[libcxx-commits] [libcxx] [libc++] Avoid constructing additional objects when using map::at (PR #157866)
Nikolas Klauser via libcxx-commits
libcxx-commits at lists.llvm.org
Thu Sep 18 07:13:52 PDT 2025
https://github.com/philnik777 updated https://github.com/llvm/llvm-project/pull/157866
>From 56c972197d819b566ace4eec26f431fe1923ea10 Mon Sep 17 00:00:00 2001
From: Nikolas Klauser <nikolasklauser at berlin.de>
Date: Wed, 10 Sep 2025 16:27:03 +0200
Subject: [PATCH] [libc++] Avoid constructing additional opjects when using
map::at
---
libcxx/include/CMakeLists.txt | 2 +
libcxx/include/__algorithm/comp.h | 4 ++
libcxx/include/__functional/is_transparent.h | 9 ++++
libcxx/include/__functional/operations.h | 18 +++++++
.../include/__functional/ranges_operations.h | 7 +++
libcxx/include/__tree | 4 +-
.../is_generic_transparent_comparator.h | 26 +++++++++
.../include/__type_traits/make_transparent.h | 48 +++++++++++++++++
libcxx/include/map | 54 ++++++++++++++++---
libcxx/include/module.modulemap.in | 2 +
libcxx/include/string | 16 ++++++
.../containers/associative/map.bench.cpp | 13 +++++
12 files changed, 195 insertions(+), 8 deletions(-)
create mode 100644 libcxx/include/__type_traits/is_generic_transparent_comparator.h
create mode 100644 libcxx/include/__type_traits/make_transparent.h
diff --git a/libcxx/include/CMakeLists.txt b/libcxx/include/CMakeLists.txt
index db918a16e9a61..c68250b80cf95 100644
--- a/libcxx/include/CMakeLists.txt
+++ b/libcxx/include/CMakeLists.txt
@@ -838,6 +838,7 @@ 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
@@ -880,6 +881,7 @@ 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
diff --git a/libcxx/include/__algorithm/comp.h b/libcxx/include/__algorithm/comp.h
index ab3c598418828..f6d36757ee3b8 100644
--- a/libcxx/include/__algorithm/comp.h
+++ b/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
diff --git a/libcxx/include/__functional/is_transparent.h b/libcxx/include/__functional/is_transparent.h
index 567df1a662f54..ea9715da17d49 100644
--- a/libcxx/include/__functional/is_transparent.h
+++ b/libcxx/include/__functional/is_transparent.h
@@ -29,6 +29,15 @@ 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
diff --git a/libcxx/include/__functional/operations.h b/libcxx/include/__functional/operations.h
index 7b0ea11db5844..7f315ca851c08 100644
--- a/libcxx/include/__functional/operations.h
+++ b/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
diff --git a/libcxx/include/__functional/ranges_operations.h b/libcxx/include/__functional/ranges_operations.h
index df95843e7c9af..dc9da061af264 100644
--- a/libcxx/include/__functional/ranges_operations.h
+++ b/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
diff --git a/libcxx/include/__tree b/libcxx/include/__tree
index 61c910c52c536..ef960d481cb7b 100644
--- a/libcxx/include/__tree
+++ b/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());
diff --git a/libcxx/include/__type_traits/is_generic_transparent_comparator.h b/libcxx/include/__type_traits/is_generic_transparent_comparator.h
new file mode 100644
index 0000000000000..cf2024b57abd7
--- /dev/null
+++ b/libcxx/include/__type_traits/is_generic_transparent_comparator.h
@@ -0,0 +1,26 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+
+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
diff --git a/libcxx/include/__type_traits/make_transparent.h b/libcxx/include/__type_traits/make_transparent.h
new file mode 100644
index 0000000000000..4d3207a807fa7
--- /dev/null
+++ b/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
diff --git a/libcxx/include/map b/libcxx/include/map
index e5085076faf02..b7e4757463ac2 100644
--- a/libcxx/include/map
+++ b/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, bool __b>
+struct __make_transparent<__map_value_compare<_Key, _MapValueT, _Compare, __b> > {
+ 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);
}
diff --git a/libcxx/include/module.modulemap.in b/libcxx/include/module.modulemap.in
index 63cf8e847751f..647a3b62a9e49 100644
--- a/libcxx/include/module.modulemap.in
+++ b/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" }
diff --git a/libcxx/include/string b/libcxx/include/string
index bbd7b98f112a6..8fa4c9fa4ada5 100644
--- a/libcxx/include/string
+++ b/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 <__ios/fpos.h>
@@ -628,6 +629,7 @@ 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>
@@ -2564,6 +2566,20 @@ 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>,
diff --git a/libcxx/test/benchmarks/containers/associative/map.bench.cpp b/libcxx/test/benchmarks/containers/associative/map.bench.cpp
index bd664dbb56ee7..142229ae64cad 100644
--- a/libcxx/test/benchmarks/containers/associative/map.bench.cpp
+++ b/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;
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