Detecting schizophrenia in imperative synchronous languages

Abstract

When a compiler synthesizes circuits from high-level descriptions, it duplicates functional units that must be used twice in a single clock to prevent misbehavior caused by conflicts. In imperative synchronous languages, a statement may be executed more than once in a single clock due to the combination of loop constructs and the perfect synchrony hypothesis; which is a schizophrenic statement. To cure schizophrenic statements, a compiler has to duplicate them to avoid multiple executions in a clock. However, if the corresponding circuit of a schizophrenic statement can perform multiple semantic behaviors in a single clock, it is harmless and thus curing is not necessary. A schizophrenic problem has to be cured when imperative synchronous languages are compiled to other computation models (software or propositional logic). Esterel is an imperative synchronous language. Imperative features and preemption mechanisms of Esterel help to describe control-intensive embedded systems and protocols. When a program in Esterel is translated to hardware circuit, functional units in a loop statement that can be executed more than once in a clock may cause schizophrenic problems: unstable cycle or wrong behavior. Schizophrenic problems have been previously defined by the instantaneous reentrance to local signal declarations and parallel statements. Compilers cure the problems in circuits by breaking combinational cycles with circuit duplication. But, we observe that the reentrance of two block statements are neither necessary nor sufficient conditions for harmful schizophrenic statements. Exact schizophrenia detectors can reduce the number of statements to be cured. If compilers can reduce the number of statements to apply curing techniques, the resultant circuits can be smaller due to avoiding unnecessary logic duplications. The accuracy of schizophrenia detectors is just as important for the synthesis of smaller circuits as the optimization of curing techniques. All schizo...