return strings.HasPrefix(s.Name, "libc_")
	}
	return ctxt.Arch.Family == sys.I386 && !ctxt.Flag_shared
}

// single-instruction no-ops of various lengths.
// constructed by hand and disassembled with gdb to verify.
// see http://www.agner.org/optimize/optimizing_assembly.pdf for discussion.
var nop = [][16]uint8{
	{0x90},
	{0x66, 0x90},
	{0x0F, 0x1F, 0x00},
	{0x0F, 0x1F, 0x40, 0x00},

	case 45: /* sxt/uxt[bhw] R,R; movT R,R -> sxtT R,R */
		as := p.As
		rt, rf := p.To.Reg, p.From.Reg
		if rf == REGZERO {
			as = AMOVWU /* clearer in disassembly */
		}
		switch as {
		case AMOVB, ASXTB:
			o1 = c.opbfm(p, ASBFM, 0, 7, rf, rt)

		case AMOVH, ASXTH:
			o1 = c.opbfm(p, ASBFM, 0, 15, rf, rt)

		case AMOVW, ASXTW: