This is the mail archive of the mailing list for the GCC project.

Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]
Other format: [Raw text]

Re: Re: Re: Adding a new thread model to GCC

One more question about block-scoped static objects:

>From compiled assembly I have learned that GCC uses a 64-bit integer guard to ensure once-initialization of static objects with block scopes.
Code in gcc/libstdc++-v3/libsupc++/ uses a global mutex to protect multiple threads from racing, which, as described in that file, could potentially cause a deadlock that can be avoided if a condition variable is used.

This, however, is unnecessary with mcfgthread. The mcfgthread library uses the once_flag itself as both the mutex and condition variable, eliminating the need of a global mutex and condition variable.
Code using mcfgthread might look like this:

class foo { ... };

static ::_MCFCRT_OnceFlag flag;
static alignas(foo) char obj[sizeof(foo)];

foo *get_obj(){
    const auto construct_obj = []{ ::new(static_cast<void *>(obj)) foo(); };
    const auto destruct_obj  = []{ reinterpret_cast<foo *>(obj)->~foo(); };

    // Lock the once flag as if it were a mutex.
    const auto result = ::_MCFCRT_WaitForOnceFlagForever(&flag); 
    // This never times out, so result can be either _MCFCRT_kOnceResultInitial or _MCFCRT_kOnceResultFinished.
    if(result == ::_MCFCRT_kOnceResultInitial){
        try {
            // Construct the object in-place.
            if(std::atexit(&destruct_obj) != 0){
                // Assume we have run out of memory.
                throw std::bad_alloc();
        } catch(...){
            // Unlock the once flag, allowing other threads to retry the initialization.
            // This works like pthread_cond_signal().
        // We are done here. Unlock the once flag and other threads will not be blocked any more.
        // This works like pthread_cond_broadcast().
    return reinterpret_cast<foo *>(obj);

Now here is my question:
That '__cxxabiv1::__guard' thing seems built-in of GCC. Does it have anything to do with code generation? And where is it involved?

Best regards,

Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]