[ABI] Rework protocol descriptor metadata. #18163
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…overlap. As a small step toward splitting apart the Swift and Objective-C protocol descriptors, eliminate most places where we rely on a shared representation.
Introduce TargetObjCProtocolPrefix, which describes just enough of the Objective-C runtime’s Protocol structure to extract the name without having to call Objective-C’s protocol_getName().
When reading the protocol metadata from existential type metadata, check the “isObjC” bit and handle the reading of the Objective-C protocol name (using TargetObjCProtocolPrefix) separately from the reading the name of a Swift protocol (using TargetProtocolDescriptor). More preparation for separating the layout of these two entities.
Reimplement protocol descriptors for Swift protocols as a kind of context descriptor, dropping the Objective-C protocol compatibility layout. The new protocol descriptors have several advantages over the current implementation: * They drop all of the unused fields required for layout-compatibility with Objective-C protocols. * They encode the full requirement signature of the protocol. This maintains more information about the protocol itself, including (e.g.) correctly encoding superclass requirements. * They fit within the general scheme of context descriptors, rather than being their own thing, which allows us to share more code with nominal type descriptors. * They only use relative pointers, so they’re smaller and can be placed in read-only memory Implements rdar://problem/38815359.
DougGregor
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From the macOS testing: |
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Rather than storing a mangled name in a Swift protocol descriptor, which encodes information that is redundant with the context of the protocol, store an unmangled name as in nominal type descriptors. Update the various places where this name is used to extract the demangle tree from the context descriptors.
These were used to encode the “inherited protocols” for Objective-C Protocol structures. These are no longer a part of either Swift protocol descriptors or the part of Objective-C Protocol structures that the Swift runtime inspects.
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…ects. Protocols are never generic, so don't inherit from TrailingGenericContextObjects. Instead, directly use swift::ABI::TrailingObjects. Since protocols are never generic, this doesn't actually affect layout at all; it's cleanup.
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Reimplement protocol descriptors for Swift protocols as a kind of
context descriptor, dropping the Objective-C protocol compatibility
layout. The new protocol descriptors have several advantages over the
current implementation:
with Objective-C protocols.
maintains more information about the protocol itself, including
(e.g.) correctly encoding superclass requirements.
being their own thing, which allows us to share more code with
nominal type descriptors.
in read-only memory
_Ttmangled names.
Implements rdar://problem/38815359.