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1//===----------------------------------------------------------------------===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8
9#ifndef LIBCXXABI_SRC_INCLUDE_CXA_GUARD_IMPL_H
10#define LIBCXXABI_SRC_INCLUDE_CXA_GUARD_IMPL_H
11
12/* cxa_guard_impl.h - Implements the C++ runtime support for function local
13 * static guards.
14 * The layout of the guard object is the same across ARM and Itanium.
15 *
16 * The first "guard byte" (which is checked by the compiler) is set only upon
17 * the completion of cxa release.
18 *
19 * The second "init byte" does the rest of the bookkeeping. It tracks if
20 * initialization is complete or pending, and if there are waiting threads.
21 *
22 * If the guard variable is 64-bits and the platforms supplies a 32-bit thread
23 * identifier, it is used to detect recursive initialization. The thread ID of
24 * the thread currently performing initialization is stored in the second word.
25 *
26 * Guard Object Layout:
27 * ---------------------------------------------------------------------------
28 * | a+0: guard byte | a+1: init byte | a+2: unused ... | a+4: thread-id ... |
29 * ---------------------------------------------------------------------------
30 *
31 * Note that we don't do what the ABI docs suggest (put a mutex in the guard
32 * object which we acquire in cxa_guard_acquire and release in
33 * cxa_guard_release). Instead we use the init byte to imitate that behaviour,
34 * but without actually holding anything mutex related between aquire and
35 * release/abort.
36 *
37 * Access Protocol:
38 * For each implementation the guard byte is checked and set before accessing
39 * the init byte.
40 *
41 * Overall Design:
42 * The implementation was designed to allow each implementation to be tested
43 * independent of the C++ runtime or platform support.
44 *
45 */
46
47#include "__cxxabi_config.h"
48#include "include/atomic_support.h" // from libc++
49#if defined(__has_include)
50# if __has_include(<sys/futex.h>)
51# include <sys/futex.h>
52# endif
53# if __has_include(<sys/syscall.h>)
54# include <sys/syscall.h>
55# endif
56# if __has_include(<unistd.h>)
57# include <unistd.h>
58# endif
59#endif
60
61#include <__thread/support.h>
62#include <cstdint>
63#include <cstring>
64#include <limits.h>
65#include <stdlib.h>
66
67#ifndef _LIBCXXABI_HAS_NO_THREADS
68# if defined(__ELF__) && defined(_LIBCXXABI_LINK_PTHREAD_LIB)
69# pragma comment(lib, "pthread")
70# endif
71#endif
72
73#if defined(__clang__)
74# pragma clang diagnostic push
75# pragma clang diagnostic ignored "-Wtautological-pointer-compare"
76#elif defined(__GNUC__)
77# pragma GCC diagnostic push
78# pragma GCC diagnostic ignored "-Waddress"
79#endif
80
81// To make testing possible, this header is included from both cxa_guard.cpp
82// and a number of tests.
83//
84// For this reason we place everything in an anonymous namespace -- even though
85// we're in a header. We want the actual implementation and the tests to have
86// unique definitions of the types in this header (since the tests may depend
87// on function local statics).
88//
89// To enforce this either `BUILDING_CXA_GUARD` or `TESTING_CXA_GUARD` must be
90// defined when including this file. Only `src/cxa_guard.cpp` should define
91// the former.
92#ifdef BUILDING_CXA_GUARD
93# include "abort_message.h"
94# define ABORT_WITH_MESSAGE(...) ::__abort_message(__VA_ARGS__)
95#elif defined(TESTING_CXA_GUARD)
96# define ABORT_WITH_MESSAGE(...) ::abort()
97#else
98# error "Either BUILDING_CXA_GUARD or TESTING_CXA_GUARD must be defined"
99#endif
100
101#if __has_feature(thread_sanitizer)
102extern "C" void __tsan_acquire(void*);
103extern "C" void __tsan_release(void*);
104#else
105# define __tsan_acquire(addr) ((void)0)
106# define __tsan_release(addr) ((void)0)
107#endif
108
109namespace __cxxabiv1 {
110// Use an anonymous namespace to ensure that the tests and actual implementation
111// have unique definitions of these symbols.
112namespace {
113
114//===----------------------------------------------------------------------===//
115// Misc Utilities
116//===----------------------------------------------------------------------===//
117
118template <class T, T (*Init)()>
119struct LazyValue {
120 LazyValue() : is_init(false) {}
121
122 T& get() {
123 if (!is_init) {
124 value = Init();
125 is_init = true;
126 }
127 return value;
128 }
129
130private:
131 T value;
132 bool is_init = false;
133};
134
135template <class IntType>
136class AtomicInt {
137public:
138 using MemoryOrder = std::__libcpp_atomic_order;
139
140 explicit AtomicInt(IntType* b) : b_(b) {}
141 AtomicInt(AtomicInt const&) = delete;
142 AtomicInt& operator=(AtomicInt const&) = delete;
143
144 IntType load(MemoryOrder ord) { return std::__libcpp_atomic_load(b_, ord); }
145 void store(IntType val, MemoryOrder ord) { std::__libcpp_atomic_store(b_, val, ord); }
146 IntType exchange(IntType new_val, MemoryOrder ord) { return std::__libcpp_atomic_exchange(b_, new_val, ord); }
147 bool compare_exchange(IntType* expected, IntType desired, MemoryOrder ord_success, MemoryOrder ord_failure) {
148 return std::__libcpp_atomic_compare_exchange(b_, expected, desired, ord_success, ord_failure);
149 }
150
151private:
152 IntType* b_;
153};
154
155//===----------------------------------------------------------------------===//
156// PlatformGetThreadID
157//===----------------------------------------------------------------------===//
158
159#if defined(__APPLE__) && _LIBCPP_HAS_THREAD_API_PTHREAD
160uint32_t PlatformThreadID() {
161 static_assert(sizeof(mach_port_t) == sizeof(uint32_t), "");
162 return static_cast<uint32_t>(pthread_mach_thread_np(std::__libcpp_thread_get_current_id()));
163}
164#elif defined(SYS_gettid) && _LIBCPP_HAS_THREAD_API_PTHREAD
165uint32_t PlatformThreadID() {
166 static_assert(sizeof(pid_t) == sizeof(uint32_t), "");
167 return static_cast<uint32_t>(syscall(SYS_gettid));
168}
169#else
170constexpr uint32_t (*PlatformThreadID)() = nullptr;
171#endif
172
173//===----------------------------------------------------------------------===//
174// GuardByte
175//===----------------------------------------------------------------------===//
176
177static constexpr uint8_t UNSET = 0;
178static constexpr uint8_t COMPLETE_BIT = (1 << 0);
179static constexpr uint8_t PENDING_BIT = (1 << 1);
180static constexpr uint8_t WAITING_BIT = (1 << 2);
181
182/// Manages reads and writes to the guard byte.
183struct GuardByte {
184 GuardByte() = delete;
185 GuardByte(GuardByte const&) = delete;
186 GuardByte& operator=(GuardByte const&) = delete;
187
188 explicit GuardByte(uint8_t* const guard_byte_address) : guard_byte(guard_byte_address) {}
189
190public:
191 /// The guard byte portion of cxa_guard_acquire. Returns true if
192 /// initialization has already been completed.
193 bool acquire() {
194 // if guard_byte is non-zero, we have already completed initialization
195 // (i.e. release has been called)
196 return guard_byte.load(std::_AO_Acquire) != UNSET;
197 }
198
199 /// The guard byte portion of cxa_guard_release.
200 void release() { guard_byte.store(COMPLETE_BIT, std::_AO_Release); }
201
202 /// The guard byte portion of cxa_guard_abort.
203 void abort() {} // Nothing to do
204
205private:
206 AtomicInt<uint8_t> guard_byte;
207};
208
209//===----------------------------------------------------------------------===//
210// InitByte Implementations
211//===----------------------------------------------------------------------===//
212//
213// Each initialization byte implementation supports the following methods:
214//
215// InitByte(uint8_t* _init_byte_address, uint32_t* _thread_id_address)
216// Construct the InitByte object, initializing our member variables
217//
218// bool acquire()
219// Called before we start the initialization. Check if someone else has already started, and if
220// not to signal our intent to start it ourselves. We determine the current status from the init
221// byte, which is one of 4 possible values:
222// COMPLETE: Initialization was finished by somebody else. Return true.
223// PENDING: Somebody has started the initialization already, set the WAITING bit,
224// then wait for the init byte to get updated with a new value.
225// (PENDING|WAITING): Somebody has started the initialization already, and we're not the
226// first one waiting. Wait for the init byte to get updated.
227// UNSET: Initialization hasn't successfully completed, and nobody is currently
228// performing the initialization. Set the PENDING bit to indicate our
229// intention to start the initialization, and return false.
230// The return value indicates whether initialization has already been completed.
231//
232// void release()
233// Called after successfully completing the initialization. Update the init byte to reflect
234// that, then if anybody else is waiting, wake them up.
235//
236// void abort()
237// Called after an error is thrown during the initialization. Reset the init byte to UNSET to
238// indicate that we're no longer performing the initialization, then if anybody is waiting, wake
239// them up so they can try performing the initialization.
240//
241
242//===----------------------------------------------------------------------===//
243// Single Threaded Implementation
244//===----------------------------------------------------------------------===//
245
246/// InitByteNoThreads - Doesn't use any inter-thread synchronization when
247/// managing reads and writes to the init byte.
248struct InitByteNoThreads {
249 InitByteNoThreads() = delete;
250 InitByteNoThreads(InitByteNoThreads const&) = delete;
251 InitByteNoThreads& operator=(InitByteNoThreads const&) = delete;
252
253 explicit InitByteNoThreads(uint8_t* _init_byte_address, uint32_t*) : init_byte_address(_init_byte_address) {}
254
255 /// The init byte portion of cxa_guard_acquire. Returns true if
256 /// initialization has already been completed.
257 bool acquire() {
258 if (*init_byte_address == COMPLETE_BIT)
259 return true;
260 if (*init_byte_address & PENDING_BIT)
261 ABORT_WITH_MESSAGE("__cxa_guard_acquire detected recursive initialization: do you have a function-local static variable whose initialization depends on that function?");
262 *init_byte_address = PENDING_BIT;
263 return false;
264 }
265
266 /// The init byte portion of cxa_guard_release.
267 void release() { *init_byte_address = COMPLETE_BIT; }
268 /// The init byte portion of cxa_guard_abort.
269 void abort() { *init_byte_address = UNSET; }
270
271private:
272 /// The address of the byte used during initialization.
273 uint8_t* const init_byte_address;
274};
275
276//===----------------------------------------------------------------------===//
277// Global Mutex Implementation
278//===----------------------------------------------------------------------===//
279
280struct LibcppMutex;
281struct LibcppCondVar;
282
283#ifndef _LIBCXXABI_HAS_NO_THREADS
284struct LibcppMutex {
285 LibcppMutex() = default;
286 LibcppMutex(LibcppMutex const&) = delete;
287 LibcppMutex& operator=(LibcppMutex const&) = delete;
288
289 bool lock() { return std::__libcpp_mutex_lock(&mutex); }
290 bool unlock() { return std::__libcpp_mutex_unlock(&mutex); }
291
292private:
293 friend struct LibcppCondVar;
294 std::__libcpp_mutex_t mutex = _LIBCPP_MUTEX_INITIALIZER;
295};
296
297struct LibcppCondVar {
298 LibcppCondVar() = default;
299 LibcppCondVar(LibcppCondVar const&) = delete;
300 LibcppCondVar& operator=(LibcppCondVar const&) = delete;
301
302 bool wait(LibcppMutex& mut) { return std::__libcpp_condvar_wait(&cond, &mut.mutex); }
303 bool broadcast() { return std::__libcpp_condvar_broadcast(&cond); }
304
305private:
306 std::__libcpp_condvar_t cond = _LIBCPP_CONDVAR_INITIALIZER;
307};
308#else
309struct LibcppMutex {};
310struct LibcppCondVar {};
311#endif // !defined(_LIBCXXABI_HAS_NO_THREADS)
312
313/// InitByteGlobalMutex - Uses a global mutex and condition variable (common to
314/// all static local variables) to manage reads and writes to the init byte.
315template <class Mutex, class CondVar, Mutex& global_mutex, CondVar& global_cond,
316 uint32_t (*GetThreadID)() = PlatformThreadID>
317struct InitByteGlobalMutex {
318
319 explicit InitByteGlobalMutex(uint8_t* _init_byte_address, uint32_t* _thread_id_address)
320 : init_byte_address(_init_byte_address), thread_id_address(_thread_id_address),
321 has_thread_id_support(_thread_id_address != nullptr && GetThreadID != nullptr) {}
322
323public:
324 /// The init byte portion of cxa_guard_acquire. Returns true if
325 /// initialization has already been completed.
326 bool acquire() {
327 LockGuard g("__cxa_guard_acquire");
328 // Check for possible recursive initialization.
329 if (has_thread_id_support && (*init_byte_address & PENDING_BIT)) {
330 if (*thread_id_address == current_thread_id.get())
331 ABORT_WITH_MESSAGE("__cxa_guard_acquire detected recursive initialization: do you have a function-local static variable whose initialization depends on that function?");
332 }
333
334 // Wait until the pending bit is not set.
335 while (*init_byte_address & PENDING_BIT) {
336 *init_byte_address |= WAITING_BIT;
337 global_cond.wait(global_mutex);
338 }
339
340 if (*init_byte_address == COMPLETE_BIT)
341 return true;
342
343 if (has_thread_id_support)
344 *thread_id_address = current_thread_id.get();
345
346 *init_byte_address = PENDING_BIT;
347 return false;
348 }
349
350 /// The init byte portion of cxa_guard_release.
351 void release() {
352 bool has_waiting;
353 {
354 LockGuard g("__cxa_guard_release");
355 has_waiting = *init_byte_address & WAITING_BIT;
356 *init_byte_address = COMPLETE_BIT;
357 }
358 if (has_waiting) {
359 if (global_cond.broadcast()) {
360 ABORT_WITH_MESSAGE("%s failed to broadcast", "__cxa_guard_release");
361 }
362 }
363 }
364
365 /// The init byte portion of cxa_guard_abort.
366 void abort() {
367 bool has_waiting;
368 {
369 LockGuard g("__cxa_guard_abort");
370 if (has_thread_id_support)
371 *thread_id_address = 0;
372 has_waiting = *init_byte_address & WAITING_BIT;
373 *init_byte_address = UNSET;
374 }
375 if (has_waiting) {
376 if (global_cond.broadcast()) {
377 ABORT_WITH_MESSAGE("%s failed to broadcast", "__cxa_guard_abort");
378 }
379 }
380 }
381
382private:
383 /// The address of the byte used during initialization.
384 uint8_t* const init_byte_address;
385 /// An optional address storing an identifier for the thread performing initialization.
386 /// It's used to detect recursive initialization.
387 uint32_t* const thread_id_address;
388
389 const bool has_thread_id_support;
390 LazyValue<uint32_t, GetThreadID> current_thread_id;
391
392private:
393 struct LockGuard {
394 LockGuard() = delete;
395 LockGuard(LockGuard const&) = delete;
396 LockGuard& operator=(LockGuard const&) = delete;
397
398 explicit LockGuard(const char* calling_func) : calling_func_(calling_func) {
399 if (global_mutex.lock())
400 ABORT_WITH_MESSAGE("%s failed to acquire mutex", calling_func_);
401 }
402
403 ~LockGuard() {
404 if (global_mutex.unlock())
405 ABORT_WITH_MESSAGE("%s failed to release mutex", calling_func_);
406 }
407
408 private:
409 const char* const calling_func_;
410 };
411};
412
413//===----------------------------------------------------------------------===//
414// Futex Implementation
415//===----------------------------------------------------------------------===//
416
417#if defined(__OpenBSD__)
418void PlatformFutexWait(int* addr, int expect) {
419 constexpr int WAIT = 0;
420 futex(reinterpret_cast<volatile uint32_t*>(addr), WAIT, expect, NULL, NULL);
421 __tsan_acquire(addr);
422}
423void PlatformFutexWake(int* addr) {
424 constexpr int WAKE = 1;
425 __tsan_release(addr);
426 futex(reinterpret_cast<volatile uint32_t*>(addr), WAKE, INT_MAX, NULL, NULL);
427}
428#elif defined(SYS_futex)
429void PlatformFutexWait(int* addr, int expect) {
430 constexpr int WAIT = 0;
431 syscall(SYS_futex, addr, WAIT, expect, 0);
432 __tsan_acquire(addr);
433}
434void PlatformFutexWake(int* addr) {
435 constexpr int WAKE = 1;
436 __tsan_release(addr);
437 syscall(SYS_futex, addr, WAKE, INT_MAX);
438}
439#else
440constexpr void (*PlatformFutexWait)(int*, int) = nullptr;
441constexpr void (*PlatformFutexWake)(int*) = nullptr;
442#endif
443
444constexpr bool PlatformSupportsFutex() { return +PlatformFutexWait != nullptr; }
445
446/// InitByteFutex - Uses a futex to manage reads and writes to the init byte.
447template <void (*Wait)(int*, int) = PlatformFutexWait, void (*Wake)(int*) = PlatformFutexWake,
448 uint32_t (*GetThreadIDArg)() = PlatformThreadID>
449struct InitByteFutex {
450
451 explicit InitByteFutex(uint8_t* _init_byte_address, uint32_t* _thread_id_address)
452 : init_byte(_init_byte_address),
453 has_thread_id_support(_thread_id_address != nullptr && GetThreadIDArg != nullptr),
454 thread_id(_thread_id_address),
455 base_address(reinterpret_cast<int*>(/*_init_byte_address & ~0x3*/ _init_byte_address - 1)) {}
456
457public:
458 /// The init byte portion of cxa_guard_acquire. Returns true if
459 /// initialization has already been completed.
460 bool acquire() {
461 while (true) {
462 uint8_t last_val = UNSET;
463 if (init_byte.compare_exchange(&last_val, PENDING_BIT, std::_AO_Acq_Rel, std::_AO_Acquire)) {
464 if (has_thread_id_support) {
465 thread_id.store(current_thread_id.get(), std::_AO_Relaxed);
466 }
467 return false;
468 }
469
470 if (last_val == COMPLETE_BIT)
471 return true;
472
473 if (last_val & PENDING_BIT) {
474
475 // Check for recursive initialization
476 if (has_thread_id_support && thread_id.load(std::_AO_Relaxed) == current_thread_id.get()) {
477 ABORT_WITH_MESSAGE("__cxa_guard_acquire detected recursive initialization: do you have a function-local static variable whose initialization depends on that function?");
478 }
479
480 if ((last_val & WAITING_BIT) == 0) {
481 // This compare exchange can fail for several reasons
482 // (1) another thread finished the whole thing before we got here
483 // (2) another thread set the waiting bit we were trying to thread
484 // (3) another thread had an exception and failed to finish
485 if (!init_byte.compare_exchange(&last_val, PENDING_BIT | WAITING_BIT, std::_AO_Acq_Rel, std::_AO_Release)) {
486 // (1) success, via someone else's work!
487 if (last_val == COMPLETE_BIT)
488 return true;
489
490 // (3) someone else, bailed on doing the work, retry from the start!
491 if (last_val == UNSET)
492 continue;
493
494 // (2) the waiting bit got set, so we are happy to keep waiting
495 }
496 }
497 wait_on_initialization();
498 }
499 }
500 }
501
502 /// The init byte portion of cxa_guard_release.
503 void release() {
504 uint8_t old = init_byte.exchange(COMPLETE_BIT, std::_AO_Acq_Rel);
505 if (old & WAITING_BIT)
506 wake_all();
507 }
508
509 /// The init byte portion of cxa_guard_abort.
510 void abort() {
511 if (has_thread_id_support)
512 thread_id.store(0, std::_AO_Relaxed);
513
514 uint8_t old = init_byte.exchange(UNSET, std::_AO_Acq_Rel);
515 if (old & WAITING_BIT)
516 wake_all();
517 }
518
519private:
520 /// Use the futex to wait on the current guard variable. Futex expects a
521 /// 32-bit 4-byte aligned address as the first argument, so we use the 4-byte
522 /// aligned address that encompasses the init byte (i.e. the address of the
523 /// raw guard object that was passed to __cxa_guard_acquire/release/abort).
524 void wait_on_initialization() { Wait(base_address, expected_value_for_futex(PENDING_BIT | WAITING_BIT)); }
525 void wake_all() { Wake(base_address); }
526
527private:
528 AtomicInt<uint8_t> init_byte;
529
530 const bool has_thread_id_support;
531 // Unsafe to use unless has_thread_id_support
532 AtomicInt<uint32_t> thread_id;
533 LazyValue<uint32_t, GetThreadIDArg> current_thread_id;
534
535 /// the 4-byte-aligned address that encompasses the init byte (i.e. the
536 /// address of the raw guard object).
537 int* const base_address;
538
539 /// Create the expected integer value for futex `wait(int* addr, int expected)`.
540 /// We pass the base address as the first argument, So this function creates
541 /// an zero-initialized integer with `b` copied at the correct offset.
542 static int expected_value_for_futex(uint8_t b) {
543 int dest_val = 0;
544 std::memcpy(reinterpret_cast<char*>(&dest_val) + 1, &b, 1);
545 return dest_val;
546 }
547
548 static_assert(Wait != nullptr && Wake != nullptr, "");
549};
550
551//===----------------------------------------------------------------------===//
552// GuardObject
553//===----------------------------------------------------------------------===//
554
555enum class AcquireResult {
556 INIT_IS_DONE,
557 INIT_IS_PENDING,
558};
559constexpr AcquireResult INIT_IS_DONE = AcquireResult::INIT_IS_DONE;
560constexpr AcquireResult INIT_IS_PENDING = AcquireResult::INIT_IS_PENDING;
561
562/// Co-ordinates between GuardByte and InitByte.
563template <class InitByteT>
564struct GuardObject {
565 GuardObject() = delete;
566 GuardObject(GuardObject const&) = delete;
567 GuardObject& operator=(GuardObject const&) = delete;
568
569private:
570 GuardByte guard_byte;
571 InitByteT init_byte;
572
573public:
574 /// ARM Constructor
575 explicit GuardObject(uint32_t* raw_guard_object)
576 : guard_byte(reinterpret_cast<uint8_t*>(raw_guard_object)),
577 init_byte(reinterpret_cast<uint8_t*>(raw_guard_object) + 1, nullptr) {}
578
579 /// Itanium Constructor
580 explicit GuardObject(uint64_t* raw_guard_object)
581 : guard_byte(reinterpret_cast<uint8_t*>(raw_guard_object)),
582 init_byte(reinterpret_cast<uint8_t*>(raw_guard_object) + 1, reinterpret_cast<uint32_t*>(raw_guard_object) + 1) {
583 }
584
585 /// Implements __cxa_guard_acquire.
586 AcquireResult cxa_guard_acquire() {
587 // Use short-circuit evaluation to avoid calling init_byte.acquire when
588 // guard_byte.acquire returns true. (i.e. don't call it when we know from
589 // the guard byte that initialization has already been completed)
590 if (guard_byte.acquire() || init_byte.acquire())
591 return INIT_IS_DONE;
592 return INIT_IS_PENDING;
593 }
594
595 /// Implements __cxa_guard_release.
596 void cxa_guard_release() {
597 // Update guard byte first, so if somebody is woken up by init_byte.release
598 // and comes all the way back around to __cxa_guard_acquire again, they see
599 // it as having completed initialization.
600 guard_byte.release();
601 init_byte.release();
602 }
603
604 /// Implements __cxa_guard_abort.
605 void cxa_guard_abort() {
606 guard_byte.abort();
607 init_byte.abort();
608 }
609};
610
611//===----------------------------------------------------------------------===//
612// Convenience Classes
613//===----------------------------------------------------------------------===//
614
615/// NoThreadsGuard - Manages initialization without performing any inter-thread
616/// synchronization.
617using NoThreadsGuard = GuardObject<InitByteNoThreads>;
618
619/// GlobalMutexGuard - Manages initialization using a global mutex and
620/// condition variable.
621template <class Mutex, class CondVar, Mutex& global_mutex, CondVar& global_cond,
622 uint32_t (*GetThreadID)() = PlatformThreadID>
623using GlobalMutexGuard = GuardObject<InitByteGlobalMutex<Mutex, CondVar, global_mutex, global_cond, GetThreadID>>;
624
625/// FutexGuard - Manages initialization using atomics and the futex syscall for
626/// waiting and waking.
627template <void (*Wait)(int*, int) = PlatformFutexWait, void (*Wake)(int*) = PlatformFutexWake,
628 uint32_t (*GetThreadIDArg)() = PlatformThreadID>
629using FutexGuard = GuardObject<InitByteFutex<Wait, Wake, GetThreadIDArg>>;
630
631//===----------------------------------------------------------------------===//
632//
633//===----------------------------------------------------------------------===//
634
635template <class T>
636struct GlobalStatic {
637 static T instance;
638};
639template <class T>
640_LIBCPP_CONSTINIT T GlobalStatic<T>::instance = {};
641
642enum class Implementation { NoThreads, GlobalMutex, Futex };
643
644template <Implementation Impl>
645struct SelectImplementation;
646
647template <>
648struct SelectImplementation<Implementation::NoThreads> {
649 using type = NoThreadsGuard;
650};
651
652template <>
653struct SelectImplementation<Implementation::GlobalMutex> {
654 using type = GlobalMutexGuard<LibcppMutex, LibcppCondVar, GlobalStatic<LibcppMutex>::instance,
655 GlobalStatic<LibcppCondVar>::instance, PlatformThreadID>;
656};
657
658template <>
659struct SelectImplementation<Implementation::Futex> {
660 using type = FutexGuard<PlatformFutexWait, PlatformFutexWake, PlatformThreadID>;
661};
662
663// TODO(EricWF): We should prefer the futex implementation when available. But
664// it should be done in a separate step from adding the implementation.
665constexpr Implementation CurrentImplementation =
666#if defined(_LIBCXXABI_HAS_NO_THREADS)
667 Implementation::NoThreads;
668#elif defined(_LIBCXXABI_USE_FUTEX)
669 Implementation::Futex;
670#else
671 Implementation::GlobalMutex;
672#endif
673
674static_assert(CurrentImplementation != Implementation::Futex || PlatformSupportsFutex(),
675 "Futex selected but not supported");
676
677using SelectedImplementation = SelectImplementation<CurrentImplementation>::type;
678
679} // namespace
680} // namespace __cxxabiv1
681
682#if defined(__clang__)
683# pragma clang diagnostic pop
684#elif defined(__GNUC__)
685# pragma GCC diagnostic pop
686#endif
687
688#endif // LIBCXXABI_SRC_INCLUDE_CXA_GUARD_IMPL_H