runtime: add a new runtime function swift_setDeallocating.

It's to be used by code produced by the ReleaseDevirtualizer.
As the function is only used for non-escaping objects, the deallocating bit is set non-atomically.

Author: eeckstein

docs/Runtime.md

@@ -160,6 +160,7 @@ Returns a random number. Only used by allocation profiling tools.
 000000000002b290 T _swift_isUniquelyReferencedOrPinned_nonNull_native
 000000000002af00 T _swift_isUniquelyReferenced_native
 000000000002aea0 T _swift_isUniquelyReferenced_nonNull_native
+00000000000????? T _swift_setDeallocating
 000000000001d280 T _swift_isDeallocating
 ```
 

include/swift/Runtime/HeapObject.h

@@ -292,6 +292,12 @@ SWIFT_RUNTIME_EXPORT
 extern "C" void (*SWIFT_CC(RegisterPreservingCC)
                      _swift_release_n)(HeapObject *object, uint32_t n);
 
+/// Sets the RC_DEALLOCATING_FLAG flag. This is done non-atomically.
+/// The strong reference count of \p object must be 1 and no other thread may
+/// retain the object during executing this function.
+SWIFT_RUNTIME_EXPORT
+extern "C" void swift_setDeallocating(HeapObject *object);
+
 // Refcounting observation hooks for memory tools. Don't use these.
 SWIFT_RUNTIME_EXPORT
 extern "C" size_t swift_retainCount(HeapObject *object);

stdlib/public/SwiftShims/RefCount.h

@@ -158,6 +158,14 @@ class StrongRefCount {
     return doDecrementShouldDeallocateN<false>(n);
   }
 
+  // Set the RC_DEALLOCATING_FLAG flag non-atomically.
+  //
+  // Precondition: the reference count must be 1
+  void decrementFromOneAndDeallocateNonAtomic() {
+    assert(refCount == RC_ONE && "Expect a count of 1");
+    refCount = RC_DEALLOCATING_FLAG;
+  }
+
   // Return the reference count.
   // During deallocation the reference count is undefined.
   uint32_t getCount() const {

stdlib/public/runtime/HeapObject.cpp

@@ -344,6 +344,10 @@ void SWIFT_RT_ENTRY_IMPL(swift_release_n)(HeapObject *object, uint32_t n)
   }
 }
 
+void swift::swift_setDeallocating(HeapObject *object) {
+  object->refCount.decrementFromOneAndDeallocateNonAtomic();
+}
+
 size_t swift::swift_retainCount(HeapObject *object) {
   return object->refCount.getCount();
 }