Transfer tensors between devices.

src/cpu_backend.rs

@@ -4,6 +4,8 @@ use gemm::{gemm, Parallelism};
 
 // TODO: Think about whether we would be better off with a dtype and
 // a buffer as an owned slice of bytes.
+// TODO: Maybe we should not implement [Clone] here and instead have an explicit allocator +
+// intercept the oom errors to avoid panicking and provide a proper error.
 #[derive(Debug, Clone)]
 pub enum CpuStorage {
     F32(Vec<f32>),

src/cuda_backend.rs

@@ -28,8 +28,22 @@ pub enum CudaError {
 
 type Result<T> = std::result::Result<T, CudaError>;
 
+/// Unique identifier for cuda devices.
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
+pub(crate) struct DeviceId(usize);
+
+impl DeviceId {
+    fn new() -> Self {
+        // https://users.rust-lang.org/t/idiomatic-rust-way-to-generate-unique-id/33805
+        use std::sync::atomic;
+        static COUNTER: atomic::AtomicUsize = atomic::AtomicUsize::new(1);
+        Self(COUNTER.fetch_add(1, atomic::Ordering::Relaxed))
+    }
+}
+
 #[derive(Debug, Clone)]
 pub struct CudaDevice {
+    id: DeviceId,
     device: Arc<cudarc::driver::CudaDevice>,
     #[allow(dead_code)]
     blas: Arc<cudarc::cublas::CudaBlas>,
@@ -48,11 +62,16 @@ impl CudaDevice {
         let device = cudarc::driver::CudaDevice::new(ordinal)?;
         let blas = cudarc::cublas::CudaBlas::new(device.clone())?;
         Ok(Self {
+            id: DeviceId::new(),
             device,
             blas: Arc::new(blas),
         })
     }
 
+    pub(crate) fn same_id(&self, rhs: &Self) -> bool {
+        self.id == rhs.id
+    }
+
     pub(crate) fn ordinal(&self) -> usize {
         self.device.ordinal()
     }

src/device.rs

@@ -66,6 +66,14 @@ impl Device {
         Ok(Self::Cuda(crate::CudaDevice::new(ordinal)?))
     }
 
+    pub fn same_id(&self, rhs: &Self) -> bool {
+        match (self, rhs) {
+            (Self::Cpu, Self::Cpu) => true,
+            (Self::Cuda(lhs), Self::Cuda(rhs)) => lhs.same_id(rhs),
+            _ => false,
+        }
+    }
+
     pub fn location(&self) -> DeviceLocation {
         match self {
             Self::Cpu => DeviceLocation::Cpu,

src/dummy_cuda_backend.rs

@@ -17,6 +17,10 @@ impl CudaDevice {
         Err(Error::NotCompiledWithCudaSupport)
     }
 
+    pub(crate) fn same_id(&self, _: &Self) -> bool {
+        true
+    }
+
     pub(crate) fn ordinal(&self) -> usize {
         fail!()
     }

src/tensor.rs

@@ -504,6 +504,36 @@ impl Tensor {
         Ok(Tensor(Arc::new(tensor_)))
     }
 
+    /// If the target device is the same as the tensor device, only a shallow copy is performed.
+    pub fn to_device(&self, device: &Device) -> Result<Tensor> {
+        if self.device().same_id(device) {
+            Ok(self.clone())
+        } else {
+            let storage = match (&self.storage, device) {
+                (Storage::Cpu(storage), Device::Cuda(cuda)) => {
+                    Storage::Cuda(cuda.cuda_from_cpu_storage(storage)?)
+                }
+                (Storage::Cuda(storage), Device::Cpu) => Storage::Cpu(storage.to_cpu_storage()?),
+                (Storage::Cuda(storage), Device::Cuda(cuda)) => {
+                    // TODO: Avoid passing through the cpu storage here, especially if the gpu ids
+                    // are the same.
+                    let cpu_storage = storage.to_cpu_storage()?;
+                    Storage::Cuda(cuda.cuda_from_cpu_storage(&cpu_storage)?)
+                }
+                (Storage::Cpu(storage), Device::Cpu) => Storage::Cpu(storage.clone()),
+            };
+            let tensor_ = Tensor_ {
+                id: TensorId::new(),
+                storage,
+                shape: self.shape.clone(),
+                stride: self.stride.clone(),
+                op: None, // TODO: Have a proper op here.
+                is_variable: self.is_variable,
+            };
+            Ok(Tensor(Arc::new(tensor_)))
+        }
+    }
+
     /// Return all the nodes that lead to this value in a topologically sorted vec, the first
     /// elements having dependencies on the latter ones, e.g. the first element if any is the
     /// argument.