Abstract: A method of source code instrumentation for computer program performance profiling includes generating (14) and inserting (19) instrumentation code around a call site of a child function in a parent function. The instrumentation code may use a reference to an unique instrumentation record (13), such as a timing record. The instrumentation code may be optimised (15) to use the exit time of a preceding call site in the parent function as the entry time of the call site. It may be inserted depending on the level in the call hierarchy of the child function and its execution at run time may depend on the state of an enable flag, which can be set via a viewing interface. Two versions of the child function may be generated (18), one being instrumented and other being non-instrumented and which one is run depends on the enable flag.