[libcxx-commits] [libcxx] [libc++] Optimize the std::mismatch tail (PR #83440)

[Mark de Wever via libcxx-commits]

================
@@ -0,0 +1,114 @@
+//===----------------------------------------------------------------------===//
+//
+// 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___ALGORITHM_SIMD_UTILS_H
+#define _LIBCPP___ALGORITHM_SIMD_UTILS_H
+
+#include <__bit/bit_cast.h>
+#include <__bit/countr.h>
+#include <__config>
+#include <__type_traits/is_arithmetic.h>
+#include <__type_traits/is_same.h>
+#include <__utility/integer_sequence.h>
+#include <cstddef>
+#include <cstdint>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+#if _LIBCPP_STD_VER >= 14 && __has_attribute(__ext_vector_type__) && __has_builtin(__builtin_reduce_and) &&            \
+    __has_builtin(__builtin_convertvector)
+#  define _LIBCPP_HAS_ALGORITHM_VECTOR_UTILS 1
+#else
+#  define _LIBCPP_HAS_ALGORITHM_VECTOR_UTILS 0
+#endif
+
+#if _LIBCPP_HAS_ALGORITHM_VECTOR_UTILS && !defined(__OPTIMIZE_SIZE__)
+#  define _LIBCPP_VECTORIZE_ALGORIHTMS 1
+#else
+#  define _LIBCPP_VECTORIZE_ALGORIHTMS 0
+#endif
+
+#if _LIBCPP_HAS_ALGORITHM_VECTOR_UTILS
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#  if defined(__AVX__)
+template <class _Tp>
+inline constexpr size_t __native_vector_size = 32 / sizeof(_Tp);
+#  elif defined(__SSE__) || defined(__ARM_NEON__)
+template <class _Tp>
+inline constexpr size_t __native_vector_size = 16 / sizeof(_Tp);
+#  elif defined(__MMX__)
+template <class _Tp>
+inline constexpr size_t __native_vector_size = 8 / sizeof(_Tp);
+#  else
+template <class _Tp>
+inline constexpr size_t __native_vector_size = 1;
+#  endif
+
+template <class _Tp, size_t _Np>
+using __simd_vector __attribute__((__ext_vector_type__(_Np))) = _Tp;
+
+template <class _VecT>
+inline constexpr size_t __simd_vector_size_v = []() -> size_t { static_assert(false, "Not a vector!"); }();
+
+template <class _Tp, size_t _Np>
+inline constexpr size_t __simd_vector_size_v<__simd_vector<_Tp, _Np>> = _Np;
+
+template <class _VecT>
+using __simd_vector_underlying_type_t =
+    decltype([]<class _Tp, size_t _Np>(__simd_vector<_Tp, _Np>) { return _Tp{}; }(_VecT{}));
+
+// This isn't inlined without always_inline when loading chars.
+template <class _VecT>
+[[__gnu__::__always_inline__]] _LIBCPP_NODISCARD _LIBCPP_HIDE_FROM_ABI _VecT __load_vector(const auto* __ptr) noexcept {
+  return []<size_t... _Indices> [[__gnu__::__always_inline__]] (
+             const auto* __lptr, index_sequence<_Indices...>) static noexcept {
+    return _VecT{__lptr[_Indices]...};
+  }(__ptr, make_index_sequence<__simd_vector_size_v<_VecT>>{});
+}
+
+template <class _Tp, size_t _Np>
+_LIBCPP_NODISCARD _LIBCPP_HIDE_FROM_ABI bool __all_of(__simd_vector<_Tp, _Np> __vec) noexcept {
+  return __builtin_reduce_and(__builtin_convertvector(__vec, __simd_vector<bool, _Np>));
+}
+
+template <class _Tp, size_t _Np>
----------------
mordante wrote:

In general I like the idea of this patch. However I really like this new header in its own patch with tests and documentation. I've no idea what this function does without starting to look at its implementation. The same holds true for other functions. I also feel this should have documentation in the design docs.

https://github.com/llvm/llvm-project/pull/83440