[VK] Fix crash on exit when using VVL (#219)

VVL has internal state which conflicts requires a specific cleanup
ordering. Because the VKContext relied on the global dtor, this order
was not respected.

There are 2 solutions:
 - use atexit
 - require an explicit call to vkDestroyDevice.

Thing is, users of `Device` except the object to be usable as a simple
singleton, with no explicit `destroy` to call.
For this reasons, it seems using `atexit` is the cleanest option from
the user perspective.

Author: Keenuts

include/API/Device.h

@@ -62,7 +62,8 @@ class Device {
 #endif
 
 #ifdef OFFLOADTEST_ENABLE_VULKAN
-  static llvm::Error initializeVXDevices();
+  static llvm::Error initializeVKDevices();
+  static void cleanupVKDevices();
 #endif
 
 #ifdef OFFLOADTEST_ENABLE_METAL

lib/API/Device.cpp

@@ -15,6 +15,7 @@
 
 #include "llvm/Support/Error.h"
 
+#include <cstdlib>
 #include <memory>
 
 using namespace offloadtest;
@@ -59,8 +60,14 @@ llvm::Error Device::initialize() {
     return Err;
 #endif
 #ifdef OFFLOADTEST_ENABLE_VULKAN
-  if (auto Err = initializeVXDevices())
+  if (auto Err = initializeVKDevices())
     return Err;
+  // Validation layers have internal state which require a specific destruction
+  // ordering. Relying on the global dtor call for this is unreliable and can
+  // cause a null-deref in the validation layers during the final
+  // vkDestroyInstance. This is a known limitation of the validation layers
+  // which explicitely requires using atexit.
+  atexit(Device::cleanupVKDevices);
 #endif
 #ifdef OFFLOADTEST_ENABLE_METAL
   if (auto Err = initializeMtlDevices())

lib/API/VK/Device.cpp

@@ -770,7 +770,7 @@ class VKContext {
   llvm::SmallVector<std::shared_ptr<VKDevice>> Devices;
 
   VKContext() = default;
-  ~VKContext() { vkDestroyInstance(Instance, NULL); }
+  ~VKContext() { cleanup(); }
   VKContext(const VKContext &) = delete;
 
 public:
@@ -779,6 +779,11 @@ class VKContext {
     return Ctx;
   }
 
+  void cleanup() {
+    vkDestroyInstance(Instance, NULL);
+    Instance = VK_NULL_HANDLE;
+  }
+
   llvm::Error initialize() {
     // Create a Vulkan 1.1 instance to determine the API version
     VkApplicationInfo AppInfo = {};
@@ -851,6 +856,8 @@ class VKContext {
 };
 } // namespace
 
-llvm::Error Device::initializeVXDevices() {
+llvm::Error Device::initializeVKDevices() {
   return VKContext::instance().initialize();
 }
+
+void Device::cleanupVKDevices() { VKContext::instance().cleanup(); }