internal/gocore/dwarf.go - debug - Git at Google

 // Copyright 2017 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.

 package gocore

 import (
 	"debug/dwarf"
 	"fmt"
 	"regexp"
 	"sort"
 	"strings"

 	"golang.org/x/debug/internal/core"
 )

 // read DWARF types from core dump.
 func (p *Process) readDWARFTypes() {
 	d, _ := p.proc.DWARF()

 	// Make one of our own Types for each dwarf type.
 	r := d.Reader()
 	var types []*Type
 	for e, err := r.Next(); e != nil && err == nil; e, err = r.Next() {
 		if isNonGoCU(e) {
 			r.SkipChildren()
 			continue
 		}
 		switch e.Tag {
 		case dwarf.TagArrayType, dwarf.TagPointerType, dwarf.TagStructType, dwarf.TagBaseType, dwarf.TagSubroutineType, dwarf.TagTypedef:
 			dt, err := d.Type(e.Offset)
 			if err != nil {
 				continue
 			}
 			t := &Type{Name: gocoreName(dt), Size: dwarfSize(dt, p.proc.PtrSize())}
 			p.dwarfMap[dt] = t
 			types = append(types, t)
 		}
 	}

 	p.runtimeNameMap = map[string][]*Type{}

 	// Fill in fields of types. Postponed until now so we're sure
 	// we have all the Types allocated and available.
 	for dt, t := range p.dwarfMap {
 		switch x := dt.(type) {
 		case *dwarf.ArrayType:
 			t.Kind = KindArray
 			t.Elem = p.dwarfMap[x.Type]
 			t.Count = x.Count
 		case *dwarf.PtrType:
 			t.Kind = KindPtr
 			// unsafe.Pointer has a void base type.
 			if _, ok := x.Type.(*dwarf.VoidType); !ok {
 				t.Elem = p.dwarfMap[x.Type]
 			}
 		case *dwarf.StructType:
 			t.Kind = KindStruct
 			for _, f := range x.Field {
 				fType := p.dwarfMap[f.Type]

 				// Work around issue 21094. There's no guarantee that the
 				// pointer type is in the DWARF, so just invent a Type.
 				if strings.HasPrefix(t.Name, "sudog<") && f.Name == "elem" &&
 					strings.Count(t.Name, "*")+1 != strings.Count(gocoreName(f.Type), "*") {
 					ptrName := "*" + gocoreName(f.Type)
 					fType = &Type{Name: ptrName, Kind: KindPtr, Size: p.proc.PtrSize(), Elem: fType}
 					p.runtimeNameMap[ptrName] = []*Type{fType}
 				}

 				t.Fields = append(t.Fields, Field{Name: f.Name, Type: fType, Off: f.ByteOffset})
 			}
 		case *dwarf.BoolType:
 			t.Kind = KindBool
 		case *dwarf.IntType:
 			t.Kind = KindInt
 		case *dwarf.UintType:
 			t.Kind = KindUint
 		case *dwarf.FloatType:
 			t.Kind = KindFloat
 		case *dwarf.ComplexType:
 			t.Kind = KindComplex
 		case *dwarf.FuncType:
 			t.Kind = KindFunc
 		case *dwarf.TypedefType:
 			// handle these types in the loop below
 		default:
 			panic(fmt.Sprintf("unknown type %s %T", dt, dt))
 		}
 	}

 	// Detect strings & slices
 	for _, t := range types {
 		if t.Kind != KindStruct {
 			continue
 		}
 		if t.Name == "string" { // TODO: also "struct runtime.stringStructDWARF" ?
 			t.Kind = KindString
 			t.Elem = t.Fields[0].Type.Elem // TODO: check that it is always uint8.
 			t.Fields = nil
 		}
 		if len(t.Name) >= 9 && t.Name[:9] == "struct []" ||
 			len(t.Name) >= 2 && t.Name[:2] == "[]" {
 			t.Kind = KindSlice
 			t.Elem = t.Fields[0].Type.Elem
 			t.Fields = nil
 		}
 	}

 	// Copy info from base types into typedefs.
 	for dt, t := range p.dwarfMap {
 		tt, ok := dt.(*dwarf.TypedefType)
 		if !ok {
 			continue
 		}
 		base := tt.Type
 		// Walk typedef chain until we reach a non-typedef type.
 		for {
 			if x, ok := base.(*dwarf.TypedefType); ok {
 				base = x.Type
 				continue
 			}
 			break
 		}
 		bt := p.dwarfMap[base]

 		// Copy type info from base. Everything except the name.
 		name := t.Name
 		*t = *bt
 		t.Name = name

 		// Detect some special types. If the base is some particular type,
 		// then the alias gets marked as special.
 		// We have aliases like:
 		//   interface {}              -> struct runtime.eface
 		//   error                     -> struct runtime.iface
 		// Note: the base itself does not get marked as special.
 		// (Unlike strings and slices, where they do.)
 		if bt.Name == "runtime.eface" {
 			t.Kind = KindEface
 			t.Fields = nil
 		}
 		if bt.Name == "runtime.iface" {
 			t.Kind = KindIface
 			t.Fields = nil
 		}
 	}

 	// Make a runtime name -> Type map for existing DWARF types.
 	for dt, t := range p.dwarfMap {
 		name := runtimeName(dt)
 		p.runtimeNameMap[name] = append(p.runtimeNameMap[name], t)
 	}

 	// Construct the runtime.specialfinalizer type.  It won't be found
 	// in DWARF before 1.10 because it does not appear in the type of any variable.
 	// type specialfinalizer struct {
 	//      special special
 	//      fn      *funcval
 	//      nret    uintptr
 	//      fint    *_type
 	//      ot      *ptrtype
 	// }
 	if p.runtimeNameMap["runtime.specialfinalizer"] == nil {
 		special := p.findType("runtime.special")
 		p.runtimeNameMap["runtime.specialfinalizer"] = []*Type{
 			&Type{
 				Name: "runtime.specialfinalizer",
 				Size: special.Size + 4*p.proc.PtrSize(),
 				Kind: KindStruct,
 				Fields: []Field{
 					Field{
 						Name: "special",
 						Off:  0,
 						Type: special,
 					},
 					Field{
 						Name: "fn",
 						Off:  special.Size,
 						Type: p.findType("*runtime.funcval"),
 					},
 					Field{
 						Name: "nret",
 						Off:  special.Size + p.proc.PtrSize(),
 						Type: p.findType("uintptr"),
 					},
 					Field{
 						Name: "fint",
 						Off:  special.Size + 2*p.proc.PtrSize(),
 						Type: p.findType("*runtime._type"),
 					},
 					Field{
 						Name: "fn",
 						Off:  special.Size + 3*p.proc.PtrSize(),
 						Type: p.findType("*runtime.ptrtype"),
 					},
 				},
 			},
 		}
 	}
 }

 func isNonGoCU(e *dwarf.Entry) bool {
 	if e.Tag != dwarf.TagCompileUnit {
 		return false
 	}
 	prod, ok := e.Val(dwarf.AttrProducer).(string)
 	if !ok {
 		return true
 	}
 	return !strings.Contains(prod, "Go cmd/compile")
 }

 // dwarfSize is used to compute the size of a DWARF type.
 // dt.Size() is wrong when it returns a negative number.
 // This function implements just enough to correct the bad behavior.
 func dwarfSize(dt dwarf.Type, ptrSize int64) int64 {
 	s := dt.Size()
 	if s >= 0 {
 		return s
 	}
 	switch x := dt.(type) {
 	case *dwarf.FuncType:
 		return ptrSize // Fix for issue 21097.
 	case *dwarf.ArrayType:
 		return x.Count * dwarfSize(x.Type, ptrSize)
 	case *dwarf.TypedefType:
 		return dwarfSize(x.Type, ptrSize)
 	default:
 		panic(fmt.Sprintf("unhandled: %s, %T", x, x))
 	}
 }

 // gocoreName generates the name this package uses to refer to a dwarf type.
 // This name differs from the dwarf name in that it stays closer to the Go name for the type.
 // For instance (dwarf name -> gocoreName)
 //   struct runtime.siginfo -> runtime.siginfo
 //   *void -> unsafe.Pointer
 //   struct struct { runtime.signalLock uint32; runtime.hz int32 } -> struct { signalLock uint32; hz int32 }
 func gocoreName(dt dwarf.Type) string {
 	switch x := dt.(type) {
 	case *dwarf.PtrType:
 		if _, ok := x.Type.(*dwarf.VoidType); ok {
 			return "unsafe.Pointer"
 		}
 		return "*" + gocoreName(x.Type)
 	case *dwarf.ArrayType:
 		return fmt.Sprintf("[%d]%s", x.Count, gocoreName(x.Type))
 	case *dwarf.StructType:
 		if !strings.HasPrefix(x.StructName, "struct {") {
 			// This is a named type, return that name.
 			return x.StructName
 		}
 		// Build gocore name from the DWARF fields.
 		s := "struct {"
 		first := true
 		for _, f := range x.Field {
 			if !first {
 				s += ";"
 			}
 			name := f.Name
 			if i := strings.Index(name, "."); i >= 0 {
 				// Remove pkg path from field names.
 				name = name[i+1:]
 			}
 			s += fmt.Sprintf(" %s %s", name, gocoreName(f.Type))
 			first = false
 		}
 		s += " }"
 		return s
 	default:
 		return dt.String()
 	}
 }

 // Generate the name the runtime uses for a dwarf type. The DWARF generator
 // and the runtime use slightly different names for the same underlying type.
 func runtimeName(dt dwarf.Type) string {
 	switch x := dt.(type) {
 	case *dwarf.PtrType:
 		if _, ok := x.Type.(*dwarf.VoidType); ok {
 			return "unsafe.Pointer"
 		}
 		return "*" + runtimeName(x.Type)
 	case *dwarf.ArrayType:
 		return fmt.Sprintf("[%d]%s", x.Count, runtimeName(x.Type))
 	case *dwarf.StructType:
 		if !strings.HasPrefix(x.StructName, "struct {") {
 			// This is a named type, return that name.
 			return stripPackagePath(x.StructName)
 		}
 		// Figure out which fields have anonymous names.
 		var anon []bool
 		for _, f := range strings.Split(x.StructName[8:len(x.StructName)-1], ";") {
 			f = strings.TrimSpace(f)
 			anon = append(anon, !strings.Contains(f, " "))
 			// TODO: this isn't perfect. If the field type has a space in it,
 			// then this logic doesn't work. Need to search for keyword for
 			// field type, like "interface", "struct", ...
 		}
 		// Make sure anon is long enough. This probably never triggers.
 		for len(anon) < len(x.Field) {
 			anon = append(anon, false)
 		}

 		// Build runtime name from the DWARF fields.
 		s := "struct {"
 		first := true
 		for _, f := range x.Field {
 			if !first {
 				s += ";"
 			}
 			name := f.Name
 			if i := strings.Index(name, "."); i >= 0 {
 				name = name[i+1:]
 			}
 			if anon[0] {
 				s += fmt.Sprintf(" %s", runtimeName(f.Type))
 			} else {
 				s += fmt.Sprintf(" %s %s", name, runtimeName(f.Type))
 			}
 			first = false
 			anon = anon[1:]
 		}
 		s += " }"
 		return s
 	default:
 		return stripPackagePath(dt.String())
 	}
 }

 var pathRegexp = regexp.MustCompile(`([\w.]+/)+\w+`)

 func stripPackagePath(name string) string {
 	// The runtime uses just package names. DWARF uses whole package paths.
 	// To convert from the latter to the former, get rid of the package paths.
 	// Examples:
 	//   text/template.Template -> template.Template
 	//   map[string]compress/gzip.Writer -> map[string]gzip.Writer
 	return pathRegexp.ReplaceAllStringFunc(name, func(path string) string {
 		return path[strings.LastIndex(path, "/")+1:]
 	})
 }

 // readRuntimeConstants populates the p.rtConstants map.
 func (p *Process) readRuntimeConstants() {
 	p.rtConstants = map[string]int64{}

 	// Hardcoded values for Go 1.9.
 	// (Go did not have constants in DWARF before 1.10.)
 	m := p.rtConstants
 	m["_MSpanDead"] = 0
 	m["_MSpanInUse"] = 1
 	m["_MSpanManual"] = 2
 	m["_MSpanFree"] = 3
 	m["_Gidle"] = 0
 	m["_Grunnable"] = 1
 	m["_Grunning"] = 2
 	m["_Gsyscall"] = 3
 	m["_Gwaiting"] = 4
 	m["_Gdead"] = 6
 	m["_Gscan"] = 0x1000
 	m["_PCDATA_StackMapIndex"] = 0
 	m["_FUNCDATA_LocalsPointerMaps"] = 1
 	m["_FUNCDATA_ArgsPointerMaps"] = 0
 	m["tflagExtraStar"] = 1 << 1
 	m["kindGCProg"] = 1 << 6
 	m["kindDirectIface"] = 1 << 5
 	m["_PageSize"] = 1 << 13
 	m["_KindSpecialFinalizer"] = 1

 	// From 1.10, these constants are recorded in DWARF records.
 	d, _ := p.proc.DWARF()
 	r := d.Reader()
 	for e, err := r.Next(); e != nil && err == nil; e, err = r.Next() {
 		if e.Tag != dwarf.TagConstant {
 			continue
 		}
 		f := e.AttrField(dwarf.AttrName)
 		if f == nil {
 			continue
 		}
 		name := f.Val.(string)
 		if !strings.HasPrefix(name, "runtime.") {
 			continue
 		}
 		name = name[8:]
 		c := e.AttrField(dwarf.AttrConstValue)
 		if c == nil {
 			continue
 		}
 		p.rtConstants[name] = c.Val.(int64)
 	}
 }

 const (
 	_DW_OP_addr           = 0x03
 	_DW_OP_call_frame_cfa = 0x9c
 	_DW_OP_plus           = 0x22
 	_DW_OP_consts         = 0x11
 )

 func (p *Process) readGlobals() {
 	d, _ := p.proc.DWARF()
 	r := d.Reader()
 	for e, err := r.Next(); e != nil && err == nil; e, err = r.Next() {
 		if isNonGoCU(e) {
 			r.SkipChildren()
 			continue
 		}

 		if e.Tag != dwarf.TagVariable {
 			continue
 		}
 		f := e.AttrField(dwarf.AttrLocation)
 		if f == nil {
 			continue
 		}
 		if f.Class != dwarf.ClassExprLoc {
 			// Globals are all encoded with this class.
 			continue
 		}
 		loc := f.Val.([]byte)
 		if len(loc) == 0 || loc[0] != _DW_OP_addr {
 			continue
 		}
 		var a core.Address
 		if p.proc.PtrSize() == 8 {
 			a = core.Address(p.proc.ByteOrder().Uint64(loc[1:]))
 		} else {
 			a = core.Address(p.proc.ByteOrder().Uint32(loc[1:]))
 		}
 		if !p.proc.Writeable(a) {
 			// Read-only globals can't have heap pointers.
 			// TODO: keep roots around anyway?
 			continue
 		}
 		f = e.AttrField(dwarf.AttrType)
 		if f == nil {
 			continue
 		}
 		dt, err := d.Type(f.Val.(dwarf.Offset))
 		if err != nil {
 			panic(err)
 		}
 		if _, ok := dt.(*dwarf.UnspecifiedType); ok {
 			continue // Ignore markers like data/edata.
 		}
 		nf := e.AttrField(dwarf.AttrName)
 		if nf == nil {
 			continue
 		}
 		p.globals = append(p.globals, &Root{
 			Name:  nf.Val.(string),
 			Addr:  a,
 			Type:  p.dwarfMap[dt],
 			Frame: nil,
 		})
 	}
 }

 func (p *Process) readStackVars() {
 	type Var struct {
 		name string
 		off  int64
 		typ  *Type
 	}
 	vars := map[*Func][]Var{}
 	var curfn *Func
 	d, _ := p.proc.DWARF()
 	r := d.Reader()
 	for e, err := r.Next(); e != nil && err == nil; e, err = r.Next() {
 		if isNonGoCU(e) {
 			r.SkipChildren()
 			continue
 		}

 		if e.Tag == dwarf.TagSubprogram {
 			lowpc := e.AttrField(dwarf.AttrLowpc)
 			highpc := e.AttrField(dwarf.AttrHighpc)
 			if lowpc == nil || highpc == nil {
 				continue
 			}
 			min := core.Address(lowpc.Val.(uint64))
 			max := core.Address(highpc.Val.(uint64))
 			f := p.funcTab.find(min)
 			if f == nil {
 				// some func Go doesn't know about. C?
 				curfn = nil
 			} else {
 				if f.entry != min {
 					panic("dwarf and runtime don't agree about start of " + f.name)
 				}
 				if p.funcTab.find(max-1) != f {
 					panic("function ranges don't match for " + f.name)
 				}
 				curfn = f
 			}
 			continue
 		}
 		if e.Tag != dwarf.TagVariable && e.Tag != dwarf.TagFormalParameter {
 			continue
 		}
 		aloc := e.AttrField(dwarf.AttrLocation)
 		if aloc == nil {
 			continue
 		}
 		if aloc.Class != dwarf.ClassExprLoc {
 			// TODO: handle ClassLocListPtr here.
 			// As of go 1.11, locals are encoded this way.
 			// Until we fix this TODO, viewcore will not be able to
 			// show local variables.
 			continue
 		}
 		// Interpret locations of the form
 		//    DW_OP_call_frame_cfa
 		//    DW_OP_consts <off>
 		//    DW_OP_plus
 		// (with possibly missing DW_OP_consts & DW_OP_plus for the zero offset.)
 		// TODO: handle other possible locations (e.g. register locations).
 		loc := aloc.Val.([]byte)
 		if len(loc) == 0 || loc[0] != _DW_OP_call_frame_cfa {
 			continue
 		}
 		loc = loc[1:]
 		var off int64
 		if len(loc) != 0 && loc[0] == _DW_OP_consts {
 			loc = loc[1:]
 			var s uint
 			for len(loc) > 0 {
 				b := loc[0]
 				loc = loc[1:]
 				off += int64(b&0x7f) << s
 				s += 7
 				if b&0x80 == 0 {
 					break
 				}
 			}
 			off = off << (64 - s) >> (64 - s)
 			if len(loc) == 0 || loc[0] != _DW_OP_plus {
 				continue
 			}
 			loc = loc[1:]
 		}
 		if len(loc) != 0 {
 			continue // more stuff we don't recognize
 		}
 		f := e.AttrField(dwarf.AttrType)
 		if f == nil {
 			continue
 		}
 		dt, err := d.Type(f.Val.(dwarf.Offset))
 		if err != nil {
 			panic(err)
 		}
 		nf := e.AttrField(dwarf.AttrName)
 		if nf == nil {
 			continue
 		}
 		name := nf.Val.(string)
 		vars[curfn] = append(vars[curfn], Var{name: name, off: off, typ: p.dwarfMap[dt]})
 	}

 	// Get roots from goroutine stacks.
 	for _, g := range p.goroutines {
 		for _, f := range g.frames {
 			// Start with all pointer slots as unnamed.
 			unnamed := map[core.Address]bool{}
 			for a := range f.Live {
 				unnamed[a] = true
 			}
 			// Emit roots for DWARF entries.
 			for _, v := range vars[f.f] {
 				r := &Root{
 					Name:  v.name,
 					Addr:  f.max.Add(v.off),
 					Type:  v.typ,
 					Frame: f,
 				}
 				f.roots = append(f.roots, r)
 				// Remove this variable from the set of unnamed pointers.
 				for a := r.Addr; a < r.Addr.Add(r.Type.Size); a = a.Add(p.proc.PtrSize()) {
 					delete(unnamed, a)
 				}
 			}
 			// Emit roots for unnamed pointer slots in the frame.
 			// Make deterministic by sorting first.
 			s := make([]core.Address, 0, len(unnamed))
 			for a := range unnamed {
 				s = append(s, a)
 			}
 			sort.Slice(s, func(i, j int) bool { return s[i] < s[j] })
 			for _, a := range s {
 				r := &Root{
 					Name:  "unk",
 					Addr:  a,
 					Type:  p.findType("unsafe.Pointer"),
 					Frame: f,
 				}
 				f.roots = append(f.roots, r)
 			}
 		}
 	}
 }

 /* Dwarf encoding notes

 type XXX sss

 translates to a dwarf type pkg.XXX of the type of sss (uint, float, ...)

 exception: if sss is a struct or array, then we get two types, the "unnamed" and "named" type.
 The unnamed type is a dwarf struct type with name "struct pkg.XXX" or a dwarf array type with
 name [N]elem.
 Then there is a typedef with pkg.XXX pointing to "struct pkg.XXX" or [N]elem.

 For structures, lowercase field names are prepended with the package name (pkg path?).

 type XXX interface{}
 pkg.XXX is a typedef to "struct runtime.eface"
 type XXX interface{f()}
 pkg.XXX is a typedef to "struct runtime.iface"

 Sometimes there is even a chain of identically-named typedefs. I have no idea why.
 main.XXX -> main.XXX -> struct runtime.iface

 */
	// Copyright 2017 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.

	package gocore

	import (
	"debug/dwarf"
	"fmt"
	"regexp"
	"sort"
	"strings"

	"golang.org/x/debug/internal/core"
	)

	// read DWARF types from core dump.
	func (p *Process) readDWARFTypes() {
	d, _ := p.proc.DWARF()

	// Make one of our own Types for each dwarf type.
	r := d.Reader()
	var types []*Type
	for e, err := r.Next(); e != nil && err == nil; e, err = r.Next() {
	if isNonGoCU(e) {
	r.SkipChildren()
	continue
	}
	switch e.Tag {
	case dwarf.TagArrayType, dwarf.TagPointerType, dwarf.TagStructType, dwarf.TagBaseType, dwarf.TagSubroutineType, dwarf.TagTypedef:
	dt, err := d.Type(e.Offset)
	if err != nil {
	continue
	}
	t := &Type{Name: gocoreName(dt), Size: dwarfSize(dt, p.proc.PtrSize())}
	p.dwarfMap[dt] = t
	types = append(types, t)
	}
	}

	p.runtimeNameMap = map[string][]*Type{}

	// Fill in fields of types. Postponed until now so we're sure
	// we have all the Types allocated and available.
	for dt, t := range p.dwarfMap {
	switch x := dt.(type) {
	case *dwarf.ArrayType:
	t.Kind = KindArray
	t.Elem = p.dwarfMap[x.Type]
	t.Count = x.Count
	case *dwarf.PtrType:
	t.Kind = KindPtr
	// unsafe.Pointer has a void base type.
	if _, ok := x.Type.(*dwarf.VoidType); !ok {
	t.Elem = p.dwarfMap[x.Type]
	}
	case *dwarf.StructType:
	t.Kind = KindStruct
	for _, f := range x.Field {
	fType := p.dwarfMap[f.Type]

	// Work around issue 21094. There's no guarantee that the
	// pointer type is in the DWARF, so just invent a Type.
	if strings.HasPrefix(t.Name, "sudog<") && f.Name == "elem" &&
	strings.Count(t.Name, "")+1 != strings.Count(gocoreName(f.Type), "") {
	ptrName := "*" + gocoreName(f.Type)
	fType = &Type{Name: ptrName, Kind: KindPtr, Size: p.proc.PtrSize(), Elem: fType}
	p.runtimeNameMap[ptrName] = []*Type{fType}
	}

	t.Fields = append(t.Fields, Field{Name: f.Name, Type: fType, Off: f.ByteOffset})
	}
	case *dwarf.BoolType:
	t.Kind = KindBool
	case *dwarf.IntType:
	t.Kind = KindInt
	case *dwarf.UintType:
	t.Kind = KindUint
	case *dwarf.FloatType:
	t.Kind = KindFloat
	case *dwarf.ComplexType:
	t.Kind = KindComplex
	case *dwarf.FuncType:
	t.Kind = KindFunc
	case *dwarf.TypedefType:
	// handle these types in the loop below
	default:
	panic(fmt.Sprintf("unknown type %s %T", dt, dt))
	}
	}

	// Detect strings & slices
	for _, t := range types {
	if t.Kind != KindStruct {
	continue
	}
	if t.Name == "string" { // TODO: also "struct runtime.stringStructDWARF" ?
	t.Kind = KindString
	t.Elem = t.Fields[0].Type.Elem // TODO: check that it is always uint8.
	t.Fields = nil
	}
	if len(t.Name) >= 9 && t.Name[:9] == "struct []" \|\|
	len(t.Name) >= 2 && t.Name[:2] == "[]" {
	t.Kind = KindSlice
	t.Elem = t.Fields[0].Type.Elem
	t.Fields = nil
	}
	}

	// Copy info from base types into typedefs.
	for dt, t := range p.dwarfMap {
	tt, ok := dt.(*dwarf.TypedefType)
	if !ok {
	continue
	}
	base := tt.Type
	// Walk typedef chain until we reach a non-typedef type.
	for {
	if x, ok := base.(*dwarf.TypedefType); ok {
	base = x.Type
	continue
	}
	break
	}
	bt := p.dwarfMap[base]

	// Copy type info from base. Everything except the name.
	name := t.Name
	t = bt
	t.Name = name

	// Detect some special types. If the base is some particular type,
	// then the alias gets marked as special.
	// We have aliases like:
	// interface {} -> struct runtime.eface
	// error -> struct runtime.iface
	// Note: the base itself does not get marked as special.
	// (Unlike strings and slices, where they do.)
	if bt.Name == "runtime.eface" {
	t.Kind = KindEface
	t.Fields = nil
	}
	if bt.Name == "runtime.iface" {
	t.Kind = KindIface
	t.Fields = nil
	}
	}

	// Make a runtime name -> Type map for existing DWARF types.
	for dt, t := range p.dwarfMap {
	name := runtimeName(dt)
	p.runtimeNameMap[name] = append(p.runtimeNameMap[name], t)
	}

	// Construct the runtime.specialfinalizer type. It won't be found
	// in DWARF before 1.10 because it does not appear in the type of any variable.
	// type specialfinalizer struct {
	// special special
	// fn *funcval
	// nret uintptr
	// fint *_type
	// ot *ptrtype
	// }
	if p.runtimeNameMap["runtime.specialfinalizer"] == nil {
	special := p.findType("runtime.special")
	p.runtimeNameMap["runtime.specialfinalizer"] = []*Type{
	&Type{
	Name: "runtime.specialfinalizer",
	Size: special.Size + 4*p.proc.PtrSize(),
	Kind: KindStruct,
	Fields: []Field{
	Field{
	Name: "special",
	Off: 0,
	Type: special,
	},
	Field{
	Name: "fn",
	Off: special.Size,
	Type: p.findType("*runtime.funcval"),
	},
	Field{
	Name: "nret",
	Off: special.Size + p.proc.PtrSize(),
	Type: p.findType("uintptr"),
	},
	Field{
	Name: "fint",
	Off: special.Size + 2*p.proc.PtrSize(),
	Type: p.findType("*runtime._type"),
	},
	Field{
	Name: "fn",
	Off: special.Size + 3*p.proc.PtrSize(),
	Type: p.findType("*runtime.ptrtype"),
	},
	},
	},
	}
	}
	}

	func isNonGoCU(e *dwarf.Entry) bool {
	if e.Tag != dwarf.TagCompileUnit {
	return false
	}
	prod, ok := e.Val(dwarf.AttrProducer).(string)
	if !ok {
	return true
	}
	return !strings.Contains(prod, "Go cmd/compile")
	}

	// dwarfSize is used to compute the size of a DWARF type.
	// dt.Size() is wrong when it returns a negative number.
	// This function implements just enough to correct the bad behavior.
	func dwarfSize(dt dwarf.Type, ptrSize int64) int64 {
	s := dt.Size()
	if s >= 0 {
	return s
	}
	switch x := dt.(type) {
	case *dwarf.FuncType:
	return ptrSize // Fix for issue 21097.
	case *dwarf.ArrayType:
	return x.Count * dwarfSize(x.Type, ptrSize)
	case *dwarf.TypedefType:
	return dwarfSize(x.Type, ptrSize)
	default:
	panic(fmt.Sprintf("unhandled: %s, %T", x, x))
	}
	}

	// gocoreName generates the name this package uses to refer to a dwarf type.
	// This name differs from the dwarf name in that it stays closer to the Go name for the type.
	// For instance (dwarf name -> gocoreName)
	// struct runtime.siginfo -> runtime.siginfo
	// *void -> unsafe.Pointer
	// struct struct { runtime.signalLock uint32; runtime.hz int32 } -> struct { signalLock uint32; hz int32 }
	func gocoreName(dt dwarf.Type) string {
	switch x := dt.(type) {
	case *dwarf.PtrType:
	if _, ok := x.Type.(*dwarf.VoidType); ok {
	return "unsafe.Pointer"
	}
	return "*" + gocoreName(x.Type)
	case *dwarf.ArrayType:
	return fmt.Sprintf("[%d]%s", x.Count, gocoreName(x.Type))
	case *dwarf.StructType:
	if !strings.HasPrefix(x.StructName, "struct {") {
	// This is a named type, return that name.
	return x.StructName
	}
	// Build gocore name from the DWARF fields.
	s := "struct {"
	first := true
	for _, f := range x.Field {
	if !first {
	s += ";"
	}
	name := f.Name
	if i := strings.Index(name, "."); i >= 0 {
	// Remove pkg path from field names.
	name = name[i+1:]
	}
	s += fmt.Sprintf(" %s %s", name, gocoreName(f.Type))
	first = false
	}
	s += " }"
	return s
	default:
	return dt.String()
	}
	}

	// Generate the name the runtime uses for a dwarf type. The DWARF generator
	// and the runtime use slightly different names for the same underlying type.
	func runtimeName(dt dwarf.Type) string {
	switch x := dt.(type) {
	case *dwarf.PtrType:
	if _, ok := x.Type.(*dwarf.VoidType); ok {
	return "unsafe.Pointer"
	}
	return "*" + runtimeName(x.Type)
	case *dwarf.ArrayType:
	return fmt.Sprintf("[%d]%s", x.Count, runtimeName(x.Type))
	case *dwarf.StructType:
	if !strings.HasPrefix(x.StructName, "struct {") {
	// This is a named type, return that name.
	return stripPackagePath(x.StructName)
	}
	// Figure out which fields have anonymous names.
	var anon []bool
	for _, f := range strings.Split(x.StructName[8:len(x.StructName)-1], ";") {
	f = strings.TrimSpace(f)
	anon = append(anon, !strings.Contains(f, " "))
	// TODO: this isn't perfect. If the field type has a space in it,
	// then this logic doesn't work. Need to search for keyword for
	// field type, like "interface", "struct", ...
	}
	// Make sure anon is long enough. This probably never triggers.
	for len(anon) < len(x.Field) {
	anon = append(anon, false)
	}

	// Build runtime name from the DWARF fields.
	s := "struct {"
	first := true
	for _, f := range x.Field {
	if !first {
	s += ";"
	}
	name := f.Name
	if i := strings.Index(name, "."); i >= 0 {
	name = name[i+1:]
	}
	if anon[0] {
	s += fmt.Sprintf(" %s", runtimeName(f.Type))
	} else {
	s += fmt.Sprintf(" %s %s", name, runtimeName(f.Type))
	}
	first = false
	anon = anon[1:]
	}
	s += " }"
	return s
	default:
	return stripPackagePath(dt.String())
	}
	}

	var pathRegexp = regexp.MustCompile(`([\w.]+/)+\w+`)

	func stripPackagePath(name string) string {
	// The runtime uses just package names. DWARF uses whole package paths.
	// To convert from the latter to the former, get rid of the package paths.
	// Examples:
	// text/template.Template -> template.Template
	// map[string]compress/gzip.Writer -> map[string]gzip.Writer
	return pathRegexp.ReplaceAllStringFunc(name, func(path string) string {
	return path[strings.LastIndex(path, "/")+1:]
	})
	}

	// readRuntimeConstants populates the p.rtConstants map.
	func (p *Process) readRuntimeConstants() {
	p.rtConstants = map[string]int64{}

	// Hardcoded values for Go 1.9.
	// (Go did not have constants in DWARF before 1.10.)
	m := p.rtConstants
	m["_MSpanDead"] = 0
	m["_MSpanInUse"] = 1
	m["_MSpanManual"] = 2
	m["_MSpanFree"] = 3
	m["_Gidle"] = 0
	m["_Grunnable"] = 1
	m["_Grunning"] = 2
	m["_Gsyscall"] = 3
	m["_Gwaiting"] = 4
	m["_Gdead"] = 6
	m["_Gscan"] = 0x1000
	m["_PCDATA_StackMapIndex"] = 0
	m["_FUNCDATA_LocalsPointerMaps"] = 1
	m["_FUNCDATA_ArgsPointerMaps"] = 0
	m["tflagExtraStar"] = 1 << 1
	m["kindGCProg"] = 1 << 6
	m["kindDirectIface"] = 1 << 5
	m["_PageSize"] = 1 << 13
	m["_KindSpecialFinalizer"] = 1

	// From 1.10, these constants are recorded in DWARF records.
	d, _ := p.proc.DWARF()
	r := d.Reader()
	for e, err := r.Next(); e != nil && err == nil; e, err = r.Next() {
	if e.Tag != dwarf.TagConstant {
	continue
	}
	f := e.AttrField(dwarf.AttrName)
	if f == nil {
	continue
	}
	name := f.Val.(string)
	if !strings.HasPrefix(name, "runtime.") {
	continue
	}
	name = name[8:]
	c := e.AttrField(dwarf.AttrConstValue)
	if c == nil {
	continue
	}
	p.rtConstants[name] = c.Val.(int64)
	}
	}

	const (
	_DW_OP_addr = 0x03
	_DW_OP_call_frame_cfa = 0x9c
	_DW_OP_plus = 0x22
	_DW_OP_consts = 0x11
	)

	func (p *Process) readGlobals() {
	d, _ := p.proc.DWARF()
	r := d.Reader()
	for e, err := r.Next(); e != nil && err == nil; e, err = r.Next() {
	if isNonGoCU(e) {
	r.SkipChildren()
	continue
	}

	if e.Tag != dwarf.TagVariable {
	continue
	}
	f := e.AttrField(dwarf.AttrLocation)
	if f == nil {
	continue
	}
	if f.Class != dwarf.ClassExprLoc {
	// Globals are all encoded with this class.
	continue
	}
	loc := f.Val.([]byte)
	if len(loc) == 0 \|\| loc[0] != _DW_OP_addr {
	continue
	}
	var a core.Address
	if p.proc.PtrSize() == 8 {
	a = core.Address(p.proc.ByteOrder().Uint64(loc[1:]))
	} else {
	a = core.Address(p.proc.ByteOrder().Uint32(loc[1:]))
	}
	if !p.proc.Writeable(a) {
	// Read-only globals can't have heap pointers.
	// TODO: keep roots around anyway?
	continue
	}
	f = e.AttrField(dwarf.AttrType)
	if f == nil {
	continue
	}
	dt, err := d.Type(f.Val.(dwarf.Offset))
	if err != nil {
	panic(err)
	}
	if _, ok := dt.(*dwarf.UnspecifiedType); ok {
	continue // Ignore markers like data/edata.
	}
	nf := e.AttrField(dwarf.AttrName)
	if nf == nil {
	continue
	}
	p.globals = append(p.globals, &Root{
	Name: nf.Val.(string),
	Addr: a,
	Type: p.dwarfMap[dt],
	Frame: nil,
	})
	}
	}

	func (p *Process) readStackVars() {
	type Var struct {
	name string
	off int64
	typ *Type
	}
	vars := map[*Func][]Var{}
	var curfn *Func
	d, _ := p.proc.DWARF()
	r := d.Reader()
	for e, err := r.Next(); e != nil && err == nil; e, err = r.Next() {
	if isNonGoCU(e) {
	r.SkipChildren()
	continue
	}

	if e.Tag == dwarf.TagSubprogram {
	lowpc := e.AttrField(dwarf.AttrLowpc)
	highpc := e.AttrField(dwarf.AttrHighpc)
	if lowpc == nil \|\| highpc == nil {
	continue
	}
	min := core.Address(lowpc.Val.(uint64))
	max := core.Address(highpc.Val.(uint64))
	f := p.funcTab.find(min)
	if f == nil {
	// some func Go doesn't know about. C?
	curfn = nil
	} else {
	if f.entry != min {
	panic("dwarf and runtime don't agree about start of " + f.name)
	}
	if p.funcTab.find(max-1) != f {
	panic("function ranges don't match for " + f.name)
	}
	curfn = f
	}
	continue
	}
	if e.Tag != dwarf.TagVariable && e.Tag != dwarf.TagFormalParameter {
	continue
	}
	aloc := e.AttrField(dwarf.AttrLocation)
	if aloc == nil {
	continue
	}
	if aloc.Class != dwarf.ClassExprLoc {
	// TODO: handle ClassLocListPtr here.
	// As of go 1.11, locals are encoded this way.
	// Until we fix this TODO, viewcore will not be able to
	// show local variables.
	continue
	}
	// Interpret locations of the form
	// DW_OP_call_frame_cfa
	// DW_OP_consts <off>
	// DW_OP_plus
	// (with possibly missing DW_OP_consts & DW_OP_plus for the zero offset.)
	// TODO: handle other possible locations (e.g. register locations).
	loc := aloc.Val.([]byte)
	if len(loc) == 0 \|\| loc[0] != _DW_OP_call_frame_cfa {
	continue
	}
	loc = loc[1:]
	var off int64
	if len(loc) != 0 && loc[0] == _DW_OP_consts {
	loc = loc[1:]
	var s uint
	for len(loc) > 0 {
	b := loc[0]
	loc = loc[1:]
	off += int64(b&0x7f) << s
	s += 7
	if b&0x80 == 0 {
	break
	}
	}
	off = off << (64 - s) >> (64 - s)
	if len(loc) == 0 \|\| loc[0] != _DW_OP_plus {
	continue
	}
	loc = loc[1:]
	}
	if len(loc) != 0 {
	continue // more stuff we don't recognize
	}
	f := e.AttrField(dwarf.AttrType)
	if f == nil {
	continue
	}
	dt, err := d.Type(f.Val.(dwarf.Offset))
	if err != nil {
	panic(err)
	}
	nf := e.AttrField(dwarf.AttrName)
	if nf == nil {
	continue
	}
	name := nf.Val.(string)
	vars[curfn] = append(vars[curfn], Var{name: name, off: off, typ: p.dwarfMap[dt]})
	}

	// Get roots from goroutine stacks.
	for _, g := range p.goroutines {
	for _, f := range g.frames {
	// Start with all pointer slots as unnamed.
	unnamed := map[core.Address]bool{}
	for a := range f.Live {
	unnamed[a] = true
	}
	// Emit roots for DWARF entries.
	for _, v := range vars[f.f] {
	r := &Root{
	Name: v.name,
	Addr: f.max.Add(v.off),
	Type: v.typ,
	Frame: f,
	}
	f.roots = append(f.roots, r)
	// Remove this variable from the set of unnamed pointers.
	for a := r.Addr; a < r.Addr.Add(r.Type.Size); a = a.Add(p.proc.PtrSize()) {
	delete(unnamed, a)
	}
	}
	// Emit roots for unnamed pointer slots in the frame.
	// Make deterministic by sorting first.
	s := make([]core.Address, 0, len(unnamed))
	for a := range unnamed {
	s = append(s, a)
	}
	sort.Slice(s, func(i, j int) bool { return s[i] < s[j] })
	for _, a := range s {
	r := &Root{
	Name: "unk",
	Addr: a,
	Type: p.findType("unsafe.Pointer"),
	Frame: f,
	}
	f.roots = append(f.roots, r)
	}
	}
	}
	}

	/* Dwarf encoding notes

	type XXX sss

	translates to a dwarf type pkg.XXX of the type of sss (uint, float, ...)

	exception: if sss is a struct or array, then we get two types, the "unnamed" and "named" type.
	The unnamed type is a dwarf struct type with name "struct pkg.XXX" or a dwarf array type with
	name [N]elem.
	Then there is a typedef with pkg.XXX pointing to "struct pkg.XXX" or [N]elem.

	For structures, lowercase field names are prepended with the package name (pkg path?).

	type XXX interface{}
	pkg.XXX is a typedef to "struct runtime.eface"
	type XXX interface{f()}
	pkg.XXX is a typedef to "struct runtime.iface"

	Sometimes there is even a chain of identically-named typedefs. I have no idea why.
	main.XXX -> main.XXX -> struct runtime.iface

	*/