Use the -xbuiltin[=(%all|%none)] command when you want to improve the optimization of code that calls standard library functions. Many standard library functions, such as the ones defined in math.h and stdio.h, are commonly used by various programs. This command lets the compiler substitute intrinsic functions or inline system functions where profitable for performance. See the er_src(1) man page for an explanation of how to read compiler commentary in object files to determine for which functions the compiler actually makes a substitution.
However, these substitutions can cause the setting of errno to become unreliable. If your program depends on the value of errno, avoid this option. See also 2.10 The Value of errno.
If you do not specify -xbuiltin, the default is -xbuiltin=%none, which means no functions from the standard libraries are substituted or inlined. If you specify -xbuiltin, but do not provide any argument, the default is -xbuiltin%all, which means the compiler substitutes intrinsics or inlines standard library functions as it determines the optimization benefit.
If you compile with -fast, then -xbuiltin is set to %all.
-xbuiltin only inlines global functions defined in system header files, never static functions defined by the user.