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From: Jonathan Wakely <redi@gcc.gnu.org> To: gcc-cvs@gcc.gnu.org, libstdc++-cvs@gcc.gnu.org Subject: [gcc r11-7631] libstdc++: Make barrier::arrival_token a move-only class type Date: Thu, 11 Mar 2021 17:54:03 +0000 (GMT) [thread overview] Message-ID: <20210311175403.D3A9B3896C19@sourceware.org> (raw) https://gcc.gnu.org/g:5643f6f396ef7f60d317aef07dd98978cec6afd0 commit r11-7631-g5643f6f396ef7f60d317aef07dd98978cec6afd0 Author: Jonathan Wakely <jwakely@redhat.com> Date: Thu Mar 11 16:57:20 2021 +0000 libstdc++: Make barrier::arrival_token a move-only class type The standard only specifies that barrier::arrival_token is a move constructible and move assignable type. We originally used a scoped enum type, but that means we do not diagnose non-portable code that makes copies of arrival tokens (or compares them for equality, or uses them as keys in map!) This wraps the enum in a move-only class type, so that users are forced to pass it correctly. The move constructor and move assignment operator of the new class do not zero out the moved-from token, as that would add additional instructions. That means that passing a moved-from token will work with our implementation, despite being a bug in the user code. We could consider doing that zeroing out in debug mode. libstdc++-v3/ChangeLog: * include/std/barrier (barrier::arrival_token): New move-only class that encapsulates the underlying token value. Diff: --- libstdc++-v3/include/std/barrier | 24 ++++++++++++++++++------ 1 file changed, 18 insertions(+), 6 deletions(-) diff --git a/libstdc++-v3/include/std/barrier b/libstdc++-v3/include/std/barrier index e09212dfcb9..6f2b9873500 100644 --- a/libstdc++-v3/include/std/barrier +++ b/libstdc++-v3/include/std/barrier @@ -209,15 +209,27 @@ It looks different from literature pseudocode for two main reasons: __algorithm_t _M_b; public: - using arrival_token = typename __tree_barrier<_CompletionF>::arrival_token; + class arrival_token final + { + public: + arrival_token(arrival_token&&) = default; + arrival_token& operator=(arrival_token&&) = default; + ~arrival_token() = default; + + private: + friend class barrier; + using __token = typename __algorithm_t::arrival_token; + explicit arrival_token(__token __tok) noexcept : _M_tok(__tok) { } + __token _M_tok; + }; static constexpr ptrdiff_t max() noexcept { return __algorithm_t::max(); } - explicit barrier(ptrdiff_t __count, - _CompletionF __completion = _CompletionF()) - : _M_b(__count, std::move(__completion)) + explicit + barrier(ptrdiff_t __count, _CompletionF __completion = _CompletionF()) + : _M_b(__count, std::move(__completion)) { } barrier(barrier const&) = delete; @@ -225,11 +237,11 @@ It looks different from literature pseudocode for two main reasons: [[nodiscard]] arrival_token arrive(ptrdiff_t __update = 1) - { return _M_b.arrive(__update); } + { return arrival_token{_M_b.arrive(__update)}; } void wait(arrival_token&& __phase) const - { _M_b.wait(std::move(__phase)); } + { _M_b.wait(std::move(__phase._M_tok)); } void arrive_and_wait()
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