Sun Studio 12: C User's Guide

B.2.16 -fast

This option is a macro that can be effectively used as a starting point for tuning an executable for maximum runtime performance. -fast is a macro that can change from one release of the compiler to the next and expands to options that are target platform specific. Use the -# option or -xdryrun to examine the expansion of -fast, and incorporate the appropriate options of -fast into the ongoing process of tuning the executable.

The expansion of -fast now includes the new -xlibmopt option. This option enables the compiler to use a library of optimized math routines. For more information, see B.2.99 -xlibmopt.

The -fast option impacts the value of errno. See 2.10 The Value of errno for more information.

Modules that are compiled with -fast must also be linked with -fast. For a complete list of all compiler options that must be specified at both compile time and at link time, see A.1.2 Compile-Time and Link-Time Options.

The –fast option is unsuitable for programs intended to run on a different target than the compilation machine. In such cases, follow -fast with the appropriate -xtarget option. For example:

cc -fast -xtarget=ultra ...

For C modules that depend on exception handling specified by SUID, follow -fast by -xnolibmil:

% cc -fast -xnolibmil

With -xlibmil, exceptions cannot be noted by setting errno or calling matherr(3m).

The –fast option is unsuitable for programs that require strict conformance to the IEEE 754 Standard.

The following table lists the set of options selected by -fast across platforms.

Table B–5 The -fast Expansion Flags


















Note –

Some optimizations make certain assumptions about program behavior. If the program does not conform to these assumptions, the application may crash or produce incorrect results. Please refer to the description of the individual options to determine if your program is suitable for compilation with -fast.

The optimizations performed by these options may alter the behavior of programs from that defined by the ISO C and IEEE standards. See the description of the specific option for details.

–fast acts like a macro expansion on the command line. Therefore, you can override the optimization level and code generation option aspects by following -fast with the desired optimization level or code generation option. Compiling with the -fast -xO4 pair is like compiling with the -xO2 -xO4 pair. The latter specification takes precedence.

Do not use this option for programs that depend on IEEE standard exception handling; you can get different numerical results, premature program termination, or unexpected SIGFPE signals.