[libcxxabi] Re-organized inheritance structure to remove CRTP in cxa_guard
Currently, the `InitByte...` classes inherit from `GuardObject` so they can
access the `base_address`, `init_byte_address` and `thread_id_address`. Then,
since `GuardObject` needs to call `acquire`/`release`/`abort_init_byte`, it uses
the curiously recurring template pattern (CRTP). This is rather messy.
Instead, we'll have `GuardObject` contain an instance of `InitByte`, and pass it
the addresses it needs in the constructor. `GuardObject` doesn't need the
addresses anyways, so it makes more sense for `InitByte` to keep them instead of
`GuardObject`. Then, `GuardObject` can call `acquire`/`release`/`abort` as one
of `InitByte`'s member functions.
Organizing things this way not only gets rid of the use of the CRTP, but also
improves separation of concerns a bit since the `InitByte` classes are no longer
indirectly responsible for things because of their inheritance from
`GuardObject`. This means we no longer have strange things like calling
`InitByteFutex.cxa_guard_acquire`, instead we call
`GuardObject<InitByteFutex>.cxa_guard_acquire`.
This is the 4th of 5 changes to overhaul cxa_guard.
See D108343 for what the final result will be.
Depends on D115367
Reviewed By: ldionne, #libc_abi
Differential Revision: https://reviews.llvm.org/D115368
GitOrigin-RevId: 29be7c9c4f5d03be21ac80dc178a730aa10452f9
diff --git a/src/cxa_guard_impl.h b/src/cxa_guard_impl.h
index 36477e4..d8d991b 100644
--- a/src/cxa_guard_impl.h
+++ b/src/cxa_guard_impl.h
@@ -23,9 +23,15 @@
* the thread currently performing initialization is stored in the second word.
*
* Guard Object Layout:
- * -------------------------------------------------------------------------
- * |a: guard byte | a+1: init byte | a+2 : unused ... | a+4: thread-id ... |
- * -------------------------------------------------------------------------
+ * ---------------------------------------------------------------------------
+ * | a+0: guard byte | a+1: init byte | a+2: unused ... | a+4: thread-id ... |
+ * ---------------------------------------------------------------------------
+ *
+ * Note that we don't do what the ABI docs suggest (put a mutex in the guard
+ * object which we acquire in cxa_guard_acquire and release in
+ * cxa_guard_release). Instead we use the init byte to imitate that behaviour,
+ * but without actually holding anything mutex related between aquire and
+ * release/abort.
*
* Access Protocol:
* For each implementation the guard byte is checked and set before accessing
@@ -176,8 +182,6 @@
public:
/// The guard byte portion of cxa_guard_acquire. Returns true if
/// initialization has already been completed.
- /// Note: On completion, we haven't 'acquired' ownership of anything mutex
- /// related. The name is simply referring to __cxa_guard_acquire.
bool acquire() {
// if guard_byte is non-zero, we have already completed initialization
// (i.e. release has been called)
@@ -185,8 +189,6 @@
}
/// The guard byte portion of cxa_guard_release.
- /// Note: On completion, we haven't 'released' ownership of anything mutex
- /// related. The name is simply referring to __cxa_guard_release.
void release() { guard_byte.store(COMPLETE_BIT, std::_AO_Release); }
/// The guard byte portion of cxa_guard_abort.
@@ -197,89 +199,70 @@
};
//===----------------------------------------------------------------------===//
-// GuardBase
+// InitByte Implementations
//===----------------------------------------------------------------------===//
-
-enum class AcquireResult {
- INIT_IS_DONE,
- INIT_IS_PENDING,
-};
-constexpr AcquireResult INIT_IS_DONE = AcquireResult::INIT_IS_DONE;
-constexpr AcquireResult INIT_IS_PENDING = AcquireResult::INIT_IS_PENDING;
-
-template <class Derived>
-struct GuardObject {
- GuardObject() = delete;
- GuardObject(GuardObject const&) = delete;
- GuardObject& operator=(GuardObject const&) = delete;
-
-private:
- GuardByte guard_byte;
-
-public:
- /// ARM Constructor
- explicit GuardObject(uint32_t* g)
- : guard_byte(reinterpret_cast<uint8_t*>(g)), base_address(g),
- init_byte_address(reinterpret_cast<uint8_t*>(g) + 1), thread_id_address(nullptr) {}
-
- /// Itanium Constructor
- explicit GuardObject(uint64_t* g)
- : guard_byte(reinterpret_cast<uint8_t*>(g)), base_address(g),
- init_byte_address(reinterpret_cast<uint8_t*>(g) + 1), thread_id_address(reinterpret_cast<uint32_t*>(g) + 1) {}
-
- /// Implements __cxa_guard_acquire.
- AcquireResult cxa_guard_acquire() {
- if (guard_byte.acquire())
- return INIT_IS_DONE;
- return derived()->acquire_init_byte();
- }
-
- /// Implements __cxa_guard_release.
- void cxa_guard_release() {
- // Update guard byte first, so if somebody is woken up by release_init_byte
- // and comes all the way back around to __cxa_guard_acquire again, they see
- // it as having completed initialization.
- guard_byte.release();
- derived()->release_init_byte();
- }
-
- /// Implements __cxa_guard_abort.
- void cxa_guard_abort() {
- guard_byte.abort();
- derived()->abort_init_byte();
- }
-
-public:
- /// base_address - the address of the original guard object.
- void* const base_address;
- /// The address of the byte used by the implementation during initialization.
- uint8_t* const init_byte_address;
- /// An optional address storing an identifier for the thread performing initialization.
- /// It's used to detect recursive initialization.
- uint32_t* const thread_id_address;
-
-private:
- Derived* derived() { return static_cast<Derived*>(this); }
-};
+//
+// Each initialization byte implementation supports the following methods:
+//
+// InitByte(uint8_t* _init_byte_address, uint32_t* _thread_id_address)
+// Construct the InitByte object, initializing our member variables
+//
+// bool acquire()
+// Called before we start the initialization. Check if someone else has already started, and if
+// not to signal our intent to start it ourselves. We determine the current status from the init
+// byte, which is one of 4 possible values:
+// COMPLETE: Initialization was finished by somebody else. Return true.
+// PENDING: Somebody has started the initialization already, set the WAITING bit,
+// then wait for the init byte to get updated with a new value.
+// (PENDING|WAITING): Somebody has started the initialization already, and we're not the
+// first one waiting. Wait for the init byte to get updated.
+// UNSET: Initialization hasn't successfully completed, and nobody is currently
+// performing the initialization. Set the PENDING bit to indicate our
+// intention to start the initialization, and return false.
+// The return value indicates whether initialization has already been completed.
+//
+// void release()
+// Called after successfully completing the initialization. Update the init byte to reflect
+// that, then if anybody else is waiting, wake them up.
+//
+// void abort()
+// Called after an error is thrown during the initialization. Reset the init byte to UNSET to
+// indicate that we're no longer performing the initialization, then if anybody is waiting, wake
+// them up so they can try performing the initialization.
+//
//===----------------------------------------------------------------------===//
// Single Threaded Implementation
//===----------------------------------------------------------------------===//
-struct InitByteNoThreads : GuardObject<InitByteNoThreads> {
- using GuardObject::GuardObject;
+/// InitByteNoThreads - Doesn't use any inter-thread synchronization when
+/// managing reads and writes to the init byte.
+struct InitByteNoThreads {
+ InitByteNoThreads() = delete;
+ InitByteNoThreads(InitByteNoThreads const&) = delete;
+ InitByteNoThreads& operator=(InitByteNoThreads const&) = delete;
- AcquireResult acquire_init_byte() {
+ explicit InitByteNoThreads(uint8_t* _init_byte_address, uint32_t*) : init_byte_address(_init_byte_address) {}
+
+ /// The init byte portion of cxa_guard_acquire. Returns true if
+ /// initialization has already been completed.
+ bool acquire() {
if (*init_byte_address == COMPLETE_BIT)
- return INIT_IS_DONE;
+ return true;
if (*init_byte_address & PENDING_BIT)
ABORT_WITH_MESSAGE("__cxa_guard_acquire detected recursive initialization");
*init_byte_address = PENDING_BIT;
- return INIT_IS_PENDING;
+ return false;
}
- void release_init_byte() { *init_byte_address = COMPLETE_BIT; }
- void abort_init_byte() { *init_byte_address = UNSET; }
+ /// The init byte portion of cxa_guard_release.
+ void release() { *init_byte_address = COMPLETE_BIT; }
+ /// The init byte portion of cxa_guard_abort.
+ void abort() { *init_byte_address = UNSET; }
+
+private:
+ /// The address of the byte used during initialization.
+ uint8_t* const init_byte_address;
};
//===----------------------------------------------------------------------===//
@@ -319,18 +302,20 @@
struct LibcppCondVar {};
#endif // !defined(_LIBCXXABI_HAS_NO_THREADS)
+/// InitByteGlobalMutex - Uses a global mutex and condition variable (common to
+/// all static local variables) to manage reads and writes to the init byte.
template <class Mutex, class CondVar, Mutex& global_mutex, CondVar& global_cond,
uint32_t (*GetThreadID)() = PlatformThreadID>
-struct InitByteGlobalMutex : GuardObject<InitByteGlobalMutex<Mutex, CondVar, global_mutex, global_cond, GetThreadID>> {
+struct InitByteGlobalMutex {
- using BaseT = typename InitByteGlobalMutex::GuardObject;
- using BaseT::BaseT;
-
- explicit InitByteGlobalMutex(uint32_t* g) : BaseT(g), has_thread_id_support(false) {}
- explicit InitByteGlobalMutex(uint64_t* g) : BaseT(g), has_thread_id_support(GetThreadID != nullptr) {}
+ explicit InitByteGlobalMutex(uint8_t* _init_byte_address, uint32_t* _thread_id_address)
+ : init_byte_address(_init_byte_address), thread_id_address(_thread_id_address),
+ has_thread_id_support(_thread_id_address != nullptr && GetThreadID != nullptr) {}
public:
- AcquireResult acquire_init_byte() {
+ /// The init byte portion of cxa_guard_acquire. Returns true if
+ /// initialization has already been completed.
+ bool acquire() {
LockGuard g("__cxa_guard_acquire");
// Check for possible recursive initialization.
if (has_thread_id_support && (*init_byte_address & PENDING_BIT)) {
@@ -345,16 +330,17 @@
}
if (*init_byte_address == COMPLETE_BIT)
- return INIT_IS_DONE;
+ return true;
if (has_thread_id_support)
*thread_id_address = current_thread_id.get();
*init_byte_address = PENDING_BIT;
- return INIT_IS_PENDING;
+ return false;
}
- void release_init_byte() {
+ /// The init byte portion of cxa_guard_release.
+ void release() {
bool has_waiting;
{
LockGuard g("__cxa_guard_release");
@@ -368,7 +354,8 @@
}
}
- void abort_init_byte() {
+ /// The init byte portion of cxa_guard_abort.
+ void abort() {
bool has_waiting;
{
LockGuard g("__cxa_guard_abort");
@@ -385,8 +372,12 @@
}
private:
- using BaseT::init_byte_address;
- using BaseT::thread_id_address;
+ /// The address of the byte used during initialization.
+ uint8_t* const init_byte_address;
+ /// An optional address storing an identifier for the thread performing initialization.
+ /// It's used to detect recursive initialization.
+ uint32_t* const thread_id_address;
+
const bool has_thread_id_support;
LazyValue<uint32_t, GetThreadID> current_thread_id;
@@ -433,38 +424,32 @@
constexpr bool PlatformSupportsFutex() { return +PlatformFutexWait != nullptr; }
-/// InitByteFutex - Manages initialization using atomics and the futex syscall
-/// for waiting and waking.
+/// InitByteFutex - Uses a futex to manage reads and writes to the init byte.
template <void (*Wait)(int*, int) = PlatformFutexWait, void (*Wake)(int*) = PlatformFutexWake,
uint32_t (*GetThreadIDArg)() = PlatformThreadID>
-struct InitByteFutex : GuardObject<InitByteFutex<Wait, Wake, GetThreadIDArg>> {
- using BaseT = typename InitByteFutex::GuardObject;
+struct InitByteFutex {
- /// ARM Constructor
- explicit InitByteFutex(uint32_t* g)
- : BaseT(g), init_byte(this->init_byte_address),
- has_thread_id_support(this->thread_id_address && GetThreadIDArg != nullptr),
- thread_id(this->thread_id_address) {}
-
- /// Itanium Constructor
- explicit InitByteFutex(uint64_t* g)
- : BaseT(g), init_byte(this->init_byte_address),
- has_thread_id_support(this->thread_id_address && GetThreadIDArg != nullptr),
- thread_id(this->thread_id_address) {}
+ explicit InitByteFutex(uint8_t* _init_byte_address, uint32_t* _thread_id_address)
+ : init_byte(_init_byte_address),
+ has_thread_id_support(_thread_id_address != nullptr && GetThreadIDArg != nullptr),
+ thread_id(_thread_id_address),
+ base_address(reinterpret_cast<int*>(/*_init_byte_address & ~0x3*/ _init_byte_address - 1)) {}
public:
- AcquireResult acquire_init_byte() {
+ /// The init byte portion of cxa_guard_acquire. Returns true if
+ /// initialization has already been completed.
+ bool acquire() {
while (true) {
uint8_t last_val = UNSET;
if (init_byte.compare_exchange(&last_val, PENDING_BIT, std::_AO_Acq_Rel, std::_AO_Acquire)) {
if (has_thread_id_support) {
thread_id.store(current_thread_id.get(), std::_AO_Relaxed);
}
- return INIT_IS_PENDING;
+ return false;
}
if (last_val == COMPLETE_BIT)
- return INIT_IS_DONE;
+ return true;
if (last_val & PENDING_BIT) {
@@ -481,7 +466,7 @@
if (!init_byte.compare_exchange(&last_val, PENDING_BIT | WAITING_BIT, std::_AO_Acq_Rel, std::_AO_Release)) {
// (1) success, via someone else's work!
if (last_val == COMPLETE_BIT)
- return INIT_IS_DONE;
+ return true;
// (3) someone else, bailed on doing the work, retry from the start!
if (last_val == UNSET)
@@ -495,13 +480,15 @@
}
}
- void release_init_byte() {
+ /// The init byte portion of cxa_guard_release.
+ void release() {
uint8_t old = init_byte.exchange(COMPLETE_BIT, std::_AO_Acq_Rel);
if (old & WAITING_BIT)
wake_all();
}
- void abort_init_byte() {
+ /// The init byte portion of cxa_guard_abort.
+ void abort() {
if (has_thread_id_support)
thread_id.store(0, std::_AO_Relaxed);
@@ -512,12 +499,11 @@
private:
/// Use the futex to wait on the current guard variable. Futex expects a
- /// 32-bit 4-byte aligned address as the first argument, so we have to use use
- /// the base address of the guard variable (not the init byte).
- void wait_on_initialization() {
- Wait(static_cast<int*>(this->base_address), expected_value_for_futex(PENDING_BIT | WAITING_BIT));
- }
- void wake_all() { Wake(static_cast<int*>(this->base_address)); }
+ /// 32-bit 4-byte aligned address as the first argument, so we use the 4-byte
+ /// aligned address that encompasses the init byte (i.e. the address of the
+ /// raw guard object that was passed to __cxa_guard_acquire/release/abort).
+ void wait_on_initialization() { Wait(base_address, expected_value_for_futex(PENDING_BIT | WAITING_BIT)); }
+ void wake_all() { Wake(base_address); }
private:
AtomicInt<uint8_t> init_byte;
@@ -527,6 +513,10 @@
AtomicInt<uint32_t> thread_id;
LazyValue<uint32_t, GetThreadIDArg> current_thread_id;
+ /// the 4-byte-aligned address that encompasses the init byte (i.e. the
+ /// address of the raw guard object).
+ int* const base_address;
+
/// Create the expected integer value for futex `wait(int* addr, int expected)`.
/// We pass the base address as the first argument, So this function creates
/// an zero-initialized integer with `b` copied at the correct offset.
@@ -540,6 +530,66 @@
};
//===----------------------------------------------------------------------===//
+// GuardObject
+//===----------------------------------------------------------------------===//
+
+enum class AcquireResult {
+ INIT_IS_DONE,
+ INIT_IS_PENDING,
+};
+constexpr AcquireResult INIT_IS_DONE = AcquireResult::INIT_IS_DONE;
+constexpr AcquireResult INIT_IS_PENDING = AcquireResult::INIT_IS_PENDING;
+
+/// Co-ordinates between GuardByte and InitByte.
+template <class InitByteT>
+struct GuardObject {
+ GuardObject() = delete;
+ GuardObject(GuardObject const&) = delete;
+ GuardObject& operator=(GuardObject const&) = delete;
+
+private:
+ GuardByte guard_byte;
+ InitByteT init_byte;
+
+public:
+ /// ARM Constructor
+ explicit GuardObject(uint32_t* raw_guard_object)
+ : guard_byte(reinterpret_cast<uint8_t*>(raw_guard_object)),
+ init_byte(reinterpret_cast<uint8_t*>(raw_guard_object) + 1, nullptr) {}
+
+ /// Itanium Constructor
+ explicit GuardObject(uint64_t* raw_guard_object)
+ : guard_byte(reinterpret_cast<uint8_t*>(raw_guard_object)),
+ init_byte(reinterpret_cast<uint8_t*>(raw_guard_object) + 1, reinterpret_cast<uint32_t*>(raw_guard_object) + 1) {
+ }
+
+ /// Implements __cxa_guard_acquire.
+ AcquireResult cxa_guard_acquire() {
+ // Use short-circuit evaluation to avoid calling init_byte.acquire when
+ // guard_byte.acquire returns true. (i.e. don't call it when we know from
+ // the guard byte that initialization has already been completed)
+ if (guard_byte.acquire() || init_byte.acquire())
+ return INIT_IS_DONE;
+ return INIT_IS_PENDING;
+ }
+
+ /// Implements __cxa_guard_release.
+ void cxa_guard_release() {
+ // Update guard byte first, so if somebody is woken up by init_byte.release
+ // and comes all the way back around to __cxa_guard_acquire again, they see
+ // it as having completed initialization.
+ guard_byte.release();
+ init_byte.release();
+ }
+
+ /// Implements __cxa_guard_abort.
+ void cxa_guard_abort() {
+ guard_byte.abort();
+ init_byte.abort();
+ }
+};
+
+//===----------------------------------------------------------------------===//
//
//===----------------------------------------------------------------------===//
@@ -555,20 +605,23 @@
template <Implementation Impl>
struct SelectImplementation;
+/// Manage initialization without performing any inter-thread synchronization.
template <>
struct SelectImplementation<Implementation::NoThreads> {
- using type = InitByteNoThreads;
+ using type = GuardObject<InitByteNoThreads>;
};
+/// Manage initialization using a global mutex and condition variable.
template <>
struct SelectImplementation<Implementation::GlobalMutex> {
- using type = InitByteGlobalMutex<LibcppMutex, LibcppCondVar, GlobalStatic<LibcppMutex>::instance,
- GlobalStatic<LibcppCondVar>::instance, PlatformThreadID>;
+ using type = GuardObject<InitByteGlobalMutex<LibcppMutex, LibcppCondVar, GlobalStatic<LibcppMutex>::instance,
+ GlobalStatic<LibcppCondVar>::instance, PlatformThreadID>>;
};
+/// Manage initialization using atomics and the futex syscall for waiting and waking.
template <>
struct SelectImplementation<Implementation::Futex> {
- using type = InitByteFutex<PlatformFutexWait, PlatformFutexWake, PlatformThreadID>;
+ using type = GuardObject<InitByteFutex<PlatformFutexWait, PlatformFutexWake, PlatformThreadID>>;
};
// TODO(EricWF): We should prefer the futex implementation when available. But
diff --git a/test/guard_test_basic.pass.cpp b/test/guard_test_basic.pass.cpp
index 588069b..aa243cc 100644
--- a/test/guard_test_basic.pass.cpp
+++ b/test/guard_test_basic.pass.cpp
@@ -119,16 +119,13 @@
{
#if defined(_LIBCXXABI_HAS_NO_THREADS)
static_assert(CurrentImplementation == Implementation::NoThreads, "");
- static_assert(
- std::is_same<SelectedImplementation, InitByteNoThreads>::value, "");
+ static_assert(std::is_same<SelectedImplementation, GuardObject<InitByteNoThreads>>::value, "");
#else
static_assert(CurrentImplementation == Implementation::GlobalMutex, "");
static_assert(
- std::is_same<
- SelectedImplementation,
- InitByteGlobalMutex<LibcppMutex, LibcppCondVar,
- GlobalStatic<LibcppMutex>::instance,
- GlobalStatic<LibcppCondVar>::instance>>::value,
+ std::is_same<SelectedImplementation,
+ GuardObject<InitByteGlobalMutex<LibcppMutex, LibcppCondVar, GlobalStatic<LibcppMutex>::instance,
+ GlobalStatic<LibcppCondVar>::instance>>>::value,
"");
#endif
}
@@ -142,19 +139,17 @@
}
}
{
- Tests<uint32_t, InitByteNoThreads>::test();
- Tests<uint64_t, InitByteNoThreads>::test();
+ Tests<uint32_t, GuardObject<InitByteNoThreads>>::test();
+ Tests<uint64_t, GuardObject<InitByteNoThreads>>::test();
}
{
using MutexImpl =
- InitByteGlobalMutex<NopMutex, NopCondVar, global_nop_mutex,
- global_nop_cond, MockGetThreadID>;
+ GuardObject<InitByteGlobalMutex<NopMutex, NopCondVar, global_nop_mutex, global_nop_cond, MockGetThreadID>>;
Tests<uint32_t, MutexImpl>::test();
Tests<uint64_t, MutexImpl>::test();
}
{
- using FutexImpl =
- InitByteFutex<&NopFutexWait, &NopFutexWake, &MockGetThreadID>;
+ using FutexImpl = GuardObject<InitByteFutex<&NopFutexWait, &NopFutexWake, &MockGetThreadID>>;
Tests<uint32_t, FutexImpl>::test();
Tests<uint64_t, FutexImpl>::test();
}
