Custom-fit processors: letting applications define architectures

In this paper we report on a system which automatically designs realistic VLIW architectures highly optimized for one given application (the input for this system), while running all other code correctly. The system uses a product-quality compiler that generates very aggressive VLIW code. We retarget the compiler until we have found a VLIW architecture idealized for the application on the basis of performance, a cost function and a hardware budget. We show that we can automatically select architectures that achieve large speedups on color and image processing codes. Specialization is shown to be very valuable: The differences between architectural choices, even among reasonable-seeming architectures having similar costs, can be very great, often a factor of 5 (and sometimes much more). We show also that specialization is also very dangerous. A reasonable choice of architecture to fit one algorithm can be a very poor choice for another even in the same domain. There is sometimes an architecture, near in cost and performance to the best, that does much better on a second algorithm.