KeyPath's getter/setter/hash/equals functions have their own calling
convention, which receives generic arguments and embedded indices from a
given KeyPath argument buffer.
The convention was previously implemented by:
1. Accepting an argument buffer as an UnsafeRawPointer and casting it to
indices tuple pointer in SIL.
2. Bind generic arguments info from the given argument buffer while emitting
prologue in IRGen by creating a new forwarding thunk.
This 2-phase lowering approach was not ideal, as it blocked KeyPath
projection optimization [^1], and also required having a target arch
specific signature lowering logic in SIL-level [^2].
This patch centralizes the KeyPath accessor calling convention logic to
IRGen, by introducing `@convention(keypath_accessor_XXX)` convention in
SIL and lowering it in IRGen. This change unblocks the KeyPath projection
optimization while capturing subscript indices, and also makes it easier
to support WebAssembly target.
[^1]: https://github.com/apple/swift/pull/28799
[^2]: https://forums.swift.org/t/wasm-support/16087/21
This removes the "optimization" where a function type, metatype or
tuple type was split up into structural components, because it seems
that in general we need this structural type metadata again.
Similarly, this no longer tries to split up dependent concrete
conformances and instead passes the witness table in the context.
This makes the context larger potentially, but it avoids calls to
metadata access functions and swift_getWitnessTable() every time the
closure is invoked.
This will permit the use of GenericRequirement in PrintAsClang.
This will allow the C++ bindings printer to print generic function signatures
correctly, by passing information about emitted GenericRequirement values
for a signature from IRGen using IRGenABIDetailsProvider to PrintAsClang
ProtocolConformanceRef already has an invalid state. Drop all of the
uses of Optional<ProtocolConformanceRef> and just use
ProtocolConformanceRef::forInvalid() to represent it. Mechanically
translate all of the callers and callsites to use this new
representation.
SubstitutionMaps are now just a trivial pointer-sized value, so
pass them by value instead.
I did have to move a couple of functors from Type.h to SubstitutionMap.h
to resolve some issues with forward declarations.
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.
Finally, remove the parent type metadata argument from type
constructors.
Now that type constructors don't take a parent metadata pointer,
we can hit some asserts concerning type constructors that do not
have any parameters. This happens when you define a concrete type
in a fully-constrained extension of a generic type.
A more efficient ABI would use concrete type metadata for these
cases, but that would be a bigger change that we can do later, so
for now just relax these assertions.
This resolves a runtime crasher since a circular metadata case is
no longer circular. I renamed the crasher to reference the more
specific radar since the more general issue of circular metadata
is still unresolved.
"minimal" is defined as the set of requirements that would be
passed to a function with the type's generic signature that
takes the thick metadata of the parent type as its only argument.
