[dart2wasm] Represent ffi compounds as actual Dart objects (they are views on the actual memory)

This fixes issues where structs are accessed dynamically
or simply accessed in a nullable fashion, e.g. with `MyStruct? foo`

dart2wasm does not yet support using structs-by-value as arguments
or return values when calling linear-memory wasm functions.

dart2wasm also doesn't support structs-by-value created in dart
that are backed by WasmGC memory (e.g. arrays).

Change-Id: I9b9c60002d2d32db3f235f1cec25fe3b35fcc17b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/365140
Commit-Queue: Martin Kustermann <kustermann@google.com>
Reviewed-by: Slava Egorov <vegorov@google.com>

pkg/dart2wasm/lib/code_generator.dart

@@ -737,7 +737,6 @@ class CodeGenerator extends ExpressionVisitor1<w.ValueType, w.ValueType>
       if (translator.needsConversion(thisLocal!.type, thisType) &&
           !(cls == translator.objectInfo.cls ||
               cls == translator.ffiPointerClass ||
-              translator.isFfiCompound(cls) ||
               translator.isWasmType(cls))) {
         preciseThisLocal = addLocal(thisType);
         b.local_get(thisLocal!);

pkg/dart2wasm/lib/intrinsics.dart

@@ -182,26 +182,6 @@ class Intrinsifier {
       return w.NumType.i64;
     }
 
-    // _Compound._typedDataBase
-    if (cls == translator.ffiCompoundClass && name == '_typedDataBase') {
-      // A compound (subclass of Struct or Union) is represented by its i32
-      // address. The _typedDataBase field contains a Pointer pointing to the
-      // compound, whose representation is the same.
-      // TODO(https://dartbug.com/55083): Implement structs backed by TypedData.
-      codeGen.wrap(receiver, w.NumType.i32);
-      return w.NumType.i32;
-    }
-
-    // _Compound._offsetInBytes
-    if (cls == translator.ffiCompoundClass && name == '_offsetInBytes') {
-      // A compound (subclass of Struct or Union) is represented by its i32
-      // address. The _offsetInBytes field contains is always 0.
-      // This also breaks nested structs, which are currently not used.
-      // TODO(https://dartbug.com/55083): Implement structs backed by TypedData.
-      b.i64_const(0);
-      return w.NumType.i64;
-    }
-
     // Pointer.address
     if (cls == translator.ffiPointerClass && name == 'address') {
       // A Pointer is represented by its i32 address.
@@ -1120,15 +1100,6 @@ class Intrinsifier {
       return w.RefType.def(arrayType, nullable: false);
     }
 
-    // _Compound.#fromTypedDataBase
-    if (name == "#fromTypedDataBase") {
-      // A compound (subclass of Struct or Union) is represented by its i32
-      // address. The argument to the #fromTypedDataBase constructor is a
-      // Pointer, whose representation is the same.
-      codeGen.wrap(node.arguments.positional.single, w.NumType.i32);
-      return w.NumType.i32;
-    }
-
     return null;
   }
 

pkg/dart2wasm/lib/translator.dart

@@ -523,8 +523,6 @@ class Translator with KernelNodes {
   bool isWasmType(Class cls) =>
       cls == wasmTypesBaseClass || _hasSuperclass(cls, wasmTypesBaseClass);
 
-  bool isFfiCompound(Class cls) => _hasSuperclass(cls, ffiCompoundClass);
-
   w.StorageType translateStorageType(DartType type) {
     bool nullable = type.isPotentiallyNullable;
     if (type is InterfaceType) {
@@ -569,12 +567,6 @@ class Translator with KernelNodes {
         return w.RefType.def(wasmType, nullable: nullable);
       }
 
-      // FFI compound?
-      if (isFfiCompound(cls)) {
-        if (nullable) throw "FFI types can't be nullable";
-        return w.NumType.i32;
-      }
-
       // Other built-in type?
       w.StorageType? builtin = builtinTypes[cls];
       if (builtin != null) {
@@ -668,9 +660,6 @@ class Translator with KernelNodes {
         if (builtin != null && builtin.isPrimitive) {
           return builtin as w.ValueType;
         }
-        if (isFfiCompound(cls)) {
-          return w.NumType.i32;
-        }
       }
     }
     // TODO(joshualitt): We'd like to use the potential nullability here too,

tests/web/wasm/ffi_native_test.dart

@@ -143,12 +143,19 @@ void main() {
   toggleBool();
   Expect.equals(boolReturn(789), false);
 
-  final struct_ = getStruct();
-  Expect.equals(struct_.ref.x, 1.0);
-  Expect.equals(struct_.ref.y, 2);
-  clearStruct(struct_);
-  Expect.equals(struct_.ref.x, 0.0);
-  Expect.equals(struct_.ref.y, 0);
+  final Pointer<MyStruct> structPointer = getStruct();
+  final MyStruct struct = structPointer.ref;
+  Expect.equals(struct.x, 1.0);
+  Expect.equals(struct.y, 2);
+  // Structs are Dart objects that are views on top of actual memory (which may
+  // be backed by C memory or typed data). The view objects can be accessed
+  // dynamically.
+  final l = <dynamic>[struct, 1];
+  Expect.equals(l[int.parse('0')].x, 1.0);
+
+  clearStruct(structPointer);
+  Expect.equals(struct.x, 0.0);
+  Expect.equals(struct.y, 0);
 
   Expect.equals(incrementChar(1), 2);
   Expect.equals(incrementUnsignedChar(3), 4);