Apparatus and method for fast filtering read and write barrier

Abstract

In a computer system that utilizes write or read barriers to perform a garbage collection function, instruction execution logic avoids unnecessary calls to the write or read barrier procedure. Each object's header includes a State flag. Each object reference also includes a State flag. Each time an instruction that is the subject of a write or read barrier (e.g., a object reference write instruction) is executed, the State flag of the object reference being processed is inspected by the instruction execution logic. If the State flag in the object reference is set, the write or read barrier procedure is not invoked, because the target object has already been processed by a previous call to the write or read barrier procedure. Otherwise the write or read barrier procedure is invoked. The write or read barrier procedure first checks the State flag in the target object's header. If it is set, the State flag in the target object reference is set and then the procedure exits. Otherwise, if the State flag in the target object header is not set, a predefined garbage collection function is performed and then the State flag in the target object's header and the State flag in the target object's reference are both set. In some embodiments the setting of the State flags is conditional on the outcome of the garbage collection operation performed by the write or read barrier procedure.