On Friday, 2 January 2015 at 23:51:05 UTC, John Colvin wrote:
The rule (in C(++) at least) is that all data is assumed to be visible and mutable from multiple other threads unless proved otherwise. However, given that you do not write a race, the compiler will provide full sequential consistency. If you do write a race though, all bets are off.
The memory is visible and mutable, but that's pretty much the only guarantee you get. Without synchronization, there's no guarantee a write made by thread A will ever be seen by thread B, and vice versa.
Analogously in D, if a thread modifies a __gshared variable, there's no guarantees another thread will ever see that modification. The variable isn't thread local, but it's almost as if the compiler to treat it that way.
These relaxed guarantees allow the compiler to keep variables in registers, and re-order memory writes. These optimizations are crucial to performance.
