The non allocating D subset

Manu turkeyman at gmail.com
Sun Jun 2 07:40:10 PDT 2013


This is super-interesting research!
Lessons learned here should attempt to be rolled back into the main
compiler/libs if they are applicable.
This sort of work is what may open D up to ' proper embedded' usage.

I'd really like to see escape analysis and lowering of small dynamic arrays
onto the stack where possible get some attention.
Kenji's recently changes for array literal assignment are a long-awaited
start! :)


On 2 June 2013 00:40, Adam D. Ruppe <destructionator at gmail.com> wrote:

> On Saturday, 1 June 2013 at 05:45:38 UTC, SomeDude wrote:
>
>> Basically it is a non allocating D subset.
>>
>
> Not necessarily nonallocating, but it doesn't use a gc. I just updated the
> zip:
>
> http://arsdnet.net/dcode/**minimal.zip<http://arsdnet.net/dcode/minimal.zip>
>
> If you make the program (only works on linux btw) and run it, you'll see a
> bunch of allocations fly by, but they are all done with malloc/free and
> their locations are pretty predictable.
>
> The file minimal.d is a test program and you can see a lot of D works,
> including features like classes, exceptions, templates, structs, and
> delegates. (Heap allocated delegates should be banned but aren't, so if you
> do one built in it will leak. The helper file, memory.d, though contains a
> HeapDelegate struct that refcounts and frees it, so the concept is still
> usable.)
>
> The other cool thing is since the library is so minimal, the generated
> executable is small too. Only about 30 KB with an empty main(), and no
> outside dependencies. A cool fact about that is you can compile it and run
> on bare metal (given a bootloader like grub) too, and it all just works.
>
> You can also make "LIBC=yes" and depend on the C library, which makes
> things work better - there's a real malloc function there! - and adds about
> 10kb to the executable. That's probably a more realistic way to use it on
> the desktop at least than totally standalone.
>
>
>
> But yeah, I haven't written any real code with this, but so far it seems
> to be pretty usable.
>
>
>
>
>
> I also talked a while on the reddit thread last night about this, so let
> me copy/paste that here too:
>
>
>
>
> Yes, certainly. And it wouldn't even necessarily be no array concats, just
> you wouldn't want to use the built-in ones.
>
> Some features that use the gc in the real druntime don't necessarily have
> to. You'll need to be aware of this most the time to free the memory in
> your app, but you can have a pretty good idea of when it will happen. One
> example is new class. If that mallocs, if you just match every new with a
> delete (or call to free_obj() or whatever), you'll be fine, just like C++.
>
> I played with one I wasn't sure would work earlier, but now think it can:
> heap closures. scope delegates are easy, since they don't allocate, but
> heap closures allocate automatically and don't give much indication that
> they do.... but, if you are careful with it, the rules can be followed (if
> it accesses an outside scope and has its address taken/reference copied or
> passed to a function, it will automatically allocate), and you can manually
> call free(dg.ptr); when you're done with it.
>
> I think it is probably safer to just disallow them, either by not
> implementing _d_allocmemory in druntime (thus if you accidentally use it,
> you'll get a linker error about the missing function), or, and this is
> tricky right now but not actually impossible, use compile time reflection
> to scan your methods and members for a non-scope delegate reference and
> throw an error.
>
> If we do the latter, a heap delegate can actually be allowed in a fairly
> safe way, by wrapping it in a struct. Usage of HeapDelegate!T will be
> pretty obvious, so you aren't going to accidentally use it the way you
> might the more sugary built in. I have a proof of concept implemented in my
> local copy of minimal.d that automatically refcounts the delegate, freeing
> it when the last reference goes out of scope.
>
> Array slices are ok the way I have them implemented now: the built in
> concat function is missing, so if you try a ~ b, it will be a linker error
> (including the source file and line number btw, easy enough to handle). No
> allocation there. The biggest risk is lifecycle management, and the rule
> there is you don't own slices (non-immutable ones at least). I'd like the
> compiler to implement a check on this, but right now it doesn't. Not a hard
> coding convention though.
>
> Built in new array[] is not implemented, meaning it is a linker error,
> because they are indistinguishable from slices type-wise. (In theory it
> could be like classes, where you just know to manually free them, but if
> you have a char[] member, are you sure that was new'd or is it holding a
> slice someone else owns? Let's just avoid it entirely.)
>
> But, this doesn't mean we can't have some of D's array convenience! In
> minimal.d, you can see a StackArray!T struct and maybe, not sure if I put
> it in that zip or not, a HeapArray!T struct. These types own their memory,
> stack of course going away with scope, and heap being automatically
> reference counted to call free() when all copies are gone, and overload a
> few operators for convenience:
>
> alias this is to a slice function, so you can do char[] slice =
> myCustomArray; You can't change the original pointer through that slice, so
> no risk of it losing the memory.
>
> The ~= operator is implemented too on the *Array containers. They know
> their length and their capacities, and you can append up to the capacity.
> (The HeapArray could also realloc() as needed, but right now I don't.) One
> important difference though with this and regular D arrays is in regular D:
>
> string a = "hey"; string b = a; b ~= " man";
> assert(a == "hey" && b == "hey man");
>
> Appending to the second one doesn't change the first one. It may allocate
> as needed (see this for details: http://dlang.org/d-array-**article.html<http://dlang.org/d-array-article.html>)
>
> Whereas with a HeapArray or StackArray, they share the same underlying
> data, so appending to one reference would append to all. I think that's OK
> though, because we have helper things like const to avoid that, and they
> are a custom type, so they are allowed to work differently than the built
> ins.
>
> Thankfully, btw, static arrays are a different type. They can be permitted
> with ease.
>
> I didn't implement a ~ b. I think that one would be too easy to lose and
> either pointlessly malloc/free in the middle of an expression, or just
> forget to free entirely, but maybe it could be done too.
>
> Another issue is strings. In phobos and druntime both, there's a lot or
> creating strings on the heap and returning them from functions. e.g.
> to!string(10) returns a brand new allocated string "10". We don't want that
> in our library, so it looks a little more like C, but slices make it easier
> to manage. The analogous function I wrote is
>
> char[] intToString(int a, char[] buffer)
>
> You pass it an area of pre-allocated buffer to write to. buffer.length
> tells it where it isn't allowed to continue (unlike a plain char* in C). It
> returns the slice of your own buffer that was actually used. So writeln(10)
> becomes:
>
> char[16] buffer;
> write(intToString(10, buffer), "\n");
>
> a little more verbose, but there's no mystery there about the memory.
> intToString knows it only has 16 spaces to work with, you know exactly
> where it is going, no allocations, and the return value conveniently has
> the length used too, so we can pass it directly to another function. (as
> long as that function doesn't store the reference!)
>
> Built in AAs? Not implemented. But we could do a library AA just as
> easily, and thanks to overloaded operators, it would be pretty too.
>
> Another issue is exceptions. They work, and must be classes. So where do
> you free them? I haven't tried this yet, but I'm pretty sure you can just
> do it when you catch() it, and no problem.
>
> Well this is turning into a real beast of a comment, so let me sum up and
> finish: a lot of D can work without the GC. It will take some custom types
> and deliberately missing druntime functions to make pretty, but it leaves
> us with a language at least as usable as C++, with the same idea of no
> surprise/hidden allocations in there. There's still a question of having
> stuff we don't necessarily want like RTTI, but their impact can be
> minimized so I think it will be ok too. (Oh btw, since I have a custom
> druntime here, I added some runtime reflection the real D doesn't have yet.
> It rox, and came cheap, but you can still version it out.)
>
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