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

Reference: /n/sources/patch/applied/6l-nop-x0
Date: Thu Jan 31 01:47:57 CET 2013
Signed-off-by: charles.forsyth@gmail.com

--- /sys/src/cmd/6l/optab.c	Thu Jan 31 01:47:06 2013
+++ /sys/src/cmd/6l/optab.c	Thu Jan 31 01:46:57 2013
@@ -15,8 +15,10 @@
 	Ynone,	Ynone,	Zpseudo,1,
 	Ynone,	Yml,	Zpseudo,1,
 	Ynone,	Yrf,	Zpseudo,1,
+	Ynone,	Yxr,	Zpseudo,1,
 	Yml,	Ynone,	Zpseudo,1,
 	Yrf,	Ynone,	Zpseudo,1,
+	Yxr,	Ynone,	Zpseudo,1,
 	0
 };
 uchar	yxorb[] =