I have created a buffer with a pinned host memory buffer BUFFER using the CL_MEM_USE_HOST_PTR flag.
When I map the buffer, can I assume that the returned pointer will always equal BUFFER ?
Yes. As AMD OpenCL Optimization guide says:
22.214.171.124 Copy Memory Objects:
For CL_MEM_USE_HOST_PTR and CL_MEM_ALLOC_HOST_PTR the same map location is used for all maps; thus, the pointer returned is always in the same memory area.
Also you can refer "OpenCL Memory Object Properties - Table 2 " in section "1.3.2 Placement"