Correctly initialise global blocks on Windows.

Summary:
Windows does not allow globals to be initialised to point to globals in
another DLL.  Exported globals may be referenced only from code.  Work
around this by creating an initialiser that runs in early library
initialisation and sets the isa pointer.

Reviewers: rjmccall

Reviewed By: rjmccall

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D50436

llvm-svn: 339317

2 files changed, 44 insertions, 2 deletions

diff --git a/clang/lib/CodeGen/CGBlocks.cpp b/clang/lib/CodeGen/CGBlocks.cpp
index 16436830b11..f8a7901dbf1 100644
--- a/clang/lib/CodeGen/CGBlocks.cpp
+++ b/clang/lib/CodeGen/CGBlocks.cpp
@@ -1213,9 +1213,13 @@ static llvm::Constant *buildGlobalBlock(CodeGenModule &CGM,
   auto fields = builder.beginStruct();
 
   bool IsOpenCL = CGM.getLangOpts().OpenCL;
+  bool IsWindows = CGM.getTarget().getTriple().isOSWindows();
   if (!IsOpenCL) {
     // isa
-    fields.add(CGM.getNSConcreteGlobalBlock());
+    if (IsWindows)
+      fields.addNullPointer(CGM.Int8PtrPtrTy);
+    else
+      fields.add(CGM.getNSConcreteGlobalBlock());
 
     // __flags
     BlockFlags flags = BLOCK_IS_GLOBAL | BLOCK_HAS_SIGNATURE;
@@ -1250,7 +1254,27 @@ static llvm::Constant *buildGlobalBlock(CodeGenModule &CGM,
 
   llvm::Constant *literal = fields.finishAndCreateGlobal(
       "__block_literal_global", blockInfo.BlockAlign,
-      /*constant*/ true, llvm::GlobalVariable::InternalLinkage, AddrSpace);
+      /*constant*/ !IsWindows, llvm::GlobalVariable::InternalLinkage, AddrSpace);
+
+  // Windows does not allow globals to be initialised to point to globals in
+  // different DLLs.  Any such variables must run code to initialise them.
+  if (IsWindows) {
+    auto *Init = llvm::Function::Create(llvm::FunctionType::get(CGM.VoidTy,
+          {}), llvm::GlobalValue::InternalLinkage, ".block_isa_init",
+        &CGM.getModule());
+    llvm::IRBuilder<> b(llvm::BasicBlock::Create(CGM.getLLVMContext(), "entry",
+          Init));
+    b.CreateAlignedStore(CGM.getNSConcreteGlobalBlock(),
+        b.CreateStructGEP(literal, 0), CGM.getPointerAlign().getQuantity());
+    b.CreateRetVoid();
+    // We can't use the normal LLVM global initialisation array, because we
+    // need to specify that this runs early in library initialisation.
+    auto *InitVar = new llvm::GlobalVariable(CGM.getModule(), Init->getType(),
+        /*isConstant*/true, llvm::GlobalValue::InternalLinkage,
+        Init, ".block_isa_init_ptr");
+    InitVar->setSection(".CRT$XCLa");
+    CGM.addUsedGlobal(InitVar);
+  }
 
   // Return a constant of the appropriately-casted type.
   llvm::Type *RequiredType =
diff --git a/clang/test/CodeGen/global-blocks-win32.c b/clang/test/CodeGen/global-blocks-win32.c
new file mode 100644
index 00000000000..7a66c924b42
--- /dev/null
+++ b/clang/test/CodeGen/global-blocks-win32.c
@@ -0,0 +1,18 @@
+// RUN: %clang_cc1 -fblocks -triple i386-pc-windows-msvc %s -emit-llvm -o - -fblocks | FileCheck %s
+
+
+int (^x)(void) = ^() { return 21; };
+
+
+// Check that the block literal is emitted with a null isa pointer
+// CHECK: @__block_literal_global = internal global { i8**, i32, i32, i8*, %struct.__block_descriptor* } { i8** null, 
+
+// Check that _NSConcreteGlobalBlock has the correct dllimport specifier.
+// CHECK: @_NSConcreteGlobalBlock = external dllimport global i8*
+// Check that we create an initialiser pointer in the correct section (early library initialisation).
+// CHECK: @.block_isa_init_ptr = internal constant void ()* @.block_isa_init, section ".CRT$XCLa"
+
+// Check that we emit an initialiser for it.
+// CHECK: define internal void @.block_isa_init() {
+// CHECK: store i8** @_NSConcreteGlobalBlock, i8*** getelementptr inbounds ({ i8**, i32, i32, i8*, %struct.__block_descriptor* }, { i8**, i32, i32, i8*, %struct.__block_descriptor* }* @__block_literal_global, i32 0, i32 0), align 4
+