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rydenf
Journeyman III

Are get_global_size and get_local_size expensive calls?

I am optimizing my kernel and I wonder if I should store the value of get_global_size(i) in a local variable or if it is as efficient just to call the function every time.

Thanks,

Fredrik

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2 Replies
nou
Exemplar

Re: Are get_global_size and get_local_size expensive calls?

IMHO this is optimized and get_global_id is not computed each time.

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realhet
Miniboss

Re: Are get_global_size and get_local_size expensive calls?

They are cheap, and calculated only once at the beginning of a kernel (only if you calling them in your kernel).

For example on the 7970 it costs only a single MAD to acquire a zero based linear thread id. All other ids are calculated based on this linear id using a hidden constant buffer filled with your NDRange's parameters.

If you use only get_global_id(0) then will use about 8 instructions.