Question

Is it possible to create a simple HashMap in OpenCL? E.g. one where all keys have type long and all values type int, and that never has to be modified (i.e. is passed read-only to the kernel).

Construction of the HashMap can take time (is it done once on the CPU and never has to be modified again), but read-access will be frequent, so get(long key, *hashmap H) should be cheap.

Are there any known implementations for this in OpenCL? I failed to find them. In case I’d have to write one from scratch, which HashMap implementation would be most suitable for this use?

Answer 1

I think that a simple hash table implementation using open addressing could fulfill your requirements here:

• By its nature it is stored on a single buffer, and thus trivial to transfer to the kernels.
• It’s then easy to write the getter logic in the kernel, especially when you don’t need any synchronization (read-only).

So, pass a buffer of long2 or a buffer of struct { long key; int val; }, when the first item is the key and the second the value, and also pass the buffer size; now write a regular open-address getter.