[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, 1 deletions

diff --git a/clang/lib/CodeGen/CGBlocks.h b/clang/lib/CodeGen/CGBlocks.h
index 80e255f7541..5a8e960ffcc 100644
--- a/clang/lib/CodeGen/CGBlocks.h
+++ b/clang/lib/CodeGen/CGBlocks.h
@@ -54,6 +54,7 @@ enum BlockByrefFlags {
 };
 
 enum BlockLiteralFlags {
+  BLOCK_IS_NOESCAPE      =  (1 << 23),
   BLOCK_HAS_COPY_DISPOSE =  (1 << 25),
   BLOCK_HAS_CXX_OBJ =       (1 << 26),
   BLOCK_IS_GLOBAL =         (1 << 28),
@@ -214,7 +215,8 @@ public:
   /// no non-constant captures.
   bool CanBeGlobal : 1;
 
-  /// True if the block needs a custom copy or dispose function.
+  /// True if the block has captures that would necessitate custom copy or
+  /// dispose helper functions if the block were escaping.
   bool NeedsCopyDispose : 1;
 
   /// HasCXXObject - True if the block's custom copy/dispose functions
@@ -276,6 +278,11 @@ public:
   }
 
   CGBlockInfo(const BlockDecl *blockDecl, StringRef Name);
+
+  // Indicates whether the block needs a custom copy or dispose function.
+  bool needsCopyDisposeHelpers() const {
+    return NeedsCopyDispose && !Block->doesNotEscape();
+  }
 };
 
 }  // end namespace CodeGen