Instruction-Isomorphism in Program Execution

This paper identifies a fundamental runtime program property: Instruction–Isomorphism. An instruction instance is said to be isomorphic if its component information derived from the instruction and its backward dynamic data dependence graph is identical to the component of an instruction executed earlier. By definition an isomorphic instruction will produce exactly the same output with the earlier instruction. This work introduces a taxonomy of isomorphic behavior, describes transformations that can change the isomorphic behavior of an instruction, characterizes empirically various aspects of instruction–isomorphism and suggests directions for improving predictors and performance. The empirical analysis shows that there is very little instruction–isomorphism in the dynamic dependence graph of a program. This is due to programming conventions and architectural semantics that introduce a lot of “overhead” nodes and dependences. However, by transforming the dynamic dependence graph closer to its dataflow form, instruction–isomorphism becomes prominent. The data show that for SPEC benchmarks, depending on the benchmark and dataset, 65 to 99.9% of the dynamic instructions are isomorphic.