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

Manuel Klimek via Phabricator via cfe-commits cfe-commits at lists.llvm.org
Mon Dec 11 07:32:44 PST 2017

klimek added inline comments.

Comment at: clangd/Context.h:65
+  Context *Parent;
+  TypedValueMap Data;
sammccall wrote:
> ilya-biryukov wrote:
> > 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.
> I don't see temporary unnamed `Context`s being any different from temporary unnamed `ContextBuilder`s.
> But we've gone around on this point a bit, and this really seems to be a question of taste. @klimek, can we have a third opinion?
> The options we're looking at are:
>   - `Context` stores a map and a parent pointer. `derive()` returns a `ContextBuilder` used to create new contexts containing 0 or more new KV pairs. `TypedValueMap` stores the payloads.
>   - `Context` stores a single KV pair and a parent pointer. `derive(K, V)` is used to create a new context with one new KV pair. A Key-pointer and AnyStorage in `Context` store the payloads, the rest of `TypedValueMap` goes away.
I'd agree that Context::derive(K, V) would be simpler here, mainly because there is only one way to pass around contexts while we're building them up; specifically, there's no temptation to pass around ContextBuilders.

Comment at: clangd/TypedValueMap.h:38
+/// A type-safe map from Key<T> to T.
+class TypedValueMap {
Should we say "from Key<T>*" or &? If you say Key<T> I expect something that compares by value.

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