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Source file src/cmd/link/internal/ld/dwarf.go

  // Copyright 2010 The Go Authors. All rights reserved.
  // Use of this source code is governed by a BSD-style
  // license that can be found in the LICENSE file.
  
  // TODO/NICETOHAVE:
  //   - eliminate DW_CLS_ if not used
  //   - package info in compilation units
  //   - assign global variables and types to their packages
  //   - gdb uses c syntax, meaning clumsy quoting is needed for go identifiers. eg
  //     ptype struct '[]uint8' and qualifiers need to be quoted away
  //   - lexical scoping is lost, so gdb gets confused as to which 'main.i' you mean.
  //   - file:line info for variables
  //   - make strings a typedef so prettyprinters can see the underlying string type
  
  package ld
  
  import (
  	"cmd/internal/dwarf"
  	"cmd/internal/obj"
  	"fmt"
  	"log"
  	"os"
  	"strings"
  )
  
  type dwctxt struct {
  	linkctxt *Link
  }
  
  func (c dwctxt) PtrSize() int {
  	return SysArch.PtrSize
  }
  func (c dwctxt) AddInt(s dwarf.Sym, size int, i int64) {
  	ls := s.(*Symbol)
  	adduintxx(c.linkctxt, ls, uint64(i), size)
  }
  func (c dwctxt) AddBytes(s dwarf.Sym, b []byte) {
  	ls := s.(*Symbol)
  	Addbytes(ls, b)
  }
  func (c dwctxt) AddString(s dwarf.Sym, v string) {
  	Addstring(s.(*Symbol), v)
  }
  func (c dwctxt) SymValue(s dwarf.Sym) int64 {
  	return s.(*Symbol).Value
  }
  
  func (c dwctxt) AddAddress(s dwarf.Sym, data interface{}, value int64) {
  	if value != 0 {
  		value -= (data.(*Symbol)).Value
  	}
  	Addaddrplus(c.linkctxt, s.(*Symbol), data.(*Symbol), value)
  }
  
  func (c dwctxt) AddSectionOffset(s dwarf.Sym, size int, t interface{}, ofs int64) {
  	ls := s.(*Symbol)
  	switch size {
  	default:
  		Errorf(ls, "invalid size %d in adddwarfref\n", size)
  		fallthrough
  	case SysArch.PtrSize:
  		Addaddr(c.linkctxt, ls, t.(*Symbol))
  	case 4:
  		addaddrplus4(c.linkctxt, ls, t.(*Symbol), 0)
  	}
  	r := &ls.R[len(ls.R)-1]
  	r.Type = obj.R_DWARFREF
  	r.Add = ofs
  }
  
  /*
   * Offsets and sizes of the debug_* sections in the cout file.
   */
  var abbrevsym *Symbol
  var arangessec *Symbol
  var framesec *Symbol
  var infosec *Symbol
  var linesec *Symbol
  
  var gdbscript string
  
  var dwarfp []*Symbol
  
  func writeabbrev(ctxt *Link, syms []*Symbol) []*Symbol {
  	s := ctxt.Syms.Lookup(".debug_abbrev", 0)
  	s.Type = obj.SDWARFSECT
  	abbrevsym = s
  	Addbytes(s, dwarf.GetAbbrev())
  	return append(syms, s)
  }
  
  /*
   * Root DIEs for compilation units, types and global variables.
   */
  var dwroot dwarf.DWDie
  
  var dwtypes dwarf.DWDie
  
  var dwglobals dwarf.DWDie
  
  func newattr(die *dwarf.DWDie, attr uint16, cls int, value int64, data interface{}) *dwarf.DWAttr {
  	a := new(dwarf.DWAttr)
  	a.Link = die.Attr
  	die.Attr = a
  	a.Atr = attr
  	a.Cls = uint8(cls)
  	a.Value = value
  	a.Data = data
  	return a
  }
  
  // Each DIE (except the root ones) has at least 1 attribute: its
  // name. getattr moves the desired one to the front so
  // frequently searched ones are found faster.
  func getattr(die *dwarf.DWDie, attr uint16) *dwarf.DWAttr {
  	if die.Attr.Atr == attr {
  		return die.Attr
  	}
  
  	a := die.Attr
  	b := a.Link
  	for b != nil {
  		if b.Atr == attr {
  			a.Link = b.Link
  			b.Link = die.Attr
  			die.Attr = b
  			return b
  		}
  
  		a = b
  		b = b.Link
  	}
  
  	return nil
  }
  
  // Every DIE has at least a AT_name attribute (but it will only be
  // written out if it is listed in the abbrev).
  func newdie(ctxt *Link, parent *dwarf.DWDie, abbrev int, name string, version int) *dwarf.DWDie {
  	die := new(dwarf.DWDie)
  	die.Abbrev = abbrev
  	die.Link = parent.Child
  	parent.Child = die
  
  	newattr(die, dwarf.DW_AT_name, dwarf.DW_CLS_STRING, int64(len(name)), name)
  
  	if name != "" && (abbrev <= dwarf.DW_ABRV_VARIABLE || abbrev >= dwarf.DW_ABRV_NULLTYPE) {
  		if abbrev != dwarf.DW_ABRV_VARIABLE || version == 0 {
  			sym := ctxt.Syms.Lookup(dwarf.InfoPrefix+name, version)
  			sym.Attr |= AttrHidden
  			sym.Type = obj.SDWARFINFO
  			die.Sym = sym
  		}
  	}
  
  	return die
  }
  
  func walktypedef(die *dwarf.DWDie) *dwarf.DWDie {
  	if die == nil {
  		return nil
  	}
  	// Resolve typedef if present.
  	if die.Abbrev == dwarf.DW_ABRV_TYPEDECL {
  		for attr := die.Attr; attr != nil; attr = attr.Link {
  			if attr.Atr == dwarf.DW_AT_type && attr.Cls == dwarf.DW_CLS_REFERENCE && attr.Data != nil {
  				return attr.Data.(*dwarf.DWDie)
  			}
  		}
  	}
  
  	return die
  }
  
  func walksymtypedef(ctxt *Link, s *Symbol) *Symbol {
  	if t := ctxt.Syms.ROLookup(s.Name+"..def", int(s.Version)); t != nil {
  		return t
  	}
  	return s
  }
  
  // Find child by AT_name using hashtable if available or linear scan
  // if not.
  func findchild(die *dwarf.DWDie, name string) *dwarf.DWDie {
  	var prev *dwarf.DWDie
  	for ; die != prev; prev, die = die, walktypedef(die) {
  		for a := die.Child; a != nil; a = a.Link {
  			if name == getattr(a, dwarf.DW_AT_name).Data {
  				return a
  			}
  		}
  		continue
  	}
  	return nil
  }
  
  // Used to avoid string allocation when looking up dwarf symbols
  var prefixBuf = []byte(dwarf.InfoPrefix)
  
  func find(ctxt *Link, name string) *Symbol {
  	n := append(prefixBuf, name...)
  	// The string allocation below is optimized away because it is only used in a map lookup.
  	s := ctxt.Syms.ROLookup(string(n), 0)
  	prefixBuf = n[:len(dwarf.InfoPrefix)]
  	if s != nil && s.Type == obj.SDWARFINFO {
  		return s
  	}
  	return nil
  }
  
  func mustFind(ctxt *Link, name string) *Symbol {
  	r := find(ctxt, name)
  	if r == nil {
  		Exitf("dwarf find: cannot find %s", name)
  	}
  	return r
  }
  
  func adddwarfref(ctxt *Link, s *Symbol, t *Symbol, size int) int64 {
  	var result int64
  	switch size {
  	default:
  		Errorf(s, "invalid size %d in adddwarfref\n", size)
  		fallthrough
  	case SysArch.PtrSize:
  		result = Addaddr(ctxt, s, t)
  	case 4:
  		result = addaddrplus4(ctxt, s, t, 0)
  	}
  	r := &s.R[len(s.R)-1]
  	r.Type = obj.R_DWARFREF
  	return result
  }
  
  func newrefattr(die *dwarf.DWDie, attr uint16, ref *Symbol) *dwarf.DWAttr {
  	if ref == nil {
  		return nil
  	}
  	return newattr(die, attr, dwarf.DW_CLS_REFERENCE, 0, ref)
  }
  
  func putdies(linkctxt *Link, ctxt dwarf.Context, syms []*Symbol, die *dwarf.DWDie) []*Symbol {
  	for ; die != nil; die = die.Link {
  		syms = putdie(linkctxt, ctxt, syms, die)
  	}
  	Adduint8(linkctxt, syms[len(syms)-1], 0)
  
  	return syms
  }
  
  func dtolsym(s dwarf.Sym) *Symbol {
  	if s == nil {
  		return nil
  	}
  	return s.(*Symbol)
  }
  
  func putdie(linkctxt *Link, ctxt dwarf.Context, syms []*Symbol, die *dwarf.DWDie) []*Symbol {
  	s := dtolsym(die.Sym)
  	if s == nil {
  		s = syms[len(syms)-1]
  	} else {
  		if s.Attr.OnList() {
  			log.Fatalf("symbol %s listed multiple times", s.Name)
  		}
  		s.Attr |= AttrOnList
  		syms = append(syms, s)
  	}
  	dwarf.Uleb128put(ctxt, s, int64(die.Abbrev))
  	dwarf.PutAttrs(ctxt, s, die.Abbrev, die.Attr)
  	if dwarf.HasChildren(die) {
  		return putdies(linkctxt, ctxt, syms, die.Child)
  	}
  	return syms
  }
  
  func reverselist(list **dwarf.DWDie) {
  	curr := *list
  	var prev *dwarf.DWDie
  	for curr != nil {
  		var next *dwarf.DWDie = curr.Link
  		curr.Link = prev
  		prev = curr
  		curr = next
  	}
  
  	*list = prev
  }
  
  func reversetree(list **dwarf.DWDie) {
  	reverselist(list)
  	for die := *list; die != nil; die = die.Link {
  		if dwarf.HasChildren(die) {
  			reversetree(&die.Child)
  		}
  	}
  }
  
  func newmemberoffsetattr(die *dwarf.DWDie, offs int32) {
  	var block [20]byte
  	b := append(block[:0], dwarf.DW_OP_plus_uconst)
  	b = dwarf.AppendUleb128(b, uint64(offs))
  	newattr(die, dwarf.DW_AT_data_member_location, dwarf.DW_CLS_BLOCK, int64(len(b)), b)
  }
  
  // GDB doesn't like FORM_addr for AT_location, so emit a
  // location expression that evals to a const.
  func newabslocexprattr(die *dwarf.DWDie, addr int64, sym *Symbol) {
  	newattr(die, dwarf.DW_AT_location, dwarf.DW_CLS_ADDRESS, addr, sym)
  	// below
  }
  
  // Lookup predefined types
  func lookupOrDiag(ctxt *Link, n string) *Symbol {
  	s := ctxt.Syms.ROLookup(n, 0)
  	if s == nil || s.Size == 0 {
  		Exitf("dwarf: missing type: %s", n)
  	}
  
  	return s
  }
  
  func dotypedef(ctxt *Link, parent *dwarf.DWDie, name string, def *dwarf.DWDie) {
  	// Only emit typedefs for real names.
  	if strings.HasPrefix(name, "map[") {
  		return
  	}
  	if strings.HasPrefix(name, "struct {") {
  		return
  	}
  	if strings.HasPrefix(name, "chan ") {
  		return
  	}
  	if name[0] == '[' || name[0] == '*' {
  		return
  	}
  	if def == nil {
  		Errorf(nil, "dwarf: bad def in dotypedef")
  	}
  
  	sym := ctxt.Syms.Lookup(dtolsym(def.Sym).Name+"..def", 0)
  	sym.Attr |= AttrHidden
  	sym.Type = obj.SDWARFINFO
  	def.Sym = sym
  
  	// The typedef entry must be created after the def,
  	// so that future lookups will find the typedef instead
  	// of the real definition. This hooks the typedef into any
  	// circular definition loops, so that gdb can understand them.
  	die := newdie(ctxt, parent, dwarf.DW_ABRV_TYPEDECL, name, 0)
  
  	newrefattr(die, dwarf.DW_AT_type, sym)
  }
  
  // Define gotype, for composite ones recurse into constituents.
  func defgotype(ctxt *Link, gotype *Symbol) *Symbol {
  	if gotype == nil {
  		return mustFind(ctxt, "<unspecified>")
  	}
  
  	if !strings.HasPrefix(gotype.Name, "type.") {
  		Errorf(gotype, "dwarf: type name doesn't start with \"type.\"")
  		return mustFind(ctxt, "<unspecified>")
  	}
  
  	name := gotype.Name[5:] // could also decode from Type.string
  
  	sdie := find(ctxt, name)
  
  	if sdie != nil {
  		return sdie
  	}
  
  	return newtype(ctxt, gotype).Sym.(*Symbol)
  }
  
  func newtype(ctxt *Link, gotype *Symbol) *dwarf.DWDie {
  	name := gotype.Name[5:] // could also decode from Type.string
  	kind := decodetypeKind(gotype)
  	bytesize := decodetypeSize(ctxt.Arch, gotype)
  
  	var die *dwarf.DWDie
  	switch kind {
  	case obj.KindBool:
  		die = newdie(ctxt, &dwtypes, dwarf.DW_ABRV_BASETYPE, name, 0)
  		newattr(die, dwarf.DW_AT_encoding, dwarf.DW_CLS_CONSTANT, dwarf.DW_ATE_boolean, 0)
  		newattr(die, dwarf.DW_AT_byte_size, dwarf.DW_CLS_CONSTANT, bytesize, 0)
  
  	case obj.KindInt,
  		obj.KindInt8,
  		obj.KindInt16,
  		obj.KindInt32,
  		obj.KindInt64:
  		die = newdie(ctxt, &dwtypes, dwarf.DW_ABRV_BASETYPE, name, 0)
  		newattr(die, dwarf.DW_AT_encoding, dwarf.DW_CLS_CONSTANT, dwarf.DW_ATE_signed, 0)
  		newattr(die, dwarf.DW_AT_byte_size, dwarf.DW_CLS_CONSTANT, bytesize, 0)
  
  	case obj.KindUint,
  		obj.KindUint8,
  		obj.KindUint16,
  		obj.KindUint32,
  		obj.KindUint64,
  		obj.KindUintptr:
  		die = newdie(ctxt, &dwtypes, dwarf.DW_ABRV_BASETYPE, name, 0)
  		newattr(die, dwarf.DW_AT_encoding, dwarf.DW_CLS_CONSTANT, dwarf.DW_ATE_unsigned, 0)
  		newattr(die, dwarf.DW_AT_byte_size, dwarf.DW_CLS_CONSTANT, bytesize, 0)
  
  	case obj.KindFloat32,
  		obj.KindFloat64:
  		die = newdie(ctxt, &dwtypes, dwarf.DW_ABRV_BASETYPE, name, 0)
  		newattr(die, dwarf.DW_AT_encoding, dwarf.DW_CLS_CONSTANT, dwarf.DW_ATE_float, 0)
  		newattr(die, dwarf.DW_AT_byte_size, dwarf.DW_CLS_CONSTANT, bytesize, 0)
  
  	case obj.KindComplex64,
  		obj.KindComplex128:
  		die = newdie(ctxt, &dwtypes, dwarf.DW_ABRV_BASETYPE, name, 0)
  		newattr(die, dwarf.DW_AT_encoding, dwarf.DW_CLS_CONSTANT, dwarf.DW_ATE_complex_float, 0)
  		newattr(die, dwarf.DW_AT_byte_size, dwarf.DW_CLS_CONSTANT, bytesize, 0)
  
  	case obj.KindArray:
  		die = newdie(ctxt, &dwtypes, dwarf.DW_ABRV_ARRAYTYPE, name, 0)
  		dotypedef(ctxt, &dwtypes, name, die)
  		newattr(die, dwarf.DW_AT_byte_size, dwarf.DW_CLS_CONSTANT, bytesize, 0)
  		s := decodetypeArrayElem(gotype)
  		newrefattr(die, dwarf.DW_AT_type, defgotype(ctxt, s))
  		fld := newdie(ctxt, die, dwarf.DW_ABRV_ARRAYRANGE, "range", 0)
  
  		// use actual length not upper bound; correct for 0-length arrays.
  		newattr(fld, dwarf.DW_AT_count, dwarf.DW_CLS_CONSTANT, decodetypeArrayLen(ctxt.Arch, gotype), 0)
  
  		newrefattr(fld, dwarf.DW_AT_type, mustFind(ctxt, "uintptr"))
  
  	case obj.KindChan:
  		die = newdie(ctxt, &dwtypes, dwarf.DW_ABRV_CHANTYPE, name, 0)
  		newattr(die, dwarf.DW_AT_byte_size, dwarf.DW_CLS_CONSTANT, bytesize, 0)
  		s := decodetypeChanElem(gotype)
  		newrefattr(die, dwarf.DW_AT_go_elem, defgotype(ctxt, s))
  		// Save elem type for synthesizechantypes. We could synthesize here
  		// but that would change the order of DIEs we output.
  		newrefattr(die, dwarf.DW_AT_type, s)
  
  	case obj.KindFunc:
  		die = newdie(ctxt, &dwtypes, dwarf.DW_ABRV_FUNCTYPE, name, 0)
  		dotypedef(ctxt, &dwtypes, name, die)
  		newrefattr(die, dwarf.DW_AT_type, mustFind(ctxt, "void"))
  		nfields := decodetypeFuncInCount(ctxt.Arch, gotype)
  		var fld *dwarf.DWDie
  		var s *Symbol
  		for i := 0; i < nfields; i++ {
  			s = decodetypeFuncInType(gotype, i)
  			fld = newdie(ctxt, die, dwarf.DW_ABRV_FUNCTYPEPARAM, s.Name[5:], 0)
  			newrefattr(fld, dwarf.DW_AT_type, defgotype(ctxt, s))
  		}
  
  		if decodetypeFuncDotdotdot(ctxt.Arch, gotype) {
  			newdie(ctxt, die, dwarf.DW_ABRV_DOTDOTDOT, "...", 0)
  		}
  		nfields = decodetypeFuncOutCount(ctxt.Arch, gotype)
  		for i := 0; i < nfields; i++ {
  			s = decodetypeFuncOutType(ctxt.Arch, gotype, i)
  			fld = newdie(ctxt, die, dwarf.DW_ABRV_FUNCTYPEPARAM, s.Name[5:], 0)
  			newrefattr(fld, dwarf.DW_AT_type, defptrto(ctxt, defgotype(ctxt, s)))
  		}
  
  	case obj.KindInterface:
  		die = newdie(ctxt, &dwtypes, dwarf.DW_ABRV_IFACETYPE, name, 0)
  		dotypedef(ctxt, &dwtypes, name, die)
  		newattr(die, dwarf.DW_AT_byte_size, dwarf.DW_CLS_CONSTANT, bytesize, 0)
  		nfields := int(decodetypeIfaceMethodCount(ctxt.Arch, gotype))
  		var s *Symbol
  		if nfields == 0 {
  			s = lookupOrDiag(ctxt, "type.runtime.eface")
  		} else {
  			s = lookupOrDiag(ctxt, "type.runtime.iface")
  		}
  		newrefattr(die, dwarf.DW_AT_type, defgotype(ctxt, s))
  
  	case obj.KindMap:
  		die = newdie(ctxt, &dwtypes, dwarf.DW_ABRV_MAPTYPE, name, 0)
  		s := decodetypeMapKey(gotype)
  		newrefattr(die, dwarf.DW_AT_go_key, defgotype(ctxt, s))
  		s = decodetypeMapValue(gotype)
  		newrefattr(die, dwarf.DW_AT_go_elem, defgotype(ctxt, s))
  		// Save gotype for use in synthesizemaptypes. We could synthesize here,
  		// but that would change the order of the DIEs.
  		newrefattr(die, dwarf.DW_AT_type, gotype)
  
  	case obj.KindPtr:
  		die = newdie(ctxt, &dwtypes, dwarf.DW_ABRV_PTRTYPE, name, 0)
  		dotypedef(ctxt, &dwtypes, name, die)
  		s := decodetypePtrElem(gotype)
  		newrefattr(die, dwarf.DW_AT_type, defgotype(ctxt, s))
  
  	case obj.KindSlice:
  		die = newdie(ctxt, &dwtypes, dwarf.DW_ABRV_SLICETYPE, name, 0)
  		dotypedef(ctxt, &dwtypes, name, die)
  		newattr(die, dwarf.DW_AT_byte_size, dwarf.DW_CLS_CONSTANT, bytesize, 0)
  		s := decodetypeArrayElem(gotype)
  		elem := defgotype(ctxt, s)
  		newrefattr(die, dwarf.DW_AT_go_elem, elem)
  
  	case obj.KindString:
  		die = newdie(ctxt, &dwtypes, dwarf.DW_ABRV_STRINGTYPE, name, 0)
  		newattr(die, dwarf.DW_AT_byte_size, dwarf.DW_CLS_CONSTANT, bytesize, 0)
  
  	case obj.KindStruct:
  		die = newdie(ctxt, &dwtypes, dwarf.DW_ABRV_STRUCTTYPE, name, 0)
  		dotypedef(ctxt, &dwtypes, name, die)
  		newattr(die, dwarf.DW_AT_byte_size, dwarf.DW_CLS_CONSTANT, bytesize, 0)
  		nfields := decodetypeStructFieldCount(ctxt.Arch, gotype)
  		var f string
  		var fld *dwarf.DWDie
  		var s *Symbol
  		for i := 0; i < nfields; i++ {
  			f = decodetypeStructFieldName(gotype, i)
  			s = decodetypeStructFieldType(gotype, i)
  			if f == "" {
  				f = s.Name[5:] // skip "type."
  			}
  			fld = newdie(ctxt, die, dwarf.DW_ABRV_STRUCTFIELD, f, 0)
  			newrefattr(fld, dwarf.DW_AT_type, defgotype(ctxt, s))
  			newmemberoffsetattr(fld, int32(decodetypeStructFieldOffs(ctxt.Arch, gotype, i)))
  		}
  
  	case obj.KindUnsafePointer:
  		die = newdie(ctxt, &dwtypes, dwarf.DW_ABRV_BARE_PTRTYPE, name, 0)
  
  	default:
  		Errorf(gotype, "dwarf: definition of unknown kind %d", kind)
  		die = newdie(ctxt, &dwtypes, dwarf.DW_ABRV_TYPEDECL, name, 0)
  		newrefattr(die, dwarf.DW_AT_type, mustFind(ctxt, "<unspecified>"))
  	}
  
  	newattr(die, dwarf.DW_AT_go_kind, dwarf.DW_CLS_CONSTANT, int64(kind), 0)
  
  	if _, ok := prototypedies[gotype.Name]; ok {
  		prototypedies[gotype.Name] = die
  	}
  
  	return die
  }
  
  func nameFromDIESym(dwtype *Symbol) string {
  	return strings.TrimSuffix(dwtype.Name[len(dwarf.InfoPrefix):], "..def")
  }
  
  // Find or construct *T given T.
  func defptrto(ctxt *Link, dwtype *Symbol) *Symbol {
  	ptrname := "*" + nameFromDIESym(dwtype)
  	die := find(ctxt, ptrname)
  	if die == nil {
  		pdie := newdie(ctxt, &dwtypes, dwarf.DW_ABRV_PTRTYPE, ptrname, 0)
  		newrefattr(pdie, dwarf.DW_AT_type, dwtype)
  		return dtolsym(pdie.Sym)
  	}
  
  	return die
  }
  
  // Copies src's children into dst. Copies attributes by value.
  // DWAttr.data is copied as pointer only. If except is one of
  // the top-level children, it will not be copied.
  func copychildrenexcept(ctxt *Link, dst *dwarf.DWDie, src *dwarf.DWDie, except *dwarf.DWDie) {
  	for src = src.Child; src != nil; src = src.Link {
  		if src == except {
  			continue
  		}
  		c := newdie(ctxt, dst, src.Abbrev, getattr(src, dwarf.DW_AT_name).Data.(string), 0)
  		for a := src.Attr; a != nil; a = a.Link {
  			newattr(c, a.Atr, int(a.Cls), a.Value, a.Data)
  		}
  		copychildrenexcept(ctxt, c, src, nil)
  	}
  
  	reverselist(&dst.Child)
  }
  
  func copychildren(ctxt *Link, dst *dwarf.DWDie, src *dwarf.DWDie) {
  	copychildrenexcept(ctxt, dst, src, nil)
  }
  
  // Search children (assumed to have TAG_member) for the one named
  // field and set its AT_type to dwtype
  func substitutetype(structdie *dwarf.DWDie, field string, dwtype *Symbol) {
  	child := findchild(structdie, field)
  	if child == nil {
  		Exitf("dwarf substitutetype: %s does not have member %s",
  			getattr(structdie, dwarf.DW_AT_name).Data, field)
  		return
  	}
  
  	a := getattr(child, dwarf.DW_AT_type)
  	if a != nil {
  		a.Data = dwtype
  	} else {
  		newrefattr(child, dwarf.DW_AT_type, dwtype)
  	}
  }
  
  func findprotodie(ctxt *Link, name string) *dwarf.DWDie {
  	die, ok := prototypedies[name]
  	if ok && die == nil {
  		defgotype(ctxt, lookupOrDiag(ctxt, name))
  		die = prototypedies[name]
  	}
  	return die
  }
  
  func synthesizestringtypes(ctxt *Link, die *dwarf.DWDie) {
  	prototype := walktypedef(findprotodie(ctxt, "type.runtime.stringStructDWARF"))
  	if prototype == nil {
  		return
  	}
  
  	for ; die != nil; die = die.Link {
  		if die.Abbrev != dwarf.DW_ABRV_STRINGTYPE {
  			continue
  		}
  		copychildren(ctxt, die, prototype)
  	}
  }
  
  func synthesizeslicetypes(ctxt *Link, die *dwarf.DWDie) {
  	prototype := walktypedef(findprotodie(ctxt, "type.runtime.slice"))
  	if prototype == nil {
  		return
  	}
  
  	for ; die != nil; die = die.Link {
  		if die.Abbrev != dwarf.DW_ABRV_SLICETYPE {
  			continue
  		}
  		copychildren(ctxt, die, prototype)
  		elem := getattr(die, dwarf.DW_AT_go_elem).Data.(*Symbol)
  		substitutetype(die, "array", defptrto(ctxt, elem))
  	}
  }
  
  func mkinternaltypename(base string, arg1 string, arg2 string) string {
  	var buf string
  
  	if arg2 == "" {
  		buf = fmt.Sprintf("%s<%s>", base, arg1)
  	} else {
  		buf = fmt.Sprintf("%s<%s,%s>", base, arg1, arg2)
  	}
  	n := buf
  	return n
  }
  
  // synthesizemaptypes is way too closely married to runtime/hashmap.c
  const (
  	MaxKeySize = 128
  	MaxValSize = 128
  	BucketSize = 8
  )
  
  func mkinternaltype(ctxt *Link, abbrev int, typename, keyname, valname string, f func(*dwarf.DWDie)) *Symbol {
  	name := mkinternaltypename(typename, keyname, valname)
  	symname := dwarf.InfoPrefix + name
  	s := ctxt.Syms.ROLookup(symname, 0)
  	if s != nil && s.Type == obj.SDWARFINFO {
  		return s
  	}
  	die := newdie(ctxt, &dwtypes, abbrev, name, 0)
  	f(die)
  	return dtolsym(die.Sym)
  }
  
  func synthesizemaptypes(ctxt *Link, die *dwarf.DWDie) {
  	hash := walktypedef(findprotodie(ctxt, "type.runtime.hmap"))
  	bucket := walktypedef(findprotodie(ctxt, "type.runtime.bmap"))
  
  	if hash == nil {
  		return
  	}
  
  	for ; die != nil; die = die.Link {
  		if die.Abbrev != dwarf.DW_ABRV_MAPTYPE {
  			continue
  		}
  		gotype := getattr(die, dwarf.DW_AT_type).Data.(*Symbol)
  		keytype := decodetypeMapKey(gotype)
  		valtype := decodetypeMapValue(gotype)
  		keysize, valsize := decodetypeSize(ctxt.Arch, keytype), decodetypeSize(ctxt.Arch, valtype)
  		keytype, valtype = walksymtypedef(ctxt, defgotype(ctxt, keytype)), walksymtypedef(ctxt, defgotype(ctxt, valtype))
  
  		// compute size info like hashmap.c does.
  		indirectKey, indirectVal := false, false
  		if keysize > MaxKeySize {
  			keysize = int64(SysArch.PtrSize)
  			indirectKey = true
  		}
  		if valsize > MaxValSize {
  			valsize = int64(SysArch.PtrSize)
  			indirectVal = true
  		}
  
  		// Construct type to represent an array of BucketSize keys
  		keyname := nameFromDIESym(keytype)
  		dwhks := mkinternaltype(ctxt, dwarf.DW_ABRV_ARRAYTYPE, "[]key", keyname, "", func(dwhk *dwarf.DWDie) {
  			newattr(dwhk, dwarf.DW_AT_byte_size, dwarf.DW_CLS_CONSTANT, BucketSize*keysize, 0)
  			t := keytype
  			if indirectKey {
  				t = defptrto(ctxt, keytype)
  			}
  			newrefattr(dwhk, dwarf.DW_AT_type, t)
  			fld := newdie(ctxt, dwhk, dwarf.DW_ABRV_ARRAYRANGE, "size", 0)
  			newattr(fld, dwarf.DW_AT_count, dwarf.DW_CLS_CONSTANT, BucketSize, 0)
  			newrefattr(fld, dwarf.DW_AT_type, mustFind(ctxt, "uintptr"))
  		})
  
  		// Construct type to represent an array of BucketSize values
  		valname := nameFromDIESym(valtype)
  		dwhvs := mkinternaltype(ctxt, dwarf.DW_ABRV_ARRAYTYPE, "[]val", valname, "", func(dwhv *dwarf.DWDie) {
  			newattr(dwhv, dwarf.DW_AT_byte_size, dwarf.DW_CLS_CONSTANT, BucketSize*valsize, 0)
  			t := valtype
  			if indirectVal {
  				t = defptrto(ctxt, valtype)
  			}
  			newrefattr(dwhv, dwarf.DW_AT_type, t)
  			fld := newdie(ctxt, dwhv, dwarf.DW_ABRV_ARRAYRANGE, "size", 0)
  			newattr(fld, dwarf.DW_AT_count, dwarf.DW_CLS_CONSTANT, BucketSize, 0)
  			newrefattr(fld, dwarf.DW_AT_type, mustFind(ctxt, "uintptr"))
  		})
  
  		// Construct bucket<K,V>
  		dwhbs := mkinternaltype(ctxt, dwarf.DW_ABRV_STRUCTTYPE, "bucket", keyname, valname, func(dwhb *dwarf.DWDie) {
  			// Copy over all fields except the field "data" from the generic
  			// bucket. "data" will be replaced with keys/values below.
  			copychildrenexcept(ctxt, dwhb, bucket, findchild(bucket, "data"))
  
  			fld := newdie(ctxt, dwhb, dwarf.DW_ABRV_STRUCTFIELD, "keys", 0)
  			newrefattr(fld, dwarf.DW_AT_type, dwhks)
  			newmemberoffsetattr(fld, BucketSize)
  			fld = newdie(ctxt, dwhb, dwarf.DW_ABRV_STRUCTFIELD, "values", 0)
  			newrefattr(fld, dwarf.DW_AT_type, dwhvs)
  			newmemberoffsetattr(fld, BucketSize+BucketSize*int32(keysize))
  			fld = newdie(ctxt, dwhb, dwarf.DW_ABRV_STRUCTFIELD, "overflow", 0)
  			newrefattr(fld, dwarf.DW_AT_type, defptrto(ctxt, dtolsym(dwhb.Sym)))
  			newmemberoffsetattr(fld, BucketSize+BucketSize*(int32(keysize)+int32(valsize)))
  			if SysArch.RegSize > SysArch.PtrSize {
  				fld = newdie(ctxt, dwhb, dwarf.DW_ABRV_STRUCTFIELD, "pad", 0)
  				newrefattr(fld, dwarf.DW_AT_type, mustFind(ctxt, "uintptr"))
  				newmemberoffsetattr(fld, BucketSize+BucketSize*(int32(keysize)+int32(valsize))+int32(SysArch.PtrSize))
  			}
  
  			newattr(dwhb, dwarf.DW_AT_byte_size, dwarf.DW_CLS_CONSTANT, BucketSize+BucketSize*keysize+BucketSize*valsize+int64(SysArch.RegSize), 0)
  		})
  
  		// Construct hash<K,V>
  		dwhs := mkinternaltype(ctxt, dwarf.DW_ABRV_STRUCTTYPE, "hash", keyname, valname, func(dwh *dwarf.DWDie) {
  			copychildren(ctxt, dwh, hash)
  			substitutetype(dwh, "buckets", defptrto(ctxt, dwhbs))
  			substitutetype(dwh, "oldbuckets", defptrto(ctxt, dwhbs))
  			newattr(dwh, dwarf.DW_AT_byte_size, dwarf.DW_CLS_CONSTANT, getattr(hash, dwarf.DW_AT_byte_size).Value, nil)
  		})
  
  		// make map type a pointer to hash<K,V>
  		newrefattr(die, dwarf.DW_AT_type, defptrto(ctxt, dwhs))
  	}
  }
  
  func synthesizechantypes(ctxt *Link, die *dwarf.DWDie) {
  	sudog := walktypedef(findprotodie(ctxt, "type.runtime.sudog"))
  	waitq := walktypedef(findprotodie(ctxt, "type.runtime.waitq"))
  	hchan := walktypedef(findprotodie(ctxt, "type.runtime.hchan"))
  	if sudog == nil || waitq == nil || hchan == nil {
  		return
  	}
  
  	sudogsize := int(getattr(sudog, dwarf.DW_AT_byte_size).Value)
  
  	for ; die != nil; die = die.Link {
  		if die.Abbrev != dwarf.DW_ABRV_CHANTYPE {
  			continue
  		}
  		elemgotype := getattr(die, dwarf.DW_AT_type).Data.(*Symbol)
  		elemsize := decodetypeSize(ctxt.Arch, elemgotype)
  		elemname := elemgotype.Name[5:]
  		elemtype := walksymtypedef(ctxt, defgotype(ctxt, elemgotype))
  
  		// sudog<T>
  		dwss := mkinternaltype(ctxt, dwarf.DW_ABRV_STRUCTTYPE, "sudog", elemname, "", func(dws *dwarf.DWDie) {
  			copychildren(ctxt, dws, sudog)
  			substitutetype(dws, "elem", elemtype)
  			if elemsize > 8 {
  				elemsize -= 8
  			} else {
  				elemsize = 0
  			}
  			newattr(dws, dwarf.DW_AT_byte_size, dwarf.DW_CLS_CONSTANT, int64(sudogsize)+elemsize, nil)
  		})
  
  		// waitq<T>
  		dwws := mkinternaltype(ctxt, dwarf.DW_ABRV_STRUCTTYPE, "waitq", elemname, "", func(dww *dwarf.DWDie) {
  
  			copychildren(ctxt, dww, waitq)
  			substitutetype(dww, "first", defptrto(ctxt, dwss))
  			substitutetype(dww, "last", defptrto(ctxt, dwss))
  			newattr(dww, dwarf.DW_AT_byte_size, dwarf.DW_CLS_CONSTANT, getattr(waitq, dwarf.DW_AT_byte_size).Value, nil)
  		})
  
  		// hchan<T>
  		dwhs := mkinternaltype(ctxt, dwarf.DW_ABRV_STRUCTTYPE, "hchan", elemname, "", func(dwh *dwarf.DWDie) {
  			copychildren(ctxt, dwh, hchan)
  			substitutetype(dwh, "recvq", dwws)
  			substitutetype(dwh, "sendq", dwws)
  			newattr(dwh, dwarf.DW_AT_byte_size, dwarf.DW_CLS_CONSTANT, getattr(hchan, dwarf.DW_AT_byte_size).Value, nil)
  		})
  
  		newrefattr(die, dwarf.DW_AT_type, defptrto(ctxt, dwhs))
  	}
  }
  
  // For use with pass.c::genasmsym
  func defdwsymb(ctxt *Link, sym *Symbol, s string, t SymbolType, v int64, gotype *Symbol) {
  	if strings.HasPrefix(s, "go.string.") {
  		return
  	}
  	if strings.HasPrefix(s, "runtime.gcbits.") {
  		return
  	}
  
  	if strings.HasPrefix(s, "type.") && s != "type.*" && !strings.HasPrefix(s, "type..") {
  		defgotype(ctxt, sym)
  		return
  	}
  
  	var dv *dwarf.DWDie
  
  	var dt *Symbol
  	switch t {
  	default:
  		return
  
  	case DataSym, BSSSym:
  		dv = newdie(ctxt, &dwglobals, dwarf.DW_ABRV_VARIABLE, s, int(sym.Version))
  		newabslocexprattr(dv, v, sym)
  		if sym.Version == 0 {
  			newattr(dv, dwarf.DW_AT_external, dwarf.DW_CLS_FLAG, 1, 0)
  		}
  		fallthrough
  
  	case AutoSym, ParamSym:
  		dt = defgotype(ctxt, gotype)
  	}
  
  	if dv != nil {
  		newrefattr(dv, dwarf.DW_AT_type, dt)
  	}
  }
  
  func movetomodule(parent *dwarf.DWDie) {
  	die := dwroot.Child.Child
  	if die == nil {
  		dwroot.Child.Child = parent.Child
  		return
  	}
  	for die.Link != nil {
  		die = die.Link
  	}
  	die.Link = parent.Child
  }
  
  // If the pcln table contains runtime/runtime.go, use that to set gdbscript path.
  func finddebugruntimepath(s *Symbol) {
  	if gdbscript != "" {
  		return
  	}
  
  	for i := range s.FuncInfo.File {
  		f := s.FuncInfo.File[i]
  		if i := strings.Index(f.Name, "runtime/runtime.go"); i >= 0 {
  			gdbscript = f.Name[:i] + "runtime/runtime-gdb.py"
  			break
  		}
  	}
  }
  
  /*
   * Generate a sequence of opcodes that is as short as possible.
   * See section 6.2.5
   */
  const (
  	LINE_BASE   = -4
  	LINE_RANGE  = 10
  	PC_RANGE    = (255 - OPCODE_BASE) / LINE_RANGE
  	OPCODE_BASE = 10
  )
  
  func putpclcdelta(linkctxt *Link, ctxt dwarf.Context, s *Symbol, deltaPC uint64, deltaLC int64) {
  	// Choose a special opcode that minimizes the number of bytes needed to
  	// encode the remaining PC delta and LC delta.
  	var opcode int64
  	if deltaLC < LINE_BASE {
  		if deltaPC >= PC_RANGE {
  			opcode = OPCODE_BASE + (LINE_RANGE * PC_RANGE)
  		} else {
  			opcode = OPCODE_BASE + (LINE_RANGE * int64(deltaPC))
  		}
  	} else if deltaLC < LINE_BASE+LINE_RANGE {
  		if deltaPC >= PC_RANGE {
  			opcode = OPCODE_BASE + (deltaLC - LINE_BASE) + (LINE_RANGE * PC_RANGE)
  			if opcode > 255 {
  				opcode -= LINE_RANGE
  			}
  		} else {
  			opcode = OPCODE_BASE + (deltaLC - LINE_BASE) + (LINE_RANGE * int64(deltaPC))
  		}
  	} else {
  		if deltaPC <= PC_RANGE {
  			opcode = OPCODE_BASE + (LINE_RANGE - 1) + (LINE_RANGE * int64(deltaPC))
  			if opcode > 255 {
  				opcode = 255
  			}
  		} else {
  			// Use opcode 249 (pc+=23, lc+=5) or 255 (pc+=24, lc+=1).
  			//
  			// Let x=deltaPC-PC_RANGE.  If we use opcode 255, x will be the remaining
  			// deltaPC that we need to encode separately before emitting 255.  If we
  			// use opcode 249, we will need to encode x+1.  If x+1 takes one more
  			// byte to encode than x, then we use opcode 255.
  			//
  			// In all other cases x and x+1 take the same number of bytes to encode,
  			// so we use opcode 249, which may save us a byte in encoding deltaLC,
  			// for similar reasons.
  			switch deltaPC - PC_RANGE {
  			// PC_RANGE is the largest deltaPC we can encode in one byte, using
  			// DW_LNS_const_add_pc.
  			//
  			// (1<<16)-1 is the largest deltaPC we can encode in three bytes, using
  			// DW_LNS_fixed_advance_pc.
  			//
  			// (1<<(7n))-1 is the largest deltaPC we can encode in n+1 bytes for
  			// n=1,3,4,5,..., using DW_LNS_advance_pc.
  			case PC_RANGE, (1 << 7) - 1, (1 << 16) - 1, (1 << 21) - 1, (1 << 28) - 1,
  				(1 << 35) - 1, (1 << 42) - 1, (1 << 49) - 1, (1 << 56) - 1, (1 << 63) - 1:
  				opcode = 255
  			default:
  				opcode = OPCODE_BASE + LINE_RANGE*PC_RANGE - 1 // 249
  			}
  		}
  	}
  	if opcode < OPCODE_BASE || opcode > 255 {
  		panic(fmt.Sprintf("produced invalid special opcode %d", opcode))
  	}
  
  	// Subtract from deltaPC and deltaLC the amounts that the opcode will add.
  	deltaPC -= uint64((opcode - OPCODE_BASE) / LINE_RANGE)
  	deltaLC -= int64((opcode-OPCODE_BASE)%LINE_RANGE + LINE_BASE)
  
  	// Encode deltaPC.
  	if deltaPC != 0 {
  		if deltaPC <= PC_RANGE {
  			// Adjust the opcode so that we can use the 1-byte DW_LNS_const_add_pc
  			// instruction.
  			opcode -= LINE_RANGE * int64(PC_RANGE-deltaPC)
  			if opcode < OPCODE_BASE {
  				panic(fmt.Sprintf("produced invalid special opcode %d", opcode))
  			}
  			Adduint8(linkctxt, s, dwarf.DW_LNS_const_add_pc)
  		} else if (1<<14) <= deltaPC && deltaPC < (1<<16) {
  			Adduint8(linkctxt, s, dwarf.DW_LNS_fixed_advance_pc)
  			Adduint16(linkctxt, s, uint16(deltaPC))
  		} else {
  			Adduint8(linkctxt, s, dwarf.DW_LNS_advance_pc)
  			dwarf.Uleb128put(ctxt, s, int64(deltaPC))
  		}
  	}
  
  	// Encode deltaLC.
  	if deltaLC != 0 {
  		Adduint8(linkctxt, s, dwarf.DW_LNS_advance_line)
  		dwarf.Sleb128put(ctxt, s, deltaLC)
  	}
  
  	// Output the special opcode.
  	Adduint8(linkctxt, s, uint8(opcode))
  }
  
  /*
   * Walk prog table, emit line program and build DIE tree.
   */
  
  func getCompilationDir() string {
  	if dir, err := os.Getwd(); err == nil {
  		return dir
  	}
  	return "/"
  }
  
  func writelines(ctxt *Link, syms []*Symbol) ([]*Symbol, []*Symbol) {
  	var dwarfctxt dwarf.Context = dwctxt{ctxt}
  	if linesec == nil {
  		linesec = ctxt.Syms.Lookup(".debug_line", 0)
  	}
  	linesec.Type = obj.SDWARFSECT
  	linesec.R = linesec.R[:0]
  
  	ls := linesec
  	syms = append(syms, ls)
  	var funcs []*Symbol
  
  	unitstart := int64(-1)
  	headerstart := int64(-1)
  	headerend := int64(-1)
  	epc := int64(0)
  	var epcs *Symbol
  	var dwinfo *dwarf.DWDie
  
  	lang := dwarf.DW_LANG_Go
  
  	s := ctxt.Textp[0]
  	if ctxt.DynlinkingGo() && Headtype == obj.Hdarwin {
  		s = ctxt.Textp[1] // skip runtime.text
  	}
  
  	dwinfo = newdie(ctxt, &dwroot, dwarf.DW_ABRV_COMPUNIT, "go", 0)
  	newattr(dwinfo, dwarf.DW_AT_language, dwarf.DW_CLS_CONSTANT, int64(lang), 0)
  	newattr(dwinfo, dwarf.DW_AT_stmt_list, dwarf.DW_CLS_PTR, 0, linesec)
  	newattr(dwinfo, dwarf.DW_AT_low_pc, dwarf.DW_CLS_ADDRESS, s.Value, s)
  	// OS X linker requires compilation dir or absolute path in comp unit name to output debug info.
  	compDir := getCompilationDir()
  	newattr(dwinfo, dwarf.DW_AT_comp_dir, dwarf.DW_CLS_STRING, int64(len(compDir)), compDir)
  
  	// Write .debug_line Line Number Program Header (sec 6.2.4)
  	// Fields marked with (*) must be changed for 64-bit dwarf
  	unitLengthOffset := ls.Size
  	Adduint32(ctxt, ls, 0) // unit_length (*), filled in at end.
  	unitstart = ls.Size
  	Adduint16(ctxt, ls, 2) // dwarf version (appendix F)
  	headerLengthOffset := ls.Size
  	Adduint32(ctxt, ls, 0) // header_length (*), filled in at end.
  	headerstart = ls.Size
  
  	// cpos == unitstart + 4 + 2 + 4
  	Adduint8(ctxt, ls, 1)              // minimum_instruction_length
  	Adduint8(ctxt, ls, 1)              // default_is_stmt
  	Adduint8(ctxt, ls, LINE_BASE&0xFF) // line_base
  	Adduint8(ctxt, ls, LINE_RANGE)     // line_range
  	Adduint8(ctxt, ls, OPCODE_BASE)    // opcode_base
  	Adduint8(ctxt, ls, 0)              // standard_opcode_lengths[1]
  	Adduint8(ctxt, ls, 1)              // standard_opcode_lengths[2]
  	Adduint8(ctxt, ls, 1)              // standard_opcode_lengths[3]
  	Adduint8(ctxt, ls, 1)              // standard_opcode_lengths[4]
  	Adduint8(ctxt, ls, 1)              // standard_opcode_lengths[5]
  	Adduint8(ctxt, ls, 0)              // standard_opcode_lengths[6]
  	Adduint8(ctxt, ls, 0)              // standard_opcode_lengths[7]
  	Adduint8(ctxt, ls, 0)              // standard_opcode_lengths[8]
  	Adduint8(ctxt, ls, 1)              // standard_opcode_lengths[9]
  	Adduint8(ctxt, ls, 0)              // include_directories  (empty)
  
  	for _, f := range ctxt.Filesyms {
  		Addstring(ls, f.Name)
  		Adduint8(ctxt, ls, 0)
  		Adduint8(ctxt, ls, 0)
  		Adduint8(ctxt, ls, 0)
  	}
  
  	// 4 zeros: the string termination + 3 fields.
  	Adduint8(ctxt, ls, 0)
  	// terminate file_names.
  	headerend = ls.Size
  
  	Adduint8(ctxt, ls, 0) // start extended opcode
  	dwarf.Uleb128put(dwarfctxt, ls, 1+int64(SysArch.PtrSize))
  	Adduint8(ctxt, ls, dwarf.DW_LNE_set_address)
  
  	pc := s.Value
  	line := 1
  	file := 1
  	Addaddr(ctxt, ls, s)
  
  	var pcfile Pciter
  	var pcline Pciter
  	for _, s := range ctxt.Textp {
  
  		epc = s.Value + s.Size
  		epcs = s
  
  		dsym := ctxt.Syms.Lookup(dwarf.InfoPrefix+s.Name, int(s.Version))
  		dsym.Attr |= AttrHidden | AttrReachable
  		dsym.Type = obj.SDWARFINFO
  		for _, r := range dsym.R {
  			if r.Type == obj.R_DWARFREF && r.Sym.Size == 0 {
  				if Buildmode == BuildmodeShared {
  					// These type symbols may not be present in BuildmodeShared. Skip.
  					continue
  				}
  				n := nameFromDIESym(r.Sym)
  				defgotype(ctxt, ctxt.Syms.Lookup("type."+n, 0))
  			}
  		}
  		funcs = append(funcs, dsym)
  
  		if s.FuncInfo == nil {
  			continue
  		}
  
  		finddebugruntimepath(s)
  
  		pciterinit(ctxt, &pcfile, &s.FuncInfo.Pcfile)
  		pciterinit(ctxt, &pcline, &s.FuncInfo.Pcline)
  		epc = pc
  		for pcfile.done == 0 && pcline.done == 0 {
  			if epc-s.Value >= int64(pcfile.nextpc) {
  				pciternext(&pcfile)
  				continue
  			}
  
  			if epc-s.Value >= int64(pcline.nextpc) {
  				pciternext(&pcline)
  				continue
  			}
  
  			if int32(file) != pcfile.value {
  				Adduint8(ctxt, ls, dwarf.DW_LNS_set_file)
  				dwarf.Uleb128put(dwarfctxt, ls, int64(pcfile.value))
  				file = int(pcfile.value)
  			}
  
  			putpclcdelta(ctxt, dwarfctxt, ls, uint64(s.Value+int64(pcline.pc)-pc), int64(pcline.value)-int64(line))
  
  			pc = s.Value + int64(pcline.pc)
  			line = int(pcline.value)
  			if pcfile.nextpc < pcline.nextpc {
  				epc = int64(pcfile.nextpc)
  			} else {
  				epc = int64(pcline.nextpc)
  			}
  			epc += s.Value
  		}
  	}
  
  	Adduint8(ctxt, ls, 0) // start extended opcode
  	dwarf.Uleb128put(dwarfctxt, ls, 1)
  	Adduint8(ctxt, ls, dwarf.DW_LNE_end_sequence)
  
  	newattr(dwinfo, dwarf.DW_AT_high_pc, dwarf.DW_CLS_ADDRESS, epc+1, epcs)
  
  	setuint32(ctxt, ls, unitLengthOffset, uint32(ls.Size-unitstart))
  	setuint32(ctxt, ls, headerLengthOffset, uint32(headerend-headerstart))
  
  	return syms, funcs
  }
  
  /*
   *  Emit .debug_frame
   */
  const (
  	dataAlignmentFactor = -4
  )
  
  // appendPCDeltaCFA appends per-PC CFA deltas to b and returns the final slice.
  func appendPCDeltaCFA(b []byte, deltapc, cfa int64) []byte {
  	b = append(b, dwarf.DW_CFA_def_cfa_offset_sf)
  	b = dwarf.AppendSleb128(b, cfa/dataAlignmentFactor)
  
  	switch {
  	case deltapc < 0x40:
  		b = append(b, uint8(dwarf.DW_CFA_advance_loc+deltapc))
  	case deltapc < 0x100:
  		b = append(b, dwarf.DW_CFA_advance_loc1)
  		b = append(b, uint8(deltapc))
  	case deltapc < 0x10000:
  		b = append(b, dwarf.DW_CFA_advance_loc2)
  		b = Thearch.Append16(b, uint16(deltapc))
  	default:
  		b = append(b, dwarf.DW_CFA_advance_loc4)
  		b = Thearch.Append32(b, uint32(deltapc))
  	}
  	return b
  }
  
  func writeframes(ctxt *Link, syms []*Symbol) []*Symbol {
  	var dwarfctxt dwarf.Context = dwctxt{ctxt}
  	if framesec == nil {
  		framesec = ctxt.Syms.Lookup(".debug_frame", 0)
  	}
  	framesec.Type = obj.SDWARFSECT
  	framesec.R = framesec.R[:0]
  	fs := framesec
  	syms = append(syms, fs)
  
  	// Emit the CIE, Section 6.4.1
  	cieReserve := uint32(16)
  	if haslinkregister(ctxt) {
  		cieReserve = 32
  	}
  	Adduint32(ctxt, fs, cieReserve)                            // initial length, must be multiple of thearch.ptrsize
  	Adduint32(ctxt, fs, 0xffffffff)                            // cid.
  	Adduint8(ctxt, fs, 3)                                      // dwarf version (appendix F)
  	Adduint8(ctxt, fs, 0)                                      // augmentation ""
  	dwarf.Uleb128put(dwarfctxt, fs, 1)                         // code_alignment_factor
  	dwarf.Sleb128put(dwarfctxt, fs, dataAlignmentFactor)       // all CFI offset calculations include multiplication with this factor
  	dwarf.Uleb128put(dwarfctxt, fs, int64(Thearch.Dwarfreglr)) // return_address_register
  
  	Adduint8(ctxt, fs, dwarf.DW_CFA_def_cfa)                   // Set the current frame address..
  	dwarf.Uleb128put(dwarfctxt, fs, int64(Thearch.Dwarfregsp)) // ...to use the value in the platform's SP register (defined in l.go)...
  	if haslinkregister(ctxt) {
  		dwarf.Uleb128put(dwarfctxt, fs, int64(0)) // ...plus a 0 offset.
  
  		Adduint8(ctxt, fs, dwarf.DW_CFA_same_value) // The platform's link register is unchanged during the prologue.
  		dwarf.Uleb128put(dwarfctxt, fs, int64(Thearch.Dwarfreglr))
  
  		Adduint8(ctxt, fs, dwarf.DW_CFA_val_offset)                // The previous value...
  		dwarf.Uleb128put(dwarfctxt, fs, int64(Thearch.Dwarfregsp)) // ...of the platform's SP register...
  		dwarf.Uleb128put(dwarfctxt, fs, int64(0))                  // ...is CFA+0.
  	} else {
  		dwarf.Uleb128put(dwarfctxt, fs, int64(SysArch.PtrSize)) // ...plus the word size (because the call instruction implicitly adds one word to the frame).
  
  		Adduint8(ctxt, fs, dwarf.DW_CFA_offset_extended)                             // The previous value...
  		dwarf.Uleb128put(dwarfctxt, fs, int64(Thearch.Dwarfreglr))                   // ...of the return address...
  		dwarf.Uleb128put(dwarfctxt, fs, int64(-SysArch.PtrSize)/dataAlignmentFactor) // ...is saved at [CFA - (PtrSize/4)].
  	}
  
  	// 4 is to exclude the length field.
  	pad := int64(cieReserve) + 4 - fs.Size
  
  	if pad < 0 {
  		Exitf("dwarf: cieReserve too small by %d bytes.", -pad)
  	}
  
  	Addbytes(fs, zeros[:pad])
  
  	var deltaBuf []byte
  	var pcsp Pciter
  	for _, s := range ctxt.Textp {
  		if s.FuncInfo == nil {
  			continue
  		}
  
  		// Emit a FDE, Section 6.4.1.
  		// First build the section contents into a byte buffer.
  		deltaBuf = deltaBuf[:0]
  		for pciterinit(ctxt, &pcsp, &s.FuncInfo.Pcsp); pcsp.done == 0; pciternext(&pcsp) {
  			nextpc := pcsp.nextpc
  
  			// pciterinit goes up to the end of the function,
  			// but DWARF expects us to stop just before the end.
  			if int64(nextpc) == s.Size {
  				nextpc--
  				if nextpc < pcsp.pc {
  					continue
  				}
  			}
  
  			if haslinkregister(ctxt) {
  				// TODO(bryanpkc): This is imprecise. In general, the instruction
  				// that stores the return address to the stack frame is not the
  				// same one that allocates the frame.
  				if pcsp.value > 0 {
  					// The return address is preserved at (CFA-frame_size)
  					// after a stack frame has been allocated.
  					deltaBuf = append(deltaBuf, dwarf.DW_CFA_offset_extended_sf)
  					deltaBuf = dwarf.AppendUleb128(deltaBuf, uint64(Thearch.Dwarfreglr))
  					deltaBuf = dwarf.AppendSleb128(deltaBuf, -int64(pcsp.value)/dataAlignmentFactor)
  				} else {
  					// The return address is restored into the link register
  					// when a stack frame has been de-allocated.
  					deltaBuf = append(deltaBuf, dwarf.DW_CFA_same_value)
  					deltaBuf = dwarf.AppendUleb128(deltaBuf, uint64(Thearch.Dwarfreglr))
  				}
  				deltaBuf = appendPCDeltaCFA(deltaBuf, int64(nextpc)-int64(pcsp.pc), int64(pcsp.value))
  			} else {
  				deltaBuf = appendPCDeltaCFA(deltaBuf, int64(nextpc)-int64(pcsp.pc), int64(SysArch.PtrSize)+int64(pcsp.value))
  			}
  		}
  		pad := int(Rnd(int64(len(deltaBuf)), int64(SysArch.PtrSize))) - len(deltaBuf)
  		deltaBuf = append(deltaBuf, zeros[:pad]...)
  
  		// Emit the FDE header, Section 6.4.1.
  		//	4 bytes: length, must be multiple of thearch.ptrsize
  		//	4 bytes: Pointer to the CIE above, at offset 0
  		//	ptrsize: initial location
  		//	ptrsize: address range
  		Adduint32(ctxt, fs, uint32(4+2*SysArch.PtrSize+len(deltaBuf))) // length (excludes itself)
  		if Linkmode == LinkExternal {
  			adddwarfref(ctxt, fs, framesec, 4)
  		} else {
  			Adduint32(ctxt, fs, 0) // CIE offset
  		}
  		Addaddr(ctxt, fs, s)
  		adduintxx(ctxt, fs, uint64(s.Size), SysArch.PtrSize) // address range
  		Addbytes(fs, deltaBuf)
  	}
  	return syms
  }
  
  /*
   *  Walk DWarfDebugInfoEntries, and emit .debug_info
   */
  const (
  	COMPUNITHEADERSIZE = 4 + 2 + 4 + 1
  )
  
  func writeinfo(ctxt *Link, syms []*Symbol, funcs []*Symbol) []*Symbol {
  	if infosec == nil {
  		infosec = ctxt.Syms.Lookup(".debug_info", 0)
  	}
  	infosec.R = infosec.R[:0]
  	infosec.Type = obj.SDWARFINFO
  	infosec.Attr |= AttrReachable
  	syms = append(syms, infosec)
  
  	if arangessec == nil {
  		arangessec = ctxt.Syms.Lookup(".dwarfaranges", 0)
  	}
  	arangessec.R = arangessec.R[:0]
  
  	var dwarfctxt dwarf.Context = dwctxt{ctxt}
  
  	for compunit := dwroot.Child; compunit != nil; compunit = compunit.Link {
  		s := dtolsym(compunit.Sym)
  
  		// Write .debug_info Compilation Unit Header (sec 7.5.1)
  		// Fields marked with (*) must be changed for 64-bit dwarf
  		// This must match COMPUNITHEADERSIZE above.
  		Adduint32(ctxt, s, 0) // unit_length (*), will be filled in later.
  		Adduint16(ctxt, s, 2) // dwarf version (appendix F)
  
  		// debug_abbrev_offset (*)
  		adddwarfref(ctxt, s, abbrevsym, 4)
  
  		Adduint8(ctxt, s, uint8(SysArch.PtrSize)) // address_size
  
  		dwarf.Uleb128put(dwarfctxt, s, int64(compunit.Abbrev))
  		dwarf.PutAttrs(dwarfctxt, s, compunit.Abbrev, compunit.Attr)
  
  		cu := []*Symbol{s}
  		if funcs != nil {
  			cu = append(cu, funcs...)
  			funcs = nil
  		}
  		cu = putdies(ctxt, dwarfctxt, cu, compunit.Child)
  		var cusize int64
  		for _, child := range cu {
  			cusize += child.Size
  		}
  		cusize -= 4 // exclude the length field.
  		setuint32(ctxt, s, 0, uint32(cusize))
  		newattr(compunit, dwarf.DW_AT_byte_size, dwarf.DW_CLS_CONSTANT, cusize, 0)
  		syms = append(syms, cu...)
  	}
  	return syms
  }
  
  /*
   *  Emit .debug_pubnames/_types.  _info must have been written before,
   *  because we need die->offs and infoo/infosize;
   */
  func ispubname(die *dwarf.DWDie) bool {
  	switch die.Abbrev {
  	case dwarf.DW_ABRV_FUNCTION, dwarf.DW_ABRV_VARIABLE:
  		a := getattr(die, dwarf.DW_AT_external)
  		return a != nil && a.Value != 0
  	}
  
  	return false
  }
  
  func ispubtype(die *dwarf.DWDie) bool {
  	return die.Abbrev >= dwarf.DW_ABRV_NULLTYPE
  }
  
  func writepub(ctxt *Link, sname string, ispub func(*dwarf.DWDie) bool, syms []*Symbol) []*Symbol {
  	s := ctxt.Syms.Lookup(sname, 0)
  	s.Type = obj.SDWARFSECT
  	syms = append(syms, s)
  
  	for compunit := dwroot.Child; compunit != nil; compunit = compunit.Link {
  		sectionstart := s.Size
  		culength := uint32(getattr(compunit, dwarf.DW_AT_byte_size).Value) + 4
  
  		// Write .debug_pubnames/types	Header (sec 6.1.1)
  		Adduint32(ctxt, s, 0)                          // unit_length (*), will be filled in later.
  		Adduint16(ctxt, s, 2)                          // dwarf version (appendix F)
  		adddwarfref(ctxt, s, dtolsym(compunit.Sym), 4) // debug_info_offset (of the Comp unit Header)
  		Adduint32(ctxt, s, culength)                   // debug_info_length
  
  		for die := compunit.Child; die != nil; die = die.Link {
  			if !ispub(die) {
  				continue
  			}
  			dwa := getattr(die, dwarf.DW_AT_name)
  			name := dwa.Data.(string)
  			if die.Sym == nil {
  				fmt.Println("Missing sym for ", name)
  			}
  			adddwarfref(ctxt, s, dtolsym(die.Sym), 4)
  			Addstring(s, name)
  		}
  
  		Adduint32(ctxt, s, 0)
  
  		setuint32(ctxt, s, sectionstart, uint32(s.Size-sectionstart)-4) // exclude the length field.
  	}
  
  	return syms
  }
  
  /*
   *  emit .debug_aranges.  _info must have been written before,
   *  because we need die->offs of dwarf.DW_globals.
   */
  func writearanges(ctxt *Link, syms []*Symbol) []*Symbol {
  	s := ctxt.Syms.Lookup(".debug_aranges", 0)
  	s.Type = obj.SDWARFSECT
  	// The first tuple is aligned to a multiple of the size of a single tuple
  	// (twice the size of an address)
  	headersize := int(Rnd(4+2+4+1+1, int64(SysArch.PtrSize*2))) // don't count unit_length field itself
  
  	for compunit := dwroot.Child; compunit != nil; compunit = compunit.Link {
  		b := getattr(compunit, dwarf.DW_AT_low_pc)
  		if b == nil {
  			continue
  		}
  		e := getattr(compunit, dwarf.DW_AT_high_pc)
  		if e == nil {
  			continue
  		}
  
  		// Write .debug_aranges	 Header + entry	 (sec 6.1.2)
  		unitlength := uint32(headersize) + 4*uint32(SysArch.PtrSize) - 4
  		Adduint32(ctxt, s, unitlength) // unit_length (*)
  		Adduint16(ctxt, s, 2)          // dwarf version (appendix F)
  
  		adddwarfref(ctxt, s, dtolsym(compunit.Sym), 4)
  
  		Adduint8(ctxt, s, uint8(SysArch.PtrSize)) // address_size
  		Adduint8(ctxt, s, 0)                      // segment_size
  		padding := headersize - (4 + 2 + 4 + 1 + 1)
  		for i := 0; i < padding; i++ {
  			Adduint8(ctxt, s, 0)
  		}
  
  		Addaddrplus(ctxt, s, b.Data.(*Symbol), b.Value-(b.Data.(*Symbol)).Value)
  		adduintxx(ctxt, s, uint64(e.Value-b.Value), SysArch.PtrSize)
  		adduintxx(ctxt, s, 0, SysArch.PtrSize)
  		adduintxx(ctxt, s, 0, SysArch.PtrSize)
  	}
  	if s.Size > 0 {
  		syms = append(syms, s)
  	}
  	return syms
  }
  
  func writegdbscript(ctxt *Link, syms []*Symbol) []*Symbol {
  
  	if gdbscript != "" {
  		s := ctxt.Syms.Lookup(".debug_gdb_scripts", 0)
  		s.Type = obj.SDWARFSECT
  		syms = append(syms, s)
  		Adduint8(ctxt, s, 1) // magic 1 byte?
  		Addstring(s, gdbscript)
  	}
  
  	return syms
  }
  
  var prototypedies map[string]*dwarf.DWDie
  
  /*
   * This is the main entry point for generating dwarf.  After emitting
   * the mandatory debug_abbrev section, it calls writelines() to set up
   * the per-compilation unit part of the DIE tree, while simultaneously
   * emitting the debug_line section.  When the final tree contains
   * forward references, it will write the debug_info section in 2
   * passes.
   *
   */
  func dwarfgeneratedebugsyms(ctxt *Link) {
  	if *FlagW { // disable dwarf
  		return
  	}
  	if *FlagS && Headtype != obj.Hdarwin {
  		return
  	}
  	if Headtype == obj.Hplan9 {
  		return
  	}
  
  	if Linkmode == LinkExternal {
  		if !Iself && Headtype != obj.Hdarwin {
  			return
  		}
  	}
  
  	if ctxt.Debugvlog != 0 {
  		ctxt.Logf("%5.2f dwarf\n", obj.Cputime())
  	}
  
  	// Forctxt.Diagnostic messages.
  	newattr(&dwtypes, dwarf.DW_AT_name, dwarf.DW_CLS_STRING, int64(len("dwtypes")), "dwtypes")
  
  	// Some types that must exist to define other ones.
  	newdie(ctxt, &dwtypes, dwarf.DW_ABRV_NULLTYPE, "<unspecified>", 0)
  
  	newdie(ctxt, &dwtypes, dwarf.DW_ABRV_NULLTYPE, "void", 0)
  	newdie(ctxt, &dwtypes, dwarf.DW_ABRV_BARE_PTRTYPE, "unsafe.Pointer", 0)
  
  	die := newdie(ctxt, &dwtypes, dwarf.DW_ABRV_BASETYPE, "uintptr", 0) // needed for array size
  	newattr(die, dwarf.DW_AT_encoding, dwarf.DW_CLS_CONSTANT, dwarf.DW_ATE_unsigned, 0)
  	newattr(die, dwarf.DW_AT_byte_size, dwarf.DW_CLS_CONSTANT, int64(SysArch.PtrSize), 0)
  	newattr(die, dwarf.DW_AT_go_kind, dwarf.DW_CLS_CONSTANT, obj.KindUintptr, 0)
  
  	// Prototypes needed for type synthesis.
  	prototypedies = map[string]*dwarf.DWDie{
  		"type.runtime.stringStructDWARF": nil,
  		"type.runtime.slice":             nil,
  		"type.runtime.hmap":              nil,
  		"type.runtime.bmap":              nil,
  		"type.runtime.sudog":             nil,
  		"type.runtime.waitq":             nil,
  		"type.runtime.hchan":             nil,
  	}
  
  	// Needed by the prettyprinter code for interface inspection.
  	defgotype(ctxt, lookupOrDiag(ctxt, "type.runtime._type"))
  
  	defgotype(ctxt, lookupOrDiag(ctxt, "type.runtime.interfacetype"))
  	defgotype(ctxt, lookupOrDiag(ctxt, "type.runtime.itab"))
  
  	genasmsym(ctxt, defdwsymb)
  
  	syms := writeabbrev(ctxt, nil)
  	syms, funcs := writelines(ctxt, syms)
  	syms = writeframes(ctxt, syms)
  
  	synthesizestringtypes(ctxt, dwtypes.Child)
  	synthesizeslicetypes(ctxt, dwtypes.Child)
  	synthesizemaptypes(ctxt, dwtypes.Child)
  	synthesizechantypes(ctxt, dwtypes.Child)
  
  	reversetree(&dwroot.Child)
  	reversetree(&dwtypes.Child)
  	reversetree(&dwglobals.Child)
  
  	movetomodule(&dwtypes)
  	movetomodule(&dwglobals)
  
  	// Need to reorder symbols so SDWARFINFO is after all SDWARFSECT
  	// (but we need to generate dies before writepub)
  	infosyms := writeinfo(ctxt, nil, funcs)
  
  	syms = writepub(ctxt, ".debug_pubnames", ispubname, syms)
  	syms = writepub(ctxt, ".debug_pubtypes", ispubtype, syms)
  	syms = writearanges(ctxt, syms)
  	syms = writegdbscript(ctxt, syms)
  	syms = append(syms, infosyms...)
  	dwarfp = syms
  }
  
  /*
   *  Elf.
   */
  func dwarfaddshstrings(ctxt *Link, shstrtab *Symbol) {
  	if *FlagW { // disable dwarf
  		return
  	}
  
  	Addstring(shstrtab, ".debug_abbrev")
  	Addstring(shstrtab, ".debug_aranges")
  	Addstring(shstrtab, ".debug_frame")
  	Addstring(shstrtab, ".debug_info")
  	Addstring(shstrtab, ".debug_line")
  	Addstring(shstrtab, ".debug_pubnames")
  	Addstring(shstrtab, ".debug_pubtypes")
  	Addstring(shstrtab, ".debug_gdb_scripts")
  	if Linkmode == LinkExternal {
  		Addstring(shstrtab, elfRelType+".debug_info")
  		Addstring(shstrtab, elfRelType+".debug_aranges")
  		Addstring(shstrtab, elfRelType+".debug_line")
  		Addstring(shstrtab, elfRelType+".debug_frame")
  		Addstring(shstrtab, elfRelType+".debug_pubnames")
  		Addstring(shstrtab, elfRelType+".debug_pubtypes")
  	}
  }
  
  // Add section symbols for DWARF debug info.  This is called before
  // dwarfaddelfheaders.
  func dwarfaddelfsectionsyms(ctxt *Link) {
  	if *FlagW { // disable dwarf
  		return
  	}
  	if Linkmode != LinkExternal {
  		return
  	}
  	sym := ctxt.Syms.Lookup(".debug_info", 0)
  	putelfsectionsym(sym, sym.Sect.Elfsect.shnum)
  	sym = ctxt.Syms.Lookup(".debug_abbrev", 0)
  	putelfsectionsym(sym, sym.Sect.Elfsect.shnum)
  	sym = ctxt.Syms.Lookup(".debug_line", 0)
  	putelfsectionsym(sym, sym.Sect.Elfsect.shnum)
  	sym = ctxt.Syms.Lookup(".debug_frame", 0)
  	putelfsectionsym(sym, sym.Sect.Elfsect.shnum)
  }
  
  /*
   * Windows PE
   */
  func dwarfaddpeheaders(ctxt *Link) {
  	if *FlagW { // disable dwarf
  		return
  	}
  	for sect := Segdwarf.Sect; sect != nil; sect = sect.Next {
  		h := newPEDWARFSection(ctxt, sect.Name, int64(sect.Length))
  		fileoff := sect.Vaddr - Segdwarf.Vaddr + Segdwarf.Fileoff
  		if uint64(h.PointerToRawData) != fileoff {
  			Exitf("%s.PointerToRawData = %#x, want %#x", sect.Name, h.PointerToRawData, fileoff)
  		}
  	}
  }
  

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