Dynamic Code Footprint Optimization for the IBM Cell Broadband Engine

Multicore designers often add a small local memory close to each core to speed up access and to reduce off-chip IO. But this approach puts a burden on the programmer, the compiler, and the runtime system, since this memory lacks hardware support (cache logic, MMU,...) and hence needs to be managed in software to exploit its performance poten-tial. The IBM Cell Broadband Engine (Cell B. E.) is extreme in this respect, since each of the parallel cores can only ad-dress code and data in its own local memory directly. Over-lay techniques from the 70ies solve this problem with the well-known drawbacks: The programmer must manually di-vide the program into overlays and the largest overlay de-termines how much data the application can work with. In our approach, programmers do no longer need to cut overlays. Instead, we automatically and at runtime frag-ment and load small code snippets into a code cache located in the local stores and supervised by a garbage collector. Since our loader does not load code that is not needed for execution, the code cache can be much smaller (up to 70%) than the original program size. Applications can therefore work on larger data sets, i. e., bigger problems. Our loader is highly efficient and slows down applications by less than 5 % on average. It can load any native code without pre-processing or changes in the software tool chain. 1