ namespace __detail
      typedef const _Value&                         reference;

      _Node_const_iterator() noexcept
-      : __base_type(0) { }
+      : __base_type(nullptr) { }

      explicit
      _Node_const_iterator(__node_type* __p) noexcept
@@ -1426,9 +1423,11 @@ namespace __detail
    struct _Local_iterator_base<_Key, _Value, _ExtractKey,
                                _H1, _H2, _Hash, __hash_cached_t>
    : private _Hashtable_ebo_helper<0, _H2>
+    , _Node_iterator_base<_Value, __hash_cached_t>

I was initially worried this is an ABI break, but it just moves the
_M_cur member into a new base class, which will be at the same offset
into the class, and won't change the sie or alignment. So I think it's
OK.

@@ -1576,33 +1572,10 @@ namespace __detail
      _M_destroy() { this->_M_h()->~__hash_code_base(); }

    public:
-      const void*
-      _M_curr() const { return _M_cur; }  // for equality ops and debug mode
-
      std::size_t
      _M_get_bucket() const { return _M_bucket; }  // for debug mode
    };

-  template<typename _Key, typename _Value, typename _ExtractKey,
-          typename _H1, typename _H2, typename _Hash,
-          typename _Cache_hash_code>
-    inline bool
-    operator==(const _Local_iterator_base<_Key, _Value, _ExtractKey,
-              _H1, _H2, _Hash, _Cache_hash_code>& __x,
-              const _Local_iterator_base<_Key, _Value, _ExtractKey,
-              _H1, _H2, _Hash, _Cache_hash_code>& __y)
-    { return __x._M_curr() == __y._M_curr(); }
-
-  template<typename _Key, typename _Value, typename _ExtractKey,
-          typename _H1, typename _H2, typename _Hash,
-          typename _Cache_hash_code>
-    inline bool
-    operator!=(const _Local_iterator_base<_Key, _Value, _ExtractKey,
-              _H1, _H2, _Hash, _Cache_hash_code>& __x,
-              const _Local_iterator_base<_Key, _Value, _ExtractKey,
-              _H1, _H2, _Hash, _Cache_hash_code>& __y)
-    { return __x._M_curr() != __y._M_curr(); }
-

Removing these operators (and making the ones for the base hidden
friends) is a nice improvement.