This chapter makes frequent reference to the compiler options that are grouped together in Options for Floating-Point Processing, especially -qfloat Option.

The use of the compiler options for floating-point calculations affects the accuracy, performance, and possibly the correctness of floating-point calculations. Although the default values for the options were chosen to provide efficient and correct execution of most programs, you may need to specify nondefault options for your applications to work the way you want. We strongly advise you to read this chapter before using these options.
Note: The discussions of single-, double-, and extended-precision calculations in this chapter all refer to the default situation, with -qrealsize=4 and no -qautodbl specified. If you change these settings, keep in mind that the size of a Fortran REAL, DOUBLE PRECISION, and so on may change, but single precision, double precision, and extended precision (in lowercase) still refer to 4-, 8-, and 16-byte entities respectively.
Most of the information in this chapter relates to floating-point processing on the PowerPC family of processors. The section Floating-Point Processing on the POWER and POWER2 Architectures describes the differences between floating-point processing on the PowerPC processors and floating-point processing on the POWER and POWER2 processors.

An easy way to choose options that reflect your goals is to run the command xxlf. This features a point-and-click interface to different categories of compiler options.

Figure 19. Floating-Point Window of the xxlf Application