Index: streamexecutor/include/streamexecutor/DeviceMemory.h
===================================================================
--- streamexecutor/include/streamexecutor/DeviceMemory.h
+++ streamexecutor/include/streamexecutor/DeviceMemory.h
@@ -18,9 +18,9 @@
 /// and a byte count to tell how much memory is pointed to by that void*.
 ///
 /// GlobalDeviceMemory<T> is a subclass of GlobalDeviceMemoryBase which keeps
-/// track of the type of element to be stored in the device array. It is similar
-/// to a pair of a T* pointer and an element count to tell how many elements of
-/// type T fit in the memory pointed to by that T*.
+/// track of the type of element to be stored in the device memory. It is
+/// similar to a pair of a T* pointer and an element count to tell how many
+/// elements of type T fit in the memory pointed to by that T*.
 ///
 /// SharedDeviceMemoryBase is just the size in bytes of a shared memory buffer.
 ///
@@ -38,6 +38,7 @@
 #ifndef STREAMEXECUTOR_DEVICEMEMORY_H
 #define STREAMEXECUTOR_DEVICEMEMORY_H
 
+#include <cassert>
 #include <cstddef>
 
 namespace streamexecutor {
@@ -91,6 +92,71 @@
   size_t ByteCount;   // Size in bytes of this allocation.
 };
 
+template <typename ElemT> class GlobalDeviceMemory;
+
+/// Reference to a slice of device memory.
+///
+/// Contains a base memory handle, an element count offset into that base
+/// memory, and an element count for the size of the slice.
+template <typename ElemT> class GlobalDeviceMemorySlice {
+public:
+  /// Intentionally implicit so GlobalDeviceMemory<T> can be passed to functions
+  /// expecting GlobalDeviceMemorySlice<T> arguments.
+  GlobalDeviceMemorySlice(const GlobalDeviceMemory<ElemT> &Memory)
+      : BaseMemory(Memory), ElementOffset(0),
+        ElementCount(Memory.getElementCount()) {}
+
+  GlobalDeviceMemorySlice(const GlobalDeviceMemory<ElemT> &BaseMemory,
+                          size_t ElementOffset, size_t ElementCount)
+      : BaseMemory(BaseMemory), ElementOffset(ElementOffset),
+        ElementCount(ElementCount) {
+    assert(ElementOffset + ElementCount <= BaseMemory.getElementCount() &&
+           "slicing past the end of a GlobalDeviceMemory buffer");
+  }
+
+  /// Gets the GlobalDeviceMemory backing this slice.
+  GlobalDeviceMemory<ElemT> getBaseMemory() const { return BaseMemory; }
+
+  /// Gets the offset of this slice from the base memory.
+  ///
+  /// The offset is measured in elements, not bytes.
+  size_t getElementOffset() const { return ElementOffset; }
+
+  /// Gets the number of elements in this slice.
+  size_t getElementCount() const { return ElementCount; }
+
+  /// Creates a slice of the memory with the first DropCount elements removed.
+  GlobalDeviceMemorySlice<ElemT> drop_front(size_t DropCount) const {
+    assert(DropCount <= ElementCount &&
+           "dropping more than the size of a slice");
+    return GlobalDeviceMemorySlice<ElemT>(BaseMemory, ElementOffset + DropCount,
+                                          ElementCount - DropCount);
+  }
+
+  /// Creates a slice of the memory with the last DropCount elements removed.
+  GlobalDeviceMemorySlice<ElemT> drop_back(size_t DropCount) const {
+    assert(DropCount <= ElementCount &&
+           "dropping more than the size of a slice");
+    return GlobalDeviceMemorySlice<ElemT>(BaseMemory, ElementOffset,
+                                          ElementCount - DropCount);
+  }
+
+  /// Creates a slice of the memory that chops off the first DropCount elements
+  /// and keeps the next TakeCount elements.
+  GlobalDeviceMemorySlice<ElemT> slice(size_t DropCount,
+                                       size_t TakeCount) const {
+    assert(DropCount + TakeCount <= ElementCount &&
+           "sub-slice operation overruns slice bounds");
+    return GlobalDeviceMemorySlice<ElemT>(BaseMemory, ElementOffset + DropCount,
+                                          TakeCount);
+  }
+
+private:
+  GlobalDeviceMemory<ElemT> BaseMemory;
+  size_t ElementOffset;
+  size_t ElementCount;
+};
+
 /// Typed wrapper around the "void *"-like GlobalDeviceMemoryBase class.
 ///
 /// For example, GlobalDeviceMemory<int> is a simple wrapper around
@@ -125,6 +191,11 @@
   /// allocation.
   size_t getElementCount() const { return getByteCount() / sizeof(ElemT); }
 
+  /// Converts this memory object into a slice.
+  GlobalDeviceMemorySlice<ElemT> asSlice() {
+    return GlobalDeviceMemorySlice<ElemT>(*this);
+  }
+
 private:
   /// Constructs a GlobalDeviceMemory instance from an opaque handle and an
   /// element count.
Index: streamexecutor/include/streamexecutor/Executor.h
===================================================================
--- streamexecutor/include/streamexecutor/Executor.h
+++ streamexecutor/include/streamexecutor/Executor.h
@@ -16,12 +16,12 @@
 #define STREAMEXECUTOR_EXECUTOR_H
 
 #include "streamexecutor/KernelSpec.h"
+#include "streamexecutor/PlatformInterfaces.h"
 #include "streamexecutor/Utils/Error.h"
 
 namespace streamexecutor {
 
 class KernelInterface;
-class PlatformExecutor;
 class Stream;
 
 class Executor {
@@ -38,6 +38,144 @@
 
   Expected<std::unique_ptr<Stream>> createStream();
 
+  /// Allocates an array of ElementCount entries of type T in device memory.
+  template <typename T>
+  Expected<GlobalDeviceMemory<T>> allocateDeviceMemory(size_t ElementCount) {
+    return PExecutor->allocateDeviceMemory(ElementCount * sizeof(T));
+  }
+
+  /// Frees memory previously allocated with allocateDeviceMemory.
+  template <typename T> Error freeDeviceMemory(GlobalDeviceMemory<T> *Memory) {
+    return PExecutor->freeDeviceMemory(Memory);
+  }
+
+  /// Allocates an array of ElementCount entries of type T in host memory.
+  ///
+  /// Host memory allocated by this function can be used for asynchronous memory
+  /// copies on streams. See Stream::thenMemcpyD2H and Stream::thenMemcpyH2D.
+  template <typename T> Expected<T *> allocateHostMemory(size_t ElementCount) {
+    return PExecutor->allocateHostMemory(ElementCount * sizeof(T));
+  }
+
+  /// Frees memory previously allocated with allocateHostMemory.
+  template <typename T> Error freeHostMemory(T *Memory) {
+    return PExecutor->freeHostMemory(Memory);
+  }
+
+  /// Registers a previously allocated host array of type T for asynchronous
+  /// memory operations.
+  ///
+  /// Host memory registered by this function can be used for asynchronous
+  /// memory copies on streams. See Stream::thenMemcpyD2H and
+  /// Stream::thenMemcpyH2D.
+  template <typename T>
+  Error registerHostMemory(T *Memory, size_t ElementCount) {
+    return PExecutor->registerHostMemory(Memory, ElementCount * sizeof(T));
+  }
+
+  /// Unregisters host memory previously registered by registerHostMemory.
+  template <typename T> Error unregisterHostMemory(T *Memory) {
+    return PExecutor->unregisterHostMemory(Memory);
+  }
+
+  /// Host-synchronously copies a slice of an array of elements of type T from
+  /// host to device memory.
+  ///
+  /// Returns an error if ElementCount is too large for the source slice or the
+  /// destination.
+  ///
+  /// The calling host thread is blocked until the copy completes. Can be used
+  /// with any host memory, the host memory does not have to be allocated with
+  /// allocateHostMemory or registered with registerHostMemory.
+  template <typename T>
+  Error synchronousCopyD2H(GlobalDeviceMemorySlice<T> Src,
+                           llvm::MutableArrayRef<T> Dst, size_t ElementCount) {
+    if (ElementCount > Src.getElementCount())
+      return make_error("copying too many elements, " +
+                        llvm::Twine(ElementCount) +
+                        ", from a device array sloce of element count " +
+                        llvm::Twine(Src.getElementCount()));
+    if (Dst.size() < ElementCount)
+      return make_error(
+          "copying too many elements, " + llvm::Twine(ElementCount) +
+          ", to a host array of element count " + llvm::Twine(Dst.size()));
+    return PExecutor->synchronousCopyD2H(
+        Src.getBaseMemory(), Src.getElementOffset() * sizeof(T), Dst.data(), 0,
+        ElementCount * sizeof(T));
+  }
+
+  /// Host-synchronously copies a slice of an array of elements of type T from
+  /// host to device memory.
+  ///
+  /// Returns an error if the Src and Dst sizes do not match.
+  ///
+  /// The calling host thread is blocked until the copy completes. Can be used
+  /// with any host memory, the host memory does not have to be allocated with
+  /// allocateHostMemory or registered with registerHostMemory.
+  template <typename T>
+  Error synchronousCopyD2H(GlobalDeviceMemorySlice<T> Src,
+                           llvm::MutableArrayRef<T> Dst) {
+    return synchronousCopyD2H(Src, Dst, Src.getElementCount());
+  }
+
+  /// Host-synchronously copies a slice of an array of elements of type T from
+  /// host to device memory.
+  ///
+  /// Returns an error if ElementCount is too large for the source slice.
+  ///
+  /// The calling host thread is blocked until the copy completes. Can be used
+  /// with any host memory, the host memory does not have to be allocated with
+  /// allocateHostMemory or registered with registerHostMemory.
+  template <typename T>
+  Error synchronousCopyD2H(GlobalDeviceMemorySlice<T> Src, T *Dst,
+                           size_t ElementCount) {
+    return synchronousCopyD2H(Src, llvm::MutableArrayRef<T>(Dst, ElementCount),
+                              ElementCount);
+  }
+
+  /// Host-synchronously copies an array of elements of type T from host to
+  /// device memory.
+  ///
+  /// Returns an error if ElementCount is too large for the source slice or the
+  /// destination.
+  ///
+  /// The calling host thread is blocked until the copy completes. Can be used
+  /// with any host memory, the host memory does not have to be allocated with
+  /// allocateHostMemory or registered with registerHostMemory.
+  template <typename T>
+  Error synchronousCopyD2H(GlobalDeviceMemory<T> Src,
+                           llvm::MutableArrayRef<T> Dst, size_t ElementCount) {
+    return synchronousCopyD2H(Src.asSlice(), Dst, ElementCount);
+  }
+
+  /// Host-synchronously copies an array of elements of type T from host to
+  /// device memory.
+  ///
+  /// Returns an error if the Src and Dst sizes do not match.
+  ///
+  /// The calling host thread is blocked until the copy completes. Can be used
+  /// with any host memory, the host memory does not have to be allocated with
+  /// allocateHostMemory or registered with registerHostMemory.
+  template <typename T>
+  Error synchronousCopyD2H(GlobalDeviceMemory<T> Src,
+                           llvm::MutableArrayRef<T> Dst) {
+    return synchronousCopyD2H(Src.asSlice(), Dst);
+  }
+
+  /// Host-synchronously copies an array of elements of type T from host to
+  /// device memory.
+  ///
+  /// Returns an error if ElementCount is too large for the source slice.
+  ///
+  /// The calling host thread is blocked until the copy completes. Can be used
+  /// with any host memory, the host memory does not have to be allocated with
+  /// allocateHostMemory or registered with registerHostMemory.
+  template <typename T>
+  Error synchronousCopyD2H(GlobalDeviceMemory<T> Src, T *Dst,
+                           size_t ElementCount) {
+    return synchronousCopyD2H(Src.asSlice(), Dst, ElementCount);
+  }
+
 private:
   PlatformExecutor *PExecutor;
 };
Index: streamexecutor/include/streamexecutor/PlatformInterfaces.h
===================================================================
--- streamexecutor/include/streamexecutor/PlatformInterfaces.h
+++ streamexecutor/include/streamexecutor/PlatformInterfaces.h
@@ -76,22 +76,32 @@
   }
 
   /// Copies data from the device to the host.
-  virtual Error memcpyD2H(PlatformStreamHandle *S,
+  ///
+  /// HostDst should have been allocated by allocateHostMemory or registered
+  /// with registerHostMemory.
+  virtual Error memcpyD2H(PlatformStreamHandle *S, void *HostDst,
                           const GlobalDeviceMemoryBase &DeviceSrc,
-                          void *HostDst, size_t ByteCount) {
+                          size_t ByteCount, size_t SrcByteOffset = 0) {
     return make_error("memcpyD2H not implemented for platform " + getName());
   }
 
   /// Copies data from the host to the device.
-  virtual Error memcpyH2D(PlatformStreamHandle *S, const void *HostSrc,
-                          GlobalDeviceMemoryBase *DeviceDst, size_t ByteCount) {
+  ///
+  /// HostSrc should have been allocated by allocateHostMemory or registered
+  /// with registerHostMemory.
+  virtual Error memcpyH2D(PlatformStreamHandle *S,
+                          GlobalDeviceMemoryBase *DeviceDst,
+                          const void *HostSrc, size_t ByteCount,
+                          size_t DstByteOffset = 0) {
     return make_error("memcpyH2D not implemented for platform " + getName());
   }
 
   /// Copies data from one device location to another.
   virtual Error memcpyD2D(PlatformStreamHandle *S,
+                          GlobalDeviceMemoryBase *DeviceDst,
                           const GlobalDeviceMemoryBase &DeviceSrc,
-                          GlobalDeviceMemoryBase *DeviceDst, size_t ByteCount) {
+                          size_t ByteCount, size_t SrcByteOffset = 0,
+                          size_t DstByteOffset = 0) {
     return make_error("memcpyD2D not implemented for platform " + getName());
   }
 
@@ -101,6 +111,81 @@
     return make_error("blockHostUntilDone not implemented for platform " +
                       getName());
   }
+
+  /// Allocates untyped device memory of a given size in bytes.
+  virtual Expected<GlobalDeviceMemoryBase>
+  allocateDeviceMemory(size_t ByteCount) {
+    return make_error("allocateDeviceMemory not implemented for platform " +
+                      getName());
+  }
+
+  /// Frees device memory previously allocated by allocateDeviceMemory.
+  virtual Error freeDeviceMemory(GlobalDeviceMemoryBase *Memory) {
+    return make_error("freeDeviceMemory not implemented for platform " +
+                      getName());
+  }
+
+  /// Allocates untyped host memory of a given size in bytes.
+  ///
+  /// Host memory allocated via this method is suitable for use with memcpyH2D
+  /// and memcpyD2H.
+  virtual Expected<void *> allocateHostMemory(size_t ByteCount) {
+    return make_error("allocateHostMemory not implemented for platform " +
+                      getName());
+  }
+
+  /// Frees host memory allocated by allocateHostMemory.
+  virtual Error freeHostMemory(void *Memory) {
+    return make_error("freeHostMemory not implemented for platform " +
+                      getName());
+  }
+
+  /// Registers previously allocated host memory so it can be used with
+  /// memcpyH2D and memcpyD2H.
+  virtual Error registerHostMemory(void *Memory, size_t ByteCount) {
+    return make_error("registerHostMemory not implemented for platform " +
+                      getName());
+  }
+
+  /// Unregisters host memory previously registered with registerHostMemory.
+  virtual Error unregisterHostMemory(void *Memory) {
+    return make_error("unregisterHostMemory not implemented for platform " +
+                      getName());
+  }
+
+  /// Copies the given number of bytes from device memory to host memory.
+  ///
+  /// Blocks the calling host thread until the copy is completed. Can operate on
+  /// any host memory, not just registered host memory or host memory allocated
+  /// by allocateHostMemory.
+  virtual Error synchronousCopyD2H(const GlobalDeviceMemoryBase &DeviceSrc,
+                                   size_t SrcByteOffset, void *HostDst,
+                                   size_t DstByteOffset, size_t ByteCount) {
+    return make_error("synchronousCopyD2H not implemented for platform " +
+                      getName());
+  }
+
+  /// Copies the given number of bytes from host memory to device memory.
+  ///
+  /// Blocks the calling host thread until the copy is completed. Can operate on
+  /// any host memory, not just registered host memory or host memory allocated
+  /// by allocateHostMemory.
+  virtual Error synchronousCopyH2D(const void *HostSrc, size_t SrcByteOffset,
+                                   GlobalDeviceMemoryBase DeviceDst,
+                                   size_t DstByteOffset, size_t ByteCount) {
+    return make_error("synchronousCopyH2D not implemented for platform " +
+                      getName());
+  }
+
+  /// Copies the given number of bytes from one location to another in device
+  /// memory.
+  virtual Error synchronousCopyD2D(GlobalDeviceMemoryBase DeviceDst,
+                                   size_t DstByteOffset,
+                                   const GlobalDeviceMemoryBase &DeviceSrc,
+                                   size_t SrcByteOffset, size_t ByteCount) {
+    return make_error("synchronousCopyD2D not implemented for platform " +
+                      getName());
+  }
 };
 
 } // namespace streamexecutor
Index: streamexecutor/include/streamexecutor/Stream.h
===================================================================
--- streamexecutor/include/streamexecutor/Stream.h
+++ streamexecutor/include/streamexecutor/Stream.h
@@ -106,27 +106,38 @@
   /// Executor::allocateHostMemory or otherwise allocated and then
   /// registered with Executor::registerHostMemory.
   template <typename T>
-  Stream &thenMemcpyD2H(const GlobalDeviceMemory<T> &DeviceSrc,
-                        llvm::MutableArrayRef<T> HostDst, size_t ElementCount) {
-    if (ElementCount > DeviceSrc.getElementCount())
+  Stream &thenMemcpyD2H(llvm::MutableArrayRef<T> HostDst,
+                        const GlobalDeviceMemory<T> &DeviceSrc,
+                        size_t ElementCount, size_t SrcElementOffset = 0) {
+    if (ElementCount + SrcElementOffset > DeviceSrc.getElementCount())
       setError("copying too many elements, " + llvm::Twine(ElementCount) +
-               ", from device memory array of size " +
+               ", at element offset " + llvm::Twine(SrcElementOffset) +
+               " from device memory array of element count " +
                llvm::Twine(DeviceSrc.getElementCount()));
     else if (ElementCount > HostDst.size())
       setError("copying too many elements, " + llvm::Twine(ElementCount) +
-               ", to host array of size " + llvm::Twine(HostDst.size()));
+               ", to host array of element count " +
+               llvm::Twine(HostDst.size()));
     else
-      setError(PExecutor->memcpyD2H(ThePlatformStream.get(), DeviceSrc,
-                                    HostDst.data(), ElementCount * sizeof(T)));
+      setError(PExecutor->memcpyD2H(ThePlatformStream.get(), HostDst.data(),
+                                    DeviceSrc, ElementCount * sizeof(T),
+                                    SrcElementOffset * sizeof(T)));
     return *this;
   }
 
   /// Same as thenMemcpyD2H above, but copies the entire source to the
   /// destination.
   template <typename T>
-  Stream &thenMemcpyD2H(const GlobalDeviceMemory<T> &DeviceSrc,
-                        llvm::MutableArrayRef<T> HostDst) {
-    return thenMemcpyD2H(DeviceSrc, HostDst, DeviceSrc.getElementCount());
+  Stream &thenMemcpyD2H(llvm::MutableArrayRef<T> HostDst,
+                        const GlobalDeviceMemory<T> &DeviceSrc) {
+    if (DeviceSrc.getElementCount() != HostDst.size()) {
+      setError("thenMemcpyD2H device source element count, " +
+               llvm::Twine(DeviceSrc.getElementCount()) +
+               " ,does not match host destination element count, " +
+               llvm::Twine(HostDst.size()));
+      return *this;
+    }
+    return thenMemcpyD2H(HostDst, DeviceSrc, DeviceSrc.getElementCount());
   }
 
   /// Entrain onto the stream a memcpy of a given number of elements from a host
@@ -136,54 +147,78 @@
   /// Executor::allocateHostMemory or otherwise allocated and then
   /// registered with Executor::registerHostMemory.
   template <typename T>
-  Stream &thenMemcpyH2D(llvm::ArrayRef<T> HostSrc,
-                        GlobalDeviceMemory<T> *DeviceDst, size_t ElementCount) {
+  Stream &thenMemcpyH2D(GlobalDeviceMemory<T> *DeviceDst,
+                        llvm::ArrayRef<T> HostSrc, size_t ElementCount,
+                        size_t DstElementOffset = 0) {
     if (ElementCount > HostSrc.size())
       setError("copying too many elements, " + llvm::Twine(ElementCount) +
-               ", from host array of size " + llvm::Twine(HostSrc.size()));
-    else if (ElementCount > DeviceDst->getElementCount())
+               ", from host array of element count " +
+               llvm::Twine(HostSrc.size()));
+    else if (DstElementOffset + ElementCount > DeviceDst->getElementCount())
       setError("copying too many elements, " + llvm::Twine(ElementCount) +
-               ", to device memory array of size " +
-               llvm::Twine(DeviceDst->getElementCount()));
+               ", to device memory array of element count " +
+               llvm::Twine(DeviceDst->getElementCount()) +
+               " at element offset " + llvm::Twine(DstElementOffset));
     else
-      setError(PExecutor->memcpyH2D(ThePlatformStream.get(), HostSrc.data(),
-                                    DeviceDst, ElementCount * sizeof(T)));
+      setError(PExecutor->memcpyH2D(ThePlatformStream.get(), DeviceDst,
+                                    HostSrc.data(), ElementCount * sizeof(T),
+                                    DstElementOffset * sizeof(T)));
     return *this;
   }
 
   /// Same as thenMemcpyH2D above, but copies the entire source to the
   /// destination.
   template <typename T>
-  Stream &thenMemcpyH2D(llvm::ArrayRef<T> HostSrc,
-                        GlobalDeviceMemory<T> *DeviceDst) {
-    return thenMemcpyH2D(HostSrc, DeviceDst, HostSrc.size());
+  Stream &thenMemcpyH2D(GlobalDeviceMemory<T> *DeviceDst,
+                        llvm::ArrayRef<T> HostSrc) {
+    if (HostSrc.size() != DeviceDst->getElementCount()) {
+      setError("thenMemcpyH2D host source element count, " +
+               llvm::Twine(HostSrc.size()) +
+               " ,does not match device destination element count, " +
+               llvm::Twine(DeviceDst->getElementCount()));
+      return *this;
+    }
+    return thenMemcpyH2D(DeviceDst, HostSrc, HostSrc.size());
   }
 
   /// Entrain onto the stream a memcpy of a given number of elements from a
   /// device source to a device destination.
   template <typename T>
-  Stream &thenMemcpyD2D(const GlobalDeviceMemory<T> &DeviceSrc,
-                        GlobalDeviceMemory<T> *DeviceDst, size_t ElementCount) {
-    if (ElementCount > DeviceSrc.getElementCount())
+  Stream &thenMemcpyD2D(GlobalDeviceMemory<T> *DeviceDst,
+                        const GlobalDeviceMemory<T> &DeviceSrc,
+                        size_t ElementCount, size_t SrcElementOffset = 0,
+                        size_t DstElementOffset = 0) {
+    if (SrcElementOffset + ElementCount > DeviceSrc.getElementCount())
       setError("copying too many elements, " + llvm::Twine(ElementCount) +
-               ", from device memory array of size " +
+               ", at element offset " + llvm::Twine(SrcElementOffset) +
+               " from device memory array of element count " +
                llvm::Twine(DeviceSrc.getElementCount()));
-    else if (ElementCount > DeviceDst->getElementCount())
+    else if (DstElementOffset + ElementCount > DeviceDst->getElementCount())
       setError("copying too many elements, " + llvm::Twine(ElementCount) +
-               ", to device memory array of size " +
-               llvm::Twine(DeviceDst->getElementCount()));
+               ", to device memory array of element count " +
+               llvm::Twine(DeviceDst->getElementCount()) +
+               " at element offset " + llvm::Twine(DstElementOffset));
     else
-      setError(PExecutor->memcpyD2D(ThePlatformStream.get(), DeviceSrc,
-                                    DeviceDst, ElementCount * sizeof(T)));
+      setError(PExecutor->memcpyD2D(ThePlatformStream.get(), DeviceDst,
+                                    DeviceSrc, ElementCount * sizeof(T),
+                                    SrcElementOffset * sizeof(T),
+                                    DstElementOffset * sizeof(T)));
     return *this;
   }
 
   /// Same as thenMemcpyD2D above, but copies the entire source to the
   /// destination.
   template <typename T>
-  Stream &thenMemcpyD2D(const GlobalDeviceMemory<T> &DeviceSrc,
-                        GlobalDeviceMemory<T> *DeviceDst) {
-    return thenMemcpyD2D(DeviceSrc, DeviceDst, DeviceSrc.getElementCount());
+  Stream &thenMemcpyD2D(GlobalDeviceMemory<T> *DeviceDst,
+                        const GlobalDeviceMemory<T> &DeviceSrc) {
+    if (DeviceSrc.getElementCount() != DeviceDst->getElementCount()) {
+      setError("thenMemcpyH2D device source element count, " +
+               llvm::Twine(DeviceSrc.getElementCount()) +
+               " ,does not match device destination element count, " +
+               llvm::Twine(DeviceDst->getElementCount()));
+      return *this;
+    }
+    return thenMemcpyD2D(DeviceDst, DeviceSrc, DeviceSrc.getElementCount());
   }
 
   /// Blocks the host code, waiting for the operations entrained on the stream
Index: streamexecutor/include/streamexecutor/Utils/Error.h
===================================================================
--- streamexecutor/include/streamexecutor/Utils/Error.h
+++ streamexecutor/include/streamexecutor/Utils/Error.h
@@ -169,10 +169,10 @@
 using llvm::consumeError;
 using llvm::Error;
 using llvm::Expected;
-using llvm::StringRef;
+using llvm::Twine;
 
 // Makes an Error object from an error message.
-Error make_error(StringRef Message);
+Error make_error(Twine Message);
 
 // Consumes the input error and returns its error message.
 //
Index: streamexecutor/lib/Utils/Error.cpp
===================================================================
--- streamexecutor/lib/Utils/Error.cpp
+++ streamexecutor/lib/Utils/Error.cpp
@@ -44,8 +44,8 @@
 
 namespace streamexecutor {
 
-Error make_error(StringRef Message) {
-  return llvm::make_error<StreamExecutorError>(Message);
+Error make_error(Twine Message) {
+  return llvm::make_error<StreamExecutorError>(Message.str());
 }
 
 std::string consumeAndGetMessage(Error &&E) {
Index: streamexecutor/lib/unittests/CMakeLists.txt
===================================================================
--- streamexecutor/lib/unittests/CMakeLists.txt
+++ streamexecutor/lib/unittests/CMakeLists.txt
@@ -1,4 +1,14 @@
 add_executable(
+    executor_test
+    ExecutorTest.cpp)
+target_link_libraries(
+    executor_test
+    streamexecutor
+    ${GTEST_BOTH_LIBRARIES}
+    ${CMAKE_THREAD_LIBS_INIT})
+add_test(ExecutorTest executor_test)
+
+add_executable(
     kernel_test
     KernelTest.cpp)
 target_link_libraries(
Index: streamexecutor/lib/unittests/ExecutorTest.cpp
===================================================================
--- /dev/null
+++ streamexecutor/lib/unittests/ExecutorTest.cpp
@@ -0,0 +1,177 @@
+//===-- ExecutorTest.cpp - Tests for Executor -----------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+///
+/// \file
+/// This file contains the unit tests for Executor code.
+///
+//===----------------------------------------------------------------------===//
+
+#include <cstdlib>
+#include <cstring>
+
+#include "streamexecutor/Executor.h"
+#include "streamexecutor/PlatformInterfaces.h"
+
+#include "gtest/gtest.h"
+
+namespace {
+
+namespace se = ::streamexecutor;
+
+class MockPlatformExecutor : public se::PlatformExecutor {
+public:
+  ~MockPlatformExecutor() override {}
+
+  std::string getName() const override { return "MockPlatformExecutor"; }
+
+  se::Expected<std::unique_ptr<se::PlatformStreamHandle>>
+  createStream() override {
+    return se::make_error("not implemented");
+  }
+
+  se::Expected<se::GlobalDeviceMemoryBase>
+  allocateDeviceMemory(size_t ByteCount) override {
+    return se::GlobalDeviceMemoryBase(std::malloc(ByteCount));
+  }
+
+  se::Error freeDeviceMemory(se::GlobalDeviceMemoryBase *Memory) override {
+    std::free(const_cast<void *>(Memory->getHandle()));
+    return se::Error::success();
+  }
+
+  se::Expected<void *> allocateHostMemory(size_t ByteCount) override {
+    return std::malloc(ByteCount);
+  }
+
+  se::Error freeHostMemory(void *Memory) override {
+    std::free(Memory);
+    return se::Error::success();
+  }
+
+  se::Error synchronousCopyD2H(const se::GlobalDeviceMemoryBase &DeviceSrc,
+                               size_t SrcByteOffset, void *HostDst,
+                               size_t DstByteOffset,
+                               size_t ByteCount) override {
+    std::memcpy(static_cast<char *>(HostDst) + DstByteOffset,
+                static_cast<const char *>(DeviceSrc.getHandle()) +
+                    SrcByteOffset,
+                ByteCount);
+    return se::Error::success();
+  }
+
+  se::Error synchronousCopyH2D(const void *HostSrc, size_t SrcByteOffset,
+                               se::GlobalDeviceMemoryBase DeviceDst,
+                               size_t DstByteOffset,
+                               size_t ByteCount) override {
+    std::memcpy(static_cast<char *>(const_cast<void *>(DeviceDst.getHandle())) +
+                    DstByteOffset,
+                static_cast<const char *>(HostSrc) + SrcByteOffset, ByteCount);
+    return se::Error::success();
+  }
+
+  se::Error synchronousCopyD2D(se::GlobalDeviceMemoryBase DeviceDst,
+                               size_t DstByteOffset,
+                               const se::GlobalDeviceMemoryBase &DeviceSrc,
+                               size_t SrcByteOffset,
+                               size_t ByteCount) override {
+    std::memcpy(static_cast<char *>(const_cast<void *>(DeviceDst.getHandle())) +
+                    DstByteOffset,
+                static_cast<const char *>(DeviceSrc.getHandle()) +
+                    SrcByteOffset,
+                ByteCount);
+    return se::Error::success();
+  }
+};
+
+/// Test fixture to hold objects used by tests.
+class ExecutorTest : public ::testing::Test {
+public:
+  ExecutorTest()
+      : HostA{0, 1, 2, 3, 4},
+        DeviceA(se::GlobalDeviceMemory<int>::makeFromElementCount(HostA, 5)),
+        HostB{5, 6, 7, 8, 9},
+        DeviceB(se::GlobalDeviceMemory<int>::makeFromElementCount(HostB, 5)),
+        Host{10, 11, 12, 13, 14}, Executor(&PExecutor) {}
+
+  // Device memory is backed by host arrays.
+  int HostA[5];
+  se::GlobalDeviceMemory<int> DeviceA;
+  int HostB[5];
+  se::GlobalDeviceMemory<int> DeviceB;
+
+  // Host memory to be used as actual host memory.
+  int Host[5];
+
+  MockPlatformExecutor PExecutor;
+  se::Executor Executor;
+};
+
+TEST_F(ExecutorTest, SyncCopyD2HFullArrayEqualSize) {
+  EXPECT_FALSE(static_cast<bool>(
+      Executor.synchronousCopyD2H(DeviceA, llvm::MutableArrayRef<int>(Host))));
+  for (int i = 0; i < 5; ++i) {
+    EXPECT_EQ(HostA[i], Host[i]);
+  }
+}
+
+TEST_F(ExecutorTest, SyncCopyD2HSliceEqualSize) {
+  EXPECT_FALSE(static_cast<bool>(Executor.synchronousCopyD2H(
+      DeviceA.asSlice().slice(1, 2), llvm::MutableArrayRef<int>(Host + 1, 2))));
+  for (int i = 1; i < 3; ++i) {
+    EXPECT_EQ(HostA[i], Host[i]);
+  }
+}
+
+TEST_F(ExecutorTest, SyncCopyD2HArraySpecifiedSize) {
+  EXPECT_FALSE(static_cast<bool>(Executor.synchronousCopyD2H(
+      DeviceA, llvm::MutableArrayRef<int>(Host), 2)));
+  for (int i = 0; i < 2; ++i) {
+    EXPECT_EQ(HostA[i], Host[i]);
+  }
+}
+
+TEST_F(ExecutorTest, SyncCopyD2HSliceSpecifiedSize) {
+  EXPECT_FALSE(static_cast<bool>(Executor.synchronousCopyD2H(
+      DeviceA.asSlice().slice(0, 2), llvm::MutableArrayRef<int>(Host), 2)));
+  for (int i = 0; i < 2; ++i) {
+    EXPECT_EQ(HostA[i], Host[i]);
+  }
+}
+
+TEST_F(ExecutorTest, SyncCopyD2HArrayToPointerSpecifiedSize) {
+  EXPECT_FALSE(
+      static_cast<bool>(Executor.synchronousCopyD2H(DeviceA, Host, 2)));
+  for (int i = 0; i < 2; ++i) {
+    EXPECT_EQ(HostA[i], Host[i]);
+  }
+}
+
+TEST_F(ExecutorTest, SyncCopyD2HSliceToPointerSpecifiedSize) {
+  EXPECT_FALSE(static_cast<bool>(
+      Executor.synchronousCopyD2H(DeviceA.asSlice().slice(0, 2), Host, 2)));
+  for (int i = 0; i < 2; ++i) {
+    EXPECT_EQ(HostA[i], Host[i]);
+  }
+}
+
+TEST_F(ExecutorTest, SyncCopyD2HArrayTooManyElements) {
+  se::Error E = Executor.synchronousCopyD2H(
+      DeviceA, llvm::MutableArrayRef<int>(Host), 20);
+  EXPECT_TRUE(static_cast<bool>(E));
+  se::consumeError(std::move(E));
+}
+
+TEST_F(ExecutorTest, SyncCopyD2HSliceTooManyElements) {
+  se::Error E = Executor.synchronousCopyD2H(
+      DeviceA.asSlice(), llvm::MutableArrayRef<int>(Host), 20);
+  EXPECT_TRUE(static_cast<bool>(E));
+  se::consumeError(std::move(E));
+}
+
+} // namespace
Index: streamexecutor/lib/unittests/StreamTest.cpp
===================================================================
--- streamexecutor/lib/unittests/StreamTest.cpp
+++ streamexecutor/lib/unittests/StreamTest.cpp
@@ -40,26 +40,36 @@
     return nullptr;
   }
 
-  se::Error memcpyD2H(se::PlatformStreamHandle *,
+  se::Error memcpyD2H(se::PlatformStreamHandle *, void *HostDst,
                       const se::GlobalDeviceMemoryBase &DeviceSrc,
-                      void *HostDst, size_t ByteCount) override {
-    std::memcpy(HostDst, DeviceSrc.getHandle(), ByteCount);
+                      size_t ByteCount, size_t SrcByteOffset) override {
+    std::memcpy(HostDst, static_cast<const char *>(DeviceSrc.getHandle()) +
+                             SrcByteOffset,
+                ByteCount);
     return se::Error::success();
   }
 
-  se::Error memcpyH2D(se::PlatformStreamHandle *, const void *HostSrc,
+  se::Error memcpyH2D(se::PlatformStreamHandle *,
                       se::GlobalDeviceMemoryBase *DeviceDst,
-                      size_t ByteCount) override {
-    std::memcpy(const_cast<void *>(DeviceDst->getHandle()), HostSrc, ByteCount);
+                      const void *HostSrc, size_t ByteCount,
+                      size_t DstByteOffset) override {
+    std::memcpy(
+        static_cast<char *>(const_cast<void *>(DeviceDst->getHandle())) +
+            DstByteOffset,
+        HostSrc, ByteCount);
     return se::Error::success();
   }
 
   se::Error memcpyD2D(se::PlatformStreamHandle *,
-                      const se::GlobalDeviceMemoryBase &DeviceSrc,
                       se::GlobalDeviceMemoryBase *DeviceDst,
-                      size_t ByteCount) override {
-    std::memcpy(const_cast<void *>(DeviceDst->getHandle()),
-                DeviceSrc.getHandle(), ByteCount);
+                      const se::GlobalDeviceMemoryBase &DeviceSrc,
+                      size_t ByteCount, size_t SrcByteOffset,
+                      size_t DstByteOffset) override {
+    std::memcpy(
+        static_cast<char *>(const_cast<void *>(DeviceDst->getHandle())) +
+            SrcByteOffset,
+        static_cast<const char *>(DeviceSrc.getHandle()) + DstByteOffset,
+        ByteCount);
     return se::Error::success();
   }
 };
@@ -87,28 +97,28 @@
 };
 
 TEST_F(StreamTest, MemcpyCorrectSize) {
-  Stream.thenMemcpyH2D(llvm::ArrayRef<int>(Host), &DeviceA);
+  Stream.thenMemcpyH2D(&DeviceA, llvm::ArrayRef<int>(Host));
   EXPECT_TRUE(Stream.isOK());
 
-  Stream.thenMemcpyD2H(DeviceA, llvm::MutableArrayRef<int>(Host));
+  Stream.thenMemcpyD2H(llvm::MutableArrayRef<int>(Host), DeviceA);
   EXPECT_TRUE(Stream.isOK());
 
-  Stream.thenMemcpyD2D(DeviceA, &DeviceB);
+  Stream.thenMemcpyD2D(&DeviceB, DeviceA);
   EXPECT_TRUE(Stream.isOK());
 }
 
 TEST_F(StreamTest, MemcpyH2DTooManyElements) {
-  Stream.thenMemcpyH2D(llvm::ArrayRef<int>(Host), &DeviceA, 20);
+  Stream.thenMemcpyH2D(&DeviceA, llvm::ArrayRef<int>(Host), 20);
   EXPECT_FALSE(Stream.isOK());
 }
 
 TEST_F(StreamTest, MemcpyD2HTooManyElements) {
-  Stream.thenMemcpyD2H(DeviceA, llvm::MutableArrayRef<int>(Host), 20);
+  Stream.thenMemcpyD2H(llvm::MutableArrayRef<int>(Host), DeviceA, 20);
   EXPECT_FALSE(Stream.isOK());
 }
 
 TEST_F(StreamTest, MemcpyD2DTooManyElements) {
-  Stream.thenMemcpyD2D(DeviceA, &DeviceB, 20);
+  Stream.thenMemcpyD2D(&DeviceB, DeviceA, 20);
   EXPECT_FALSE(Stream.isOK());
 }