Proposal

MarkDirtyResource on a buffer if contents is called and the returned pointer is not NULL (type should be MTLStorageModeShared).
MarkDirtyResource on a buffer if didModifyRange is called (type should be MTLStorageModeManaged).
MarkDirtyResource on a buffer if buffer type is MTLStorageModePrivate.
MTLBuffer type is only known at creation time.

Questions

How do dirty resources interact with frame referenced resources. Will dirty resources always be output or is it only if they are frame referenced also?
- TODO

Metal Documentation: Choosing a Resource Storage Mode in macOS
MTLBuffer Storage Modes
- MTLStorageModeShared : Stored in CPU memory accessible by the GPU.
  - Application manages synchronisation:
  - CPU->GPU: MTLCommandBuffer::commit
  - GPU->CPU: when command buffer completes execution ie. MTLCommandBuffer::status == MTLCommandBufferStatusCompleted
- MTLStorageModeManaged : Stored in CPU and GPU memory.
  - Metal driver manages synchronisation:
  - CPU->GPU: MTLBuffer::didModifyRange
  - GPU->CPU: MTLBlitCommandEncoder::synchronizeResource
- MTLStorageModePrivate : Stored in GPU memory.
  - Only accessible by copying to a MTLStorageModeShared buffer.
MTLBuffer CPU contents locked / submitted to GPU on MTLCommandBuffer:: commit.
RenderCommandEncoder has APIs to use MTLBuffer ie. setVertexBuffer, setFragmentBuffer.

To read buffer data will need to convert MTLStorageModePrivate buffers to MTLStorageModeManaged or use a temporary intermediate MTLStorageModeManaged buffer.
Mark buffer dirty if contents is called and returned pointer is not NULL : type should be MTLStorageModeShared
Mark buffer dirty if didModifyRange is called, type should be MTLStorageModeManaged
Mark buffer dirty if buffer type is MTLStorageModePrivate ie. GPU only
Should MTLStorageModeManaged buffers only be tracked if contents is called on them?
- In theory yes if the returned pointer is not NULL.
- In practice might be easier to snashot the buffer without explicit tracking.
Should buffers only be tracked if they can reside in CPU memory?
- GPU only buffers do not need to be tracked for CPU modification. They need to be tracked for GPU modification which is hard to do for all situations.
  - In practice easier to snashot the buffer without explicit tracking.
- CPU buffers with explicit synchronization can be handled in the serialization of the synchronization APIs
  - In practice might be easier to snashot the buffer without explicit tracking.
Attach the buffers to the MTLRenderCommandEncoder parent MTLCommandBuffer in setVertexBuffer, setFragmentBuffer APIs.
On MTLCommandBuffer::commit in active capture serialize attached buffer contents.
On MTLCommandBuffer::commit replay de-serialize buffer contents.