[libcxx-commits] [libcxx] [libc++] Create a `constexpr` hash to use during constant evaluation (PR #214946)
Vinay Deshmukh via libcxx-commits
libcxx-commits at lists.llvm.org
Mon Aug 10 00:43:43 PDT 2026
https://github.com/vinay-deshmukh updated https://github.com/llvm/llvm-project/pull/214946
>From 817a1a5c598f182cee778de8822cad23a6264568 Mon Sep 17 00:00:00 2001
From: Vinay Deshmukh <vinay_deshmukh at outlook.com>
Date: Sat, 8 Aug 2026 10:33:03 +0100
Subject: [PATCH 1/4] Parameterize first hash test for different hash function
---
.../unord.hash/integral.pass.cpp | 138 +++++++++---------
1 file changed, 71 insertions(+), 67 deletions(-)
diff --git a/libcxx/test/std/utilities/function.objects/unord.hash/integral.pass.cpp b/libcxx/test/std/utilities/function.objects/unord.hash/integral.pass.cpp
index c798b3aa77ea5..0c09ec5fc9f0e 100644
--- a/libcxx/test/std/utilities/function.objects/unord.hash/integral.pass.cpp
+++ b/libcxx/test/std/utilities/function.objects/unord.hash/integral.pass.cpp
@@ -30,17 +30,16 @@
# include "poisoned_hash_helper.h"
#endif
-template <class T>
-void
-test()
-{
+template <class T, template <typename> typename THash = std::hash >
+void test() {
#if TEST_STD_VER >= 11
- test_hash_disabled<const T>();
- test_hash_disabled<volatile T>();
- test_hash_disabled<const volatile T>();
+ test_hash_disabled<const T, THash<const T>>();
+ test_hash_disabled<volatile T, THash<volatile T>>();
+ test_hash_disabled<const volatile T, THash<const volatile T>>();
#endif
- typedef std::hash<T> H;
+ // typedef std::hash<T> H;
+ typedef THash<T> H;
#if TEST_STD_VER <= 17
static_assert((std::is_same<typename H::argument_type, T>::value), "");
static_assert((std::is_same<typename H::result_type, std::size_t>::value), "");
@@ -61,70 +60,75 @@ test()
}
}
-int main(int, char**)
-{
- test<bool>();
- test<char>();
- test<signed char>();
- test<unsigned char>();
- test<char16_t>();
- test<char32_t>();
+template <template <typename> typename THash >
+bool test_with_hash() {
+ test<bool, THash>();
+ test<char, THash>();
+ test<signed char, THash>();
+ test<unsigned char, THash>();
+ test<char16_t, THash>();
+ test<char32_t, THash>();
#ifndef TEST_HAS_NO_WIDE_CHARACTERS
- test<wchar_t>();
+ test<wchar_t, THash>();
#endif
- test<short>();
- test<unsigned short>();
- test<int>();
- test<unsigned int>();
- test<long>();
- test<unsigned long>();
- test<long long>();
- test<unsigned long long>();
-
-// LWG #2119
- test<std::ptrdiff_t>();
- test<std::size_t>();
-
- test<std::int8_t>();
- test<std::int16_t>();
- test<std::int32_t>();
- test<std::int64_t>();
-
- test<std::int_fast8_t>();
- test<std::int_fast16_t>();
- test<std::int_fast32_t>();
- test<std::int_fast64_t>();
-
- test<std::int_least8_t>();
- test<std::int_least16_t>();
- test<std::int_least32_t>();
- test<std::int_least64_t>();
-
- test<std::intmax_t>();
- test<std::intptr_t>();
-
- test<std::uint8_t>();
- test<std::uint16_t>();
- test<std::uint32_t>();
- test<std::uint64_t>();
-
- test<std::uint_fast8_t>();
- test<std::uint_fast16_t>();
- test<std::uint_fast32_t>();
- test<std::uint_fast64_t>();
-
- test<std::uint_least8_t>();
- test<std::uint_least16_t>();
- test<std::uint_least32_t>();
- test<std::uint_least64_t>();
-
- test<std::uintmax_t>();
- test<std::uintptr_t>();
+ test<short, THash>();
+ test<unsigned short, THash>();
+ test<int, THash>();
+ test<unsigned int, THash>();
+ test<long, THash>();
+ test<unsigned long, THash>();
+ test<long long, THash>();
+ test<unsigned long long, THash>();
+
+ // LWG #2119
+ test<std::ptrdiff_t, THash>();
+ test<std::size_t, THash>();
+
+ test<std::int8_t, THash>();
+ test<std::int16_t, THash>();
+ test<std::int32_t, THash>();
+ test<std::int64_t, THash>();
+
+ test<std::int_fast8_t, THash>();
+ test<std::int_fast16_t, THash>();
+ test<std::int_fast32_t, THash>();
+ test<std::int_fast64_t, THash>();
+
+ test<std::int_least8_t, THash>();
+ test<std::int_least16_t, THash>();
+ test<std::int_least32_t, THash>();
+ test<std::int_least64_t, THash>();
+
+ test<std::intmax_t, THash>();
+ test<std::intptr_t, THash>();
+
+ test<std::uint8_t, THash>();
+ test<std::uint16_t, THash>();
+ test<std::uint32_t, THash>();
+ test<std::uint64_t, THash>();
+
+ test<std::uint_fast8_t, THash>();
+ test<std::uint_fast16_t, THash>();
+ test<std::uint_fast32_t, THash>();
+ test<std::uint_fast64_t, THash>();
+
+ test<std::uint_least8_t, THash>();
+ test<std::uint_least16_t, THash>();
+ test<std::uint_least32_t, THash>();
+ test<std::uint_least64_t, THash>();
+
+ test<std::uintmax_t, THash>();
+ test<std::uintptr_t, THash>();
#ifndef TEST_HAS_NO_INT128
- test<__int128_t>();
- test<__uint128_t>();
+ test<__int128_t, THash>();
+ test<__uint128_t, THash>();
#endif
+ return true;
+}
+
+int main(int, char**) {
+ assert(test_with_hash<std::hash>());
return 0;
}
>From 1b6383a715716d0ba9c787097311e24640433867 Mon Sep 17 00:00:00 2001
From: Vinay Deshmukh <vinay_deshmukh at outlook.com>
Date: Sat, 8 Aug 2026 10:43:07 +0100
Subject: [PATCH 2/4] Setup a failing test for std::__constexpr_hash
---
.../function.objects/unord.hash/integral.pass.cpp | 13 +++++++++++++
1 file changed, 13 insertions(+)
diff --git a/libcxx/test/std/utilities/function.objects/unord.hash/integral.pass.cpp b/libcxx/test/std/utilities/function.objects/unord.hash/integral.pass.cpp
index 0c09ec5fc9f0e..d4d9bb4fb5041 100644
--- a/libcxx/test/std/utilities/function.objects/unord.hash/integral.pass.cpp
+++ b/libcxx/test/std/utilities/function.objects/unord.hash/integral.pass.cpp
@@ -128,7 +128,20 @@ bool test_with_hash() {
return true;
}
+// TODO: move to libcxx/include/__functional/hash.h
+namespace std {
+
+template <class _Key>
+struct __constexpr_hash {};
+
+} // namespace std
+
int main(int, char**) {
assert(test_with_hash<std::hash>());
+
+#if TEST_STD_VER >= 26
+ static_assert(test_with_hash<std::__constexpr_hash>());
+#endif
+
return 0;
}
>From ce52820da7d6dfe85ff92cfad1b69bcf07fcb33d Mon Sep 17 00:00:00 2001
From: Vinay Deshmukh <vinay_deshmukh at outlook.com>
Date: Sat, 8 Aug 2026 14:15:15 +0100
Subject: [PATCH 3/4] Pass integer.pass.cpp
---
.../unord.hash/integral.pass.cpp | 50 +++++++++++++++++--
libcxx/test/support/poisoned_hash_helper.h | 2 +-
2 files changed, 47 insertions(+), 5 deletions(-)
diff --git a/libcxx/test/std/utilities/function.objects/unord.hash/integral.pass.cpp b/libcxx/test/std/utilities/function.objects/unord.hash/integral.pass.cpp
index d4d9bb4fb5041..7bc1425cb291f 100644
--- a/libcxx/test/std/utilities/function.objects/unord.hash/integral.pass.cpp
+++ b/libcxx/test/std/utilities/function.objects/unord.hash/integral.pass.cpp
@@ -31,7 +31,7 @@
#endif
template <class T, template <typename> typename THash = std::hash >
-void test() {
+TEST_CONSTEXPR_CXX26 void test() {
#if TEST_STD_VER >= 11
test_hash_disabled<const T, THash<const T>>();
test_hash_disabled<volatile T, THash<volatile T>>();
@@ -61,7 +61,7 @@ void test() {
}
template <template <typename> typename THash >
-bool test_with_hash() {
+TEST_CONSTEXPR_CXX26 bool test_with_hash() {
test<bool, THash>();
test<char, THash>();
test<signed char, THash>();
@@ -128,13 +128,55 @@ bool test_with_hash() {
return true;
}
+#if TEST_STD_VER >= 26
// TODO: move to libcxx/include/__functional/hash.h
namespace std {
-template <class _Key>
-struct __constexpr_hash {};
+// TODO: use _ prefix
+template <typename _Tp>
+concept EnabledForHash = requires(_Tp t) {
+ { std::bool_constant<__is_unqualified_v<_Tp>>() } -> std::same_as<std::true_type>;
+};
+
+template <typename _Tp>
+concept DisabledForHash = not EnabledForHash<_Tp>;
+
+// TODO: document the constraints of using this at runtime OR make it consteval only
+template <typename _Tp>
+struct __constexpr_hash;
+
+template <DisabledForHash _Tp>
+struct __constexpr_hash<_Tp> {
+ __constexpr_hash() = delete;
+ __constexpr_hash(const __constexpr_hash&) = delete;
+ __constexpr_hash& operator=(const __constexpr_hash&) = delete;
+};
+
+template <EnabledForHash _Tp>
+struct __constexpr_hash<_Tp> {
+ [[__nodiscard__]] constexpr _LIBCPP_HIDE_FROM_ABI size_t operator()(const _Tp& __v) const noexcept {
+ if constexpr (std::is_same_v<_Tp, nullptr_t>) {
+ return 662607004ull;
+ } else if constexpr (std::is_integral_v<_Tp>) {
+ if constexpr (sizeof(_Tp) <= sizeof(size_t)) {
+ return static_cast<size_t>(__v);
+ } else {
+ constexpr size_t multiple = sizeof(_Tp) / sizeof(size_t);
+ char region[multiple];
+
+ // TODO: 0, 1, 2, 3, 4
+ return region[multiple - 1]; // TODO: hash-ing
+ }
+ }
+ __builtin_unreachable(); // todo: revisit
+ }
+
+ __constexpr_hash() noexcept = default;
+ __constexpr_hash& operator=(const __constexpr_hash&) = default;
+};
} // namespace std
+#endif
int main(int, char**) {
assert(test_with_hash<std::hash>());
diff --git a/libcxx/test/support/poisoned_hash_helper.h b/libcxx/test/support/poisoned_hash_helper.h
index cd71cd70d6a84..ec3f6e9af813f 100644
--- a/libcxx/test/support/poisoned_hash_helper.h
+++ b/libcxx/test/support/poisoned_hash_helper.h
@@ -87,7 +87,7 @@ TEST_CONSTEXPR_CXX20 void test_hash_enabled(Key const& key = Key{}) {
// Test that the specified Hash meets the requirements of a disabled hash.
template <class Key, class Hash = std::hash<Key>>
-void test_hash_disabled() {
+TEST_CONSTEXPR_CXX26 void test_hash_disabled() {
// Disabled hash requirements
static_assert(!std::is_default_constructible<Hash>::value, "");
static_assert(!std::is_copy_constructible<Hash>::value, "");
>From ae2da4414f761421c0eee17cd97cbeae03ee6c2b Mon Sep 17 00:00:00 2001
From: Vinay Deshmukh <vinay_deshmukh at outlook.com>
Date: Mon, 10 Aug 2026 08:43:19 +0100
Subject: [PATCH 4/4] -Wc++17 extensions
---
.../std/utilities/function.objects/unord.hash/integral.pass.cpp | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/libcxx/test/std/utilities/function.objects/unord.hash/integral.pass.cpp b/libcxx/test/std/utilities/function.objects/unord.hash/integral.pass.cpp
index 7bc1425cb291f..4974916ed4c80 100644
--- a/libcxx/test/std/utilities/function.objects/unord.hash/integral.pass.cpp
+++ b/libcxx/test/std/utilities/function.objects/unord.hash/integral.pass.cpp
@@ -30,7 +30,7 @@
# include "poisoned_hash_helper.h"
#endif
-template <class T, template <typename> typename THash = std::hash >
+template <class T, template <class> class THash = std::hash >
TEST_CONSTEXPR_CXX26 void test() {
#if TEST_STD_VER >= 11
test_hash_disabled<const T, THash<const T>>();
More information about the libcxx-commits
mailing list