Tags should be represented using null ptrs where possible

[nikomatsakis]

A tag with one nullary variant and one variant that accepts only a pointer should be implemented using a null pointer at runtime. This would save space and be generally nifty.

[boggle]

Hmm would be nice to have this in other cases, too, by annotating the NULLary case that should be represented by null.

Additonally, now that I think about it, might be desirable to assign explicit values to the different nullary cases for representing C enums.
Reason to do this instead of using constants would be checking of exhaustiveness in alt expressions.

[nikomatsakis]

Yes, I like the idea of adding explicit annotations to help guide the representation. This seems related to supporting C-style enums, where the value of each case can be specified explicitly.

[nikomatsakis]

We could in principle support "single-word" variant type representation (i.e., use NULL) for tags with a small number of variants, but we would have to use "tagged pointers" (i.e., set the low bits). In this way we could support up to 4 nullary variants and 4 pointer variants. I would certainly be inclined to start with NULL or not NULL (i.e., option) but I think tagged ptrs would be as general as we could get.

[graydon]

A long time ago I set out to make this part of the mandatory representation of tags in rust (including the bit-packing on unused pointer bits). I'd be willing to revive the concept, though I'll admit it feels weird to encode as a first-class language feature, and likely hard to cover all cases.

[nikomatsakis]

It doesn't seem like such a weird thing to me, though I wouldn't call it a super high priority feature. Still, I'm pretty keen on the idea of Rust giving you very good control of how data is represented in memory, and I think being able to present a null pointer as an option type can be really important for something like a big array of data where you don't want to spend an extra word per item. Tagged pointers would just be icing on the cake.

[kevina]

I just noticed this bug and was wondering about the issue.

Not directly related, but another thing I would like to see is using special values to represent the case when there is only one unary variant and the rest are nullary to avoid having to have a separate tag field. I am guessing the typestate system can be used to enforce the constrain that the value not be one of the special values. Naturally this is something the user need to request explicitly somehow.

However, I agree the special case of one unary that is a pointer and one nullary is the most important.

[pcwalton]

The only issue to me with this is that it makes the rules about memory layout more complex with tags. This matters for C interop. As an alternative, we could build option into the language.

[nikomatsakis]

Yes, it does make things more complex. Though, frankly, most C types probably use a null pointer for this case rather than a (uint, ptr*) pair! Basically, enums with data don't have a very natural C representation imo. I don't know that most unions are immediately preceded by a uint saying which variant of the union to use. Building option<> into the language would be an "ok" compromise. Worse to be sure, but is it also better?

I could also imagine some attributes that specify the layout as well.

[graydon]

I would prefer not to build option into the language. System malloc usually aligns to eight bytes; on x64 the high 16 bits of every pointer are also free (all implementations are currently 48 bit addressed). It's not too hard for us to wire these (or a similar assumptions that the compiler can "know") into our handling of boxed values, and it means a bunch of cases drop to half their size.

Issue #1704 requests a packed modifier. I'm ok with packed enum { ... } knowing enough about pointers to pull this trick when it can find the bits to stuff the tag values into. It's not like we have to maintain compatibility with C handling here. C doesn't have disjoint unions, and most C code that emulates this sort of thing does in fact do just this.

[graydon]

Oh, also, removing RFC tag. This doesn't really require "everyone nodding" consensus. It's mostly a codegen thing; users aren't even that likely to notice most of the time.

[catamorphism]

You don't need to build option into the language to do the "nullary tags are null pointers" optimization. Rather, the compiler can do that optimization for any data type that's isomorphic to option. There's a bag of tricks like this -- since I didn't see this paper mentioned in comments so far:

http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.156.3526

and also a chapter in Appel's Compiling with Continuations.

[nikomatsakis]

Agreed, building in option is more complex than is warranted. To address @pcwalton's objection, just have an annotation for specifying the layout of an enum from the various choices we have available. In the absence of such an annotation, we just select as we think best.

[catamorphism]

When I was looking at work related to this for my dissertation proposal, I found http://people.ischool.berkeley.edu/~parikh/projects/WIL/ -- a proposal that talks about just such annotations (in the context of the Whirlwind compiler) -- though the work was never implemented and I couldn't find anything close to it. So the problem of how to specify "hints" to the compiler about how to lay out data structures is novel/interesting in and of itself, IMO.