[libcxx-commits] [libcxx] [libc++] Reduce std::conjunction overhead (PR #124259)

[Nikolas Klauser via libcxx-commits]

https://github.com/philnik777 created https://github.com/llvm/llvm-project/pull/124259

The old and new implementation of `_And` are very close in terms of performance according to my testing, but the new implementation can also be used to implement `conjunction`, which make that ~50% faster.


>From 63da627beebb0a98705ba9d88b47f66e363a1a87 Mon Sep 17 00:00:00 2001
From: Nikolas Klauser <nikolasklauser at berlin.de>
Date: Fri, 24 Jan 2025 12:27:00 +0100
Subject: [PATCH] [libc++] Reduce std::conjunction overhead

---
 libcxx/include/__type_traits/conjunction.h | 42 ++++++++++------------
 1 file changed, 18 insertions(+), 24 deletions(-)

diff --git a/libcxx/include/__type_traits/conjunction.h b/libcxx/include/__type_traits/conjunction.h
index ad9656acd47ecc..5d45e221b9a975 100644
--- a/libcxx/include/__type_traits/conjunction.h
+++ b/libcxx/include/__type_traits/conjunction.h
@@ -10,8 +10,6 @@
 #define _LIBCPP___TYPE_TRAITS_CONJUNCTION_H
 
 #include <__config>
-#include <__type_traits/conditional.h>
-#include <__type_traits/enable_if.h>
 #include <__type_traits/integral_constant.h>
 #include <__type_traits/is_same.h>
 
@@ -21,22 +19,29 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-template <class...>
-using __expand_to_true _LIBCPP_NODEBUG = true_type;
+template <bool>
+struct _AndImpl;
 
-template <class... _Pred>
-__expand_to_true<__enable_if_t<_Pred::value>...> __and_helper(int);
+template <>
+struct _AndImpl<true> {
+  template <class _Res, class _First, class... _Rest>
+  using _Result _LIBCPP_NODEBUG =
+      typename _AndImpl<bool(_First::value) && sizeof...(_Rest) != 0>::template _Result<_First, _Rest...>;
+};
 
-template <class...>
-false_type __and_helper(...);
+template <>
+struct _AndImpl<false> {
+  template <class _Res, class...>
+  using _Result = _Res;
+};
 
 // _And always performs lazy evaluation of its arguments.
 //
 // However, `_And<_Pred...>` itself will evaluate its result immediately (without having to
 // be instantiated) since it is an alias, unlike `conjunction<_Pred...>`, which is a struct.
 // If you want to defer the evaluation of `_And<_Pred...>` itself, use `_Lazy<_And, _Pred...>`.
-template <class... _Pred>
-using _And _LIBCPP_NODEBUG = decltype(std::__and_helper<_Pred...>(0));
+template <class... _Args>
+using _And _LIBCPP_NODEBUG = typename _AndImpl<sizeof...(_Args) != 0>::template _Result<true_type, _Args...>;
 
 template <bool... _Preds>
 struct __all_dummy;
@@ -46,22 +51,11 @@ struct __all : _IsSame<__all_dummy<_Pred...>, __all_dummy<((void)_Pred, true)...
 
 #if _LIBCPP_STD_VER >= 17
 
-template <class...>
-struct _LIBCPP_NO_SPECIALIZATIONS conjunction : true_type {};
-
-_LIBCPP_DIAGNOSTIC_PUSH
-#  if __has_warning("-Winvalid-specialization")
-_LIBCPP_CLANG_DIAGNOSTIC_IGNORED("-Winvalid-specialization")
-#  endif
-template <class _Arg>
-struct conjunction<_Arg> : _Arg {};
-
-template <class _Arg, class... _Args>
-struct conjunction<_Arg, _Args...> : conditional_t<!bool(_Arg::value), _Arg, conjunction<_Args...>> {};
-_LIBCPP_DIAGNOSTIC_POP
+template <class... _Args>
+struct _LIBCPP_NO_SPECIALIZATIONS conjunction : _And<_Args...> {};
 
 template <class... _Args>
-_LIBCPP_NO_SPECIALIZATIONS inline constexpr bool conjunction_v = conjunction<_Args...>::value;
+_LIBCPP_NO_SPECIALIZATIONS inline constexpr bool conjunction_v = _And<_Args...>::value;
 
 #endif // _LIBCPP_STD_VER >= 17