feat(blocks runtime): Enable using libBlocksRuntime instead of the embedded runtime

.cirrus.yml

@@ -1,7 +1,7 @@
 libcxxrt_freebsd_task:
   matrix:
   - freebsd_instance:
-     image_family: freebsd-13-2
+     image_family: freebsd-13-3
   - freebsd_instance:
      image_family: freebsd-15-0-snap
   - freebsd_instance:

.github/workflows/main.yml

@@ -20,6 +20,7 @@ jobs:
         # Build each combination of OS and release/debug variants
         os: [ "ubuntu-22.04", "ubuntu-20.04" ]
         build-type: [ Release, Debug ]
+        blocks-runtime: [ "EMBEDDED", "swift-5.10-RELEASE" ]
         cxxlib: [ "libc++", "libstdc++" ]
         llvm-version: [10, 11, 12, 13, 14, 15]
         # Don't bother testing the LLVM versions that aren't in the default image for the different platforms
@@ -41,7 +42,7 @@ jobs:
       # Don't abort runners if a single one fails
       fail-fast: false
     runs-on: ${{ matrix.os }}
-    name: ${{ matrix.os }} ${{ matrix.build-type }} LLVM-${{ matrix.llvm-version }} ${{ matrix.cxxlib }}
+    name: ${{ matrix.os }} ${{ matrix.build-type }} LLVM-${{ matrix.llvm-version }} ${{ matrix.cxxlib }} BlocksRuntime-${{ matrix.blocks-runtime }}
     steps:
       - uses: actions/checkout@v3
       - name: Install dependencies 
@@ -53,11 +54,24 @@ jobs:
             sudo apt install libc++-${{matrix.llvm-version}}-dev libc++abi-${{matrix.llvm-version}}-dev 
             sudo apt install libunwind-${{matrix.llvm-version}}-dev || true
           fi
+          if [ "${{ matrix.blocks-runtime }}" != "EMBEDDED" ]; then
+            git clone --depth 1 --branch "${{ matrix.blocks-runtime }}" https://github.com/apple/swift-corelibs-libdispatch.git ${{github.workspace}}/swift-corelibs-libdispatch
+            cmake -B ${{github.workspace}}/swift-corelibs-libdispatch/build -G Ninja -DINSTALL_PRIVATE_HEADERS=ON -DCMAKE_C_COMPILER=clang-${{matrix.llvm-version}} -DCMAKE_CXX_COMPILER=clang++-${{matrix.llvm-version}} -S ${{github.workspace}}/swift-corelibs-libdispatch
+            pushd ${{github.workspace}}/swift-corelibs-libdispatch/build
+            ninja
+            sudo ninja install
+            popd
+          fi
       - name: Configure CMake
         run: |
           export LDFLAGS=-L/usr/lib/llvm-${{ matrix.llvm-version }}/lib/
+          if [ "${{ matrix.blocks-runtime }}" != "EMBEDDED" ]; then
+            export EMBEDDED_BLOCKS_RUNTIME=OFF
+          else
+            export EMBEDDED_BLOCKS_RUNTIME=ON
+          fi
           ls -lahR /usr/lib/llvm-${{ matrix.llvm-version }}/lib/
-          cmake -B ${{github.workspace}}/build -DCMAKE_BUILD_TYPE=${{matrix.build-type}} -G Ninja -DTESTS=ON -DCMAKE_C_COMPILER=clang-${{matrix.llvm-version}} -DCMAKE_OBJC_COMPILER=clang-${{matrix.llvm-version}} -DCMAKE_ASM_COMPILER=clang-${{matrix.llvm-version}} -DCMAKE_CXX_COMPILER=clang++-${{matrix.llvm-version}} -DCMAKE_OBJCXX_COMPILER=clang++-${{matrix.llvm-version}} -DCMAKE_CXX_FLAGS="-stdlib=${{matrix.cxxlib}}"
+          cmake -B ${{github.workspace}}/build -DCMAKE_BUILD_TYPE=${{matrix.build-type}} -G Ninja -DTESTS=ON -DEMBEDDED_BLOCKS_RUNTIME=$EMBEDDED_BLOCKS_RUNTIME -DCMAKE_C_COMPILER=clang-${{matrix.llvm-version}} -DCMAKE_OBJC_COMPILER=clang-${{matrix.llvm-version}} -DCMAKE_ASM_COMPILER=clang-${{matrix.llvm-version}} -DCMAKE_CXX_COMPILER=clang++-${{matrix.llvm-version}} -DCMAKE_OBJCXX_COMPILER=clang++-${{matrix.llvm-version}} -DCMAKE_CXX_FLAGS="-stdlib=${{matrix.cxxlib}}"
         # Build with a nice ninja status line
       - name: Build
         working-directory: ${{github.workspace}}/build

CMakeLists.txt

@@ -28,6 +28,7 @@ endif ()
 
 INCLUDE (CheckCXXSourceCompiles)
 INCLUDE (FetchContent)
+INCLUDE (CheckSymbolExists)
 
 set(libobjc_VERSION 4.6)
 
@@ -58,7 +59,7 @@ set(libobjc_OBJC_SRCS
 	NSBlocks.m
 	Protocol2.m
 	associate.m
-	blocks_runtime.m
+	blocks_runtime_np.m
 	properties.m)
 set(libobjc_C_SRCS 
 	alias_table.c
@@ -83,8 +84,6 @@ set(libobjc_HDRS
 	objc/Availability.h
 	objc/Object.h
 	objc/Protocol.h
-	objc/blocks_private.h
-	objc/blocks_runtime.h
 	objc/capabilities.h
 	objc/developer.h
 	objc/encoding.h
@@ -101,10 +100,7 @@ set(libobjc_HDRS
 	objc/runtime-deprecated.h
 	objc/runtime.h
 	objc/slot.h)
-set(libBlocksRuntime_COMPATIBILITY_HDRS
+
-	Block.h
-	Block_private.h
-	)
 set(libobjc_CXX_SRCS
 	selector_table.cc
 	)
@@ -143,7 +139,7 @@ option(DEBUG_ARC_COMPAT
 	"Log warnings for classes that don't hit ARC fast paths" OFF)
 option(ENABLE_OBJCXX "Enable support for Objective-C++" ON)
 option(TESTS "Enable building the tests")
-
+option(EMBEDDED_BLOCKS_RUNTIME "Include an embedded blocks runtime, rather than relying on libBlocksRuntime to supply it" ON)
 
 # For release builds, we disable spamming the terminal with warnings about
 # selector type mismatches
@@ -230,6 +226,31 @@ else ()
 	find_library(M_LIBRARY m)
 endif ()
 
+if (EMBEDDED_BLOCKS_RUNTIME)
+	set(libBlocksRuntime_COMPATIBILITY_HDRS
+		Block.h
+		Block_private.h
+		)
+	list(APPEND libobjc_OBJC_SRCS blocks_runtime.m)
+	list(APPEND libobjc_HDRS objc/blocks_private.h)
+	list(APPEND libobjc_HDRS objc/blocks_runtime.h)
+	add_definitions(-DEMBEDDED_BLOCKS_RUNTIME)
+else ()
+	find_library(BLOCKS_RUNTIME_LIBRARY BlocksRuntime)
+	if (BLOCKS_RUNTIME_LIBRARY)
+		set(CMAKE_REQUIRED_LIBRARIES ${BLOCKS_RUNTIME_LIBRARY})
+		check_symbol_exists(_Block_use_RR2 "Block_private.h" HAVE_BLOCK_USE_RR2)
+		if (HAVE_BLOCK_USE_RR2)
+			add_definitions(-DHAVE_BLOCK_USE_RR2)
+		else ()
+			message(FATAL_ERROR "libBlocksRuntime does not contain _Block_use_RR2(). Enable EMBEDDED_BLOCKS_RUNTIME to use the built-in blocks runtime.")
+		endif ()
+		unset(CMAKE_REQUIRED_LIBRARIES)
+	else ()
+		message(FATAL_ERROR "libBlocksRuntime not found. Enable EMBEDDED_BLOCKS_RUNTIME to use the built-in blocks runtime.")
+	endif ()
+endif ()
+
 add_library(objc SHARED ${libobjc_C_SRCS} ${libobjc_ASM_SRCS} ${libobjc_OBJC_SRCS} ${libobjc_OBJCXX_SRCS} ${libobjc_ASM_OBJS})
 target_compile_options(objc PRIVATE "$<$<OR:$<COMPILE_LANGUAGE:OBJC>,$<COMPILE_LANGUAGE:OBJCXX>>:-Wno-deprecated-objc-isa-usage;-Wno-objc-root-class;-fobjc-runtime=gnustep-2.0>$<$<COMPILE_LANGUAGE:C>:-Xclang;-fexceptions>")
 
@@ -268,6 +289,10 @@ if (M_LIBRARY)
 	target_link_libraries(objc PUBLIC ${M_LIBRARY})
 endif ()
 
+if (BLOCKS_RUNTIME_LIBRARY)
+	target_link_libraries(objc PUBLIC ${BLOCKS_RUNTIME_LIBRARY})
+endif ()
+
 # Make weak symbols work on OS X
 if (APPLE)
 	set(CMAKE_SHARED_LIBRARY_CREATE_C_FLAGS
@@ -340,8 +365,12 @@ install(TARGETS ${INSTALL_TARGETS}
 
 install(FILES ${libobjc_HDRS}
 	DESTINATION "${HEADER_INSTALL_PATH}/${INCLUDE_DIRECTORY}")
-install(FILES ${libBlocksRuntime_COMPATIBILITY_HDRS}
+
-	DESTINATION "${HEADER_INSTALL_PATH}")
+if (EMBEDDED_BLOCKS_RUNTIME)
+	install(FILES ${libBlocksRuntime_COMPATIBILITY_HDRS}
+		DESTINATION "${HEADER_INSTALL_PATH}")
+endif ()
+
 
 set(CPACK_GENERATOR TGZ CACHE STRING
 	"Installer types to generate.  Sensible options include TGZ, RPM and DEB")
@@ -386,6 +415,9 @@ set(PC_LIBS_PRIVATE ${CMAKE_CXX_IMPLICIT_LINK_LIBRARIES})
 if (M_LIBRARY)
 	list(APPEND PC_LIBS_PRIVATE ${M_LIBRARY})
 endif ()
+if (BLOCKS_RUNTIME_LIBRARY)
+	list(APPEND PC_LIBS_PRIVATE ${BLOCKS_RUNTIME_LIBRARY})
+endif ()
 list(REMOVE_DUPLICATES PC_LIBS_PRIVATE)
 string(REPLACE  ";" " -l" PC_LIBS_PRIVATE "${PC_LIBS_PRIVATE}")
 set(PC_LIBS_PRIVATE "Libs.private: -l${PC_LIBS_PRIVATE}")

INSTALL

@@ -79,6 +79,20 @@ this configuration, we provide a separate libobjcxx.so, which avoids the need
 for the Objective-C runtime to depend on the STL implementation just to be able
 to interoperate with C++ exceptions.
 
+Blocks Runtime Integration
+--------------------------
+
+libobjc2 ships with a runtime for the blocks C extension (i.e. closures/lambdas) and
+will install compatibility headers for the libBlocksRuntime library that ships with
+LLVM's compiler-rt or Swift's libdispatch. Alternatively, libobjc2 can be built without
+the embedded blocks runtime and utilise the one from libdispatch instead. This can be
+enabled by adding `-DEMBEDDED_BLOCKS_RUNTIME=OFF` to the `cmake` command. It's required
+that your version of libBlocksRuntime provides the `Blocks_private.h` header.
+(enabled with `-DINSTALL_PRIVATE_HEADERS=ON` when building libdispatch from source)
+
+Regardless of the chosen blocks runtime implementation, blocks will be fully integrated
+into the Objective-C runtime.
+
 Installation Location
 ---------------------
 

NSBlocks.m

@@ -6,13 +6,23 @@
 #include "dtable.h"
 #include <assert.h>
 
-OBJC_PUBLIC struct objc_class _NSConcreteGlobalBlock;
+#ifdef EMBEDDED_BLOCKS_RUNTIME
-OBJC_PUBLIC struct objc_class _NSConcreteStackBlock;
+#define BLOCK_STORAGE OBJC_PUBLIC
-OBJC_PUBLIC struct objc_class _NSConcreteMallocBlock;
+#else
+#define BLOCK_STORAGE extern
+#endif
+
+BLOCK_STORAGE struct objc_class _NSConcreteGlobalBlock;
+BLOCK_STORAGE struct objc_class _NSConcreteStackBlock;
+BLOCK_STORAGE struct objc_class _NSConcreteMallocBlock;
+BLOCK_STORAGE struct objc_class _NSConcreteAutoBlock;
+BLOCK_STORAGE struct objc_class _NSConcreteFinalizingBlock;
 
 static struct objc_class _NSConcreteGlobalBlockMeta;
 static struct objc_class _NSConcreteStackBlockMeta;
 static struct objc_class _NSConcreteMallocBlockMeta;
+static struct objc_class _NSConcreteAutoBlockMeta;
+static struct objc_class _NSConcreteFinalizingBlockMeta;
 
 static struct objc_class _NSBlock;
 static struct objc_class _NSBlockMeta;
@@ -31,6 +41,7 @@ static void createNSBlockSubclass(Class superclass, Class newClass,
 	newClass->super_class = superclass;
 	newClass->name = name;
 	newClass->dtable = uninstalled_dtable;
+	newClass->info = objc_class_flag_is_block;
 
 	LOCK_RUNTIME_FOR_SCOPE();
 	objc_load_class(newClass);
@@ -49,8 +60,20 @@ BOOL objc_create_block_classes_as_subclasses_of(Class super)
 	NEW_CLASS(&_NSBlock, _NSConcreteStackBlock);
 	NEW_CLASS(&_NSBlock, _NSConcreteGlobalBlock);
 	NEW_CLASS(&_NSBlock, _NSConcreteMallocBlock);
+	NEW_CLASS(&_NSBlock, _NSConcreteAutoBlock);
+	NEW_CLASS(&_NSBlock, _NSConcreteFinalizingBlock);
 	// Global blocks never need refcount manipulation.
 	objc_set_class_flag(&_NSConcreteGlobalBlock,
 	                    objc_class_flag_permanent_instances);
 	return YES;
 }
+
+PRIVATE void init_early_blocks(void)
+{
+	if (_NSBlock.super_class != NULL) { return; }
+	_NSConcreteStackBlock.info = objc_class_flag_is_block;
+	_NSConcreteGlobalBlock.info = objc_class_flag_is_block | objc_class_flag_permanent_instances;
+	_NSConcreteMallocBlock.info = objc_class_flag_is_block;
+	_NSConcreteAutoBlock.info = objc_class_flag_is_block;
+	_NSConcreteFinalizingBlock.info = objc_class_flag_is_block;
+}

arc.mm

@@ -10,14 +10,19 @@
 #include <tsl/robin_map.h>
 #import "lock.h"
 #import "objc/runtime.h"
+#ifdef EMBEDDED_BLOCKS_RUNTIME
+#import "objc/blocks_private.h"
 #import "objc/blocks_runtime.h"
+#else
+#include <Block.h>
+#include <Block_private.h>
+#endif
 #import "nsobject.h"
 #import "class.h"
 #import "selector.h"
 #import "visibility.h"
 #import "objc/hooks.h"
 #import "objc/objc-arc.h"
-#import "objc/blocks_runtime.h"
 #include "objc/message.h"
 
 /**
@@ -75,12 +80,16 @@ static inline arc_tls_key_t arc_tls_key_create(arc_cleanup_function_t cleanupFun
 arc_tls_key_t ARCThreadKey;
 #endif
 
+#ifndef HAVE_BLOCK_USE_RR2
 extern "C"
 {
 	extern struct objc_class _NSConcreteMallocBlock;
 	extern struct objc_class _NSConcreteStackBlock;
 	extern struct objc_class _NSConcreteGlobalBlock;
+	extern struct objc_class _NSConcreteAutoBlock;
+	extern struct objc_class _NSConcreteFinalizingBlock;
 }
+#endif
 
 @interface NSAutoreleasePool
 + (Class)class;
@@ -315,8 +324,7 @@ static inline id retain(id obj, BOOL isWeak)
 {
 	if (isPersistentObject(obj)) { return obj; }
 	Class cls = obj->isa;
-	if ((Class)&_NSConcreteMallocBlock == cls ||
+	if (UNLIKELY(objc_test_class_flag(cls, objc_class_flag_is_block)))
-	    (Class)&_NSConcreteStackBlock == cls)
 	{
 		return Block_copy(obj);
 	}
@@ -376,15 +384,15 @@ static inline void release(id obj)
 {
 	if (isPersistentObject(obj)) { return; }
 	Class cls = obj->isa;
-	if (cls == static_cast<Class>(&_NSConcreteMallocBlock))
+	if (UNLIKELY(objc_test_class_flag(cls, objc_class_flag_is_block)))
 	{
+		if (cls == static_cast<void*>(&_NSConcreteStackBlock))
+		{
+			return;
+		}
 		_Block_release(obj);
 		return;
 	}
-	if (cls == static_cast<Class>(&_NSConcreteStackBlock))
-	{
-		return;
-	}
 	if (objc_test_class_flag(cls, objc_class_flag_fast_arc))
 	{
 		objc_release_fast_np(obj);
@@ -702,12 +710,24 @@ mutex_t weakRefLock;
 
 }
 
+#ifdef HAVE_BLOCK_USE_RR2
+static const struct Block_callbacks_RR blocks_runtime_callbacks = {
+		sizeof(Block_callbacks_RR),
+		(void (*)(const void*))objc_retain,
+		(void (*)(const void*))objc_release,
+		(void (*)(const void*))objc_delete_weak_refs
+	};
+#endif
+
 PRIVATE extern "C" void init_arc(void)
 {
 	INIT_LOCK(weakRefLock);
 #ifdef arc_tls_store
 	ARCThreadKey = arc_tls_key_create((arc_cleanup_function_t)cleanupPools);
 #endif
+#ifdef HAVE_BLOCK_USE_RR2
+	_Block_use_RR2(&blocks_runtime_callbacks);
+#endif
 }
 
 /**
@@ -843,9 +863,18 @@ extern "C" OBJC_PUBLIC id objc_storeWeak(id *addr, id obj)
 		*addr = obj;
 		return obj;
 	}
-	// If the object is being deallocated return nil.
+	Class cls = classForObject(obj);
-	if (object_getRetainCount_np(obj) == 0)
+	if (UNLIKELY(objc_test_class_flag(cls, objc_class_flag_is_block)))
 	{
+		// Check whether the block is being deallocated and return nil if so
+		if (_Block_isDeallocating(obj)) {
+			*addr = nil;
+			return nil;
+		}
+	}
+	else if (object_getRetainCount_np(obj) == 0)
+	{
+		// If the object is being deallocated return nil.
 		*addr = nil;
 		return nil;
 	}
@@ -916,19 +945,31 @@ extern "C" OBJC_PUBLIC id objc_loadWeakRetained(id* addr)
 		return nil;
 	}
 	Class cls = classForObject(obj);
-	if (static_cast<Class>(&_NSConcreteMallocBlock) == cls)
+	if (objc_test_class_flag(cls, objc_class_flag_permanent_instances))
 	{
-		obj = static_cast<id>(block_load_weak(obj));
+		return obj;
 	}
-	else if (objc_test_class_flag(cls, objc_class_flag_permanent_instances))
+	else if (UNLIKELY(objc_test_class_flag(cls, objc_class_flag_is_block)))
 	{
-		return obj;
+		obj = static_cast<id>(block_load_weak(obj));
+		if (obj == nil)
+		{
+			return nil;
+		}
+		// This is a defeasible retain operation that protects against another thread concurrently
+		// starting to deallocate the block.
+		if (_Block_tryRetain(obj))
+		{
+			return obj;
+		}
+		return nil;
+
 	}
 	else if (!objc_test_class_flag(cls, objc_class_flag_fast_arc))
 	{
 		obj = _objc_weak_load(obj);
 	}
-	// block_load_weak() or _objc_weak_load() can return nil
+	// _objc_weak_load() can return nil
 	if (obj == nil) { return nil; }
 	return retain(obj, YES);
 }

blocks_runtime.m

@@ -24,6 +24,7 @@
  *  OTHER DEALINGS IN THE SOFTWARE.
  */
 #import "objc/blocks_runtime.h"
+#include "objc/blocks_private.h"
 #import "objc/runtime.h"
 #import "objc/objc-arc.h"
 #include "blocks_runtime.h"
@@ -60,10 +61,6 @@ OBJC_PUBLIC const char * _Block_signature(void *b)
 	}
 	return block->descriptor->encoding;
 }
-OBJC_PUBLIC const char *block_getType_np(const void *b)
-{
-	return _Block_signature((void*)b);
-}
 
 static int increment24(int *ref)
 {
@@ -296,8 +293,32 @@ OBJC_PUBLIC void _Block_release(const void *src)
 	}
 }
 
-PRIVATE void* block_load_weak(void *block)
+OBJC_PUBLIC bool _Block_isDeallocating(const void* arg)
 {
-	struct Block_layout *self = block;
+	struct Block_layout *block = (struct Block_layout*)arg;
-	return (self->reserved) > 0 ? block : 0;
+	int *refCountPtr = &((struct Block_layout*)arg)->reserved;
+	int refCount = __sync_fetch_and_add(refCountPtr, 0);
+	return refCount == 0;
 }
+
+OBJC_PUBLIC bool _Block_tryRetain(const void* arg)
+{
+	/* This is used by the weak reference management in ARC. The implementation
+	 * follows the reasoning of `retain_fast()` in arc.mm: We want to abandon the
+	 * retain operation if another thread has started deallocating the object between
+	 * loading the weak pointer and executing the retain operation.
+	 */
+	struct Block_layout *block = (struct Block_layout*)arg;
+	int *refCountPtr = &block->reserved;
+	int refCountVal = __sync_fetch_and_add(refCountPtr, 0);
+	int newVal = refCountVal;
+	do {
+		refCountVal = newVal;
+		if (refCountVal <= 0)
+		{
+			return false;
+		}
+		newVal = __sync_val_compare_and_swap(refCountPtr, refCountVal, newVal + 1);
+	} while (newVal != refCountVal);
+	return true;
+}

blocks_runtime_np.m

@@ -0,0 +1,57 @@
+/*
+ * Copyright (c) 2009 Remy Demarest
+ * Portions Copyright (c) 2009 David Chisnall
+ *
+ *  Permission is hereby granted, free of charge, to any person
+ *  obtaining a copy of this software and associated documentation
+ *  files (the "Software"), to deal in the Software without
+ *  restriction, including without limitation the rights to use,
+ *  copy, modify, merge, publish, distribute, sublicense, and/or sell
+ *  copies of the Software, and to permit persons to whom the
+ *  Software is furnished to do so, subject to the following
+ *  conditions:
+ *
+ *  The above copyright notice and this permission notice shall be
+ *  included in all copies or substantial portions of the Software.
+ *
+ *  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+ *  EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
+ *  OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+ *  NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
+ *  HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
+ *  WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+ *  FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
+ *  OTHER DEALINGS IN THE SOFTWARE.
+ */
+
+
+#ifdef EMBEDDED_BLOCKS_RUNTIME
+#import "objc/blocks_runtime.h"
+#include "blocks_runtime.h"
+#else
+#import <Block.h>
+#import <Block_private.h>
+#endif
+#include "visibility.h"
+
+
+OBJC_PUBLIC const char *block_getType_np(const void *b)
+{
+	return _Block_signature((void*)b);
+}
+
+/**
+ * Returns the block pointer, or NULL if the block is already
+ * being deallocated. The implementation does not employ atomic 
+ * operations, so this function must only be called by the ARC
+ * subsystem after obtaining the weak-reference lock.
+ */
+PRIVATE void* block_load_weak(void *block)
+{
+	struct Block_layout *self = block;
+	#ifdef EMBEDDED_BLOCKS_RUNTIME
+	return (self->reserved) > 0 ? block : 0;
+	#else
+	return (self->flags) & BLOCK_REFCOUNT_MASK ? block : 0;
+	#endif
+}

class.h

@@ -366,6 +366,11 @@ enum objc_class_flags
 	 * On a class, guarantees that `+init` is trivial.
 	 */
 	objc_class_flag_fast_alloc_init = (1<<15),
+	/**
+	 * The class is a block class. Reference count management must be done by
+	 * the underlying blocks runtime.
+	 */
+	objc_class_flag_is_block = (1 << 16),
 };
 
 /**

loader.c

@@ -27,6 +27,7 @@ void init_gc(void);
 void init_protocol_table(void);
 void init_selector_tables(void);
 void init_trampolines(void);
+void init_early_blocks(void);
 void objc_send_load_message(Class class);
 
 void log_selector_memory_usage(void);
@@ -76,6 +77,7 @@ static void init_runtime(void)
 		init_protocol_table();
 		init_class_tables();
 		init_alias_table();
+		init_early_blocks();
 		init_arc();
 		init_trampolines();
 		first_run = NO;

objc/blocks_private.h

@@ -3,7 +3,14 @@
 #if defined(__clang__) && !defined(__OBJC_RUNTIME_INTERNAL__)
 #pragma clang system_header
 #endif
+#ifdef __cplusplus
+#define BLOCKS_EXPORT extern "C"
+#else
+#define BLOCKS_EXPORT extern
+#endif
 
+#include <stdbool.h>
+#include "Availability.h"
 
 /*
  * This header file exposes some implementation details of the blocks runtime
@@ -38,6 +45,23 @@ struct Block_descriptor
 	const char *encoding;
 };
 
+/** 
+  * Checks whether the block is currently being deallocated.
+  *
+  * Used by ARC weak reference management. Only call this after the weak
+  * reference lock is acquired. 
+  */
+OBJC_PUBLIC BLOCKS_EXPORT bool _Block_isDeallocating(const void *aBlock);
+/**
+ *  Atomically increments the reference count of the block.
+ *  Returns true if the block was retained, and false if it is already 
+ *  being deallocated.
+ *
+ * Used by ARC weak reference management. Only call this after the weak
+ * reference lock is acquired.
+ */
+OBJC_PUBLIC BLOCKS_EXPORT bool _Block_tryRetain(const void *aBlock);
+
 // Helper structure
 struct Block_layout
 {