There are multiple reasons to do this. Primarily this is
useful as an optimization. Whenever analysis can determine that no
potentially conflicting access occurs within the scope, the access can
be demoted to "nontracking". It is also useful as an escape hatch for
future code deploying to older runtimes. For example, if a future access
scope may cross threads, and the older runtime doesn't know how to
migrate threads.
See <rdar://problem/37507434> add a flag to swift_beginAccess to inform
the runtime that an access might migrate between threads
I de-templated MetadataState and MetadataRequest because we weren't
relying on the template and because using the template was causing
conversion problems due to the inability to directly template an enum
in C++.
Rename it to swift_initClassMetadata() just like we recently did
swift_initStructMetadata(), and add a StructLayoutFlags parameter
so we can version calls to this function in the future.
Maybe at some point this will become a separate ClassLayoutFlags
type, but at this point it doesn't matter because IRGen always
passes a value of 0.
This includes global generic and non-generic global access
functions, protocol associated type access functions,
swift_getGenericMetadata, and generic type completion functions.
The main part of this change is that the functions now need to take
a MetadataRequest and return a MetadataResponse, which is capable
of expressing that the request can fail. The state of the returned
metadata is reported as an second, independent return value; this
allows the caller to easily check the possibility of failure without
having to mask it out from the returned metadata pointer, as well
as allowing it to be easily ignored.
Also, change metadata access functions to use swiftcc to ensure that
this return value is indeed returned in two separate registers.
Also, change protocol associated conformance access functions to use
swiftcc. This isn't really related, but for some reason it snuck in.
Since it's clearly the right thing to do, and since I really didn't
want to retroactively tease that back out from all the rest of the
test changes, I've left it in.
Also, change generic metadata access functions to either pass all
the generic arguments directly or pass them all indirectly. I don't
know how we ended up with the hybrid approach. I needed to change all
the code-generation and calls here anyway in order to pass the request
parameter, and I figured I might as well change the ABI to something
sensible.
I was trying to make the entry-delegation thing do *way* too much.
Just give the entry access to the lock/queue and introduce subclasses
which simplify most of the work.
Also, fix some bad reasoning around the attempts to avoid acquiring
locks in the absence of waiters. It really is always necessary to
acquire the lock when notifying; waiters cannot atomically set the
has-waiters flag and wait, so we have to protect against the
possibility that we notify before they can wait.
