The math routine library is optimized for performance and usually generates faster code. The results may be slightly different from those produced by the normal math library. If so, they usually differ in the last bit.
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.
The order on the command line for this library option is not significant.
This option is set by the -fast option.