A performance counter is that part of a microprocessor that measures and gathers performance-relevant events on the microprocessor. The number and type of available events differ significantly between existing microprocessors, because there is no commonly accepted specification, and because each manufacturer has different priorities on analyzing the performance of architectures and programs. Looking at the supported events on the different microprocessors, it can be observed that the functionality of these events differs from the requirements of an expert application programmer or a performance tool writer.PCL, the Performance Counter Library, establishes a common platform for performance measurements on a wide range of computer systems. With a common interface on all systems and a set of application-oriented events defined, the application programmer is able to do program optimization in a portable way and the performance tool writer is able to rely on a common interface on different systems. PCL has functions to query the functionality, to start and to stop counters, and to read the values of counters. PCL supports nested calls to PCL functions thus allowing hierarchical performance measurements. Counting may be done either in system or in user mode. All interface functions are callable in C, C++, and Fortran.
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