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When we initially implemented the C++20 synchronization library, we reluctantly accepted for the implementation to be backported to C++03 upon request from the person who provided the patch. This was when we were only starting to have experience with the issues this can create, so we flinched. Nowadays, we have a much stricter stance about not backporting features to previous standards. We have recently started fixing several bugs (and near bugs) in our implementation of the synchronization library. A recurring theme during these reviews has been how difficult to understand the current code is, and upon inspection it becomes clear that being able to use a few recent C++ features (in particular lambdas) would help a great deal. The code would still be pretty intricate, but it would be a lot easier to reason about the flow of callbacks through things like __thread_poll_with_backoff. As a result, this patch drops support for the synchronization library before C++20. This makes us more strictly conforming and opens the door to major simplifications, in particular around atomic_wait which was supported all the way to C++03. This change will probably have some impact on downstream users, however since the C++20 synchronization library was added only in LLVM 10 (~3 years ago) and it's quite a niche feature, the set of people trying to use this part of the library before C++20 should be reasonably small.
130 lines
3.7 KiB
C++
130 lines
3.7 KiB
C++
// -*- C++ -*-
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//===----------------------------------------------------------------------===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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#ifndef _LIBCPP_LATCH
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#define _LIBCPP_LATCH
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/*
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latch synopsis
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namespace std
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{
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class latch // since C++20
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{
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public:
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static constexpr ptrdiff_t max() noexcept;
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constexpr explicit latch(ptrdiff_t __expected);
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~latch();
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latch(const latch&) = delete;
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latch& operator=(const latch&) = delete;
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void count_down(ptrdiff_t __update = 1);
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bool try_wait() const noexcept;
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void wait() const;
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void arrive_and_wait(ptrdiff_t __update = 1);
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private:
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ptrdiff_t __counter; // exposition only
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};
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}
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*/
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#include <__config>
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#if !defined(_LIBCPP_HAS_NO_THREADS)
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# include <__assert>
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# include <__atomic/atomic_base.h>
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# include <__atomic/atomic_sync.h>
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# include <__atomic/memory_order.h>
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# include <cstddef>
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# include <limits>
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# include <version>
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# if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
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# pragma GCC system_header
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# endif
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_LIBCPP_PUSH_MACROS
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# include <__undef_macros>
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# if _LIBCPP_STD_VER >= 20
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_LIBCPP_BEGIN_NAMESPACE_STD
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class latch {
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__atomic_base<ptrdiff_t> __a_;
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public:
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static _LIBCPP_HIDE_FROM_ABI constexpr ptrdiff_t max() noexcept { return numeric_limits<ptrdiff_t>::max(); }
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inline _LIBCPP_HIDE_FROM_ABI constexpr explicit latch(ptrdiff_t __expected) : __a_(__expected) {
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_LIBCPP_ASSERT_ARGUMENT_WITHIN_DOMAIN(
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__expected >= 0,
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"latch::latch(ptrdiff_t): latch cannot be "
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"initialized with a negative value");
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_LIBCPP_ASSERT_ARGUMENT_WITHIN_DOMAIN(
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__expected <= max(),
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"latch::latch(ptrdiff_t): latch cannot be "
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"initialized with a value greater than max()");
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}
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_LIBCPP_HIDE_FROM_ABI ~latch() = default;
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latch(const latch&) = delete;
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latch& operator=(const latch&) = delete;
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inline _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void count_down(ptrdiff_t __update = 1) {
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_LIBCPP_ASSERT_ARGUMENT_WITHIN_DOMAIN(__update >= 0, "latch::count_down called with a negative value");
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auto const __old = __a_.fetch_sub(__update, memory_order_release);
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_LIBCPP_ASSERT_ARGUMENT_WITHIN_DOMAIN(
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__update <= __old,
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"latch::count_down called with a value greater "
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"than the internal counter");
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if (__old == __update)
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__a_.notify_all();
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}
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inline _LIBCPP_HIDE_FROM_ABI bool try_wait() const noexcept {
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auto __value = __a_.load(memory_order_acquire);
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return try_wait_impl(__value);
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}
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inline _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void wait() const {
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std::__atomic_wait_unless(
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__a_, [this](ptrdiff_t& __value) -> bool { return try_wait_impl(__value); }, memory_order_acquire);
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}
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inline _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void arrive_and_wait(ptrdiff_t __update = 1) {
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_LIBCPP_ASSERT_ARGUMENT_WITHIN_DOMAIN(__update >= 0, "latch::arrive_and_wait called with a negative value");
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// other preconditions on __update are checked in count_down()
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count_down(__update);
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wait();
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}
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private:
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_LIBCPP_HIDE_FROM_ABI bool try_wait_impl(ptrdiff_t& __value) const noexcept { return __value == 0; }
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};
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_LIBCPP_END_NAMESPACE_STD
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# endif // _LIBCPP_STD_VER >= 20
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_LIBCPP_POP_MACROS
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#endif // !defined(_LIBCPP_HAS_NO_THREADS)
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#if !defined(_LIBCPP_REMOVE_TRANSITIVE_INCLUDES) && _LIBCPP_STD_VER <= 20
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# include <atomic>
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#endif
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#endif // _LIBCPP_LATCH
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