-mno-fancy-math-387 disables generation of fsqrt, fsin, and several other instructions. It was needed long ago when FreeBSD supported FPUless 80386 and 80486 systems via a partial in-kernel emulator that couldn't cope with these instructions. Support for FPUless x86 machines was dropped many years ago, and the emulator is gone, so -mfancy-math-387 should be the default for FreeBSD.
http://www.freebsd.org/cgi/cvsweb.cgi/src/contrib/gcc/config/i386/freebsd.h.diff?r1=1.75;r2=1.76 -- Summary: -mfancy-math-387 should be the default in FreeBSD Product: gcc Version: 4.2.1 Status: UNCONFIRMED Severity: normal Priority: P3 Component: target AssignedTo: unassigned at gcc dot gnu dot org ReportedBy: das at FreeBSD dot ORG GCC build triplet: amd64-undermydesk-freebsd GCC host triplet: amd64-undermydesk-freebsd GCC target triplet: i386-undermydesk-freebsd http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37072