[libcxx-commits] [PATCH] D88599: [SystemZ][ZOS] Porting pthread_t related functionality within libc++ to z/OS

[Marshall Clow via Phabricator via libcxx-commits]

mclow.lists added inline comments.


================
Comment at: libcxx/include/__threading_support:495
 {
-  return pthread_equal(t1, t2) != 0;
+  return t1 == t2;
 }
----------------
zibi wrote:
> zibi wrote:
> > uweigand wrote:
> > > zibi wrote:
> > > > mclow.lists wrote:
> > > > > This is what this code used to be. It's wrong, because `pthread_t` is not a type that we control, and there's no guarantee that it has an `operator==`.  That's why there's a call `pthread_equal`
> > > > > This is what this code used to be. It's wrong, because `pthread_t` is not a type that we control, and there's no guarantee that it has an `operator==`.  That's why there's a call `pthread_equal`
> > > > 
> > > > 
> > > > Thx Marshall, the problem with the original call to `'thread_equal` is that it was passing wrong parameters by mixing thread_id with thread_t. We need clear separation between them and don't use them interchangeably.
> > > Note that even in the current code, `__libcpp_thread_id` is implicitly assumed to be a numeric type, see in particular the routine `__libcpp_thread_is_less` immediately below.
> > > 
> > > My suggestion has been to use `__libcpp_thread_t` for the platform-specific opaque thread handle (e.g. `pthread_t`), and to use `__libcpp_thread_id` for a numeric ID.  On platforms where `pthread_t` happens to be numeric itself, the types can be the same; other platforms need to provide a different definition for `__libcpp_thread_id` (as done by this patch for z/OS).
> > > 
> > > Note that even in the current code, `__libcpp_thread_id` is implicitly assumed to be a numeric type, see in particular the routine `__libcpp_thread_is_less` immediately below.
> > 
> > Yes Ulrich, I noticed that as well.  
> > 
> > If we want to use `pthread_equal()` we will have to operate on `__libcpp_thread_t` in both `__libcpp_thread_id_less` and `__libcpp_thread_id_equal` like we do already do in `__libcpp_thread_isnull'.
> > However, that will get us to a ripple effect of changing template `__thread_id`. Furthermore `__thread_id` is being used in `hash<>`.
> > 
> > **Marshall**, let me know if you want me to pursue this road.
> One of the usage of  `__thread_id` is `'recursive_timed_mutex` which does retrieves the id of the thread before calling checking if it's the same as the current thread. Here is the snippet of this:
> 
> ```
>     __thread_id __id = this_thread::get_id();
>     unique_lock<mutex> lk(__m_);
>     if (__id == __id_)
> ```
> 
> Therefore we are assuming that thread_id is integral type and using == is perfectly fine.
> Alternatively, we could change the code to use pthread_equal(), but that would require the argument type to change and other ripple effects.
> 
Now you're mixing types. `__thread_id` is a different type than `__libcpp_thread_t`

`__thread_id` has a full complement of comparison operators, and `hash` support as well.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D88599/new/

https://reviews.llvm.org/D88599