A study of source-level compiler algorithms for automatic construction of pre-execution code

Pre-execution is a promising latency tolerance technique that uses one or more helper threads running in spare hardware contexts ahead of the main computation to trigger long-latency memory operations early, hence absorbing their latency on behalf of the main computation. This article investigates several source-to-source C compilers for extracting pre-execution thread code automatically, thus relieving the programmer or hardware from this onerous task. We present an aggressive profile-driven compiler that employs three powerful algorithms for code extraction. First, program slicing removes non-critical code for computing cache-missing memory references. Second, prefetch conversion replaces blocking memory references with non-blocking prefetch instructions to minimize pre-execution thread stalls. Finally, speculative loop parallelization generates thread-level parallelism to tolerate the latency of blocking loads. In addition, we present four "reduced" compilers that employ less aggressive algorithms to simplify compiler implementation. Our reduced compilers rely on back-end code optimizations rather than program slicing to remove non-critical code, and use compile-time heuristics rather than profiling to approximate runtime information (e.g., cache-miss and loop-trip counts).