Memory management

This section explains how pocl supports multiple address spaces and host-side memory management of device memory.

Multiple logical address spaces

By default, Clang (at least version 3.3 and older) converts the OpenCL C address space qualifiers to target specific address space identifiers. That is, e.g., for the common CPU targets with single uniform address space, all of the OpenCL address spaces are mapped to the address space identifier 0 (the default C address space). For multiple address space LLVM backends such as AMD GPUs there are different ids produced for the OpenCL C address spaces, but they differ from those of the TCE backend, etc. Thus, after the Clang processing of the kernel source, the information of the original OpenCL C address spaces is lost or is target specific, preventing or complicating the special treatment of the pointers pointing to (logically) different address spaces (e.g. OpenCL disjoint address space alias analysis, see Other OpenCL-specific optimizations).

pocl’s kernel compiler needs to know the original logical address spaces in the kernel during some of its processing steps. In order to unify these parts of the kernel compiler, pocl uses the “fake address space map” mechanism of Clang to force pocl-known separate ids to be produced for each of the OpenCL C logical address spaces in the frontend.

Before the code generation, the forced OpenCL C logical address space ids should be mapped to the backend understood ones. This can be done in the kernel compiler pass TargetAddressSpaces. It goes through all the memory references in the bitcode and maps their address space ids to the target specific ones. In case it is known that the targeted backend either understands the logical address space ids (or simply maps everything to 0 there aswell), this processing is skipped (and left for the backend).

Managing the device memories

When a buffer is allocated on the device, the device layer implementation is responsible for making sure the device has enough free space on the memory the given address space is mapped to and for returning a handle for later referring to that memory.

When all the memories are mapped to a single address space shared with the host memory (the case with CPU host+device setups), one could simply use malloc() for this. However, for the heterogeneous device setup where the device has separate memories, one cannot use the host’s malloc function for managing the memory spaces. For this, pocl implements a simple memory allocator called bufalloc. With bufalloc it is possible to manage chunks of memory allocated from a region of addresses. The allocator is optimized for speed and to minimize fragmentation assuming largish chunks of memory (the input/output buffers) are allocated and freed at once.

Bufalloc can be used for host-side management of continuous ranges of memories on the device side. Bufalloc is used for managing the memory also in the pthread/basic CPU device implementations for testing and optimization purposes. For an example of its use for managing memory in the heterogeneous separated memory setup, one should take a look at the TCE device layer code (lib/CL/devices/tce/tce_common.cc). For TCE devices it is assumed there are actual separated physical address spaces for both the local and global address spaces. The device layer implementation manages allocations from both of these spaces using two instances of bufalloc memory regions.

When passing buffer pointers to the kernel/work-group launchers, the memory addresses are passed as integer values. The values passed from the host are casted to the actual address-space qualified LLVM IR pointers for calling the kernels with correct types by the work-group function (see Creating the work-group function launchers).

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