device_ptr

rocThrust Device Pointer Example

Description

This simple program showcases the usage of the thrust::device_ptr template.

Application flow

1. A thrust::device_ptr<int> is instantiated, and memory for ten elements is allocated.
2. Two more thrust::device_ptr<int> are instantiated and set to the start- and end-point of the allocated memory region.
3. Normal pointer arithmetic is used on the thrust::device_ptr<int>s to calculate the number of elements allocated in step 1.
4. The elements pointed to by thrust::device_ptr<int> are initialized using thrust::sequence. The values of the elements are printed to the standard output.
5. The first three elements are modified directly in host-code with thrust::device_ptr::operator[] and all elements are printed to the standard output.
6. The raw pointer to the device memory is obtained by calling thrust::device_pointer_cast
7. The raw pointer is wrapped back to a thrust::device_ptr<int>. It is asserted that the original and the wrapped thrust::device_ptr<int> are equal.
8. The sum of the wrapped pointer is calculated with thrust::reduce and printed to the standard output.
9. The device memory is freed using thrust::device_free.

Key APIs and Concepts

• Thrust's device_ptr is a simple and transparent way of handling device memory the same way one would handle host memory with normal pointers.
• Unlike a normal pointer to device memory device_ptr adds type safety, and the underlying device memory is transparently accessible on the host.
• The device_ptr can be used in Thrust algorithms like a normal pointer to device memory.
• The "raw" normal pointer to the device memory for usage in kernels or other APIs can be obtained from a device_ptr by using thrust::raw_pointer_cast.
• device_ptr is not a smart pointer. Allocating and freeing memory lies in the responsibility of the programmer.

Demonstrated API Calls

rocThrust

• thrust::device_ptr<T>::operator=
• thrust::device_ptr<T>::operator[]
• thrust::device_malloc<T>
• thrust::sequence
• thrust::raw_pointer_cast
• thrust::device_pointer_cast
• thrust::reduce
• thrust::device_free