[libc++] Vectorize std::mismatch with trivially equality comparable types

libcxx/include/CMakeLists.txt

@@ -446,6 +446,7 @@ set(files
   __ios/fpos.h
   __iterator/access.h
   __iterator/advance.h
+  __iterator/aliasing_iterator.h
   __iterator/back_insert_iterator.h
   __iterator/bounded_iter.h
   __iterator/common_iterator.h

libcxx/include/__algorithm/mismatch.h

@@ -16,6 +16,7 @@
 #include <__algorithm/unwrap_iter.h>
 #include <__config>
 #include <__functional/identity.h>
+#include <__iterator/aliasing_iterator.h>
 #include <__type_traits/desugars_to.h>
 #include <__type_traits/invoke.h>
 #include <__type_traits/is_constant_evaluated.h>
@@ -55,18 +56,13 @@ __mismatch(_Iter1 __first1, _Sent1 __last1, _Iter2 __first2, _Pred& __pred, _Pro
 
 #if _LIBCPP_VECTORIZE_ALGORITHMS
 
-template <class _Tp,
-          class _Pred,
-          class _Proj1,
-          class _Proj2,
-          __enable_if_t<is_integral<_Tp>::value && __desugars_to_v<__equal_tag, _Pred, _Tp, _Tp> &&
-                            __is_identity<_Proj1>::value && __is_identity<_Proj2>::value,
-                        int> = 0>
-_LIBCPP_NODISCARD _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 pair<_Tp*, _Tp*>
-__mismatch(_Tp* __first1, _Tp* __last1, _Tp* __first2, _Pred& __pred, _Proj1& __proj1, _Proj2& __proj2) {
+template <class _Iter>
+_LIBCPP_NODISCARD _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 pair<_Iter, _Iter>
+__mismatch_vectorized(_Iter __first1, _Iter __last1, _Iter __first2) {
+  using __value_type              = __iter_value_type<_Iter>;
   constexpr size_t __unroll_count = 4;
-  constexpr size_t __vec_size     = __native_vector_size<_Tp>;
-  using __vec                     = __simd_vector<_Tp, __vec_size>;
+  constexpr size_t __vec_size     = __native_vector_size<__value_type>;
+  using __vec                     = __simd_vector<__value_type, __vec_size>;
 
   if (!__libcpp_is_constant_evaluated()) {
     auto __orig_first1 = __first1;
@@ -116,9 +112,41 @@ __mismatch(_Tp* __first1, _Tp* __last1, _Tp* __first2, _Pred& __pred, _Proj1& __
     } // else loop over the elements individually
   }
 
-  return std::__mismatch_loop(__first1, __last1, __first2, __pred, __proj1, __proj2);
+  __equal_to __pred;
+  __identity __proj;
+  return std::__mismatch_loop(__first1, __last1, __first2, __pred, __proj, __proj);
+}
+
+template <class _Tp,
+          class _Pred,
+          class _Proj1,
+          class _Proj2,
+          __enable_if_t<is_integral<_Tp>::value && __desugars_to_v<__equal_tag, _Pred, _Tp, _Tp> &&
+                            __is_identity<_Proj1>::value && __is_identity<_Proj2>::value,
+                        int> = 0>
+_LIBCPP_NODISCARD _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 pair<_Tp*, _Tp*>
+__mismatch(_Tp* __first1, _Tp* __last1, _Tp* __first2, _Pred&, _Proj1&, _Proj2&) {
+  return std::__mismatch_vectorized(__first1, __last1, __first2);
 }
 
+template <class _Tp,
+          class _Pred,
+          class _Proj1,
+          class _Proj2,
+          __enable_if_t<!is_integral<_Tp>::value && __desugars_to_v<__equal_tag, _Pred, _Tp, _Tp> &&
+                            __is_identity<_Proj1>::value && __is_identity<_Proj2>::value &&
+                            __can_map_to_integer_v<_Tp> && __libcpp_is_trivially_equality_comparable<_Tp, _Tp>::value,
+                        int> = 0>
+_LIBCPP_NODISCARD _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 pair<_Tp*, _Tp*>
+__mismatch(_Tp* __first1, _Tp* __last1, _Tp* __first2, _Pred& __pred, _Proj1& __proj1, _Proj2& __proj2) {
+  if (__libcpp_is_constant_evaluated()) {
+    return std::__mismatch_loop(__first1, __last1, __first2, __pred, __proj1, __proj2);
+  } else {
+    using _Iter = __aliasing_iterator<_Tp*, __get_as_integer_type_t<_Tp>>;
+    auto __ret  = std::__mismatch_vectorized(_Iter(__first1), _Iter(__last1), _Iter(__first2));
+    return {__ret.first.__base(), __ret.second.__base()};
+  }
+}
 #endif // _LIBCPP_VECTORIZE_ALGORITHMS
 
 template <class _InputIterator1, class _InputIterator2, class _BinaryPredicate>

libcxx/include/__algorithm/simd_utils.h

@@ -43,6 +43,34 @@ _LIBCPP_PUSH_MACROS
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
+template <class _Tp>
+inline constexpr bool __can_map_to_integer_v =
+    sizeof(_Tp) == alignof(_Tp) && (sizeof(_Tp) == 1 || sizeof(_Tp) == 2 || sizeof(_Tp) == 4 || sizeof(_Tp) == 8);
+
+template <size_t _TypeSize>
+struct __get_as_integer_type_impl;
+
+template <>
+struct __get_as_integer_type_impl<1> {
+  using type = uint8_t;
+};
+
+template <>
+struct __get_as_integer_type_impl<2> {
+  using type = uint16_t;
+};
+template <>
+struct __get_as_integer_type_impl<4> {
+  using type = uint32_t;
+};
+template <>
+struct __get_as_integer_type_impl<8> {
+  using type = uint64_t;
+};
+
+template <class _Tp>
+using __get_as_integer_type_t = typename __get_as_integer_type_impl<sizeof(_Tp)>::type;
+
 // This isn't specialized for 64 byte vectors on purpose. They have the potential to significantly reduce performance
 // in mixed simd/non-simd workloads and don't provide any performance improvement for currently vectorized algorithms
 // as far as benchmarks are concerned.
@@ -80,10 +108,10 @@ template <class _VecT>
 using __simd_vector_underlying_type_t = decltype(std::__simd_vector_underlying_type_impl(_VecT{}));
 
 // This isn't inlined without always_inline when loading chars.
-template <class _VecT, class _Tp>
-_LIBCPP_NODISCARD _LIBCPP_ALWAYS_INLINE _LIBCPP_HIDE_FROM_ABI _VecT __load_vector(const _Tp* __ptr) noexcept {
+template <class _VecT, class _Iter>
+_LIBCPP_NODISCARD _LIBCPP_ALWAYS_INLINE _LIBCPP_HIDE_FROM_ABI _VecT __load_vector(_Iter __iter) noexcept {
   return [=]<size_t... _Indices>(index_sequence<_Indices...>) _LIBCPP_ALWAYS_INLINE noexcept {
-    return _VecT{__ptr[_Indices]...};
+    return _VecT{__iter[_Indices]...};
   }(make_index_sequence<__simd_vector_size_v<_VecT>>{});
 }
 

libcxx/include/__iterator/aliasing_iterator.h

@@ -0,0 +1,127 @@
+//===----------------------------------------------------------------------===//
+//
+// 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___ITERATOR_ALIASING_ITERATOR_H
+#define _LIBCPP___ITERATOR_ALIASING_ITERATOR_H
+
+#include <__config>
+#include <__iterator/iterator_traits.h>
+#include <__memory/pointer_traits.h>
+#include <__type_traits/is_trivial.h>
+#include <cstddef>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+// This iterator wrapper is used to type-pun an iterator to return a different type. This is done without UB by not
+// actually punning the type, but instead inspecting the object representation of the base type and copying that into
+// an instance of the alias type. For that reason the alias type has to be trivial. The alias is returned as a prvalue
+// when derferencing the iterator, since it is temporary storage. This wrapper is used to vectorize some algorithms.
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+template <class _BaseIter, class _Alias>
+struct __aliasing_iterator_wrapper {
+  class __iterator {
+    _BaseIter __base_ = nullptr;
+
+    using __iter_traits     = iterator_traits<_BaseIter>;
+    using __base_value_type = typename __iter_traits::value_type;
+
+    static_assert(__has_random_access_iterator_category<_BaseIter>::value,
+                  "The base iterator has to be a random access iterator!");
+
+  public:
+    using iterator_category = random_access_iterator_tag;
+    using value_type        = _Alias;
+    using difference_type   = ptrdiff_t;
+    using reference         = value_type&;
+    using pointer           = value_type*;
+
+    static_assert(is_trivial<value_type>::value);
+    static_assert(sizeof(__base_value_type) == sizeof(value_type));
+
+    _LIBCPP_HIDE_FROM_ABI __iterator() = default;
+    _LIBCPP_HIDE_FROM_ABI __iterator(_BaseIter __base) _NOEXCEPT : __base_(__base) {}
+
+    _LIBCPP_HIDE_FROM_ABI __iterator& operator++() _NOEXCEPT {
+      ++__base_;
+      return *this;
+    }
+
+    _LIBCPP_HIDE_FROM_ABI __iterator operator++(int) _NOEXCEPT {
+      __iterator __tmp(*this);
+      ++__base_;
+      return __tmp;
+    }
+
+    _LIBCPP_HIDE_FROM_ABI __iterator& operator--() _NOEXCEPT {
+      --__base_;
+      return *this;
+    }
+
+    _LIBCPP_HIDE_FROM_ABI __iterator operator--(int) _NOEXCEPT {
+      __iterator __tmp(*this);
+      --__base_;
+      return __tmp;
+    }
+
+    _LIBCPP_HIDE_FROM_ABI friend __iterator operator+(__iterator __iter, difference_type __n) _NOEXCEPT {
+      return __iterator(__iter.__base_ + __n);
+    }
+
+    _LIBCPP_HIDE_FROM_ABI friend __iterator operator+(difference_type __n, __iterator __iter) _NOEXCEPT {
+      return __iterator(__n + __iter.__base_);
+    }
+
+    _LIBCPP_HIDE_FROM_ABI __iterator& operator+=(difference_type __n) _NOEXCEPT {
+      __base_ += __n;
+      return *this;
+    }
+
+    _LIBCPP_HIDE_FROM_ABI friend __iterator operator-(__iterator __iter, difference_type __n) _NOEXCEPT {
+      return __iterator(__iter.__base_ - __n);
+    }
+
+    _LIBCPP_HIDE_FROM_ABI friend difference_type operator-(__iterator __lhs, __iterator __rhs) _NOEXCEPT {
+      return __lhs.__base_ - __rhs.__base_;
+    }
+
+    _LIBCPP_HIDE_FROM_ABI __iterator& operator-=(difference_type __n) _NOEXCEPT {
+      __base_ -= __n;
+      return *this;
+    }
+
+    _LIBCPP_HIDE_FROM_ABI _BaseIter __base() const _NOEXCEPT { return __base_; }
+
+    _LIBCPP_HIDE_FROM_ABI _Alias operator*() const _NOEXCEPT {
+      _Alias __val;
+      __builtin_memcpy(&__val, std::__to_address(__base_), sizeof(value_type));
+      return __val;
+    }
+
+    _LIBCPP_HIDE_FROM_ABI value_type operator[](difference_type __n) const _NOEXCEPT { return *(*this + __n); }
+
+    _LIBCPP_HIDE_FROM_ABI friend bool operator==(const __iterator& __lhs, const __iterator& __rhs) _NOEXCEPT {
+      return __lhs.__base_ == __rhs.__base_;
+    }
+
+    _LIBCPP_HIDE_FROM_ABI friend bool operator!=(const __iterator& __lhs, const __iterator& __rhs) _NOEXCEPT {
+      return __lhs.__base_ != __rhs.__base_;
+    }
+  };
+};
+
+// This is required to avoid ADL instantiations on _BaseT
+template <class _BaseT, class _Alias>
+using __aliasing_iterator = typename __aliasing_iterator_wrapper<_BaseT, _Alias>::__iterator;
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___ITERATOR_ALIASING_ITERATOR_H

libcxx/include/__type_traits/is_equality_comparable.h

@@ -44,6 +44,8 @@ struct __is_equality_comparable<_Tp, _Up, __void_t<decltype(std::declval<_Tp>()
 // pointers that don't have the same type (ignoring cv-qualifiers): pointers to virtual bases are equality comparable,
 //   but don't have the same bit-pattern. An exception to this is comparing to a void-pointer. There the bit-pattern is
 //   always compared.
+// objects with padding bytes: since objects with padding bytes may compare equal, even though their object
+//   representation may not be equivalent.
 
 template <class _Tp, class _Up, class = void>
 struct __libcpp_is_trivially_equality_comparable_impl : false_type {};

libcxx/include/module.modulemap

@@ -700,6 +700,7 @@ module std_private_algorithm_minmax_element                              [system
 module std_private_algorithm_mismatch                                    [system] {
   header "__algorithm/mismatch.h"
   export std_private_algorithm_simd_utils
+  export std_private_iterator_aliasing_iterator
 }
 module std_private_algorithm_move                                        [system] { header "__algorithm/move.h" }
 module std_private_algorithm_move_backward                               [system] { header "__algorithm/move_backward.h" }
@@ -1390,6 +1391,7 @@ module std_private_iosfwd_streambuf_fwd [system] { header "__fwd/streambuf.h" }
 
 module std_private_iterator_access                  [system] { header "__iterator/access.h" }
 module std_private_iterator_advance                 [system] { header "__iterator/advance.h" }
+module std_private_iterator_aliasing_iterator       [system] { header "__iterator/aliasing_iterator.h" }
 module std_private_iterator_back_insert_iterator    [system] { header "__iterator/back_insert_iterator.h" }
 module std_private_iterator_bounded_iter            [system] { header "__iterator/bounded_iter.h" }
 module std_private_iterator_common_iterator         [system] { header "__iterator/common_iterator.h" }

libcxx/test/libcxx/iterators/aliasing_iterator.pass.cpp

@@ -0,0 +1,45 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// ADDITIONAL_COMPILE_FLAGS(clang): -Wprivate-header
+
+#include <__iterator/aliasing_iterator.h>
+#include <cassert>
+
+struct NonTrivial {
+  int i_;
+
+  NonTrivial(int i) : i_(i) {}
+  NonTrivial(const NonTrivial& other) : i_(other.i_) {}
+
+  NonTrivial& operator=(const NonTrivial& other) {
+    i_ = other.i_;
+    return *this;
+  }
+
+  ~NonTrivial() {}
+};
+
+int main(int, char**) {
+  {
+    NonTrivial arr[] = {1, 2, 3, 4};
+    std::__aliasing_iterator<NonTrivial*, int> iter(arr);
+
+    assert(*iter == 1);
+    assert(iter[0] == 1);
+    assert(iter[1] == 2);
+    ++iter;
+    assert(*iter == 2);
+    assert(iter[-1] == 1);
+    assert(iter.__base() == arr + 1);
+    assert(iter == iter);
+    assert(iter != (iter + 1));
+  }
+
+  return 0;
+}