Although graph coloring is widely recognized as an effective technique for register allocation, memory demands can become quite high for large interference graphs that are needed in coloring. In this paper we present an algorithm that uses the notion of clique separators to improve the space overhead of coloring. The algorithm, based on a result by R. Tarjan regarding the colorability of graphs, partitions program code into code segments using clique separators. The interference graphs for the code partitions are constructed one at a time and colored independently. The colorings for the partitions are combined to obtain a register allocation for the entire program. This approach can be used to perform register allocation in a space-efficient manner. For straight-line code (e.g., local register allocation), an optimal allocation can be obtained from optimal allocations for individual code partitions. Experimental results are presented demonstrating memory demand reductions for interference graphs when allocating registers using clique separators.

Multicomputersbuiltaround a general network are now a viable alternative to multicomputersbased on a system-speci~c interconnect because of architectural improvements in two areas.First, the host-network interface overhead can be minimized by reducing copy operations and host interrupts.Second, the network can provide high bandwidth and low latency by using high-speed crossbar switches and efficient protocol implementations.While still enjoying thejexibility of general networks, the resulting network-based multicomputers achieve high performance for typical multicomputer applications that use system-specijic interconnects.We have developed a network-based mtdticomputer called Nectar that supports these claims.