Correctly initialise global blocks on Windows.

[theraven]

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.

[rjmccall]

Alright, LGTM, at least until we have that backend support you mentioned.

cfe/trunk/lib/CodeGen/CGBlocks.cpp

static llvm::Constant *buildGlobalBlock(CodeGenModule &CGM,
   assert(!CGM.getAddrOfGlobalBlockIfEmitted(blockInfo.BlockExpression) &&
          "Refusing to re-emit a global block.");
 
   // Generate the constants for the block literal initializer.
   ConstantInitBuilder builder(CGM);
   auto fields = builder.beginStruct();
 
   bool IsOpenCL = CGM.getLangOpts().OpenCL;
+  bool IsWindows = CGM.getTarget().getTriple().isOSWindows();
   if (!IsOpenCL) {
     // isa
+    if (IsWindows)
+      fields.addNullPointer(CGM.Int8PtrPtrTy);
+    else
       fields.add(CGM.getNSConcreteGlobalBlock());
 
     // __flags
     BlockFlags flags = BLOCK_IS_GLOBAL | BLOCK_HAS_SIGNATURE;
     if (blockInfo.UsesStret)
       flags |= BLOCK_USE_STRET;
 
     fields.addInt(CGM.IntTy, flags.getBitMask());
 
static llvm::Constant *buildGlobalBlock(CodeGenModule &CGM,
   }
 
   unsigned AddrSpace = 0;
   if (CGM.getContext().getLangOpts().OpenCL)
     AddrSpace = CGM.getContext().getTargetAddressSpace(LangAS::opencl_global);
 
   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 =
     CGM.getTypes().ConvertType(blockInfo.getBlockExpr()->getType());
   llvm::Constant *Result =
       llvm::ConstantExpr::getPointerCast(literal, RequiredType);
   CGM.setAddrOfGlobalBlock(blockInfo.BlockExpression, Result);
   if (CGM.getContext().getLangOpts().OpenCL)

cfe/trunk/test/CodeGen/global-blocks-win32.c

+// 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
+