122 lines
4.5 KiB
C++
122 lines
4.5 KiB
C++
/*
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* Copyright 2011-present Facebook, Inc.
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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#pragma once
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#include <cstddef>
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#include <folly/Portability.h>
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namespace folly {
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namespace detail {
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// Implemented this way because of a bug in Clang for ARMv7, which gives the
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// wrong result for `alignof` a `union` with a field of each scalar type.
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constexpr size_t max_align_(std::size_t a) {
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return a;
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}
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template <typename... Es>
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constexpr std::size_t max_align_(std::size_t a, std::size_t e, Es... es) {
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return !(a < e) ? a : max_align_(e, es...);
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}
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template <typename... Ts>
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struct max_align_t_ {
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static constexpr std::size_t value = max_align_(0u, alignof(Ts)...);
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};
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using max_align_v_ = max_align_t_<
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long double,
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double,
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float,
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long long int,
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long int,
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int,
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short int,
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bool,
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char,
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char16_t,
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char32_t,
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wchar_t,
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void*,
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std::max_align_t>;
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} // namespace detail
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// max_align_v is the alignment of max_align_t.
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//
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// max_align_t is a type which is aligned at least as strictly as the
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// most-aligned basic type (see the specification of std::max_align_t). This
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// implementation exists because 32-bit iOS platforms have a broken
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// std::max_align_t (see below).
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//
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// You should refer to this as `::folly::max_align_t` in portable code, even if
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// you have `using namespace folly;` because C11 defines a global namespace
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// `max_align_t` type.
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//
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// To be certain, we consider every non-void fundamental type specified by the
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// standard. On most platforms `long double` would be enough, but iOS 32-bit
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// has an 8-byte aligned `double` and `long long int` and a 4-byte aligned
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// `long double`.
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//
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// So far we've covered locals and other non-allocated storage, but we also need
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// confidence that allocated storage from `malloc`, `new`, etc will also be
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// suitable for objects with this alignment requirement.
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//
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// Apple document that their implementation of malloc will issue 16-byte
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// granularity chunks for small allocations (large allocations are page-size
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// granularity and page-aligned). We think that allocated storage will be
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// suitable for these objects based on the following assumptions:
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//
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// 1. 16-byte granularity also means 16-byte aligned.
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// 2. `new` and other allocators follow the `malloc` rules.
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//
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// We also have some anecdotal evidence: we don't see lots of misaligned-storage
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// crashes on 32-bit iOS apps that use `double`.
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//
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// Apple's allocation reference: http://bit.ly/malloc-small
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constexpr std::size_t max_align_v = detail::max_align_v_::value;
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struct alignas(max_align_v) max_align_t {};
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// Memory locations within the same cache line are subject to destructive
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// interference, also known as false sharing, which is when concurrent
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// accesses to these different memory locations from different cores, where at
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// least one of the concurrent accesses is or involves a store operation,
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// induce contention and harm performance.
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//
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// Microbenchmarks indicate that pairs of cache lines also see destructive
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// interference under heavy use of atomic operations, as observed for atomic
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// increment on Sandy Bridge.
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//
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// We assume a cache line size of 64, so we use a cache line pair size of 128
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// to avoid destructive interference.
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//
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// mimic: std::hardware_destructive_interference_size, C++17
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constexpr std::size_t hardware_destructive_interference_size =
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kIsArchArm ? 64 : 128;
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static_assert(hardware_destructive_interference_size >= max_align_v, "math?");
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// Memory locations within the same cache line are subject to constructive
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// interference, also known as true sharing, which is when accesses to some
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// memory locations induce all memory locations within the same cache line to
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// be cached, benefiting subsequent accesses to different memory locations
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// within the same cache line and heping performance.
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//
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// mimic: std::hardware_constructive_interference_size, C++17
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constexpr std::size_t hardware_constructive_interference_size = 64;
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static_assert(hardware_constructive_interference_size >= max_align_v, "math?");
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} // namespace folly
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