-// Copyright (c) 2011-present, Facebook, Inc. All rights reserved.
-// This source code is licensed under both the GPLv2 (found in the
-// COPYING file in the root directory) and Apache 2.0 License
-// (found in the LICENSE.Apache file in the root directory).
+/*
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
#pragma once
+#include <cstddef>
#include <cstdint>
+#include <folly/Portability.h>
+#include <folly/ConstexprMath.h>
+
+// Work around bug https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56019
+#ifdef __GNUC__
+#if __GNUC__ < 4 || (__GNUC__ == 4 && __GNUC_MINOR__ < 9)
+namespace std {
+using ::max_align_t;
+}
+#endif
+#endif
+
namespace folly {
+// has_extended_alignment
+//
+// True if it may be presumed that the platform has static extended alignment;
+// false if it may not be so presumed, even when the platform might actually
+// have it. Static extended alignment refers to extended alignment of objects
+// with automatic, static, or thread storage. Whether the there is support for
+// dynamic extended alignment is a property of the allocator which is used for
+// each given dynamic allocation.
+//
+// Currently, very heuristical - only non-mobile 64-bit linux gets the extended
+// alignment treatment. Theoretically, this could be tuned better.
+constexpr bool has_extended_alignment =
+ kIsLinux && sizeof(void*) >= sizeof(std::uint64_t);
+
+namespace detail {
+
+// Implemented this way because of a bug in Clang for ARMv7, which gives the
+// wrong result for `alignof` a `union` with a field of each scalar type.
+// Modified for RocksDB to use C++11 only
+constexpr std::size_t max_align_v = constexpr_max(
+ alignof(long double),
+ alignof(double),
+ alignof(float),
+ alignof(long long int),
+ alignof(long int),
+ alignof(int),
+ alignof(short int),
+ alignof(bool),
+ alignof(char),
+ alignof(char16_t),
+ alignof(char32_t),
+ alignof(wchar_t),
+ alignof(void*),
+ alignof(std::max_align_t));
+
+} // namespace detail
+
+// max_align_v is the alignment of max_align_t.
+//
+// max_align_t is a type which is aligned at least as strictly as the
+// most-aligned basic type (see the specification of std::max_align_t). This
+// implementation exists because 32-bit iOS platforms have a broken
+// std::max_align_t (see below).
+//
+// You should refer to this as `::folly::max_align_t` in portable code, even if
+// you have `using namespace folly;` because C11 defines a global namespace
+// `max_align_t` type.
+//
+// To be certain, we consider every non-void fundamental type specified by the
+// standard. On most platforms `long double` would be enough, but iOS 32-bit
+// has an 8-byte aligned `double` and `long long int` and a 4-byte aligned
+// `long double`.
+//
+// So far we've covered locals and other non-allocated storage, but we also need
+// confidence that allocated storage from `malloc`, `new`, etc will also be
+// suitable for objects with this alignment requirement.
+//
+// Apple document that their implementation of malloc will issue 16-byte
+// granularity chunks for small allocations (large allocations are page-size
+// granularity and page-aligned). We think that allocated storage will be
+// suitable for these objects based on the following assumptions:
+//
+// 1. 16-byte granularity also means 16-byte aligned.
+// 2. `new` and other allocators follow the `malloc` rules.
+//
+// We also have some anecdotal evidence: we don't see lots of misaligned-storage
+// crashes on 32-bit iOS apps that use `double`.
+//
+// Apple's allocation reference: http://bit.ly/malloc-small
+constexpr std::size_t max_align_v = detail::max_align_v;
+struct alignas(max_align_v) max_align_t {};
+
// Memory locations within the same cache line are subject to destructive
// interference, also known as false sharing, which is when concurrent
// accesses to these different memory locations from different cores, where at
// to avoid destructive interference.
//
// mimic: std::hardware_destructive_interference_size, C++17
-constexpr std::size_t hardware_destructive_interference_size = 128;
+constexpr std::size_t hardware_destructive_interference_size =
+ kIsArchArm ? 64 : 128;
+static_assert(hardware_destructive_interference_size >= max_align_v, "math?");
// Memory locations within the same cache line are subject to constructive
// interference, also known as true sharing, which is when accesses to some
//
// mimic: std::hardware_constructive_interference_size, C++17
constexpr std::size_t hardware_constructive_interference_size = 64;
+static_assert(hardware_constructive_interference_size >= max_align_v, "math?");
-} // namespace folly
+// A value corresponding to hardware_constructive_interference_size but which
+// may be used with alignas, since hardware_constructive_interference_size may
+// be too large on some platforms to be used with alignas.
+constexpr std::size_t cacheline_align_v = has_extended_alignment
+ ? hardware_constructive_interference_size
+ : max_align_v;
+struct alignas(cacheline_align_v) cacheline_align_t {};
+} // namespace folly
projects / ceph.git / blobdiff