The effect of instruction set complexity on program size and memory performance

Abstract

One potential disadvantage of a machine with a reduced instruction set is that object programs may be substantially larger than those for a machine with a richer, more complex instruction set. The main reason is that a small instruction set will require more instructions to implement the same function. In addition, the tendency of RISC machines to use fixed length instructions with a few instruction formats also increases object program size. It has been conjectured that the resulting larger programs could adversely affect memory performance and bus traffic. In this paper we report the results of a set of experiments to isolate and determine the effect of instruction set complexity on cache memory performance and bus traffic. Three high-level language compilers were constructed for machines with instruction sets of varying degrees of complexity. Using a set of benchmark programs, we evaluated the effect of instruction set complexity had on program size. Five of the programs were used to perform a set of trace-driven simulations to study each machine's cache and bus performance. While we found that the miss ratio is affected by object program size, it appears that this can be corrected by simplying increasing the size of the cache. Our measurements of bus traffic, however, show that even with large caches, machines with simple instruction sets can expect substantially more main memory reads than machines with dense object programs.