Register Windows vs. Register Allocation

A large register set can be exploited by keeping variables and constants in registers instead of in memory. Hardware register windows and compiletime or link-time global register allocation are ways to do this. A measure of the effectiveness of any of these register management schemes is how thoroughly they manage to remove loads and stores. This measure also must count extra loads and stores that had to be executed because of window overflow or conflicts between procedures. By combining profiling, instrumentation, and in-line simulation, we measured the effectiveness of several register management schemes. These included compile-time and link-time schemes for allocating 52 registers, and register window schemes using 128 registers organized into fixed-size or variable-sized windows. Link-time allocation based on profile information was the clear winner in some cases and did about as well as windows in the rest. Even link-time allocation based on an estimated profile was about as good as windows. Variable-sized windows sometimes did better than fixedsized windows, but the difference was usually small. Register windows require additional logic in the data path, which may slow the machine cycle slightly, and sometimes use more chip real estate for additional registers. Proponents of windows suppose that they trade these drawbacks for a reduction in the number of memory references that must be made. Our results show that this tradeoff should be made the other way: keep the hardware simple, because a link-time register allocator can effectively duplicate the improvement in memory reference frequency, and the cycle time can therefore be kept as small as possible, resulting in faster programs overall. This paper also appears in Proceedings of the SIGPLAN ’88 Conference on Programming Language Design and Implementation, June 1988. Permission to copy without fee all or part of this material is granted provided that the copies are not made or distributed for direct commercial advantage, the ACM copyright notice and the title of the publication and its date appear, and notice is given that copying is by permission of the Association for Computing machinery. To copy otherwise, or to republish, requires a fee and/or specific permission.