[PATCH] libstdc++: Optimize std::con/disjunction, __and_/__or_, etc

[Patrick Palka <ppalka@redhat.com>]

The internal type-level logical operations __and_ and __or_ are
currently quite slow to compile for a couple of reasons:

  1. They are drop-in replacements for std::con/disjunction, which
     are rigidly specified to form a type that derives from the first
     type argument that caused the overall computation to short-circuit.
     In practice this inheritance property seems to be rarely needed;
     usually all we care about is the value of the overall expression.
  2. Their recursive implementations instantiate up to ~N class templates
     and form up to a depth ~N inheritance chain.

This patch does away with this inheritance property of __and_ and __or_
(which seems to be unneeded in the library except indirectly by
std::con/disjunction) and redefines them as alias templates that yield
either false_type or true_type via SFINAE and overload resolution of a
pair of function templates.

As for std::con/disjunction, it seems we need to keep defining them in
terms of recursive class templates for sake of the inheritance property.
But in the recursive step, instead of using inheritance which would
yield a depth ~N inheritance chain, use a recursive member typedef which
gets immediately flattened.  Thus a specialization of conjunction and
disjunction now has depth ~1 instead of up to ~N.

In passing, redefine __not_ as an alias template for consistency with
__and_ and __or_, and to remove a layer of indirection.

Together these changes have a substantial effect on compile time and
memory usage for code that indirectly makes heavy use of these internal
type traits.  For example, for the following which tests constructibility
between two compatible 257-element tuple types

  #include <tuple>

  struct A { A(int); };

  #define M(x) x, x

  using ty1 = std::tuple<M(M(M(M(M(M(M(M(int)))))))), int>;
  using ty2 = std::tuple<M(M(M(M(M(M(M(M(int)))))))), A>;

  static_assert(std::is_constructible_v<ty2, ty1>);

memory usage improves by ~27% from 440MB to 320M and compile time
improves by ~20% from ~2s to ~1.6s (with -std=c++23).

Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
trunk?

libstdc++-v3/ChangeLog:

	* include/std/type_traits (enable_if, __enable_if_t): Move up
	their definitions.
	(__detail::__first_t): Define.
	(__detail::__or_fn, __detail::__and_fn): Declare.
	(__or_, __and_): Redefine as alias templates in terms of __or_fn
	and __and_fn.
	(__not_): Redefine as an alias template.
	(__detail::__disjunction_impl, __detail::__conjunction_impl):
	Define.
	(conjuction, disjunction): Redefine in terms of __disjunction_impl
	and __conjunction_impl.
---
 libstdc++-v3/include/std/type_traits | 152 ++++++++++++++++-----------
 1 file changed, 93 insertions(+), 59 deletions(-)

diff --git a/libstdc++-v3/include/std/type_traits b/libstdc++-v3/include/std/type_traits
index 14b029cec64..07a50a31e86 100644
--- a/libstdc++-v3/include/std/type_traits
+++ b/libstdc++-v3/include/std/type_traits
@@ -100,6 +100,21 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
   // Metaprogramming helper types.
 
+  // Primary template.
+  /// Define a member typedef `type` only if a boolean constant is true.
+  template<bool, typename _Tp = void>
+    struct enable_if
+    { };
+
+  // Partial specialization for true.
+  template<typename _Tp>
+    struct enable_if<true, _Tp>
+    { typedef _Tp type; };
+
+  // __enable_if_t (std::enable_if_t for C++11)
+  template<bool _Cond, typename _Tp = void>
+    using __enable_if_t = typename enable_if<_Cond, _Tp>::type;
+
   template<bool>
     struct __conditional
     {
@@ -127,56 +142,39 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   template<typename _Tp>
     using __type_identity_t = typename __type_identity<_Tp>::type;
 
-  template<typename...>
-    struct __or_;
-
-  template<>
-    struct __or_<>
-    : public false_type
-    { };
+  namespace __detail
+  {
+    // A variadic alias template that resolves to its first argument.
+    template<typename _Tp, typename...>
+      using __first_t = _Tp;
 
-  template<typename _B1>
-    struct __or_<_B1>
-    : public _B1
-    { };
+    // These are deliberately not defined.
+    template<typename... _Bn>
+      auto __or_fn(int) -> __first_t<false_type,
+				     __enable_if_t<!bool(_Bn::value)>...>;
 
-  template<typename _B1, typename _B2>
-    struct __or_<_B1, _B2>
-    : public __conditional_t<_B1::value, _B1, _B2>
-    { };
+    template<typename... _Bn>
+      auto __or_fn(...) -> true_type;
 
-  template<typename _B1, typename _B2, typename _B3, typename... _Bn>
-    struct __or_<_B1, _B2, _B3, _Bn...>
-    : public __conditional_t<_B1::value, _B1, __or_<_B2, _B3, _Bn...>>
-    { };
+    template<typename... _Bn>
+      auto __and_fn(int) -> __first_t<true_type,
+				      __enable_if_t<_Bn::value>...>;
 
-  template<typename...>
-    struct __and_;
+    template<typename... _Bn>
+      auto __and_fn(...) -> false_type;
+  } // namespace detail
 
-  template<>
-    struct __and_<>
-    : public true_type
-    { };
-
-  template<typename _B1>
-    struct __and_<_B1>
-    : public _B1
-    { };
-
-  template<typename _B1, typename _B2>
-    struct __and_<_B1, _B2>
-    : public __conditional_t<_B1::value, _B2, _B1>
-    { };
+  // Like C++17 std::dis/conjunction, but usable in C++11 and resolves
+  // to either true_type or false_type which allows for a more efficient
+  // implementation that avoids instantiating any class templates.
+  template<typename... _Bn>
+    using __or_ = decltype(__detail::__or_fn<_Bn...>(0));
 
-  template<typename _B1, typename _B2, typename _B3, typename... _Bn>
-    struct __and_<_B1, _B2, _B3, _Bn...>
-    : public __conditional_t<_B1::value, __and_<_B2, _B3, _Bn...>, _B1>
-    { };
+  template<typename... _Bn>
+    using __and_ = decltype(__detail::__and_fn<_Bn...>(0));
 
   template<typename _Pp>
-    struct __not_
-    : public __bool_constant<!bool(_Pp::value)>
-    { };
+    using __not_ = __bool_constant<!bool(_Pp::value)>;
   /// @endcond
 
 #if __cplusplus >= 201703L
@@ -186,18 +184,69 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     inline constexpr bool __or_v = __or_<_Bn...>::value;
   template<typename... _Bn>
     inline constexpr bool __and_v = __and_<_Bn...>::value;
+
+  namespace __detail
+  {
+    template<typename...>
+      struct __disjunction_impl;
+
+    template<>
+      struct __disjunction_impl<>
+      { using type = false_type; };
+
+    template<typename _B1>
+      struct __disjunction_impl<_B1>
+      { using type = _B1; };
+
+    template<typename _B1, typename _B2>
+      struct __disjunction_impl<_B1, _B2>
+      { using type = __conditional_t<_B1::value, _B1, _B2>; };
+
+    template<typename _B1, typename _B2, typename _B3, typename... _Bn>
+      struct __disjunction_impl<_B1, _B2, _B3, _Bn...>
+      {
+	using type
+	  = __conditional_t<_B1::value,
+			    _B1,
+			    typename __disjunction_impl<_B2, _B3, _Bn...>::type>;
+      };
+
+    template<typename...>
+      struct __conjunction_impl;
+
+    template<>
+      struct __conjunction_impl<>
+      { using type = true_type; };
+
+    template<typename _B1>
+      struct __conjunction_impl<_B1>
+      { using type = _B1; };
+
+    template<typename _B1, typename _B2>
+      struct __conjunction_impl<_B1, _B2>
+      { using type = __conditional_t<_B1::value, _B2, _B1>; };
+
+    template<typename _B1, typename _B2, typename _B3, typename... _Bn>
+      struct __conjunction_impl<_B1, _B2, _B3, _Bn...>
+      {
+	using type
+	  = __conditional_t<_B1::value,
+			    typename __conjunction_impl<_B2, _B3, _Bn...>::type,
+			    _B1>;
+      };
+  } // namespace __detail
   /// @endcond
 
 #define __cpp_lib_logical_traits 201510L
 
   template<typename... _Bn>
     struct conjunction
-    : __and_<_Bn...>
+    : __detail::__conjunction_impl<_Bn...>::type
     { };
 
   template<typename... _Bn>
     struct disjunction
-    : __or_<_Bn...>
+    : __detail::__disjunction_impl<_Bn...>::type
     { };
 
   template<typename _Pp>
@@ -2219,23 +2268,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     using __decay_and_strip = __strip_reference_wrapper<__decay_t<_Tp>>;
   /// @endcond
 
-  // Primary template.
-  /// Define a member typedef `type` only if a boolean constant is true.
-  template<bool, typename _Tp = void>
-    struct enable_if
-    { };
-
-  // Partial specialization for true.
-  template<typename _Tp>
-    struct enable_if<true, _Tp>
-    { typedef _Tp type; };
-
   /// @cond undocumented
 
-  // __enable_if_t (std::enable_if_t for C++11)
-  template<bool _Cond, typename _Tp = void>
-    using __enable_if_t = typename enable_if<_Cond, _Tp>::type;
-
   // Helper for SFINAE constraints
   template<typename... _Cond>
     using _Require = __enable_if_t<__and_<_Cond...>::value>;
-- 
2.37.2.382.g795ea8776b