An Instruction Filter for Time-Predictable Code Execution on Standard Processors

Abstract

Dependable cyber-physical systems usually have stringent requirements on their response time, since failure to react to changes in the system state in a timely manner might lead to catastrophic consequences. It is therefore necessary to determine reliable bounds on the execution time of tasks. However, timing analysis, whether done statically using a timing model or based on measurements, struggles with the large number of possible execution paths in typical applications. The single-path code generation paradigm makes timing analysis trivial by producing programs with a single execution path. Single-path code uses predicated execution, where individual instructions are enabled or disabled based on predicates, instead of conditional control-flow branches. Most processing architectures support a limited number of predicated instructions, such as for instance a conditional move, but single-path code benefits from fully predicated execution, where every instruction is predicated. However, few architectures support full predication, thus limiting the choice of processing platforms. We present a novel approach that adds support for fully predicated execution to existing processor cores which do not natively provide it. Single-path code is generated by restructuring regular machine code and replacing conditional control-flow branches with special instructions that control the predication of subsequent code. At runtime an instruction filter interprets these predicate-defining instructions, computes and saves predicates and filters regular instructions based on the predicate state, replacing inactive instructions with a substitute that has no effect (e.g. a NOP). We are implementing this single-path filter for the LEON3 and the IBEX processors.