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Thread Proposal |
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Java provides library support for writing multi-threaded applications, mainly through the Thread class, and Object methods for synchronization. This proposal details implementing threading support in JLang using the pthread library.
Java objects must be updated to include a mutex and condition variable. As all objects require these, but most do not use them, the object header should not contain both the mutex and condition variable, but a pointer to a struct containing them. Upon first use, the mutex and condition variable can be created. The proposed struct containing the mutex and condition variable is as follows:
struct sync_vars {
pthread_mutex_t* mutex;
pthread_cond_t *condition_variable;
};
The proposed object header then needs a single pointer the the above struct:
struct jobj_header {
dv* dispatch_vector;
struct sync_vars* synchronization_vars;
};
The translation for new object creation will set the synchronization_vars pointer to null initally. Checking if the mutex or condition variable is null, and setting them requires obtaining a global mutex. If the synchronization_vars pointer is null, it is allocated, and the mutex and condition variable are constructed. The mutex must be constructed using the PTHREAD_MUTEX_RECURSIVE type. The global lock is released before the object lock is used.
The LLVM memory model makes non-atomic memory races undefined, so it cannot introduce data races from the source program, thus store widening is dissalowed. This matches the Java Specification.
Synchronization blocks must be translated within a try-finally block. If execution of the synchronized block is completed, either abruptly or normally, the lock must be released. The lock is obtained with the pthread_mutex_lock function call on the object's mutex, and the lock is released with the pthread_mutex_unlock function call.
The Object.wait method can be implemented using the pthread_cond_wait function, the Object.notify method can be implemented using the pthread_cond_signal. and the Object.notifyAll method can be implemented using the function pthread_cond_broadcast.
To wait for a specified amount of time, the pthread_cond_timedwait function can be used.
These methods need to check if the mutex is held, which can be done using the pthread_mutex_trylock function.
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