1. A computer program runtime bottleneck diagnostic method for use on a computer platform that runs a computer program of a BIOS (Basic Input/Output System) POST (Power On Self Test) routine composed of a number of code modules for providing the computer program with a runtime bottleneck diagnostic function, which comprises: responding to a runtime execution event of the computer program by issuing a runtime-length recording enable message; responding to the runtime-length recording enable message by recording the runtime length of each of the code modules of the computer program during runtime execution; and after the completion of the execution of the computer program, generating a diagnostic report in electronic form which shows the runtime length of each of the code modules of the computer program in human-readable format, wherein recording the runtime length of each of the code modules of the computer program during runtime execution further comprises: recording a start time of the execution of each of the code modules of the computer program; recording an ending time of the execution of each of the code modules of the computer program; and determining the runtime length of the execution of each of the code modules of the computer program by subtracting the start time from the ending time.