// If it does not exist, it creates it.
func (ctxt *Link) Lookup(name string) *LSym {
	return ctxt.LookupInit(name, nil)
}

// LookupInit looks up the symbol with name name.
// If it does not exist, it creates it and
// passes it to init for one-time initialization.
func (ctxt *Link) LookupInit(name string, init func(s *LSym)) *LSym {
	ctxt.hashmu.Lock()
	s := ctxt.hash[name]
	if s == nil {

		var sym *obj.LSym
		if p.As == obj.ADUFFZERO {
			sym = c.ctxt.Lookup("runtime.duffzero")
		} else {
			sym = c.ctxt.Lookup("runtime.duffcopy")
		}
		// Retrieve or create the TOC anchor.
		symtoc := c.ctxt.LookupInit("TOC."+sym.Name, func(s *obj.LSym) {
			s.Type = objabi.SDATA
			s.Set(obj.AttrDuplicateOK, true)
			s.Set(obj.AttrStatic, true)
			c.ctxt.Data = append(c.ctxt.Data, s)
			s.WriteAddr(c.ctxt, 0, 8, sym, 0)
		})