[vlc-devel] [RFC] Atomic API

[Felix Abecassis]

Hi,

In order to compile VLC with MSVC I recently implemented atomic operations
in vlc_atomic.h using the Interlocked API
(http://msdn.microsoft.com/en-us/library/windows/desktop/ms686360%28v=vs.85%29.aspx#interlocked_functions<(http://msdn.microsoft.com/en-us/library/sbhbke0y%28v=vs.110%29.aspx>
).

However when compiling for WinRT we discovered some operations are not
implemented for all types, for instance InterlockedExchange8 is only
available for desktop apps. This mean we cannot currently provide an
implementation of atomic load/store operations on char/bool types. Since
some important files (stream_demux.c, picture_pool.c, variables.h and
win32/thread.c) are using the atomic_bool type, we need a way to solve this
problem.

I've found three options and I would like your opinion in order to decide
which one is the best.

1) Fill the holes in the Interlocked API by implementing the missing
functions. We could use locks similarly to what is currently being
implemented in vlc_atomic.h when no builtin atomic operations are used.

2) Replace the aforementioned occurrences of variables of type atomic_bool
and use the type vlc_atomic_t instead. Some modules take this approach for
similar variables. For instance android/thread.c has a variable
"vlc_atomic_t killed;" whereas win32/thread.c uses "atomic_bool killed;".
vlc_atomic_t uses the type uintptr_t (4 or 8 bytes).

3) Disable atomic types of size 1 byte for this target. We could force the
types atomic_bool/atomic_char... to be of a larger size. For instance with
something like this:
#if defined (_MSC_VER)
typedef int atomic_bool;
#else
typedef bool atomic_bool;
#endif
This approach could also be used if we encounter the same issue with a
different target.

Cheers,

-- 
Félix Abecassis
http://felix.abecassis.me
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.videolan.org/pipermail/vlc-devel/attachments/20131205/a2ee0ff8/attachment.html>