http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54893
--- Comment #2 from Mike Spear <spear at cse dot lehigh.edu> 2012-10-11 13:21:19 UTC --- There is a key difference here. The transaction in my example is /relaxed/. According to the specification, there are no restrictions on what a relaxed transaction can do. Relaxed transactions can call unsafe functions, do I/O, acquire locks, execute inline asm, etc. And access to C++11 atomics and C11 volatiles is definitely legal in relaxed transactions. If I understand the GCC TM implementation correctly, immediately before the attempt to access a volatile within the relaxed transaction, the transaction will need to become serial irrevocable, after which point it can continue to access and directly modify volatile variables. Note that a workaround for the time being is to move the volatile access into a function, to *not* mark the function [[transaction_callable]], and then to call the function instead of accessing the volatile directly. This will achieve the exact same behavior as is expected, though with the overhead of a function call on each volatile access within a relaxed transaction. However, longer term this is definitely a bug in the current implementation.
