1 /* 2 * Copyright (C) 2006 The Android Open Source Project 3 * 4 * Licensed under the Apache License, Version 2.0 (the "License"); 5 * you may not use this file except in compliance with the License. 6 * You may obtain a copy of the License at 7 * 8 * http://www.apache.org/licenses/LICENSE-2.0 9 * 10 * Unless required by applicable law or agreed to in writing, software 11 * distributed under the License is distributed on an "AS IS" BASIS, 12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 13 * See the License for the specific language governing permissions and 14 * limitations under the License. 15 */ 16 17 #include <endian.h> 18 #include <limits.h> 19 #undef _USING_LIBCXX // Prevent using of <atomic>. 20 #include <stdatomic.h> 21 22 #include <stddef.h> 23 24 #include "private/bionic_futex.h" 25 26 // This file contains C++ ABI support functions for one time 27 // constructors as defined in the "Run-time ABI for the ARM Architecture" 28 // section 4.4.2 29 // 30 // ARM C++ ABI and Itanium/x86 C++ ABI has different definition for 31 // one time construction: 32 // 33 // ARM C++ ABI defines the LSB of guard variable should be tested 34 // by compiler-generated code before calling __cxa_guard_acquire et al. 35 // 36 // The Itanium/x86 C++ ABI defines the low-order _byte_ should be 37 // tested instead. 38 // 39 // Meanwhile, guard variable are 32bit aligned for ARM, and 64bit 40 // aligned for x86. 41 // 42 // Reference documentation: 43 // 44 // section 3.2.3 of ARM IHI 0041C (for ARM) 45 // section 3.3.2 of the Itanium C++ ABI specification v1.83 (for x86). 46 // 47 // There is no C++ ABI available for other ARCH. But the gcc source 48 // shows all other ARCH follow the definition of Itanium/x86 C++ ABI. 49 50 #if defined(__arm__) 51 // The ARM C++ ABI mandates that guard variables are 32-bit aligned, 32-bit 52 // values. The LSB is tested by the compiler-generated code before calling 53 // __cxa_guard_acquire. 54 union _guard_t { 55 atomic_int state; 56 int32_t aligner; 57 }; 58 59 #else 60 // The Itanium/x86 C++ ABI (used by all other architectures) mandates that 61 // guard variables are 64-bit aligned, 64-bit values. The LSB is tested by 62 // the compiler-generated code before calling __cxa_guard_acquire. 63 union _guard_t { 64 atomic_int state; 65 int64_t aligner; 66 }; 67 68 #endif 69 70 // Set construction state values according to reference documentation. 71 // 0 is the initialization value. 72 // Arm requires ((*gv & 1) == 1) after __cxa_guard_release, ((*gv & 3) == 0) after __cxa_guard_abort. 73 // X86 requires first byte not modified by __cxa_guard_acquire, first byte is non-zero after 74 // __cxa_guard_release. 75 76 #define CONSTRUCTION_NOT_YET_STARTED 0 77 #define CONSTRUCTION_COMPLETE 1 78 #define CONSTRUCTION_UNDERWAY_WITHOUT_WAITER 0x100 79 #define CONSTRUCTION_UNDERWAY_WITH_WAITER 0x200 80 81 extern "C" int __cxa_guard_acquire(_guard_t* gv) { 82 int old_value = atomic_load_explicit(&gv->state, memory_order_acquire); 83 // In the common CONSTRUCTION_COMPLETE case we have to ensure that all the stores performed by 84 // the construction function are observable on this CPU after we exit. A similar constraint may 85 // apply in the CONSTRUCTION_NOT_YET_STARTED case with a prior abort. 86 87 while (true) { 88 if (old_value == CONSTRUCTION_COMPLETE) { 89 return 0; 90 } else if (old_value == CONSTRUCTION_NOT_YET_STARTED) { 91 if (!atomic_compare_exchange_weak_explicit(&gv->state, &old_value, 92 CONSTRUCTION_UNDERWAY_WITHOUT_WAITER, 93 memory_order_acquire /* or relaxed in C++17 */, 94 memory_order_acquire)) { 95 continue; 96 } 97 return 1; 98 } else if (old_value == CONSTRUCTION_UNDERWAY_WITHOUT_WAITER) { 99 if (!atomic_compare_exchange_weak_explicit(&gv->state, &old_value, 100 CONSTRUCTION_UNDERWAY_WITH_WAITER, 101 memory_order_acquire /* or relaxed in C++17 */, 102 memory_order_acquire)) { 103 continue; 104 } 105 } 106 107 __futex_wait_ex(&gv->state, false, CONSTRUCTION_UNDERWAY_WITH_WAITER); 108 old_value = atomic_load_explicit(&gv->state, memory_order_acquire); 109 } 110 } 111 112 extern "C" void __cxa_guard_release(_guard_t* gv) { 113 // Release fence is used to make all stores performed by the construction function 114 // visible in other threads. 115 int old_value = atomic_exchange_explicit(&gv->state, CONSTRUCTION_COMPLETE, memory_order_release); 116 if (old_value == CONSTRUCTION_UNDERWAY_WITH_WAITER) { 117 __futex_wake_ex(&gv->state, false, INT_MAX); 118 } 119 } 120 121 extern "C" void __cxa_guard_abort(_guard_t* gv) { 122 // Release fence is used to make all stores performed by the construction function 123 // visible in other threads. 124 int old_value = atomic_exchange_explicit(&gv->state, CONSTRUCTION_NOT_YET_STARTED, memory_order_release); 125 if (old_value == CONSTRUCTION_UNDERWAY_WITH_WAITER) { 126 __futex_wake_ex(&gv->state, false, INT_MAX); 127 } 128 } 129
