[vm/ffi] Inline multi-dimensional arrays

[dcharkes]

Follow up of #35763

class Matrix4x4 extends Struct {
  @CArraySize([4,4])
  CArray<CArray<Float>> data;
}

struct Matrix4x4 {
  float data[4][4];
};

Context: https://dart-review.googlesource.com/c/sdk/+/183640/21/pkg/front_end/testcases/general_nnbd_opt_out/ffi_struct_inline_array.dart#12

Or

class MyStruct { 
  @ArraySize(8)
  Array<Int8> a;
 
  @ArraySize2(2, 4)
  Array<Array<Int8>> b;

  @ArraySizeN([2,3,4,5])
  Array<Array<Array<Array<Int8>>>> c;
}

[dcharkes]

Or using the initializer:

class MyStruct { 
  Array<Int8> a = Array(8);
 
  Array<Array<Int8>> b = Array(2, 4);

  Array<Array<Array<Array<Int8>>>> c = Array.multi([2,3,4,5]);
}

external const Array(int firstDimension, [int dimension2, int dimension3, int dimension4, int dimension5]);
external const Array.multi(List<int> dimensions);

Credits to @lrhn

[dcharkes]

After some more discussion: keeping the annotations for consistency and enforcing const size arguments.

We use Array for the dimensions as well.

class MyStruct extends Struct {
  @Array(8)
  external Array<Uint8> inlineArray;

  @Array(2, 2, 2)
  external Array<Array<Array<Uint8>>> threeDimensionalInlineArray;

  @Array.multi([2, 2, 2, 2, 2, 2, 2, 2])
  external Array<Array<Array<Array<Array<Array<Array<Array<Uint8>>>>>>>> eightDimensionalInlineArray;
}

/// A fixed-sized array of [T]s.
class Array<T extends NativeType> extends NativeType {
  /// Const constructor to specify [Array] dimensions in [Struct]s.
  ///
  /// ```
  /// class MyStruct extends Struct {
  ///   @Array(8)
  ///   external Array<Uint8> inlineArray;
  ///
  ///   @Array(2, 2, 2)
  ///   external Array<Array<Array<Uint8>>> threeDimensionalInlineArray;
  /// }
  /// ```
  ///
  /// Do not invoke in normal code.
  external const factory Array(int dimension1,
      [int dimension2, int dimension3, int dimension4, int dimension5]);

  /// Const constructor to specify [Array] dimensions in [Struct]s.
  ///
  /// ```
  /// class MyStruct extends Struct {
  ///   @Array.multi([2, 2, 2])
  ///   external Array<Array<Array<Uint8>>> threeDimensionalInlineArray;
  ///
  ///   @Array.multi([2, 2, 2, 2, 2, 2, 2, 2])
  ///   external Array<Array<Array<Array<Array<Array<Array<Array<Uint8>>>>>>>> eightDimensionalInlineArray;
  /// }
  /// ```
  ///
  /// Do not invoke in normal code.
  external const factory Array.multi(List<int> dimensions);
}

[lrhn]

Shouldn't

 ///   @Array.multi([2, 2, 2])
 ///   external Array<Uint8> threeDimensionalInlineArray;

be

  ///   @Array.multi([2, 2, 2])
  ///   external Array<Array<Array<Uint8>>> threeDimensionalInlineArray;

and similarly for the other examples (good luck with fitting the eight-dimensional case on one line).

[dcharkes]

Good catch, updated!

[ishomam]

What if the size of the multi-dimensional arrays will be only available during runtime (during c++ function execution)?
Is there a way to handle this?

[dcharkes]

@ishomam that would be a Pointer.