12b Instruction Scheduling (Side-bars)R (Rice)

12b. Instruction Scheduling (Side-bars)

1.
MEASURING RUNTIME PERFORMANCE

The primary goal of instruction scheduling is to improve the running time of the generated code. Discussions of performance use many different metrics; the two most common are

Instructions per second The metric commonly used to advertise computers and to compare system performance is the number of instructions executed in a second. This can be measured as instructions issued per second or instructions retired per second.

Time to complete a fixed task This metric uses one or more programs whose behavior is known and compares the time required to complete these fixed tasks. This approach, called benchmarking, provides information about overall system performance, both hardware and software, on a particular workload.

No single metric contains enough information to allow evaluation of the quality of code generated by the compiler’s back end. For example, if the measure is instructions per second, does the compiler get extra credit for leaving extraneous (but independent) instructions in code? The simple timing metric provides no information about what is achievable for a given program. Thus, it allows one compiler to do better than another but fails to show the distance between the generated code and what is optimal for that code on the target machine.

Numbers that the compiler writer might want to measure include the percentage of executed instructions whose results are actually used and the percentage of cycles spent in stalls and interlocks. The former gives insight into some aspects of predicated execution, while the latter directly measures some aspects of schedule quality.