fix: let cppgc align to 16 bytes (#1677)

examples/cppgc-object.rs

@@ -27,7 +27,7 @@ fn main() {
   v8::V8::initialize_platform(platform.clone());
   v8::V8::initialize();
 
-  v8::cppgc::initalize_process(platform.clone());
+  v8::cppgc::initialize_process(platform.clone());
 
   {
     let heap =

examples/cppgc.rs

@@ -46,7 +46,7 @@ fn main() {
   let platform = v8::new_default_platform(0, false).make_shared();
   v8::V8::initialize_platform(platform.clone());
   v8::V8::initialize();
-  v8::cppgc::initalize_process(platform.clone());
+  v8::cppgc::initialize_process(platform.clone());
 
   {
     // Create a managed heap.

src/binding.cc

@@ -3976,9 +3976,19 @@ void cppgc__heap__collect_garbage_for_testing(
   heap->CollectGarbageForTesting(stack_state);
 }
 
-RustObj* cppgc__make_garbage_collectable(v8::CppHeap* heap, size_t size) {
-  return cppgc::MakeGarbageCollected<RustObj>(heap->GetAllocationHandle(),
-                                              cppgc::AdditionalBytes(size));
+class alignas(16) RustObjButAlign16 : public RustObj {};
+
+RustObj* cppgc__make_garbage_collectable(v8::CppHeap* heap, size_t size,
+                                         size_t alignment) {
+  if (alignment <= 8) {
+    return cppgc::MakeGarbageCollected<RustObj>(heap->GetAllocationHandle(),
+                                                cppgc::AdditionalBytes(size));
+  }
+  if (alignment <= 16) {
+    return cppgc::MakeGarbageCollected<RustObjButAlign16>(
+        heap->GetAllocationHandle(), cppgc::AdditionalBytes(size));
+  }
+  return nullptr;
 }
 
 void cppgc__Visitor__Trace__Member(cppgc::Visitor* visitor,

src/cppgc.rs

@@ -27,6 +27,7 @@ extern "C" {
   fn cppgc__make_garbage_collectable(
     heap: *mut Heap,
     size: usize,
+    alignment: usize,
   ) -> *mut RustObj;
 
   fn cppgc__heap__enable_detached_garbage_collections_for_testing(
@@ -145,7 +146,7 @@ unsafe fn get_object_from_rust_obj<T: GarbageCollected>(
 /// creating a Heap.
 ///
 /// Can be called multiple times when paired with `ShutdownProcess()`.
-pub fn initalize_process(platform: SharedRef<Platform>) {
+pub fn initialize_process(platform: SharedRef<Platform>) {
   unsafe {
     cppgc__initialize_process(&*platform as *const Platform as *mut _);
   }
@@ -155,7 +156,7 @@ pub fn initalize_process(platform: SharedRef<Platform>) {
 ///
 /// Must be called after destroying the last used heap. Some process-global
 /// metadata may not be returned and reused upon a subsequent
-/// `initalize_process()` call.
+/// `initialize_process()` call.
 pub unsafe fn shutdown_process() {
   cppgc__shutdown_process();
 }
@@ -343,6 +344,11 @@ pub unsafe fn make_garbage_collected<T: GarbageCollected + 'static>(
   heap: &Heap,
   obj: T,
 ) -> Ptr<T> {
+  const {
+    // max alignment in cppgc is 16
+    assert!(std::mem::align_of::<T>() <= 16);
+  }
+
   let additional_bytes = (object_offset_for_rust_obj::<T>()
     - std::mem::size_of::<RustObj>())
     + std::mem::size_of::<T>();
@@ -351,9 +357,12 @@ pub unsafe fn make_garbage_collected<T: GarbageCollected + 'static>(
     cppgc__make_garbage_collectable(
       heap as *const Heap as *mut _,
       additional_bytes,
+      std::mem::align_of::<T>(),
     )
   };
 
+  assert!(!pointer.is_null());
+
   unsafe {
     let inner = get_object_from_rust_obj::<T>(pointer);
     inner.write(obj);

src/support.h

@@ -193,8 +193,8 @@ struct three_pointers_t {
 
 #endif  // SUPPORT_H_
 
-class RustObj final : public cppgc::GarbageCollected<RustObj>,
-                      public cppgc::NameProvider {
+class RustObj : public cppgc::GarbageCollected<RustObj>,
+                public cppgc::NameProvider {
  public:
   ~RustObj();
   void Trace(cppgc::Visitor* visitor) const;