From fa8c75182fbfdeddb2485fc0d53baeda3f40b7a3 Mon Sep 17 00:00:00 2001 From: Abseil Team Date: Wed, 13 Nov 2019 08:54:32 -0800 Subject: Export of internal Abseil changes -- 049ac45508e335c6f010f2d28d71016b9fa65b4e by Derek Mauro : Fix librt detection PiperOrigin-RevId: 280207723 -- 6382c3a9fb2643af9dc031f92ca846c4a78e249c by Andy Getzendanner : Fix Conan builds Import of https://github.com/abseil/abseil-cpp/pull/400 PiperOrigin-RevId: 280025424 -- aebcd52b1686ac82663a8d0193b60d0122a43372 by Samuel Benzaquen : Enable the assertion in the iterator's operator== and operator!= PiperOrigin-RevId: 279998951 -- 5b61d909e2159ac6fd45e0e456818db1e725ecd1 by Derek Mauro : Add best effort support for compiling much of Abseil with MinGW. This involves disabling ABSL_ATTRIBUTE_WEAK and adding link flags. A change to CCTZ is still necessary. Tests were not run yet, but most of them now build. PiperOrigin-RevId: 279966541 -- 4336f8b10cff906e2defdd7d1d449cde4907da5d by Abseil Team : Add comments and relax memory orders in base_internal::CallOnceImpl. Add a comment to document the memory order guarantee if base_internal::SpinLockWait() is called and returns kOnceDone. Add a comment for the load/store sequence in base_internal::CallOnceImpl based on Mike Burrows' explanation. The atomic load of 'control' in the #ifndef NDEBUG block does not need std::memory_order_acquire. It can use std::memory_order_relaxed. The atomic compare_exchange_strong of 'control' does not need std::memory_order_acquire in the success case. It can use std::memory_order_relaxed. PiperOrigin-RevId: 279814155 -- 407de3a5e9af957cded54a136ca0468bde620d4d by Abseil Team : Added a script to generate abseil.podspec from all BUILD.bazel files automatically. PiperOrigin-RevId: 279811441 -- 26139497d4a363d6c7bc989c554da593e8819a07 by Derek Mauro : Add missing copyright and Apache License to //absl/functional/BUILD.bazel PiperOrigin-RevId: 279795227 -- 98ed625b02af6e5834edf52a920d8ca2dab4cd90 by Matt Kulukundis : Switch the implementation of hashtablez to *only* work on platforms that have a PER_THREAD_TLS. The old case is very slow (global mutex) and nobody collects data from that configuration anyway. PiperOrigin-RevId: 279775149 -- 07225900ef672c005c38f467ad3f92f38d0922b3 by Derek Mauro : Remove the minumum glibc version check PiperOrigin-RevId: 279750412 -- ec09956a951b4f52228ecc81968b8db7ae19ed15 by Derek Mauro : CMake only: link with -lrt to support older glibc versions PiperOrigin-RevId: 279741661 -- 97b113fb2e8246f6152c36330ba13793b37154b6 by Xiaoyi Zhang : Internal change. PiperOrigin-RevId: 279390188 -- ca8f72f2721546cc9b01bd01b2ea144962e6e0c5 by Andy Getzendanner : Expose PutTwoDigits for internal use within Abseil. PiperOrigin-RevId: 279374239 -- 14c6384cc03bbdfdefd2e4b635f104af5dd7e026 by Derek Mauro : Remove log_severity sources from the base target. They are already compiled as part of a separate library. PiperOrigin-RevId: 279372619 -- 3c5d926c718f8bf394e3bee87b6ba8d94601e0d3 by Abseil Team : s/indepdent/independent/g in SimpleAtof's documentation. PiperOrigin-RevId: 279350836 -- de2c44be8a8edf9efa1fe2007cba3564f3e5b0b8 by Abseil Team : Internal change PiperOrigin-RevId: 279346990 -- 2ba078341423fcf6d0ba5ca1831f86570a26e615 by Samuel Benzaquen : Add hash support for std::wstring, std::u16string and std::u32string. PiperOrigin-RevId: 279320672 -- 3272d3ffcfa55283a04f90e5868701912da95ef7 by Andy Soffer : Removing a bunch of __restricts that amount to no performance differences. One of these is the cause of https://github.com/abseil/abseil-cpp/issues/396. In particular, in one of the Vector128Store functions, restricts on two pointers that were indeed aliased seems to be the root cause of the issues. Closes #396 PiperOrigin-RevId: 279318999 -- 342f338ab31cc24344d5de8f28cf455bbb629a17 by Jorg Brown : Support uint128 in SimpleAtoi PiperOrigin-RevId: 279234038 -- 81cb0a04cf2dc4515d303679fc60968712191571 by Derek Mauro : Change the check for futex availability to support older Linux systems PiperOrigin-RevId: 279147079 -- cb4ca4aa4c8d2d710a5d483c56c4ce4f979e14b1 by Abseil Team : Add IWYU pragma: export for int128 .inc files. PiperOrigin-RevId: 279107098 -- b8df86ef610c366729f07326c726f3e34817b4dd by Abseil Team : An optimization for Waiter::Post() in the SEM waiter mode. Like the FUTEX waiter mode, Waiter::Post() only needs to call Poke() if it incremented the atomic variable from 0. PiperOrigin-RevId: 279086133 GitOrigin-RevId: 049ac45508e335c6f010f2d28d71016b9fa65b4e Change-Id: I4c1a4073fff62cb6a1fcb1c104aa7d62dad588c2 --- absl/base/call_once.h | 13 +++++++++++-- 1 file changed, 11 insertions(+), 2 deletions(-) (limited to 'absl/base/call_once.h') diff --git a/absl/base/call_once.h b/absl/base/call_once.h index 4aa6360..e1614e5 100644 --- a/absl/base/call_once.h +++ b/absl/base/call_once.h @@ -148,7 +148,7 @@ void CallOnceImpl(std::atomic* control, Args&&... args) { #ifndef NDEBUG { - uint32_t old_control = control->load(std::memory_order_acquire); + uint32_t old_control = control->load(std::memory_order_relaxed); if (old_control != kOnceInit && old_control != kOnceRunning && old_control != kOnceWaiter && @@ -166,14 +166,23 @@ void CallOnceImpl(std::atomic* control, // Must do this before potentially modifying control word's state. base_internal::SchedulingHelper maybe_disable_scheduling(scheduling_mode); // Short circuit the simplest case to avoid procedure call overhead. + // The base_internal::SpinLockWait() call returns either kOnceInit or + // kOnceDone. If it returns kOnceDone, it must have loaded the control word + // with std::memory_order_acquire and seen a value of kOnceDone. uint32_t old_control = kOnceInit; if (control->compare_exchange_strong(old_control, kOnceRunning, - std::memory_order_acquire, std::memory_order_relaxed) || base_internal::SpinLockWait(control, ABSL_ARRAYSIZE(trans), trans, scheduling_mode) == kOnceInit) { base_internal::Invoke(std::forward(fn), std::forward(args)...); + // The call to SpinLockWake below is an optimization, because the waiter + // in SpinLockWait is waiting with a short timeout. The atomic load/store + // sequence is slightly faster than an atomic exchange: + // old_control = control->exchange(base_internal::kOnceDone, + // std::memory_order_release); + // We opt for a slightly faster case when there are no waiters, in spite + // of longer tail latency when there are waiters. old_control = control->load(std::memory_order_relaxed); control->store(base_internal::kOnceDone, std::memory_order_release); if (old_control == base_internal::kOnceWaiter) { -- cgit v1.2.3