1. An instruction stream generation method for an instruction stream including millicoded instructions, the millicoded instructions being executed by an underlying millicode not visible to a program, the method comprising: generating a testcase for a millicoded instruction in an instruction trace pool, wherein the millicoded instruction is included in a parent instruction trace; processing the testcase to generate a millicode instruction trace snippet, the millicode instruction trace snippet comprising at least one millicode instruction for executing an instruction of the instruction stream; converting the millicode instruction trace snippet into a templatized millimode snippet by supplanting specific information located in the millicode instruction trace snippet with templatized information; processing, by a computer, the parent instruction trace one instruction at a time such that the templatized millimode snippet is accessed and updated with a value from the parent instruction case based on encountering the millicoded instruction in the parent instruction trace; writing the updated templatized millimode snippet to a millicoded instruction trace; providing the instruction stream to a performance model, wherein each occurrence of the millicoded instruction comprises the updated templatized millimode snippet from the millicoded instruction trace; and feeding the performance model the parent instruction trace included in the instruction stream, and based on encountering the millicoded instruction included in the instruction stream, feeding the performance model the millicoded instruction trace.