John McCall a906f43329 Allow type metadata to be incomplete. ...

Most of the work of this patch is just propagating metadata states
throughout the system, especially local-type-data caching and
metadata-path resolution.  It took a few design revisions to get both
DynamicMetadataRequest and MetadataResponse to a shape that felt
right and seemed to make everything easier.

The design is laid out pretty clearly (I hope) in the comments on
DynamicMetadataRequest and MetadataResponse, so I'm not going to
belabor it again here.  Instead, I'll list out the work that's still
outstanding:

- I'm sure there are places we're asking for complete metadata where
  we could be asking for something weaker.

- I need to actually test the runtime behavior to verify that it's
  breaking the cycles it's supposed to, instead of just not regressing
  anything else.

- I need to add something to the runtime to actually force all the
  generic arguments of a generic type to be complete before reporting
  completion.  I think we can get away with this for now because all
  existing types construct themselves completely on the first request,
  but there might be a race condition there if another asks for the
  type argument, gets an abstract metadata, and constructs a type with
  it without ever needing it to be completed.

- Non-generic resilient types need to be switched over to an IRGen
  pattern that supports initialization suspension.

- We should probably space out the MetadataStates so that there's some
  space between Abstract and Complete.

- The runtime just calmly sits there, never making progress and
  permanently blocking any waiting threads, if you actually form an
  unresolvable metadata dependency cycle.  It is possible to set up such
  a thing in a way that Sema can't diagnose, and we should detect it at
  runtime.  I've set up some infrastructure so that it should be
  straightforward to diagnose this, but I haven't actually implemented
  the diagnostic yet.

- It's not clear to me that swift_checkMetadataState is really cheap
  enough that it doesn't make sense to use a cache for type-fulfilled
  metadata in associated type access functions.  Fortunately this is not
  ABI-affecting, so we can evaluate it anytime.

- Type layout really seems like a lot of code now that we sometimes
  need to call swift_checkMetadataState for generic arguments.  Maybe
  we can have the runtime do this by marking low bits or something, so
  that a TypeLayoutRef is actually either (1) a TypeLayout, (2) a known
  layout-complete metadata, or (3) a metadata of unknown state.  We could
  do that later with a flag, but we'll need to at least future-proof by
  allowing the runtime functions to return a MetadataDependency.

2018-03-26 12:18:04 -04:00

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