erik found that -N left NOPs in that 6l couldn't ignore.
add Xn to the NOP table.

bonanza; cat > fp.c
#include <u.h>
#include <libc.h>
#include <stdio.h>

void
main(void)
{
        double g;

        g = -0.;

        print("%g\n", g);
        printf("%g\n", g);
        exits("");
}
bonanza; 6c -N -FVTw fp.c
bonanza; 6l -o 6.fp fp.6
main: doasm: notfound from=6f to=34 (939)       NOP     ,X0
main: doasm: notfound from=6f to=34 (939)       NOP     ,X0
main: doasm: notfound from=6f to=34 (939)       NOP     ,X0