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

[philnik777]

No description provided.

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[ldionne]

This LGTM on a high level, but I have some comments and I'd like to see this again before it ships, since it's tricky. But the approach looks fine to me. This is clever!

[llvmbot]

@llvm/pr-subscribers-libcxx

Author: Nikolas Klauser (philnik777)

Changes

---

Full diff: https://github.com/llvm/llvm-project/pull/87716.diff

7 Files Affected:

• (modified) libcxx/include/CMakeLists.txt (+1)
• (modified) libcxx/include/__algorithm/mismatch.h (+40-12)
• (modified) libcxx/include/__algorithm/simd_utils.h (+31-3)
• (added) libcxx/include/__iterator/aliasing_iterator.h (+120)
• (modified) libcxx/include/libcxx.imp (+1)
• (modified) libcxx/include/module.modulemap (+1)
• (modified) libcxx/test/std/algorithms/alg.nonmodifying/mismatch/mismatch.pass.cpp (+37-10)

diff --git a/libcxx/include/CMakeLists.txt b/libcxx/include/CMakeLists.txt
index 097a41d4c41740..5922c9106853ed 100644
--- a/libcxx/include/CMakeLists.txt
+++ b/libcxx/include/CMakeLists.txt
@@ -456,6 +456,7 @@ 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
diff --git a/libcxx/include/__algorithm/mismatch.h b/libcxx/include/__algorithm/mismatch.h
index 4ada29eabc470c..c25fae67c7cfb3 100644
--- a/libcxx/include/__algorithm/mismatch.h
+++ b/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>
diff --git a/libcxx/include/__algorithm/simd_utils.h b/libcxx/include/__algorithm/simd_utils.h
index 989a1957987e1e..7fd7b3c33ee6de 100644
--- a/libcxx/include/__algorithm/simd_utils.h
+++ b/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>>{});
 }
 
diff --git a/libcxx/include/__iterator/aliasing_iterator.h b/libcxx/include/__iterator/aliasing_iterator.h
new file mode 100644
index 00000000000000..eb2b59af188f7b
--- /dev/null
+++ b/libcxx/include/__iterator/aliasing_iterator.h
@@ -0,0 +1,120 @@
+//===----------------------------------------------------------------------===//
+//
+// 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 <__compare/strong_order.h>
+#include <__config>
+#include <__iterator/concepts.h>
+#include <__iterator/iterator_traits.h>
+#include <__type_traits/is_trivial.h>
+#include <cstddef>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+template <class _BaseT, class _Alias>
+struct __aliasing_iterator_wrapper {
+  class __iterator {
+    _BaseT* __base_ = nullptr;
+
+  public:
+    using iterator_category = random_access_iterator_tag;
+    using value_type        = _Alias;
+    using difference_type   = ptrdiff_t;
+
+    static_assert(is_trivial<_Alias>::value);
+    static_assert(sizeof(_BaseT) == sizeof(_Alias));
+
+    _LIBCPP_HIDE_FROM_ABI __iterator() = default;
+    _LIBCPP_HIDE_FROM_ABI __iterator(_BaseT* __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 _BaseT* base() _NOEXCEPT { return __base_; }
+    _LIBCPP_HIDE_FROM_ABI const _BaseT* base() const _NOEXCEPT { return __base_; }
+
+    _LIBCPP_HIDE_FROM_ABI _Alias operator*() const _NOEXCEPT {
+      _Alias __val;
+      __builtin_memcpy(&__val, __base_, sizeof(_BaseT));
+      return __val;
+    }
+
+    _LIBCPP_HIDE_FROM_ABI _Alias 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_;
+    }
+
+#if _LIBCPP_STD_VER >= 20
+    friend auto operator<=>(__iterator, __iterator) noexcept = default;
+#endif
+  };
+};
+
+// This is required to avoid ADL instantiations on _BaseT
+template <class _BaseT, class _Alias>
+using __aliasing_iterator = __aliasing_iterator_wrapper<_BaseT, _Alias>::__iterator;
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP___ITERATOR_ALIASING_ITERATOR_H
diff --git a/libcxx/include/libcxx.imp b/libcxx/include/libcxx.imp
index 607f63e6d82206..e958756004273e 100644
--- a/libcxx/include/libcxx.imp
+++ b/libcxx/include/libcxx.imp
@@ -450,6 +450,7 @@
   { include: [ "<__ios/fpos.h>", "private", "<ios>", "public" ] },
   { include: [ "<__iterator/access.h>", "private", "<iterator>", "public" ] },
   { include: [ "<__iterator/advance.h>", "private", "<iterator>", "public" ] },
+  { include: [ "<__iterator/aliasing_iterator.h>", "private", "<iterator>", "public" ] },
   { include: [ "<__iterator/back_insert_iterator.h>", "private", "<iterator>", "public" ] },
   { include: [ "<__iterator/bounded_iter.h>", "private", "<iterator>", "public" ] },
   { include: [ "<__iterator/common_iterator.h>", "private", "<iterator>", "public" ] },
diff --git a/libcxx/include/module.modulemap b/libcxx/include/module.modulemap
index ed45a1b1833893..d2ce35e3fe481e 100644
--- a/libcxx/include/module.modulemap
+++ b/libcxx/include/module.modulemap
@@ -1406,6 +1406,7 @@ 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" }
diff --git a/libcxx/test/std/algorithms/alg.nonmodifying/mismatch/mismatch.pass.cpp b/libcxx/test/std/algorithms/alg.nonmodifying/mismatch/mismatch.pass.cpp
index eb5f7cacdde34b..72df17628dad78 100644
--- a/libcxx/test/std/algorithms/alg.nonmodifying/mismatch/mismatch.pass.cpp
+++ b/libcxx/test/std/algorithms/alg.nonmodifying/mismatch/mismatch.pass.cpp
@@ -66,14 +66,27 @@ TEST_CONSTEXPR_CXX20 void check(Container1 lhs, Container2 rhs, size_t offset) {
 #endif
 }
 
-struct NonTrivial {
+// Compares modulo 4 to make sure we only forward to the vectorized version if we are trivially equality comparable
+struct NonTrivialMod4Comp {
   int i_;
 
-  TEST_CONSTEXPR_CXX20 NonTrivial(int i) : i_(i) {}
-  TEST_CONSTEXPR_CXX20 NonTrivial(NonTrivial&& other) : i_(other.i_) { other.i_ = 0; }
+  TEST_CONSTEXPR_CXX20 NonTrivialMod4Comp(int i) : i_(i) {}
+  TEST_CONSTEXPR_CXX20 NonTrivialMod4Comp(NonTrivialMod4Comp&& other) : i_(other.i_) { other.i_ = 0; }
 
-  TEST_CONSTEXPR_CXX20 friend bool operator==(const NonTrivial& lhs, const NonTrivial& rhs) { return lhs.i_ == rhs.i_; }
+  TEST_CONSTEXPR_CXX20 friend bool operator==(const NonTrivialMod4Comp& lhs, const NonTrivialMod4Comp& rhs) {
+    return lhs.i_ % 4 == rhs.i_ % 4;
+  }
+};
+
+#if TEST_STD_VER >= 20
+struct TriviallyEqualityComparable {
+  int i_;
+
+  TEST_CONSTEXPR_CXX20 TriviallyEqualityComparable(int i) : i_(i) {}
+
+  TEST_CONSTEXPR_CXX20 friend bool operator==(TriviallyEqualityComparable, TriviallyEqualityComparable) = default;
 };
+#endif // TEST_STD_VER >= 20
 
 struct ModTwoComp {
   TEST_CONSTEXPR_CXX20 bool operator()(int lhs, int rhs) { return lhs % 2 == rhs % 2; }
@@ -136,16 +149,30 @@ TEST_CONSTEXPR_CXX20 bool test() {
   types::for_each(types::cpp17_input_iterator_list<int*>(), Test());
 
   { // use a non-integer type to also test the general case - all elements match
-    std::array<NonTrivial, 8> lhs = {1, 2, 3, 4, 5, 6, 7, 8};
-    std::array<NonTrivial, 8> rhs = {1, 2, 3, 4, 5, 6, 7, 8};
-    check<NonTrivial*>(std::move(lhs), std::move(rhs), 8);
+    std::array<NonTrivialMod4Comp, 8> lhs = {1, 2, 3, 4, 5, 6, 7, 8};
+    std::array<NonTrivialMod4Comp, 8> rhs = {1, 2, 3, 4, 1, 6, 7, 8};
+    check<NonTrivialMod4Comp*>(std::move(lhs), std::move(rhs), 8);
   }
 
   { // use a non-integer type to also test the general case - not all elements match
-    std::array<NonTrivial, 8> lhs = {1, 2, 3, 4, 7, 6, 7, 8};
-    std::array<NonTrivial, 8> rhs = {1, 2, 3, 4, 5, 6, 7, 8};
-    check<NonTrivial*>(std::move(lhs), std::move(rhs), 4);
+    std::array<NonTrivialMod4Comp, 8> lhs = {1, 2, 3, 4, 7, 6, 7, 8};
+    std::array<NonTrivialMod4Comp, 8> rhs = {1, 2, 3, 4, 5, 6, 7, 8};
+    check<NonTrivialMod4Comp*>(std::move(lhs), std::move(rhs), 4);
+  }
+
+#if TEST_STD_VER >= 20
+  { // trivially equaltiy comparable class type to test forwarding to the vectorized version - all elements match
+    std::array<TriviallyEqualityComparable, 8> lhs = {1, 2, 3, 4, 5, 6, 7, 8};
+    std::array<TriviallyEqualityComparable, 8> rhs = {1, 2, 3, 4, 5, 6, 7, 8};
+    check<TriviallyEqualityComparable*>(std::move(lhs), std::move(rhs), 8);
+  }
+
+  { // trivially equaltiy comparable class type to test forwarding to the vectorized version - not all elements match
+    std::array<TriviallyEqualityComparable, 8> lhs = {1, 2, 3, 4, 7, 6, 7, 8};
+    std::array<TriviallyEqualityComparable, 8> rhs = {1, 2, 3, 4, 5, 6, 7, 8};
+    check<TriviallyEqualityComparable*>(std::move(lhs), std::move(rhs), 4);
   }
+#endif // TEST_STD_VER >= 20
 
   return true;
 }

[ldionne]

LGTM w/ nitpicks and the CI passing.