[gcc r16-284] libstdc++: Optimize removal from unique assoc containers [PR112934]

[Jonathan Wakely]

https://gcc.gnu.org/g:7c2e60f67a02d17d8c2f67ba438fdb50d51bc9f4

commit r16-284-g7c2e60f67a02d17d8c2f67ba438fdb50d51bc9f4
Author: Barnabás Pőcze <pobrn@protonmail.com>
Date:   Mon Mar 11 23:35:50 2024 +0000

    libstdc++: Optimize removal from unique assoc containers [PR112934]
    
    Previously, calling erase(key) on both std::map and std::set
    would execute that same code that std::multi{map,set} would.
    However, doing that is unnecessary because std::{map,set}
    guarantee that all elements are unique.
    
    It is reasonable to expect that erase(key) is equivalent
    or better than:
    
      auto it = m.find(key);
      if (it != m.end())
        m.erase(it);
    
    However, this was not the case. Fix that by adding a new
    function _Rb_tree<>::_M_erase_unique() that is essentially
    equivalent to the above snippet, and use this from both
    std::map and std::set.
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/112934
            * include/bits/stl_map.h (map::erase): Use _M_erase_unique.
            * include/bits/stl_set.h (set::erase): Likewise.
            * include/bits/stl_tree.h (_Rb_tree::_M_erase_unique): Add.

Diff:
---
 libstdc++-v3/include/bits/stl_map.h  |  2 +-
 libstdc++-v3/include/bits/stl_set.h  |  2 +-
 libstdc++-v3/include/bits/stl_tree.h | 17 +++++++++++++++++
 3 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/libstdc++-v3/include/bits/stl_map.h b/libstdc++-v3/include/bits/stl_map.h
index 006ff46dfb5d..68c23b8e3e71 100644
--- a/libstdc++-v3/include/bits/stl_map.h
+++ b/libstdc++-v3/include/bits/stl_map.h
@@ -1156,7 +1156,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
        */
       size_type
       erase(const key_type& __x)
-      { return _M_t.erase(__x); }
+      { return _M_t._M_erase_unique(__x); }
 
 #if __cplusplus >= 201103L
       // _GLIBCXX_RESOLVE_LIB_DEFECTS
diff --git a/libstdc++-v3/include/bits/stl_set.h b/libstdc++-v3/include/bits/stl_set.h
index 0799fd0918a3..b65d63195aa2 100644
--- a/libstdc++-v3/include/bits/stl_set.h
+++ b/libstdc++-v3/include/bits/stl_set.h
@@ -728,7 +728,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
        */
       size_type
       erase(const key_type& __x)
-      { return _M_t.erase(__x); }
+      { return _M_t._M_erase_unique(__x); }
 
 #if __cplusplus >= 201103L
       // _GLIBCXX_RESOLVE_LIB_DEFECTS
diff --git a/libstdc++-v3/include/bits/stl_tree.h b/libstdc++-v3/include/bits/stl_tree.h
index 6c12c41bba7b..4b7f482e794c 100644
--- a/libstdc++-v3/include/bits/stl_tree.h
+++ b/libstdc++-v3/include/bits/stl_tree.h
@@ -1850,6 +1850,9 @@ namespace __rb_tree
       size_type
       erase(const key_type& __x);
 
+      size_type
+      _M_erase_unique(const key_type& __x);
+
 #if __cplusplus >= 201103L
       // _GLIBCXX_RESOLVE_LIB_DEFECTS
       // DR 130. Associative erase should return an iterator.
@@ -3139,6 +3142,20 @@ namespace __rb_tree
       return __old_size - size();
     }
 
+  template<typename _Key, typename _Val, typename _KeyOfValue,
+	   typename _Compare, typename _Alloc>
+    typename _Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::size_type
+    _Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::
+    _M_erase_unique(const _Key& __x)
+    {
+      iterator __it = find(__x);
+      if (__it == end())
+	return 0;
+
+      _M_erase_aux(__it);
+      return 1;
+    }
+
   template<typename _Key, typename _Val, typename _KeyOfValue,
 	   typename _Compare, typename _Alloc>
     typename _Rb_tree<_Key, _Val, _KeyOfValue,