/*
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* Copyright (C) 2006 The Android Open Source Project
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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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#include <endian.h>
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#include <limits.h>
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#undef _USING_LIBCXX // Prevent using of <atomic>.
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#include <stdatomic.h>
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#include <stddef.h>
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#include "private/bionic_futex.h"
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// This file contains C++ ABI support functions for one time
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// constructors as defined in the "Run-time ABI for the ARM Architecture"
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// section 4.4.2
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//
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// ARM C++ ABI and Itanium/x86 C++ ABI has different definition for
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// one time construction:
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//
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// ARM C++ ABI defines the LSB of guard variable should be tested
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// by compiler-generated code before calling __cxa_guard_acquire et al.
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//
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// The Itanium/x86 C++ ABI defines the low-order _byte_ should be
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// tested instead.
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//
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// Meanwhile, guard variable are 32bit aligned for ARM, and 64bit
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// aligned for x86.
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//
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// Reference documentation:
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//
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// section 3.2.3 of ARM IHI 0041C (for ARM)
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// section 3.3.2 of the Itanium C++ ABI specification v1.83 (for x86).
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//
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// There is no C++ ABI available for other ARCH. But the gcc source
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// shows all other ARCH follow the definition of Itanium/x86 C++ ABI.
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#if defined(__arm__)
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// The ARM C++ ABI mandates that guard variables are 32-bit aligned, 32-bit
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// values. The LSB is tested by the compiler-generated code before calling
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// __cxa_guard_acquire.
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union _guard_t {
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atomic_int state;
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int32_t aligner;
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};
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#else
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// The Itanium/x86 C++ ABI (used by all other architectures) mandates that
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// guard variables are 64-bit aligned, 64-bit values. The LSB is tested by
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// the compiler-generated code before calling __cxa_guard_acquire.
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union _guard_t {
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atomic_int state;
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int64_t aligner;
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};
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#endif
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// Set construction state values according to reference documentation.
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// 0 is the initialization value.
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// Arm requires ((*gv & 1) == 1) after __cxa_guard_release, ((*gv & 3) == 0) after __cxa_guard_abort.
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// X86 requires first byte not modified by __cxa_guard_acquire, first byte is non-zero after
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// __cxa_guard_release.
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#define CONSTRUCTION_NOT_YET_STARTED 0
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#define CONSTRUCTION_COMPLETE 1
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#define CONSTRUCTION_UNDERWAY_WITHOUT_WAITER 0x100
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#define CONSTRUCTION_UNDERWAY_WITH_WAITER 0x200
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extern "C" int __cxa_guard_acquire(_guard_t* gv) {
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int old_value = atomic_load_explicit(&gv->state, memory_order_acquire);
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// In the common CONSTRUCTION_COMPLETE case we have to ensure that all the stores performed by
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// the construction function are observable on this CPU after we exit. A similar constraint may
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// apply in the CONSTRUCTION_NOT_YET_STARTED case with a prior abort.
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while (true) {
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if (old_value == CONSTRUCTION_COMPLETE) {
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return 0;
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} else if (old_value == CONSTRUCTION_NOT_YET_STARTED) {
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if (!atomic_compare_exchange_weak_explicit(&gv->state, &old_value,
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CONSTRUCTION_UNDERWAY_WITHOUT_WAITER,
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memory_order_acquire /* or relaxed in C++17 */,
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memory_order_acquire)) {
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continue;
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}
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return 1;
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} else if (old_value == CONSTRUCTION_UNDERWAY_WITHOUT_WAITER) {
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if (!atomic_compare_exchange_weak_explicit(&gv->state, &old_value,
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CONSTRUCTION_UNDERWAY_WITH_WAITER,
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memory_order_acquire /* or relaxed in C++17 */,
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memory_order_acquire)) {
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continue;
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}
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}
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__futex_wait_ex(&gv->state, false, CONSTRUCTION_UNDERWAY_WITH_WAITER);
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old_value = atomic_load_explicit(&gv->state, memory_order_acquire);
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}
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}
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extern "C" void __cxa_guard_release(_guard_t* gv) {
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// Release fence is used to make all stores performed by the construction function
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// visible in other threads.
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int old_value = atomic_exchange_explicit(&gv->state, CONSTRUCTION_COMPLETE, memory_order_release);
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if (old_value == CONSTRUCTION_UNDERWAY_WITH_WAITER) {
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__futex_wake_ex(&gv->state, false, INT_MAX);
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}
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}
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extern "C" void __cxa_guard_abort(_guard_t* gv) {
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// Release fence is used to make all stores performed by the construction function
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// visible in other threads.
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int old_value = atomic_exchange_explicit(&gv->state, CONSTRUCTION_NOT_YET_STARTED, memory_order_release);
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if (old_value == CONSTRUCTION_UNDERWAY_WITH_WAITER) {
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__futex_wake_ex(&gv->state, false, INT_MAX);
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}
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}
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