On 5/13/2012 10:34 PM, Alex Rønne Petersen wrote:
I have yet to see any compiler make sensible use of the information provided by both C++'s const and D's const.
D's const is part of purity, which is optimizable.
const in particular is completely useless to an optimizer because it does not give it any information that it can use for anything. The kind of information that an optimization pass, in general, wants to see is whether something is guaranteed to *never* change. const does not provide this information. const simply guarantees that the code working on the const data cannot alter it (but at the same time allows *other* code to alter it), which, as said, is useless to the optimizer. immutable is a different story. immutable actually opens the door to many optimization opportunities exactly because the optimizer knows that the data will not be altered, ever. This allows it to (almost) arbitrarily reorder code, fold many computations at compile time, do conditional constant propagation, dead code elimination, ...
You cannot have immutable without also having const. Or, at least, it would be impractical.
This seems reasonable. But now consider that the majority of functions *are written for const, not immutable*. Thereby, you're throwing away the immutable guarantee, which is what the *compiler* (not the *programmer*) cares about. immutable is an excellent idea in theory, but in practice, it doesn't help the compiler because you'd have to either a) templatize all functions operating on const/immutable data so the compiler can retain the immutable guarantee when the input is such, or b) explicitly duplicate code for the const and the immutable case.
strings are immutable, not just const. It's been very successful.
Both approaches clearly suck. Templates don't play nice with polymorphism, and code duplication is...well...duplication. So, most of druntime and phobos is written for const because const is the bridge between the mutable and immutable world, and writing code against that rather than explicitly against mutable/immutable data is just simpler. But this completely ruins any opportunity the compiler has to optimize!
That isn't true when it comes to purity.
(An interesting fact is that even the compiler engineers working on compilers for strictly pure functional languages have yet to take full advantage of the potential that a pure, immutable world offers. If *they* haven't done it yet, I don't think we're going to do it for a long time to come.)
It isn't just what the compiler can do, purity and immutability offer a means to prove things about code.
Now, you might argue that the compiler could simply say "okay, this data is const, which means it cannot be changed in this particular piece of code and thus nowhere else, since it is not explicitly shared, and therefore not touched by any other threads". This would be great if shared wasn't a complete design fallacy. Unfortunately, in most real world code, shared just doesn't cut it, and data is often shared between threads without using the shared qualifier (__gshared is one example).
Yes, if you're thinking like a C programmer!
shared is another can of worms entirely. I can list a few initial reasons why it's unrealistic and impractical: 1) It is extremely x86-biased; implementing it on other architectures is going to be...interesting (read: on many architectures, impossible at ISA level).
I don't see why.
2) There is no bridge between shared and unshared like there is for mutable and immutable. This means that all code operating on shared data has to be templatized (no, casts will not suffice; the compiler can't insert memory barriers then) or code has to be explicitly duplicated for the shared and unshared case. Funnily, the exact same issue mentioned above for const and immutable!
Frankly, you're doing it wrong if you're doing more than trivial things with shared types. Running an algorithm on a shared type is just a bad idea.
3) It only provides documentation value. The low-level atomicity that it is supposed to provide (but doesn't yet...) is of extremely questionable value. In my experience, I never actually access shared data from multiple threads simultaneously, but rather, transfer the data from one thread to another and use it exclusively in the other thread (i.e. handing over the ownership). In such scenarios, shared just adds overhead (memory barriers are Bad (TM) for performance).
Transferring data between threads should be done either using value types, which are copied, or references which are typed as shared only transitorially.
