r must be one of: nearest, tozero, negative, positive.
The default is -fround=nearest.
The meanings are the same as those for the ieee_flags subroutine.
When r is tozero, negative, or positive, this flag sets the rounding direction mode to round-to-zero, round-to-negative-infinity, or round-to-positive-infinity respectively when a program begins execution. When r is nearest or the -fround flag is not used, the rounding direction mode is not altered from its initial value (round-to-nearest by default).
This option is effective only if used when compiling the main program.