[PATCH] D40485: [clangd] Introduced a Context that stores implicit data

Ilya Biryukov via Phabricator via cfe-commits cfe-commits at lists.llvm.org
Mon Dec 11 02:55:56 PST 2017

ilya-biryukov added inline comments.

Comment at: clangd/Context.h:65
+  Context *Parent;
+  TypedValueMap Data;
sammccall wrote:
> ilya-biryukov wrote:
> > sammccall wrote:
> > > We add complexity here (implementation and conceptual) to allow multiple properties to be set at the same level (vs having a key and an AnyStorage and making Context a linked list).
> > > Is this for performance? I'm not convinced it'll actually be faster for our workloads, or that it matters.
> > Conceptually, a `Context` is more convenient to use when it stores multiple values. This allows to put a bunch of things and assign meaning to `Context` (i.e., a `Context` for processing a single LSP request, global context). If `Context`s were a linked list, the intermediate `Context`s would be hard to assign the meaning to.
> > 
> > That being said, storage strategy for `Context`s is an implementation detail and could be changed anytime. I don't have big preferences here, but I think that storing a linked list of maps has, in general, a better performance than storing a linked list.
> > And given that it's already there, I'd leave it this way.
> With the new shared_ptr semantics:
>      Context D = move(C).derive(K1, V1).derive(K2, V2);
> Is just as meaningful as
>     Context D = move(C).derive().add(K1, V1).add(K2, V2);
> Yeah, the list of maps in an implementation detail. It's one that comes with a bunch of complexity (`ContextBuilder` and most of `TypedValueMap`). It really doesn't seem to buy us anything (the performance is both uninteresting and seems likely to be worse in this specific case with very few entries). 
The thing I like about it is that the `Context`s are layered properly in a sense that there's a Context corresponding to the request, a Context corresponding to the forked subrequests, etc.
If we change the interface, we'll be creating a bunch of temporary Contexts that don't correspond to a nice meaningful abstraction (like request) in my head, even though we don't give those contexts any names.

I do agree we currently pay with some complexity for that. Though I'd argue it's all hidden from the users of the interface, as building and consuming contexts is still super-easy and you don't need to mention ContextBuilder or TypedValueMap. And the implementation complexity is totally manageable from my point of view, but I am the one who implemented it in the first place, so there's certainly a bias there.

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