[libcxx-commits] [libcxx] [libc++] Simplify the implementation of std::hash (PR #140407)

[Nikolas Klauser via libcxx-commits]

https://github.com/philnik777 updated https://github.com/llvm/llvm-project/pull/140407

>From 4fc3297cce581278a08cf8f1231e90bde213836c Mon Sep 17 00:00:00 2001
From: Nikolas Klauser <nikolasklauser at berlin.de>
Date: Sat, 17 May 2025 23:25:45 +0200
Subject: [PATCH] [libc++] Simplify the implementation of std::hash

---
 libcxx/include/__functional/hash.h | 143 ++++++-----------------------
 1 file changed, 26 insertions(+), 117 deletions(-)

diff --git a/libcxx/include/__functional/hash.h b/libcxx/include/__functional/hash.h
index f9f7d2c767caa..489a6f00b8a3d 100644
--- a/libcxx/include/__functional/hash.h
+++ b/libcxx/include/__functional/hash.h
@@ -19,6 +19,8 @@
 #include <__type_traits/invoke.h>
 #include <__type_traits/is_constructible.h>
 #include <__type_traits/is_enum.h>
+#include <__type_traits/is_floating_point.h>
+#include <__type_traits/is_integral.h>
 #include <__type_traits/underlying_type.h>
 #include <__utility/pair.h>
 #include <__utility/swap.h>
@@ -345,122 +347,43 @@ struct hash<_Tp*> : public __unary_function<_Tp*, size_t> {
   }
 };
 
-template <>
-struct hash<bool> : public __unary_function<bool, size_t> {
-  _LIBCPP_HIDE_FROM_ABI size_t operator()(bool __v) const _NOEXCEPT { return static_cast<size_t>(__v); }
-};
-
-template <>
-struct hash<char> : public __unary_function<char, size_t> {
-  _LIBCPP_HIDE_FROM_ABI size_t operator()(char __v) const _NOEXCEPT { return static_cast<size_t>(__v); }
-};
-
-template <>
-struct hash<signed char> : public __unary_function<signed char, size_t> {
-  _LIBCPP_HIDE_FROM_ABI size_t operator()(signed char __v) const _NOEXCEPT { return static_cast<size_t>(__v); }
-};
-
-template <>
-struct hash<unsigned char> : public __unary_function<unsigned char, size_t> {
-  _LIBCPP_HIDE_FROM_ABI size_t operator()(unsigned char __v) const _NOEXCEPT { return static_cast<size_t>(__v); }
-};
-
-#if _LIBCPP_HAS_CHAR8_T
-template <>
-struct hash<char8_t> : public __unary_function<char8_t, size_t> {
-  _LIBCPP_HIDE_FROM_ABI size_t operator()(char8_t __v) const _NOEXCEPT { return static_cast<size_t>(__v); }
-};
-#endif // _LIBCPP_HAS_CHAR8_T
-
-template <>
-struct hash<char16_t> : public __unary_function<char16_t, size_t> {
-  _LIBCPP_HIDE_FROM_ABI size_t operator()(char16_t __v) const _NOEXCEPT { return static_cast<size_t>(__v); }
-};
-
-template <>
-struct hash<char32_t> : public __unary_function<char32_t, size_t> {
-  _LIBCPP_HIDE_FROM_ABI size_t operator()(char32_t __v) const _NOEXCEPT { return static_cast<size_t>(__v); }
-};
-
-#if _LIBCPP_HAS_WIDE_CHARACTERS
-template <>
-struct hash<wchar_t> : public __unary_function<wchar_t, size_t> {
-  _LIBCPP_HIDE_FROM_ABI size_t operator()(wchar_t __v) const _NOEXCEPT { return static_cast<size_t>(__v); }
-};
-#endif // _LIBCPP_HAS_WIDE_CHARACTERS
-
-template <>
-struct hash<short> : public __unary_function<short, size_t> {
-  _LIBCPP_HIDE_FROM_ABI size_t operator()(short __v) const _NOEXCEPT { return static_cast<size_t>(__v); }
-};
-
-template <>
-struct hash<unsigned short> : public __unary_function<unsigned short, size_t> {
-  _LIBCPP_HIDE_FROM_ABI size_t operator()(unsigned short __v) const _NOEXCEPT { return static_cast<size_t>(__v); }
+template <class _Tp, class = void>
+struct __hash_impl {
+  __hash_impl()                              = delete;
+  __hash_impl(__hash_impl const&)            = delete;
+  __hash_impl& operator=(__hash_impl const&) = delete;
 };
 
-template <>
-struct hash<int> : public __unary_function<int, size_t> {
-  _LIBCPP_HIDE_FROM_ABI size_t operator()(int __v) const _NOEXCEPT { return static_cast<size_t>(__v); }
-};
-
-template <>
-struct hash<unsigned int> : public __unary_function<unsigned int, size_t> {
-  _LIBCPP_HIDE_FROM_ABI size_t operator()(unsigned int __v) const _NOEXCEPT { return static_cast<size_t>(__v); }
-};
-
-template <>
-struct hash<long> : public __unary_function<long, size_t> {
-  _LIBCPP_HIDE_FROM_ABI size_t operator()(long __v) const _NOEXCEPT { return static_cast<size_t>(__v); }
-};
-
-template <>
-struct hash<unsigned long> : public __unary_function<unsigned long, size_t> {
-  _LIBCPP_HIDE_FROM_ABI size_t operator()(unsigned long __v) const _NOEXCEPT {
-    static_assert(sizeof(size_t) >= sizeof(unsigned long),
-                  "This would be a terrible hash function on a platform where size_t is smaller than unsigned long");
-    return static_cast<size_t>(__v);
+template <class _Tp>
+struct __hash_impl<_Tp, __enable_if_t<is_enum<_Tp>::value> > : __unary_function<_Tp, size_t> {
+  _LIBCPP_HIDE_FROM_ABI size_t operator()(_Tp __v) const _NOEXCEPT {
+    using type = __underlying_type_t<_Tp>;
+    return hash<type>()(static_cast<type>(__v));
   }
 };
 
-template <>
-struct hash<long long> : public __scalar_hash<long long> {};
-
-template <>
-struct hash<unsigned long long> : public __scalar_hash<unsigned long long> {};
-
-#if _LIBCPP_HAS_INT128
-
-template <>
-struct hash<__int128_t> : public __scalar_hash<__int128_t> {};
-
-template <>
-struct hash<__uint128_t> : public __scalar_hash<__uint128_t> {};
+template <class _Tp>
+struct __hash_impl<_Tp, __enable_if_t<is_integral<_Tp>::value && (sizeof(_Tp) <= sizeof(size_t))> >
+    : __unary_function<_Tp, size_t> {
+  _LIBCPP_HIDE_FROM_ABI size_t operator()(_Tp __v) const _NOEXCEPT { return static_cast<size_t>(__v); }
+};
 
-#endif
+template <class _Tp>
+struct __hash_impl<_Tp, __enable_if_t<is_integral<_Tp>::value && (sizeof(_Tp) > sizeof(size_t))> >
+    : __scalar_hash<_Tp> {};
 
-template <>
-struct hash<float> : public __scalar_hash<float> {
-  _LIBCPP_HIDE_FROM_ABI size_t operator()(float __v) const _NOEXCEPT {
+template <class _Tp>
+struct __hash_impl<_Tp, __enable_if_t<is_floating_point<_Tp>::value> > : __scalar_hash<_Tp> {
+  _LIBCPP_HIDE_FROM_ABI size_t operator()(_Tp __v) const _NOEXCEPT {
     // -0.0 and 0.0 should return same hash
     if (__v == 0.0f)
       return 0;
-    return __scalar_hash<float>::operator()(__v);
-  }
-};
-
-template <>
-struct hash<double> : public __scalar_hash<double> {
-  _LIBCPP_HIDE_FROM_ABI size_t operator()(double __v) const _NOEXCEPT {
-    // -0.0 and 0.0 should return same hash
-    if (__v == 0.0)
-      return 0;
-    return __scalar_hash<double>::operator()(__v);
+    return __scalar_hash<_Tp>::operator()(__v);
   }
 };
 
 template <>
-struct hash<long double> : public __scalar_hash<long double> {
+struct __hash_impl<long double> : __scalar_hash<long double> {
   _LIBCPP_HIDE_FROM_ABI size_t operator()(long double __v) const _NOEXCEPT {
     // -0.0 and 0.0 should return same hash
     if (__v == 0.0L)
@@ -501,22 +424,8 @@ struct hash<long double> : public __scalar_hash<long double> {
   }
 };
 
-template <class _Tp, bool = is_enum<_Tp>::value>
-struct __enum_hash : public __unary_function<_Tp, size_t> {
-  _LIBCPP_HIDE_FROM_ABI size_t operator()(_Tp __v) const _NOEXCEPT {
-    using type = __underlying_type_t<_Tp>;
-    return hash<type>()(static_cast<type>(__v));
-  }
-};
-template <class _Tp>
-struct __enum_hash<_Tp, false> {
-  __enum_hash()                              = delete;
-  __enum_hash(__enum_hash const&)            = delete;
-  __enum_hash& operator=(__enum_hash const&) = delete;
-};
-
 template <class _Tp>
-struct hash : public __enum_hash<_Tp> {};
+struct hash : public __hash_impl<_Tp> {};
 
 #if _LIBCPP_STD_VER >= 17