alto : A Link-Time Optimizer for the DEC Alpha

Traditional optimizing compilers are limited in the scope of their optimizations by the fact that only a sin-gle function, or possibly a single module, is available for analysis and optimization. In particular, this meansthat library routines cannot be optimized to specific calling contexts. Other optimization opportunities, exploit-ing information not available before linktime such as addresses of variables and the final code layout, are oftenignored because linkers are traditionally unsophisticated. A possible solution is to carry out whole-program op-timization at link time. This paper describes alto, a link-time optimizer for the DEC Alpha architecture. It isable to realize significant performance improvements even for programs compiled with a good optimizing com-piler with a high level of optimization. The resulting code is considerably faster that that obtained using the OMlink-time optimizer, even when the latter is used in conjunction with profile-guided and inter-file compile-timeoptimizations. The work of Robert Muth, Saumya Debray and Scott Watterson was supported in part by the National Science Foundation under grantnumbers CCR-9502826 and CCR-9711166. Koen De Bosschere is a research associate with the Fund for Scientific Research – Flanders.