[vm/ffi] Support `.address.cast()` in leaf calls

[dcharkes]

Many functions with buffers take a void*, for example https://en.cppreference.com/w/c/io/fread.

However, using a Uint8List and .address yields a Pointer<Uint8>.

It would be useful to be able to pass uint8list.address.cast(). That would prevent having to modify the FFIgen generated function signature.

Thanks for the suggestion @brianquinlan!

For anyone willing to contribute, the implementation is in:

sdk/pkg/vm/lib/modular/transformations/ffi/use_sites.dart

Lines 1489 to 1504 in 05d9e5b

	/// Converts a single parameter with argument for [_replaceNativeCall].
	(
	/// '' for non-Pointer.
	/// 'P' for Pointer.
	/// 'T' for TypedData.
	/// 'C' for _Compound (TypedData/Pointer and offset in bytes).
	/// 'E' for errors.
	String methodPostFix,
	DartType parameterType,
	Expression argument,
	) _replaceNativeCallParameterAndArgument(
	VariableDeclaration parameter,
	DartType parameterType,
	Expression argument,
	int fileOffset,
	) {

The implementation should find the cast() invocation and use its receiver instead (effectively ignoring the cast expression as a no-op).

[codesculpture]

Hey @dcharkes, so we have to change the data type of address from int to Pointer<Void>, so that address always yields a Pointer<Void ? But this would introduce expression such as address.cast().address.cast().address Wdy? am not sure whether am making sense , Please feel free to correct me

[dcharkes]

Hey @dcharkes, so we have to change the data type of address from int to Pointer<Void>, so that address always yields a Pointer<Void ? But this would introduce expression such as address.cast().address.cast().address Wdy? am not sure whether am making sense , Please feel free to correct me

Hi @codesculpture!

No, the .address here refers to the various extension methods on non-Pointers which return a Pointer such as:

• https://api.dart.dev/dev/3.5.0-202.0.dev/dart-ffi/Uint8ListAddress/address.html

[codesculpture]

Hi @dcharkes , i think i don't understand the problem

It would be useful to be able to pass uint8list.address.cast(). That would prevent having to modify the FFIgen generated function signature.

can you please elaborate above with an example :)

[dcharkes]

The following function in C

size_t fread( void          *buffer, size_t size, size_t count,
              FILE          *stream );

yields the following bindings with package:ffigen:

@Native<Size Function(Pointer<Void>, Size, Size, Pointer<File>)
external int fread(Pointer<Void> buffer, int size, int count, Pointer<File> stream);

But if you want to use for buffer a someUint8List.address the type of that expression will be Pointer<Uint8> and not the required Pointer<Void>.

[codesculpture]

Sorry again @dcharkes , thanks for explaining the problem, i understood the problem, but what is the solution you are suggesting? Thanks

[dcharkes]

The following code should work:

@Native<Size Function(Pointer<Void>, Size, Size, Pointer<Void>)>()
external int fread(
    Pointer<Void> buffer, int size, int count, Pointer<Void> stream);

void main() {
  final buffer = Uint8List.fromList([1, 2, 3]);
  fread(buffer.address.cast(), 3, 3, nullptr);
}

Currently it is rejected because .address is followed by .cast(). We should change the compiler such that it is allowed.

[codesculpture]

@dcharkes am trying to debug the https://github.com/dart-lang/sdk/blob/05d9e5bf37a68ec75f8ffc56a65a89e0e86b5a1d/pkg%2Fvm%2Flib%2Fmodular%2Ftransformations%2Fffi%2Fuse_sites.dart, but i cannot find a way to do so, i already asked a related question in dart discord server long ago, what would be ideal solution to debug the internal dart codes (now am adding print statements, that would require re building dart, for every changes)

[dcharkes]

If you're in VSCode, you can run the CFE like this in the debugger:

			{
				"name": "dart gen_kernel.dart",
				"type": "dart",
				"request": "launch",
				"program": "pkg/vm/bin/gen_kernel.dart",
				"args": [
					"--platform=${workspaceFolder}/xcodebuild/ReleaseARM64/vm_platform_strong.dill",
					"pkg/vm/testcases/transformations/ffi/address_of_struct_element.dart",
				],
				"toolArgs": [],
				"enableAsserts": true,
				"cwd": "${workspaceFolder}",
			},

[codesculpture]

Hey @dcharkes , are we going to allow .address.cast() only on direct typed data (Uint8List) or we gonna allow expression such access members which are from struct and unions myStruct.myList.address.cast() ?

[codesculpture]

Also @dcharkes, one small doubt is that _replaceNativeCallParameterAndArgument replacing the address with the actual instance in the AST (internally), for ex: if i fed native(buffer.address) this would transformed into native(buffer) ?
Am i correct ?

[dcharkes]

Also @dcharkes, one small doubt is that _replaceNativeCallParameterAndArgument replacing the address with the actual instance in the AST (internally), for ex: if i fed native(buffer.address) this would transformed into native(buffer) ? Am i correct ?

Yes, this is already what is happening:

sdk/pkg/vm/lib/modular/transformations/ffi/use_sites.dart

Lines 1521 to 1530 in e9c7b2b

	if (addressOfMethodsTypedData.contains(argument.target)) {
	final subExpression = argument.arguments.positional.single;
	// Argument is `typedData.address`.
	final typedDataType = InterfaceType(
	typedDataClass,
	Nullability.nonNullable,
	const <DartType>[],
	);
	return ('T', typedDataType, subExpression);
	}

The TypedData is converted to the address that points into into the the typed data in the FFI call in the VM:

sdk/runtime/vm/compiler/backend/il.cc

Lines 7589 to 7599 in e9c7b2b

	if (marshaller_.IsTypedDataPointer(arg_index) ||
	marshaller_.IsCompoundPointer(arg_index)) {
	// Unwrap typed data before move to native location.
	__ Comment("Load typed data base address");
	if (origin.IsStackSlot()) {
	compiler->EmitMove(Location::RegisterLocation(temp0), origin,
	&temp_alloc);
	origin = Location::RegisterLocation(temp0);
	}
	ASSERT(origin.IsRegister());
	__ LoadFromSlot(origin.reg(), origin.reg(), Slot::PointerBase_data());

[dcharkes]

Hey @dcharkes , are we going to allow .address.cast() only on direct typed data (Uint8List) or we gonna allow expression such access members which are from struct and unions myStruct.myList.address.cast() ?

This should be allowable as well. 👍