Predicated Instructions for Code Compaction

Procedural abstraction, the replacement of several identical code sequences with calls to a single representative function, is a powerful tool in producing compact executables. We explore how predicated instructions can be used to allow procedural abstraction of non-identical basic blocks. A predicated instruction is one that the processor executes if a condition (specified in the opcode) is true, otherwise the instruction has no effect. Architectures such as the ARM provide predicated versions of most of their instructions. By using predicated instructions within a representative function and setting the appropriate flags prior to the call, a single function can serve to represent several different code sequences. To find representative functions, we group sequences that have a short common supersequence and use this supersequence as a representative. We report results on the use of predication for procedural abstraction on the ARM and also indicate the potential compaction benefit of allowing more predication conditions.