Slab-allocate OpenGL bind groups

Now that all backends use slab-allocated bind groups, this patch also
moves the BindGroup implementation with owned-data into the Null backend.

Bug: dawn:340
Change-Id: I08a952075b382008fb82f1fbab3f779cc05bc2a3
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/16747
Commit-Queue: Austin Eng <enga@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>

src/dawn_native/null/DeviceNull.cpp

@@ -252,6 +252,25 @@ namespace dawn_native { namespace null {
         mLastSubmittedSerial++;
     }
 
+    // BindGroupDataHolder
+
+    BindGroupDataHolder::BindGroupDataHolder(size_t size)
+        : mBindingDataAllocation(malloc(size))  // malloc is guaranteed to return a
+                                                // pointer aligned enough for the allocation
+    {
+    }
+
+    BindGroupDataHolder::~BindGroupDataHolder() {
+        free(mBindingDataAllocation);
+    }
+
+    // BindGroup
+
+    BindGroup::BindGroup(DeviceBase* device, const BindGroupDescriptor* descriptor)
+        : BindGroupDataHolder(descriptor->layout->GetBindingDataSize()),
+          BindGroupBase(device, descriptor, mBindingDataAllocation) {
+    }
+
     // Buffer
 
     struct BufferMapOperation : PendingOperation {

src/dawn_native/null/DeviceNull.h

@@ -38,7 +38,7 @@
 namespace dawn_native { namespace null {
 
     class Adapter;
-    using BindGroup = BindGroupBaseOwnBindingData;
+    class BindGroup;
     using BindGroupLayout = BindGroupLayoutBase;
     class Buffer;
     class CommandBuffer;
@@ -157,6 +157,24 @@ namespace dawn_native { namespace null {
         ResultOrError<DeviceBase*> CreateDeviceImpl(const DeviceDescriptor* descriptor) override;
     };
 
+    // Helper class so |BindGroup| can allocate memory for its binding data,
+    // before calling the BindGroupBase base class constructor.
+    class BindGroupDataHolder {
+      protected:
+        explicit BindGroupDataHolder(size_t size);
+        ~BindGroupDataHolder();
+
+        void* mBindingDataAllocation;
+    };
+
+    // We don't have the complexity of placement-allocation of bind group data in
+    // the Null backend. This class, keeps the binding data in a separate allocation for simplicity.
+    class BindGroup : private BindGroupDataHolder, public BindGroupBase {
+      public:
+        BindGroup(DeviceBase* device, const BindGroupDescriptor* descriptor);
+        ~BindGroup() override = default;
+    };
+
     class Buffer : public BufferBase {
       public:
         Buffer(Device* device, const BufferDescriptor* descriptor);