IRGen: Annotate runtime calls with willreturn attribute

[aschwaighofer]

willreturn
This function attribute indicates that a call of this function will
either exhibit undefined behavior or comes back and continues execution
at a point in the existing call stack that includes the current
invocation. Annotated functions may still raise an exception, i.a.,
nounwind is not implied. If an invocation of an annotated function does
not return control back to a point in the call stack, the behavior is
undefined.

I conservatively did not assume that the deinit is willreturn therefore
release like operations are not marked willreturn.

rdar://73574236

[aschwaighofer]

@swift-ci Please test

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - fbe7fd7e9440642f6aefbe75abc62f38fe063ba9

[aschwaighofer]

@swift-ci Please test

[mikeash]

Some of these (e.g. swift_allocObject) can fatal error when things go wrong, is that acceptable? I don't entirely understand the practicalities of willreturn.

[aschwaighofer]

If you go by the letter than we would not be allowed to mark such a function as willreturn. fatal error is a side-effect we want to observe.

Because swift_allocObject is marked as having unknown side-effects (by the virtue of not being marked as readnone, or readonly) it is okay since we already have other side-effects.

I will go and update the file to fix any confusion.

LLVM was assuming every function is willreturn up until recently.

[aschwaighofer]

Now without allocation functions and objc apis that I wasnt sure about whether they could fatalError (or similar).

[aschwaighofer]

@swift-ci Please test

[mikeash]

There are some more that can allocate (mostly things that use caches, which might insert new entries and allocate memory for them) or otherwise fatal error:

swift_makeBoxUnique
swift_getTypeByMangledNameInContextInMetadataState
swift_getTypeByMangledNameInContext
swift_conformsToProtocol
swift_dynamicCast*Unconditional
swift_get*Metadata
swift_getWitnessTable
swift_getInitializedObjCClass // can potentially invoke arbitrary code if +self is overridden
swift_objc_swift3ImplicitObjCEntrypoint // has an option to fatal error

[aschwaighofer]

@swift-ci Please test