libobjc2: use libBlocksRuntime from libdispatch

[hmelder]

Complexity of the Blocks Runtime in libdispatch is lower and only copies the block descriptor not the body. This means we do not need to mprotect a page as -W- and then R-X to execute copied instructions.

[hmelder]

Because of this, there is no need for a thunk written in assembly to jump to the block body residing in the mprotected page.

[triplef]

Sounds great! Builds are currently failing with:

CMake Error at CMakeLists.txt:252 (message):
  libBlocksRuntime does not contain _Block_use_RR2().  Enable
  EMBEDDED_BLOCKS_RUNTIME to use the built-in blocks runtime.

[hmelder]

I know. The header is not in the search path. So this is just a CMake Configuration issue :)

[hmelder]

Arggh should build now :D

[hmelder]

I will clean up the patches and upstream them into the respective repositories.

[hmelder]

Patches can be removed once swiftlang/swift-corelibs-libdispatch#916 and gnustep/libobjc2#355 are merged.

[hmelder]

Hmm seems like GNUstep tries to use the GSBlocks compatibility layer, instead of libBlocksRuntime ...

gnustep-make requires patching as well.

[hmelder]

Breaking Changes:

• All Objective-C programs and shared libraries that use blocks need to explicitly link against BlocksRuntime.lib. Alternatively, we could also link the BlocksRuntime statically into libobjc and link libdispatch against libobjc.

Open Pull Requests (Patches can be removed once merged):

• BlocksRuntime: Export all NSConcrete* globals on Win32 swiftlang/swift-corelibs-blocksruntime#7
• Cleanup CMake configuration for external blocks runtime support libobjc2#355