On Saturday, 3 January 2015 at 00:48:23 UTC, Peter Alexander
wrote:
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.
That is exactly how I understood the situation to be, yes.
2 questions I have to absolutely nail this down for good:
Does D assume that local variables are truly thread-local? I.e.
can the compiler perform optimisations on local references that
would break SC-DRF without first proving that it does not?
Another way of putting it is: can a D compiler perform
optimisations that are normally illegal in modern C(++) and Java?
If the answer to the above is yes, does the same apply to
explicit use of __gshared variables?
