[CodeGen][ObjC] Make copying and disposing of a non-escaping block

no-ops.

A non-escaping block on the stack will never be called after its
lifetime ends, so it doesn't have to be copied to the heap. To prevent
a non-escaping block from being copied to the heap, this patch sets
field 'isa' of the block object to NSConcreteGlobalBlock and sets the
BLOCK_IS_GLOBAL bit of field 'flags', which causes the runtime to treat
the block as if it were a global block (calling _Block_copy on the block
just returns the original block and calling _Block_release is a no-op).

Also, a new flag bit 'BLOCK_IS_NOESCAPE' is added, which allows the
runtime or tools to distinguish between true global blocks and
non-escaping blocks.

rdar://problem/39352313

Differential Revision: https://reviews.llvm.org/D49303

llvm-svn: 337580

1 files changed, 8 insertions, 0 deletions

diff --git a/clang/docs/Block-ABI-Apple.rst b/clang/docs/Block-ABI-Apple.rst
index 7f49bbd..e48a24b 100644
--- a/clang/docs/Block-ABI-Apple.rst
+++ b/clang/docs/Block-ABI-Apple.rst
@@ -61,6 +61,14 @@ The following flags bits are in use thusly for a possible ABI.2010.3.16:
 .. code-block:: c
 
     enum {
+        // Set to true on blocks that have captures (and thus are not true
+        // global blocks) but are known not to escape for various other
+        // reasons. For backward compatiblity with old runtimes, whenever
+        // BLOCK_IS_NOESCAPE is set, BLOCK_IS_GLOBAL is set too. Copying a
+        // non-escaping block returns the original block and releasing such a
+        // block is a no-op, which is exactly how global blocks are handled.
+        BLOCK_IS_NOESCAPE      =  (1 << 23),
+
         BLOCK_HAS_COPY_DISPOSE =  (1 << 25),
         BLOCK_HAS_CTOR =          (1 << 26), // helpers have C++ code
         BLOCK_IS_GLOBAL =         (1 << 28),