On Wednesday, 30 April 2014 at 21:51:17 UTC, Andrei Alexandrescu
wrote:
On 4/30/14, 2:47 PM, John Colvin wrote:
On Wednesday, 30 April 2014 at 20:57:26 UTC, Andrei
Alexandrescu wrote:
Finally, immutable is sharable accross thread. That mean,
even if we
bypass the type system, that RC must be atomic for immutable.
As they
convert automatically for co,st, that mean that all const
code will be
full of atomic increment/decrement. They are bad for the
CPU, and cannot
be optimized away.
Good point. I see that as a problem, albeit a solvable one.
How? Having lock; instructions implicitly appearing in normal
looking
slice code is unacceptable.
I'm thinking e.g. non-interlocked refcounts go like 1, 3, 5,
... and interlocked refcounts go like 2, 4, 6, ...
Then you do an unprotected read of the refcount. If it's odd,
then it's impossible to having originated as an interlocked
one. So proceed with simple increment. If it's even, do an
interlocked increment.
Andrei
I don't think I fully understand.
Either all RC changes for a given type need to be atomic or none
do, and that information is given by the type (everything that is
immutable/const/shared). I don't see any feasible way of escaping
this, or any advantage to a runtime convention like the odd/even
trick above.