Extending path profiling across loop backedges and procedure boundaries

Since their introduction, path profiles have been used to guide the application of aggressive code optimizations and performing instruction scheduling. However, for optimization and scheduling, it is often desirable to obtain frequency counts of paths that extend across loop iterations and cross procedure boundaries. These longer paths, referred to as interesting paths, account for over 75% of the flow in a subset of SPEC benchmarks. Although the frequency counts of interesting paths can be estimated from path profiles, the degree of imprecision of these estimates is very high. We extend Ball Larus (BL) paths to create slightly longer overlapping paths and develop an instrumentation algorithm to collect their frequencies. While these paths are slightly longer than BL paths, they enable very precise estimation of frequencies of potentially much longer interesting paths. Our experiments show that the average cost of collecting frequencies of overlapping paths is 86.8%, which is 4.2 times that of BL paths. However, while the average imprecision in estimated total flow of interesting paths derived from BL path frequencies ranges from -38 % to +138 %, the average imprecision inflow estimates derived from overlapping path frequencies ranges only from -4% to +8%.