Bruno Medeiros wrote:
On 23/09/2010 23:39, Robert Jacques wrote:
On Thu, 23 Sep 2010 16:35:23 -0400, Tomek Sowiński <j...@ask.me> wrote:
On topic: this means a pure function can take a reference to data that
can be mutated by
someone else. So we're giving up on the "can parallelize with no
dataraces" guarantee on
pure functions?
In short, No. In long; the proposal is for pure functions become broken
up into two groups (weak and strong) based on their function signatures.
This division is internal to the compiler, and isn't expressed in the
language in any way. Strongly-pure functions provide all the guarantees
that pure does today and can be automatically parallelized or cached
without consequence. Weakly-pure functions, don't provide either of
these guarantees, but allow a much larger number of functions to be
strongly-pure. In order to guarantee a function is strongly pure, one
would have to declare all its inputs immutable or use an appropriate
template constraint.
I think we need to be more realistic with what kinds of optimizations
we could expect from a D compiler and pure functions.
Caching might be done, but only a temporary sense (caching under a
limited execution scope). I doubt we would ever have something like
memoization, which would incur memory costs (potentially quite big
ones), and so the compiler almost certainly would not be able to know
(without additional metadata/anotations or compile options) if that
trade-off is acceptable.
Similarly for parallelism, how would the compiler know that it's ok to
spawn 10 or 100 new threads to parallelize the execution of some loop?
The consequences for program and whole-machine scheduling would not be
trivial and easy to understand. For this to happen, amongst other things
the compiler and OS would need to ensure that the spawned threads would
not starve the rest of the threads of that program.
I suspect all these considerations might be very difficult to guarantee
on a non-VM environment.
I agree with this, especially with regard to memoization.
However, several other very interesting optimisations are
possible with pure, especially with regard to memory allocation.
At exit from an immutably pure function, all memory allocated by that
function and its subfunctions can be collected, except for anything
which is reachable through the function return value.
Even for a weakly-pure function, the set of roots for gc is limited to
the mutable function parameters and the return value.
And for all pure functions with no mutable reference parameters, it is
guaranteed that no other thread has access to them.
The implications for gc are very interesting.
