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The OpenCL power: offloading to the CPU (AVX+SSE)

by Vincent Hindriksen

Say you have some data that needs to be used as input for a larger kernel, but needs a little preparation to get it aligned in memory (small kernel and random reads). Unluckily the efficiency of such kernel is very low and there is no speed-up or even a slowdown. When programming a GPU it is all about trade-offs, but one trade-off is forgotten a lot (especially by CUDA-programmers) once is decided to use accelerators: just use the CPU. Main problem is not the kernel that has been optimised for the GPU, but all supporting code (like the host-code) needs to be rewritten to be able to use the CPU.

Why use the CPU for vector-computations?

The CPU has support for computing vectors. Each core has a 256 bit wide vector computer. This mean a double4 (a vector of 4 times a 64-bit float) can be computed in one clock-cycle. So a 4-core CPU of 3.5GHz goes from 3.5 billion instructions to 14 billion when using all 4 cores, and to 56 billion instructions when using vectors. When using a float8, it doubles to 112 billion instructions. Using MAD-instructions (Multiply+Add), this can be doubled to even 224 billion instructions.

Say we have this CPU with 4 core and AVX/SSE, and the below code:

int* a = ...;
int* b = ...; 
for (int i = 0; i < M; i++)
   a[i] = b[i]*2;
}

How do you classify the accelerated version of above code? A parallel computation or a vector-computation? Is it is an operation using an M-wide vector or is it using M threads. The answer is both – vector-computations are a subset of parallel computations, so vector-computations can be run in parallel threads too. This is interesting, as this means the code can run on both the AVX as on the various codes.

If you have written the above code, you’d secretly hope the compiler finds out this automatically runs on all hyper-threaded cores and all vector-extensions it has. To have code made use of the separate cores, you have various options like normal threads or OpenMP/MPI. To make use of the vectors (which increases speed dramatically), you need to use vector-enhanced programming languages like OpenCL.

To learn more about the difference between vectors and parallel code, read the series on programming theories, read my first article on OpenCL-CPU, look around at this site (over 100 articles and a growing knowledge-section), ask us a direct question, use the comments, or help make this blog tick: request a full training and/or code-review.