[libcxx-commits] [libcxx] [libc++] Vectorize std::mismatch with trivially equality comparable types (PR #87716)

[Louis Dionne via libcxx-commits]

================
@@ -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));
----------------
ldionne wrote:

This is basically the part of the patch that's tricky on a UB level. After consideration, we think this is valid. We're allowed to inspect the object representation of anything using `char*`, and that's basically what `memcpy` does. Then, we copy that into a trivial type, so that object of a trivial type is also valid -- it just contains some garbage. The interesting part is that this garbage happens to be exactly the garbage that we want, since it corresponds to the object representation of `*__it_`.

We then use the property of being trivially equality comparable to tie it all together and compare these trivial objects, and bingo.

Yeah, I think this optimization is valid and it's pretty clever. I like this iteration a lot more than the previous one.

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