[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

2 files changed, 20 insertions, 5 deletions

diff --git a/clang/lib/CodeGen/CGBlocks.cpp b/clang/lib/CodeGen/CGBlocks.cpp
index 926683db93c..352a02db2b9 100644
--- a/clang/lib/CodeGen/CGBlocks.cpp
+++ b/clang/lib/CodeGen/CGBlocks.cpp
@@ -104,7 +104,7 @@ static llvm::Constant *buildBlockDescriptor(CodeGenModule &CGM,
   elements.addInt(ulong, blockInfo.BlockSize.getQuantity());
 
   // Optional copy/dispose helpers.
-  if (blockInfo.NeedsCopyDispose) {
+  if (blockInfo.needsCopyDisposeHelpers()) {
     // copy_func_helper_decl
     elements.add(buildCopyHelper(CGM, blockInfo));
 
@@ -159,6 +159,7 @@ static llvm::Constant *buildBlockDescriptor(CodeGenModule &CGM,
 
     /// These are the flags (with corresponding bit number) that the
     /// compiler is actually supposed to know about.
+    ///  23. BLOCK_IS_NOESCAPE - indicates that the block is non-escaping
     ///  25. BLOCK_HAS_COPY_DISPOSE - indicates that the block
     ///   descriptor provides copy and dispose helper functions
     ///  26. BLOCK_HAS_CXX_OBJ - indicates that there's a captured
@@ -778,8 +779,13 @@ llvm::Value *CodeGenFunction::EmitBlockLiteral(const CGBlockInfo &blockInfo) {
   llvm::Constant *descriptor;
   BlockFlags flags;
   if (!IsOpenCL) {
-    isa = llvm::ConstantExpr::getBitCast(CGM.getNSConcreteStackBlock(),
-                                         VoidPtrTy);
+    // If the block is non-escaping, set field 'isa 'to NSConcreteGlobalBlock
+    // and set the BLOCK_IS_GLOBAL bit of field 'flags'. Copying a non-escaping
+    // block just returns the original block and releasing it is a no-op.
+    llvm::Constant *blockISA = blockInfo.getBlockDecl()->doesNotEscape()
+                                   ? CGM.getNSConcreteGlobalBlock()
+                                   : CGM.getNSConcreteStackBlock();
+    isa = llvm::ConstantExpr::getBitCast(blockISA, VoidPtrTy);
 
     // Build the block descriptor.
     descriptor = buildBlockDescriptor(CGM, blockInfo);
@@ -788,12 +794,14 @@ llvm::Value *CodeGenFunction::EmitBlockLiteral(const CGBlockInfo &blockInfo) {
     flags = BLOCK_HAS_SIGNATURE;
     if (blockInfo.HasCapturedVariableLayout)
       flags |= BLOCK_HAS_EXTENDED_LAYOUT;
-    if (blockInfo.NeedsCopyDispose)
+    if (blockInfo.needsCopyDisposeHelpers())
       flags |= BLOCK_HAS_COPY_DISPOSE;
     if (blockInfo.HasCXXObject)
       flags |= BLOCK_HAS_CXX_OBJ;
     if (blockInfo.UsesStret)
       flags |= BLOCK_USE_STRET;
+    if (blockInfo.getBlockDecl()->doesNotEscape())
+      flags |= BLOCK_IS_NOESCAPE | BLOCK_IS_GLOBAL;
   }
 
   auto projectField =
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