Set Initializer ExecutionMode for global ctors

SPIR-V 1.1 adds the Initializer Execution Mode for entry points.  Set
this execution mode for entry points listed in the llvm.global_ctors
variable.

Author: svenvh

llvm-spirv/lib/SPIRV/PreprocessMetadata.cpp

@@ -68,6 +68,8 @@ class PreprocessMetadata : public ModulePass {
 
   bool runOnModule(Module &M) override;
   void visit(Module *M);
+  void preprocessCXXStructorList(SPIRVMDBuilder::NamedMDWrapper &EM,
+                                 GlobalVariable *V, ExecutionMode EMode);
   void preprocessOCLMetadata(Module *M, SPIRVMDBuilder *B, SPIRVMDWalker *W);
   void preprocessVectorComputeMetadata(Module *M, SPIRVMDBuilder *B,
                                        SPIRVMDWalker *W);
@@ -94,6 +96,24 @@ bool PreprocessMetadata::runOnModule(Module &Module) {
   return true;
 }
 
+void PreprocessMetadata::preprocessCXXStructorList(
+    SPIRVMDBuilder::NamedMDWrapper &EM, GlobalVariable *V,
+    ExecutionMode EMode) {
+  auto *List = dyn_cast_or_null<ConstantArray>(V->getInitializer());
+  if (!List)
+    return;
+
+  for (Value *V : List->operands()) {
+    auto *Structor = cast<ConstantStruct>(V);
+
+    // Each entry in the list is a struct containing 3 members:
+    // (priority, function, data), with function being the entry point.
+    auto *Kernel = cast<Function>(Structor->getOperand(1));
+
+    EM.addOp().add(Kernel).add(EMode).done();
+  }
+}
+
 void PreprocessMetadata::visit(Module *M) {
   SPIRVMDBuilder B(*M);
   SPIRVMDWalker W(*M);
@@ -105,6 +125,10 @@ void PreprocessMetadata::visit(Module *M) {
   // of OpExecutionMode instructions
   auto EM = B.addNamedMD(kSPIRVMD::ExecutionMode); // !spirv.ExecutionMode = {}
 
+  // Process special variables in LLVM IR module.
+  if (auto *GV = M->getGlobalVariable("llvm.global_ctors"))
+    preprocessCXXStructorList(EM, GV, spv::ExecutionModeInitializer);
+
   // Add execution modes for kernels. We take it from metadata attached to
   // the kernel functions.
   for (Function &Kernel : *M) {

llvm-spirv/test/CXX/global-ctor.cl

@@ -0,0 +1,21 @@
+// RUN: %clang_cc1 -cl-std=clc++ -emit-llvm-bc -triple spir -O0 %s -o %t.bc
+// RUN: llvm-spirv %t.bc -o %t.spv
+// RUN: spirv-val %t.spv
+// RUN: llvm-spirv %t.spv -to-text -o - | FileCheck %s --check-prefix=CHECK-SPIRV
+
+class Something {
+  public:
+    Something(int a) : v(a) {}
+    int v;
+};
+
+Something g(33);
+
+void kernel work(global int *out) {
+  *out = g.v;
+}
+
+// CHECK-SPIRV: EntryPoint 6 [[work:[0-9]+]] "work"
+// CHECK-SPIRV-NOT: ExecutionMode [[work]] 33
+// CHECK-SPIRV: EntryPoint 6 [[ctor:[0-9]+]] "_GLOBAL__sub_I_global_ctor.cl"
+// CHECK-SPIRV: ExecutionMode [[ctor]] 33