A built-in function is a coding extension to C and C++ that allows a programmer to use the syntax of C function calls and C variables to access the instruction set of the processor of the compiling machine. IBM® Blue Gene® architectures have special instructions that enable the development of highly optimized applications. Access to some Power® instructions cannot be generated using the standard constructs of the C and C++ languages. Other instructions can be generated through standard constructs, but using built-in functions allows exact control of the generated code. Inline assembly language programming, which uses these instructions directly, is not fully supported by XL C/C++ and other compilers. Furthermore, the technique can be time-consuming to implement.
As an alternative to managing hardware registers through assembly language, XL C/C++ built-in functions provide access to the optimized Power instruction set and allow the compiler to optimize the instruction scheduling.
To call any of the XL C/C++ built-in functions in C++, you must include the header file builtins.h
in your source code.
The following sections describe the available built-in functions for the Blue Gene®/Q platform.