Re: [Boost-users] [boost] Boost.Fiber mini-review September 4-13

On 09/06/2015 06:30 AM, Oliver Kowalke wrote:

2015-09-05 22:04 GMT+02:00 Hartmut Kaiser mailto:hartmut.kaiser@gmail.com>:

> but I was asking for the code snipped which accesses TLS variable - if all > threads share the same scheduler why has it to be stored in TLS? > a slight modification of my example above - replace > retrn_ptr_of_scheduler_thread_lcoal_var() by return_ptr_to_active_fiber() > I assume that as you mentioned in b) that the active fiber is stored in > TLS - the exmaple code prints out the address of the current active fiber > how does the code look like after compiler optimization?

I don't know. We have never ran into any problems related to this.

I did a short look at the souces from HPX - please correct me if my assumptions are false:

- HPX does not support C++11, so thread_local is not available - instead you implemented thread_specific_ptr

It does support C++11 and C++14. thread_specific_ptr is an artifact from the past but still perfectly conformant in C++11/14.

- thread_specific_ptr has an private member - static __thread T * t - so thread_specific_ptr is the TLS container

correct.

- coroutine_impl has a private member - static thread_specific_ptr< self_type > self_ which is used to access the 'active fiber/coroutine' via static member function - static self_type * get_self()

correct.

- coroutine_impl::operator() executes the function/code passed to coroutine_impl

- the implementation of coroutine::operator() looks like this (in short):

void operator() { do { self_type * old_self = coroutine_impl::get_self(); self_type self( this, old_self); reset_self_on_exit( & self, old_self);

fun(); // execute function } while ( state == running); }

I the compiler optimizes the code - self_type * old_self = coroutine_impl::get_self(); - will likely be moved out of the do-loop.

nope it won't. see my other post for an explanation of this.

void operator() { self_type * old_self = coroutine_impl::get_self(); do { self_type self( this, old_self); reset_self_on_exit( & self, old_self);

fun(); // execute function } while ( state == running); }

This happens probably in other places of HPX too - the question is why you never ran into any problems.

because we use the value, not the address of the TLS variable, which is of course constant.

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