Proposal: Relax rules for 'pure'

[Bruno Medeiros]

On 23/09/2010 23:39, Robert Jacques wrote:
> On Thu, 23 Sep 2010 16:35:23 -0400, Tomek Sowiński <just at 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.

-- 
Bruno Medeiros - Software Engineer