Add a single-threaded stdlib mode, use it for the 'minimal' stdlib

[kubamracek]

No description provided.

[kubamracek]

@swift-ci please test

[gottesmm]

@kubamracek Are you sure that all rr operations will be non-atomic? I wouldn't rely on the current support in the compiler for emitting non-atomic rr. The compiler only attempts to keep things non-atomic as a "best effort". Better in your case to just add a flag to the runtime to ensure that ARC operations always go to the non-atomic variant.

[gottesmm]

@kubamracek or you could leave everything alone and just have them forward to the non-atomic implementation if the flag is set that way we aren't changing the external ABI interface of the library.

[kubamracek]

Are you sure that all rr operations will be non-atomic?

No, and actually I'm pretty sure some will stay as atomic. However, my motivation here is just an "optimization", and if I discover places that are still using atomics when they don't need to, I plan to fix them up with separate PRs.

add a flag to the runtime to ensure that ARC operations always go to the non-atomic variant
just have them forward to the non-atomic implementation if the flag is set

That sounds reasonable. Let me try to prepare this in a separate PR.

[kubamracek]

@swift-ci please test

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 6e35f92a3c4fb29f34c64d2c17ee90aca6318155

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 6e35f92a3c4fb29f34c64d2c17ee90aca6318155

[compnerd]

The change looks reasonable to me. I really like the idea that @gottesmm has to use shims, and if the shims are static I think that could be an interesting approach as well. Since this form hasn't shipped yet it does make sense to simply do the more straightforward changes that @kubamracek has proposed.

LGTM, but please make sure that @gottesmm is okay with it as well before merging.

[kubamracek]

@gottesmm Are you okay with the state of this PR?

[kubamracek]

@swift-ci please test

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 5f5b61ff0f778cb9b9bff597170c15d210100a58

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 5f5b61ff0f778cb9b9bff597170c15d210100a58

[kubamracek]

@swift-ci please test

[kubamracek]

@swift-ci please test

[gottesmm]

I talked with Kuba offline. I asked him in follow on 1-2 follow on PRs to:

1. Eliminate NONATOMIC_RC. That was just an experimental flag from some time ago that isn't needed today.
2. Change the 10 atomic operations in HeapObject.cpp to be non-atomic if the Single threaded runtime mode is set. The change is really small. Example:

HeapObject *swift::swift_retain(HeapObject *object) {
  CALL_IMPL(swift_retain, (object));
}

becomes

HeapObject *swift::swift_retain(HeapObject *object) {
#if SINGLE_THREAD_FLAG
  CALL_IMPL(swift_nonatomic_retain, (object));
#else
  CALL_IMPL(swift_retain, (object));
#endif
}

Thanks for your work here Kuba!

[benrimmington]

@swift-ci Please test Windows platform