[libc++] Avoid constructing additional objects when using map::at

libcxx/include/CMakeLists.txt

@@ -839,6 +839,7 @@ set(files
   __type_traits/is_floating_point.h
   __type_traits/is_function.h
   __type_traits/is_fundamental.h
+  __type_traits/is_generic_transparent_comparator.h
   __type_traits/is_implicit_lifetime.h
   __type_traits/is_implicitly_default_constructible.h
   __type_traits/is_integral.h
@@ -881,6 +882,7 @@ set(files
   __type_traits/make_32_64_or_128_bit.h
   __type_traits/make_const_lvalue_ref.h
   __type_traits/make_signed.h
+  __type_traits/make_transparent.h
   __type_traits/make_unsigned.h
   __type_traits/maybe_const.h
   __type_traits/nat.h

libcxx/include/__algorithm/comp.h

@@ -11,6 +11,7 @@
 
 #include <__config>
 #include <__type_traits/desugars_to.h>
+#include <__type_traits/is_generic_transparent_comparator.h>
 #include <__type_traits/is_integral.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
@@ -48,6 +49,9 @@ inline const bool __desugars_to_v<__less_tag, __less<>, _Tp, _Tp> = true;
 template <class _Tp>
 inline const bool __desugars_to_v<__totally_ordered_less_tag, __less<>, _Tp, _Tp> = is_integral<_Tp>::value;
 
+template <>
+inline const bool __is_generic_transparent_comparator_v<__less<> > = true;
+
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP___ALGORITHM_COMP_H

libcxx/include/__functional/is_transparent.h

@@ -29,6 +29,14 @@ inline const bool __is_transparent_v<_Tp, _Key, __void_t<typename _Tp::is_transp
 
 #endif
 
+// Two types are considered transparently comparable if `comparator(key, arg)` is equivalent to `comparator(key,
+// <implicit cast to KeyT>(arg))`.
+//
+// This is different from `__is_transparent_v`, which is only a property of the comparator and doesn't provide
+// additional semantic guarantees.
+template <class _Comparator, class _KeyT, class _Arg, class = void>
+inline const bool __is_transparently_comparable_v = false;
+
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP___FUNCTIONAL_IS_TRANSPARENT

libcxx/include/__functional/operations.h

@@ -15,7 +15,9 @@
 #include <__functional/unary_function.h>
 #include <__fwd/functional.h>
 #include <__type_traits/desugars_to.h>
+#include <__type_traits/is_generic_transparent_comparator.h>
 #include <__type_traits/is_integral.h>
+#include <__type_traits/make_transparent.h>
 #include <__utility/forward.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
@@ -377,6 +379,14 @@ struct less<void> {
   typedef void is_transparent;
 };
 
+template <class _Tp>
+struct __make_transparent<less<_Tp> > {
+  using type _LIBCPP_NODEBUG = less<>;
+};
+
+template <>
+inline const bool __is_generic_transparent_comparator_v<less<>> = true;
+
 template <class _Tp, class _Up>
 inline const bool __desugars_to_v<__less_tag, less<>, _Tp, _Up> = true;
 
@@ -466,6 +476,14 @@ struct greater<void> {
 
 template <class _Tp, class _Up>
 inline const bool __desugars_to_v<__greater_tag, greater<>, _Tp, _Up> = true;
+
+template <class _Tp>
+struct __make_transparent<greater<_Tp>> {
+  using type _LIBCPP_NODEBUG = greater<>;
+};
+
+template <>
+inline const bool __is_generic_transparent_comparator_v<greater<>> = true;
 #endif
 
 // Logical operations

libcxx/include/__functional/ranges_operations.h

@@ -14,6 +14,7 @@
 #include <__concepts/totally_ordered.h>
 #include <__config>
 #include <__type_traits/desugars_to.h>
+#include <__type_traits/is_generic_transparent_comparator.h>
 #include <__utility/forward.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
@@ -108,6 +109,12 @@ inline const bool __desugars_to_v<__less_tag, ranges::less, _Tp, _Up> = true;
 template <class _Tp, class _Up>
 inline const bool __desugars_to_v<__greater_tag, ranges::greater, _Tp, _Up> = true;
 
+template <>
+inline const bool __is_generic_transparent_comparator_v<ranges::less> = true;
+
+template <>
+inline const bool __is_generic_transparent_comparator_v<ranges::greater> = true;
+
 #endif // _LIBCPP_STD_VER >= 20
 
 _LIBCPP_END_NAMESPACE_STD

libcxx/include/__tree

@@ -34,6 +34,7 @@
 #include <__type_traits/is_same.h>
 #include <__type_traits/is_specialization.h>
 #include <__type_traits/is_swappable.h>
+#include <__type_traits/make_transparent.h>
 #include <__type_traits/remove_const.h>
 #include <__utility/forward.h>
 #include <__utility/lazy_synth_three_way_comparator.h>
@@ -1749,7 +1750,8 @@ __tree<_Tp, _Compare, _Allocator>::__find_equal(const _Key& __v) {
   }
 
   __node_base_pointer* __node_ptr = __root_ptr();
-  auto __comp                     = __lazy_synth_three_way_comparator<_Compare, _Key, value_type>(value_comp());
+  auto&& __transparent            = std::__as_transparent(value_comp());
+  auto __comp = __lazy_synth_three_way_comparator<__make_transparent_t<_Compare>, _Key, value_type>(__transparent);
 
   while (true) {
     auto __comp_res = __comp(__v, __nd->__get_value());

libcxx/include/__type_traits/is_generic_transparent_comparator.h

@@ -0,0 +1,30 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___TYPE_TRAITS_IS_GENERIC_TRANSPARENT_COMPARATOR_H
+#define _LIBCPP___TYPE_TRAITS_IS_GENERIC_TRANSPARENT_COMPARATOR_H
+
+#include <__config>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+// This traits returns true if the given _Comparator is known to accept any two types for compaison. This is separate
+// from `__is_transparent_v`, since that only enables overloads of specific functions, but doesn't give any semantic
+// guarantees. This trait guarantess that the comparator simply calls the appropriate comparison functions for any two
+// types.
+
+template <class _Comparator>
+inline const bool __is_generic_transparent_comparator_v = false;
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___TYPE_TRAITS_IS_GENERIC_TRANSPARENT_COMPARATOR_H

libcxx/include/__type_traits/make_transparent.h

@@ -0,0 +1,48 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___TYPE_TRAITS_MAKE_TRANSPARENT_H
+#define _LIBCPP___TYPE_TRAITS_MAKE_TRANSPARENT_H
+
+#include <__config>
+#include <__type_traits/enable_if.h>
+#include <__type_traits/is_empty.h>
+#include <__type_traits/is_same.h>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+// __make_transparent tries to create a transparent comparator from its non-transparent counterpart, e.g. obtain
+// `less<>` from `less<T>`. This is useful in cases where conversions can be avoided (e.g. a string literal to a
+// std::string).
+
+template <class _Comparator>
+struct __make_transparent {
+  using type _LIBCPP_NODEBUG = _Comparator;
+};
+
+template <class _Comparator>
+using __make_transparent_t _LIBCPP_NODEBUG = typename __make_transparent<_Comparator>::type;
+
+template <class _Comparator, __enable_if_t<is_same<_Comparator, __make_transparent_t<_Comparator> >::value, int> = 0>
+_LIBCPP_HIDE_FROM_ABI _Comparator& __as_transparent(_Comparator& __comp) {
+  return __comp;
+}
+
+template <class _Comparator, __enable_if_t<!is_same<_Comparator, __make_transparent_t<_Comparator> >::value, int> = 0>
+_LIBCPP_HIDE_FROM_ABI __make_transparent_t<_Comparator> __as_transparent(_Comparator&) {
+  static_assert(is_empty<_Comparator>::value);
+  return __make_transparent_t<_Comparator>();
+}
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___TYPE_TRAITS_MAKE_TRANSPARENT_H

libcxx/include/map

@@ -600,7 +600,10 @@ erase_if(multimap<Key, T, Compare, Allocator>& c, Predicate pred);  // C++20
 #  include <__ranges/from_range.h>
 #  include <__tree>
 #  include <__type_traits/container_traits.h>
+#  include <__type_traits/desugars_to.h>
 #  include <__type_traits/is_allocator.h>
+#  include <__type_traits/is_convertible.h>
+#  include <__type_traits/make_transparent.h>
 #  include <__type_traits/remove_const.h>
 #  include <__type_traits/type_identity.h>
 #  include <__utility/forward.h>
@@ -666,6 +669,11 @@ public:
 #  endif
 };
 
+template <class _Key, class _MapValueT, class _Compare>
+struct __make_transparent<__map_value_compare<_Key, _MapValueT, _Compare> > {
+  using type _LIBCPP_NODEBUG = __map_value_compare<_Key, _MapValueT, __make_transparent_t<_Compare> >;
+};
+
 #  if _LIBCPP_STD_VER >= 14
 template <class _MapValueT, class _Key, class _Compare>
 struct __lazy_synth_three_way_comparator<__map_value_compare<_Key, _MapValueT, _Compare>, _MapValueT, _MapValueT> {
@@ -1048,6 +1056,24 @@ public:
   _LIBCPP_HIDE_FROM_ABI mapped_type& operator[](key_type&& __k);
 #  endif
 
+  template <class _Arg,
+            __enable_if_t<__is_transparently_comparable_v<_Compare, key_type, __remove_cvref_t<_Arg> >, int> = 0>
+  _LIBCPP_HIDE_FROM_ABI mapped_type& at(_Arg&& __arg) {
+    auto [_, __child] = __tree_.__find_equal(__arg);
+    if (__child == nullptr)
+      std::__throw_out_of_range("map::at:  key not found");
+    return static_cast<__node_pointer>(__child)->__get_value().second;
+  }
+
+  template <class _Arg,
+            __enable_if_t<__is_transparently_comparable_v<_Compare, key_type, __remove_cvref_t<_Arg> >, int> = 0>
+  _LIBCPP_HIDE_FROM_ABI const mapped_type& at(_Arg&& __arg) const {
+    auto [_, __child] = __tree_.__find_equal(__arg);
+    if (__child == nullptr)
+      std::__throw_out_of_range("map::at:  key not found");
+    return static_cast<__node_pointer>(__child)->__get_value().second;
+  }
+
   _LIBCPP_HIDE_FROM_ABI mapped_type& at(const key_type& __k);
   _LIBCPP_HIDE_FROM_ABI const mapped_type& at(const key_type& __k) const;
 
@@ -1242,11 +1268,15 @@ public:
   _LIBCPP_HIDE_FROM_ABI iterator find(const key_type& __k) { return __tree_.find(__k); }
   _LIBCPP_HIDE_FROM_ABI const_iterator find(const key_type& __k) const { return __tree_.find(__k); }
 #  if _LIBCPP_STD_VER >= 14
-  template <typename _K2, enable_if_t<__is_transparent_v<_Compare, _K2>, int> = 0>
+  template <typename _K2,
+            enable_if_t<__is_transparent_v<_Compare, _K2> || __is_transparently_comparable_v<_Compare, key_type, _K2>,
+                        int> = 0>
   _LIBCPP_HIDE_FROM_ABI iterator find(const _K2& __k) {
     return __tree_.find(__k);
   }
-  template <typename _K2, enable_if_t<__is_transparent_v<_Compare, _K2>, int> = 0>
+  template <typename _K2,
+            enable_if_t<__is_transparent_v<_Compare, _K2> || __is_transparently_comparable_v<_Compare, key_type, _K2>,
+                        int> = 0>
   _LIBCPP_HIDE_FROM_ABI const_iterator find(const _K2& __k) const {
     return __tree_.find(__k);
   }
@@ -1262,7 +1292,9 @@ public:
 
 #  if _LIBCPP_STD_VER >= 20
   _LIBCPP_HIDE_FROM_ABI bool contains(const key_type& __k) const { return find(__k) != end(); }
-  template <typename _K2, enable_if_t<__is_transparent_v<_Compare, _K2>, int> = 0>
+  template <typename _K2,
+            enable_if_t<__is_transparent_v<_Compare, _K2> || __is_transparently_comparable_v<_Compare, key_type, _K2>,
+                        int> = 0>
   _LIBCPP_HIDE_FROM_ABI bool contains(const _K2& __k) const {
     return find(__k) != end();
   }
@@ -1271,12 +1303,16 @@ public:
   _LIBCPP_HIDE_FROM_ABI iterator lower_bound(const key_type& __k) { return __tree_.lower_bound(__k); }
   _LIBCPP_HIDE_FROM_ABI const_iterator lower_bound(const key_type& __k) const { return __tree_.lower_bound(__k); }
 #  if _LIBCPP_STD_VER >= 14
-  template <typename _K2, enable_if_t<__is_transparent_v<_Compare, _K2>, int> = 0>
+  template <typename _K2,
+            enable_if_t<__is_transparent_v<_Compare, _K2> || __is_transparently_comparable_v<_Compare, key_type, _K2>,
+                        int> = 0>
   _LIBCPP_HIDE_FROM_ABI iterator lower_bound(const _K2& __k) {
     return __tree_.lower_bound(__k);
   }
 
-  template <typename _K2, enable_if_t<__is_transparent_v<_Compare, _K2>, int> = 0>
+  template <typename _K2,
+            enable_if_t<__is_transparent_v<_Compare, _K2> || __is_transparently_comparable_v<_Compare, key_type, _K2>,
+                        int> = 0>
   _LIBCPP_HIDE_FROM_ABI const_iterator lower_bound(const _K2& __k) const {
     return __tree_.lower_bound(__k);
   }
@@ -1285,11 +1321,15 @@ public:
   _LIBCPP_HIDE_FROM_ABI iterator upper_bound(const key_type& __k) { return __tree_.upper_bound(__k); }
   _LIBCPP_HIDE_FROM_ABI const_iterator upper_bound(const key_type& __k) const { return __tree_.upper_bound(__k); }
 #  if _LIBCPP_STD_VER >= 14
-  template <typename _K2, enable_if_t<__is_transparent_v<_Compare, _K2>, int> = 0>
+  template <typename _K2,
+            enable_if_t<__is_transparent_v<_Compare, _K2> || __is_transparently_comparable_v<_Compare, key_type, _K2>,
+                        int> = 0>
   _LIBCPP_HIDE_FROM_ABI iterator upper_bound(const _K2& __k) {
     return __tree_.upper_bound(__k);
   }
-  template <typename _K2, enable_if_t<__is_transparent_v<_Compare, _K2>, int> = 0>
+  template <typename _K2,
+            enable_if_t<__is_transparent_v<_Compare, _K2> || __is_transparently_comparable_v<_Compare, key_type, _K2>,
+                        int> = 0>
   _LIBCPP_HIDE_FROM_ABI const_iterator upper_bound(const _K2& __k) const {
     return __tree_.upper_bound(__k);
   }

libcxx/include/module.modulemap.in

@@ -203,6 +203,7 @@ module std_core [system] {
       header "__type_traits/is_fundamental.h"
       export std_core.type_traits.integral_constant
     }
+    module is_generic_transparent_comparator { header "__type_traits/is_generic_transparent_comparator.h" }
     module is_implicit_lifetime {
       header "__type_traits/is_implicit_lifetime.h"
       export std_core.type_traits.integral_constant
@@ -356,6 +357,7 @@ module std_core [system] {
     module make_32_64_or_128_bit                      { header "__type_traits/make_32_64_or_128_bit.h" }
     module make_const_lvalue_ref                      { header "__type_traits/make_const_lvalue_ref.h" }
     module make_signed                                { header "__type_traits/make_signed.h" }
+    module make_transparent                           { header "__type_traits/make_transparent.h" }
     module make_unsigned                              { header "__type_traits/make_unsigned.h" }
     module maybe_const                                { header "__type_traits/maybe_const.h" }
     module nat                                        { header "__type_traits/nat.h" }

libcxx/include/string

@@ -598,6 +598,7 @@ basic_string<char32_t> operator""s( const char32_t *str, size_t len );
 #  include <__debug_utils/sanitizers.h>
 #  include <__format/enable_insertable.h>
 #  include <__functional/hash.h>
+#  include <__functional/is_transparent.h>
 #  include <__functional/unary_function.h>
 #  include <__fwd/string.h>
 #  include <__iterator/bounded_iter.h>
@@ -626,6 +627,7 @@ basic_string<char32_t> operator""s( const char32_t *str, size_t len );
 #  include <__type_traits/is_allocator.h>
 #  include <__type_traits/is_array.h>
 #  include <__type_traits/is_convertible.h>
+#  include <__type_traits/is_generic_transparent_comparator.h>
 #  include <__type_traits/is_nothrow_assignable.h>
 #  include <__type_traits/is_nothrow_constructible.h>
 #  include <__type_traits/is_replaceable.h>
@@ -2560,6 +2562,20 @@ struct __default_three_way_comparator<basic_string<_CharT, _Traits, _Alloc>, bas
 };
 #  endif
 
+template <class _Comparator, class _CharT, class _Traits, class _Alloc>
+inline const bool __is_transparently_comparable_v<_Comparator,
+                                                  basic_string<_CharT, _Traits, _Alloc>,
+                                                  const _CharT*,
+                                                  __enable_if_t<__is_generic_transparent_comparator_v<_Comparator> > > =
+    true;
+
+template <class _Comparator, class _CharT, class _Traits, class _Alloc, size_t _Np>
+inline const bool __is_transparently_comparable_v<_Comparator,
+                                                  basic_string<_CharT, _Traits, _Alloc>,
+                                                  _CharT[_Np],
+                                                  __enable_if_t<__is_generic_transparent_comparator_v<_Comparator> > > =
+    true;
+
 #  if _LIBCPP_STD_VER >= 17
 template <class _InputIterator,
           class _CharT     = __iter_value_type<_InputIterator>,

libcxx/test/benchmarks/containers/associative/map.bench.cpp

@@ -16,6 +16,19 @@
 #include "../../GenerateInput.h"
 #include "benchmark/benchmark.h"
 
+static void BM_map_find_string_literal(benchmark::State& state) {
+  std::map<std::string, int> map;
+  map.emplace("Something very very long to show a long string situation", 1);
+  map.emplace("Something Else", 2);
+
+  for (auto _ : state) {
+    benchmark::DoNotOptimize(map);
+    benchmark::DoNotOptimize(map.find("Something very very long to show a long string situation"));
+  }
+}
+
+BENCHMARK(BM_map_find_string_literal);
+
 template <class K, class V>
 struct support::adapt_operations<std::map<K, V>> {
   using ValueType = typename std::map<K, V>::value_type;