Facilities specific to PowerPC that are not specific to a particular operating system are declared in sys/platform/ppc.h.
Read the current value of the Time Base Register.
The Time Base Register is a 64-bit register that stores a monotonically incremented value updated at a system-dependent frequency that may be different from the processor frequency. More information is available in Power ISA 2.06b - Book II - Section 5.2.
__ppc_get_timebase
uses the processor's time base facility directly without requiring assistance from the operating system, so it is very efficient.