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

Documentation: cmd/internal/dwarf

  // Copyright 2016 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 dwarf generates DWARF debugging information.
  // DWARF generation is split between the compiler and the linker,
  // this package contains the shared code.
  package dwarf
  
  import (
  	"errors"
  	"fmt"
  )
  
  // InfoPrefix is the prefix for all the symbols containing DWARF info entries.
  const InfoPrefix = "go.info."
  
  // RangePrefix is the prefix for all the symbols containing DWARF range lists.
  const RangePrefix = "go.range."
  
  // Sym represents a symbol.
  type Sym interface {
  	Len() int64
  }
  
  // A Var represents a local variable or a function parameter.
  type Var struct {
  	Name   string
  	Abbrev int // Either DW_ABRV_AUTO or DW_ABRV_PARAM
  	Offset int32
  	Scope  int32
  	Type   Sym
  }
  
  // A Scope represents a lexical scope. All variables declared within a
  // scope will only be visible to instructions covered by the scope.
  // Lexical scopes are contiguous in source files but can end up being
  // compiled to discontiguous blocks of instructions in the executable.
  // The Ranges field lists all the blocks of instructions that belong
  // in this scope.
  type Scope struct {
  	Parent int32
  	Ranges []Range
  	Vars   []*Var
  }
  
  // A Range represents a half-open interval [Start, End).
  type Range struct {
  	Start, End int64
  }
  
  // UnifyRanges merges the list of ranges of c into the list of ranges of s
  func (s *Scope) UnifyRanges(c *Scope) {
  	out := make([]Range, 0, len(s.Ranges)+len(c.Ranges))
  
  	i, j := 0, 0
  	for {
  		var cur Range
  		if i < len(s.Ranges) && j < len(c.Ranges) {
  			if s.Ranges[i].Start < c.Ranges[j].Start {
  				cur = s.Ranges[i]
  				i++
  			} else {
  				cur = c.Ranges[j]
  				j++
  			}
  		} else if i < len(s.Ranges) {
  			cur = s.Ranges[i]
  			i++
  		} else if j < len(c.Ranges) {
  			cur = c.Ranges[j]
  			j++
  		} else {
  			break
  		}
  
  		if n := len(out); n > 0 && cur.Start <= out[n-1].End {
  			out[n-1].End = cur.End
  		} else {
  			out = append(out, cur)
  		}
  	}
  
  	s.Ranges = out
  }
  
  // A Context specifies how to add data to a Sym.
  type Context interface {
  	PtrSize() int
  	AddInt(s Sym, size int, i int64)
  	AddBytes(s Sym, b []byte)
  	AddAddress(s Sym, t interface{}, ofs int64)
  	AddSectionOffset(s Sym, size int, t interface{}, ofs int64)
  	AddString(s Sym, v string)
  	SymValue(s Sym) int64
  }
  
  // AppendUleb128 appends v to b using DWARF's unsigned LEB128 encoding.
  func AppendUleb128(b []byte, v uint64) []byte {
  	for {
  		c := uint8(v & 0x7f)
  		v >>= 7
  		if v != 0 {
  			c |= 0x80
  		}
  		b = append(b, c)
  		if c&0x80 == 0 {
  			break
  		}
  	}
  	return b
  }
  
  // AppendSleb128 appends v to b using DWARF's signed LEB128 encoding.
  func AppendSleb128(b []byte, v int64) []byte {
  	for {
  		c := uint8(v & 0x7f)
  		s := uint8(v & 0x40)
  		v >>= 7
  		if (v != -1 || s == 0) && (v != 0 || s != 0) {
  			c |= 0x80
  		}
  		b = append(b, c)
  		if c&0x80 == 0 {
  			break
  		}
  	}
  	return b
  }
  
  // sevenbits contains all unsigned seven bit numbers, indexed by their value.
  var sevenbits = [...]byte{
  	0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f,
  	0x10, 0x11, 0x12, 0x13, 0x14, 0x15, 0x16, 0x17, 0x18, 0x19, 0x1a, 0x1b, 0x1c, 0x1d, 0x1e, 0x1f,
  	0x20, 0x21, 0x22, 0x23, 0x24, 0x25, 0x26, 0x27, 0x28, 0x29, 0x2a, 0x2b, 0x2c, 0x2d, 0x2e, 0x2f,
  	0x30, 0x31, 0x32, 0x33, 0x34, 0x35, 0x36, 0x37, 0x38, 0x39, 0x3a, 0x3b, 0x3c, 0x3d, 0x3e, 0x3f,
  	0x40, 0x41, 0x42, 0x43, 0x44, 0x45, 0x46, 0x47, 0x48, 0x49, 0x4a, 0x4b, 0x4c, 0x4d, 0x4e, 0x4f,
  	0x50, 0x51, 0x52, 0x53, 0x54, 0x55, 0x56, 0x57, 0x58, 0x59, 0x5a, 0x5b, 0x5c, 0x5d, 0x5e, 0x5f,
  	0x60, 0x61, 0x62, 0x63, 0x64, 0x65, 0x66, 0x67, 0x68, 0x69, 0x6a, 0x6b, 0x6c, 0x6d, 0x6e, 0x6f,
  	0x70, 0x71, 0x72, 0x73, 0x74, 0x75, 0x76, 0x77, 0x78, 0x79, 0x7a, 0x7b, 0x7c, 0x7d, 0x7e, 0x7f,
  }
  
  // sevenBitU returns the unsigned LEB128 encoding of v if v is seven bits and nil otherwise.
  // The contents of the returned slice must not be modified.
  func sevenBitU(v int64) []byte {
  	if uint64(v) < uint64(len(sevenbits)) {
  		return sevenbits[v : v+1]
  	}
  	return nil
  }
  
  // sevenBitS returns the signed LEB128 encoding of v if v is seven bits and nil otherwise.
  // The contents of the returned slice must not be modified.
  func sevenBitS(v int64) []byte {
  	if uint64(v) <= 63 {
  		return sevenbits[v : v+1]
  	}
  	if uint64(-v) <= 64 {
  		return sevenbits[128+v : 128+v+1]
  	}
  	return nil
  }
  
  // Uleb128put appends v to s using DWARF's unsigned LEB128 encoding.
  func Uleb128put(ctxt Context, s Sym, v int64) {
  	b := sevenBitU(v)
  	if b == nil {
  		var encbuf [20]byte
  		b = AppendUleb128(encbuf[:0], uint64(v))
  	}
  	ctxt.AddBytes(s, b)
  }
  
  // Sleb128put appends v to s using DWARF's signed LEB128 encoding.
  func Sleb128put(ctxt Context, s Sym, v int64) {
  	b := sevenBitS(v)
  	if b == nil {
  		var encbuf [20]byte
  		b = AppendSleb128(encbuf[:0], v)
  	}
  	ctxt.AddBytes(s, b)
  }
  
  /*
   * Defining Abbrevs.  This is hardcoded, and there will be
   * only a handful of them.  The DWARF spec places no restriction on
   * the ordering of attributes in the Abbrevs and DIEs, and we will
   * always write them out in the order of declaration in the abbrev.
   */
  type dwAttrForm struct {
  	attr uint16
  	form uint8
  }
  
  // Go-specific type attributes.
  const (
  	DW_AT_go_kind = 0x2900
  	DW_AT_go_key  = 0x2901
  	DW_AT_go_elem = 0x2902
  	// Attribute for DW_TAG_member of a struct type.
  	// Nonzero value indicates the struct field is an embedded field.
  	DW_AT_go_embedded_field = 0x2903
  
  	DW_AT_internal_location = 253 // params and locals; not emitted
  )
  
  // Index into the abbrevs table below.
  // Keep in sync with ispubname() and ispubtype() below.
  // ispubtype considers >= NULLTYPE public
  const (
  	DW_ABRV_NULL = iota
  	DW_ABRV_COMPUNIT
  	DW_ABRV_FUNCTION
  	DW_ABRV_VARIABLE
  	DW_ABRV_AUTO
  	DW_ABRV_PARAM
  	DW_ABRV_LEXICAL_BLOCK_RANGES
  	DW_ABRV_LEXICAL_BLOCK_SIMPLE
  	DW_ABRV_STRUCTFIELD
  	DW_ABRV_FUNCTYPEPARAM
  	DW_ABRV_DOTDOTDOT
  	DW_ABRV_ARRAYRANGE
  	DW_ABRV_NULLTYPE
  	DW_ABRV_BASETYPE
  	DW_ABRV_ARRAYTYPE
  	DW_ABRV_CHANTYPE
  	DW_ABRV_FUNCTYPE
  	DW_ABRV_IFACETYPE
  	DW_ABRV_MAPTYPE
  	DW_ABRV_PTRTYPE
  	DW_ABRV_BARE_PTRTYPE // only for void*, no DW_AT_type attr to please gdb 6.
  	DW_ABRV_SLICETYPE
  	DW_ABRV_STRINGTYPE
  	DW_ABRV_STRUCTTYPE
  	DW_ABRV_TYPEDECL
  	DW_NABRV
  )
  
  type dwAbbrev struct {
  	tag      uint8
  	children uint8
  	attr     []dwAttrForm
  }
  
  var abbrevs = [DW_NABRV]dwAbbrev{
  	/* The mandatory DW_ABRV_NULL entry. */
  	{0, 0, []dwAttrForm{}},
  
  	/* COMPUNIT */
  	{
  		DW_TAG_compile_unit,
  		DW_CHILDREN_yes,
  		[]dwAttrForm{
  			{DW_AT_name, DW_FORM_string},
  			{DW_AT_language, DW_FORM_data1},
  			{DW_AT_low_pc, DW_FORM_addr},
  			{DW_AT_high_pc, DW_FORM_addr},
  			{DW_AT_stmt_list, DW_FORM_sec_offset},
  			{DW_AT_comp_dir, DW_FORM_string},
  			{DW_AT_producer, DW_FORM_string},
  		},
  	},
  
  	/* FUNCTION */
  	{
  		DW_TAG_subprogram,
  		DW_CHILDREN_yes,
  		[]dwAttrForm{
  			{DW_AT_name, DW_FORM_string},
  			{DW_AT_low_pc, DW_FORM_addr},
  			{DW_AT_high_pc, DW_FORM_addr},
  			{DW_AT_frame_base, DW_FORM_block1},
  			{DW_AT_external, DW_FORM_flag},
  		},
  	},
  
  	/* VARIABLE */
  	{
  		DW_TAG_variable,
  		DW_CHILDREN_no,
  		[]dwAttrForm{
  			{DW_AT_name, DW_FORM_string},
  			{DW_AT_location, DW_FORM_block1},
  			{DW_AT_type, DW_FORM_ref_addr},
  			{DW_AT_external, DW_FORM_flag},
  		},
  	},
  
  	/* AUTO */
  	{
  		DW_TAG_variable,
  		DW_CHILDREN_no,
  		[]dwAttrForm{
  			{DW_AT_name, DW_FORM_string},
  			{DW_AT_location, DW_FORM_block1},
  			{DW_AT_type, DW_FORM_ref_addr},
  		},
  	},
  
  	/* PARAM */
  	{
  		DW_TAG_formal_parameter,
  		DW_CHILDREN_no,
  		[]dwAttrForm{
  			{DW_AT_name, DW_FORM_string},
  			{DW_AT_location, DW_FORM_block1},
  			{DW_AT_type, DW_FORM_ref_addr},
  		},
  	},
  	/* LEXICAL_BLOCK_RANGES */
  	{
  		DW_TAG_lexical_block,
  		DW_CHILDREN_yes,
  		[]dwAttrForm{
  			{DW_AT_ranges, DW_FORM_sec_offset},
  		},
  	},
  
  	/* LEXICAL_BLOCK_SIMPLE */
  	{
  		DW_TAG_lexical_block,
  		DW_CHILDREN_yes,
  		[]dwAttrForm{
  			{DW_AT_low_pc, DW_FORM_addr},
  			{DW_AT_high_pc, DW_FORM_addr},
  		},
  	},
  
  	/* STRUCTFIELD */
  	{
  		DW_TAG_member,
  		DW_CHILDREN_no,
  		[]dwAttrForm{
  			{DW_AT_name, DW_FORM_string},
  			{DW_AT_data_member_location, DW_FORM_block1},
  			{DW_AT_type, DW_FORM_ref_addr},
  			{DW_AT_go_embedded_field, DW_FORM_flag},
  		},
  	},
  
  	/* FUNCTYPEPARAM */
  	{
  		DW_TAG_formal_parameter,
  		DW_CHILDREN_no,
  
  		// No name!
  		[]dwAttrForm{
  			{DW_AT_type, DW_FORM_ref_addr},
  		},
  	},
  
  	/* DOTDOTDOT */
  	{
  		DW_TAG_unspecified_parameters,
  		DW_CHILDREN_no,
  		[]dwAttrForm{},
  	},
  
  	/* ARRAYRANGE */
  	{
  		DW_TAG_subrange_type,
  		DW_CHILDREN_no,
  
  		// No name!
  		[]dwAttrForm{
  			{DW_AT_type, DW_FORM_ref_addr},
  			{DW_AT_count, DW_FORM_udata},
  		},
  	},
  
  	// Below here are the types considered public by ispubtype
  	/* NULLTYPE */
  	{
  		DW_TAG_unspecified_type,
  		DW_CHILDREN_no,
  		[]dwAttrForm{
  			{DW_AT_name, DW_FORM_string},
  		},
  	},
  
  	/* BASETYPE */
  	{
  		DW_TAG_base_type,
  		DW_CHILDREN_no,
  		[]dwAttrForm{
  			{DW_AT_name, DW_FORM_string},
  			{DW_AT_encoding, DW_FORM_data1},
  			{DW_AT_byte_size, DW_FORM_data1},
  			{DW_AT_go_kind, DW_FORM_data1},
  		},
  	},
  
  	/* ARRAYTYPE */
  	// child is subrange with upper bound
  	{
  		DW_TAG_array_type,
  		DW_CHILDREN_yes,
  		[]dwAttrForm{
  			{DW_AT_name, DW_FORM_string},
  			{DW_AT_type, DW_FORM_ref_addr},
  			{DW_AT_byte_size, DW_FORM_udata},
  			{DW_AT_go_kind, DW_FORM_data1},
  		},
  	},
  
  	/* CHANTYPE */
  	{
  		DW_TAG_typedef,
  		DW_CHILDREN_no,
  		[]dwAttrForm{
  			{DW_AT_name, DW_FORM_string},
  			{DW_AT_type, DW_FORM_ref_addr},
  			{DW_AT_go_kind, DW_FORM_data1},
  			{DW_AT_go_elem, DW_FORM_ref_addr},
  		},
  	},
  
  	/* FUNCTYPE */
  	{
  		DW_TAG_subroutine_type,
  		DW_CHILDREN_yes,
  		[]dwAttrForm{
  			{DW_AT_name, DW_FORM_string},
  			// {DW_AT_type,	DW_FORM_ref_addr},
  			{DW_AT_go_kind, DW_FORM_data1},
  		},
  	},
  
  	/* IFACETYPE */
  	{
  		DW_TAG_typedef,
  		DW_CHILDREN_yes,
  		[]dwAttrForm{
  			{DW_AT_name, DW_FORM_string},
  			{DW_AT_type, DW_FORM_ref_addr},
  			{DW_AT_go_kind, DW_FORM_data1},
  		},
  	},
  
  	/* MAPTYPE */
  	{
  		DW_TAG_typedef,
  		DW_CHILDREN_no,
  		[]dwAttrForm{
  			{DW_AT_name, DW_FORM_string},
  			{DW_AT_type, DW_FORM_ref_addr},
  			{DW_AT_go_kind, DW_FORM_data1},
  			{DW_AT_go_key, DW_FORM_ref_addr},
  			{DW_AT_go_elem, DW_FORM_ref_addr},
  		},
  	},
  
  	/* PTRTYPE */
  	{
  		DW_TAG_pointer_type,
  		DW_CHILDREN_no,
  		[]dwAttrForm{
  			{DW_AT_name, DW_FORM_string},
  			{DW_AT_type, DW_FORM_ref_addr},
  			{DW_AT_go_kind, DW_FORM_data1},
  		},
  	},
  
  	/* BARE_PTRTYPE */
  	{
  		DW_TAG_pointer_type,
  		DW_CHILDREN_no,
  		[]dwAttrForm{
  			{DW_AT_name, DW_FORM_string},
  		},
  	},
  
  	/* SLICETYPE */
  	{
  		DW_TAG_structure_type,
  		DW_CHILDREN_yes,
  		[]dwAttrForm{
  			{DW_AT_name, DW_FORM_string},
  			{DW_AT_byte_size, DW_FORM_udata},
  			{DW_AT_go_kind, DW_FORM_data1},
  			{DW_AT_go_elem, DW_FORM_ref_addr},
  		},
  	},
  
  	/* STRINGTYPE */
  	{
  		DW_TAG_structure_type,
  		DW_CHILDREN_yes,
  		[]dwAttrForm{
  			{DW_AT_name, DW_FORM_string},
  			{DW_AT_byte_size, DW_FORM_udata},
  			{DW_AT_go_kind, DW_FORM_data1},
  		},
  	},
  
  	/* STRUCTTYPE */
  	{
  		DW_TAG_structure_type,
  		DW_CHILDREN_yes,
  		[]dwAttrForm{
  			{DW_AT_name, DW_FORM_string},
  			{DW_AT_byte_size, DW_FORM_udata},
  			{DW_AT_go_kind, DW_FORM_data1},
  		},
  	},
  
  	/* TYPEDECL */
  	{
  		DW_TAG_typedef,
  		DW_CHILDREN_no,
  		[]dwAttrForm{
  			{DW_AT_name, DW_FORM_string},
  			{DW_AT_type, DW_FORM_ref_addr},
  		},
  	},
  }
  
  // GetAbbrev returns the contents of the .debug_abbrev section.
  func GetAbbrev() []byte {
  	var buf []byte
  	for i := 1; i < DW_NABRV; i++ {
  		// See section 7.5.3
  		buf = AppendUleb128(buf, uint64(i))
  		buf = AppendUleb128(buf, uint64(abbrevs[i].tag))
  		buf = append(buf, byte(abbrevs[i].children))
  		for _, f := range abbrevs[i].attr {
  			buf = AppendUleb128(buf, uint64(f.attr))
  			buf = AppendUleb128(buf, uint64(f.form))
  		}
  		buf = append(buf, 0, 0)
  	}
  	return append(buf, 0)
  }
  
  /*
   * Debugging Information Entries and their attributes.
   */
  
  // DWAttr represents an attribute of a DWDie.
  //
  // For DW_CLS_string and _block, value should contain the length, and
  // data the data, for _reference, value is 0 and data is a DWDie* to
  // the referenced instance, for all others, value is the whole thing
  // and data is null.
  type DWAttr struct {
  	Link  *DWAttr
  	Atr   uint16 // DW_AT_
  	Cls   uint8  // DW_CLS_
  	Value int64
  	Data  interface{}
  }
  
  // DWDie represents a DWARF debug info entry.
  type DWDie struct {
  	Abbrev int
  	Link   *DWDie
  	Child  *DWDie
  	Attr   *DWAttr
  	Sym    Sym
  }
  
  func putattr(ctxt Context, s Sym, abbrev int, form int, cls int, value int64, data interface{}) error {
  	switch form {
  	case DW_FORM_addr: // address
  		ctxt.AddAddress(s, data, value)
  
  	case DW_FORM_block1: // block
  		if cls == DW_CLS_ADDRESS {
  			ctxt.AddInt(s, 1, int64(1+ctxt.PtrSize()))
  			ctxt.AddInt(s, 1, DW_OP_addr)
  			ctxt.AddAddress(s, data, 0)
  			break
  		}
  
  		value &= 0xff
  		ctxt.AddInt(s, 1, value)
  		p := data.([]byte)[:value]
  		ctxt.AddBytes(s, p)
  
  	case DW_FORM_block2: // block
  		value &= 0xffff
  
  		ctxt.AddInt(s, 2, value)
  		p := data.([]byte)[:value]
  		ctxt.AddBytes(s, p)
  
  	case DW_FORM_block4: // block
  		value &= 0xffffffff
  
  		ctxt.AddInt(s, 4, value)
  		p := data.([]byte)[:value]
  		ctxt.AddBytes(s, p)
  
  	case DW_FORM_block: // block
  		Uleb128put(ctxt, s, value)
  
  		p := data.([]byte)[:value]
  		ctxt.AddBytes(s, p)
  
  	case DW_FORM_data1: // constant
  		ctxt.AddInt(s, 1, value)
  
  	case DW_FORM_data2: // constant
  		ctxt.AddInt(s, 2, value)
  
  	case DW_FORM_data4: // constant, {line,loclist,mac,rangelist}ptr
  		if cls == DW_CLS_PTR { // DW_AT_stmt_list and DW_AT_ranges
  			ctxt.AddSectionOffset(s, 4, data, value)
  			break
  		}
  		ctxt.AddInt(s, 4, value)
  
  	case DW_FORM_data8: // constant, {line,loclist,mac,rangelist}ptr
  		ctxt.AddInt(s, 8, value)
  
  	case DW_FORM_sdata: // constant
  		Sleb128put(ctxt, s, value)
  
  	case DW_FORM_udata: // constant
  		Uleb128put(ctxt, s, value)
  
  	case DW_FORM_string: // string
  		str := data.(string)
  		ctxt.AddString(s, str)
  		// TODO(ribrdb): verify padded strings are never used and remove this
  		for i := int64(len(str)); i < value; i++ {
  			ctxt.AddInt(s, 1, 0)
  		}
  
  	case DW_FORM_flag: // flag
  		if value != 0 {
  			ctxt.AddInt(s, 1, 1)
  		} else {
  			ctxt.AddInt(s, 1, 0)
  		}
  
  	// As of DWARF 3 the ref_addr is always 32 bits, unless emitting a large
  	// (> 4 GB of debug info aka "64-bit") unit, which we don't implement.
  	case DW_FORM_ref_addr: // reference to a DIE in the .info section
  		fallthrough
  	case DW_FORM_sec_offset: // offset into a DWARF section other than .info
  		if data == nil {
  			return fmt.Errorf("dwarf: null reference in %d", abbrev)
  		}
  		ctxt.AddSectionOffset(s, 4, data, value)
  
  	case DW_FORM_ref1, // reference within the compilation unit
  		DW_FORM_ref2,      // reference
  		DW_FORM_ref4,      // reference
  		DW_FORM_ref8,      // reference
  		DW_FORM_ref_udata, // reference
  
  		DW_FORM_strp,     // string
  		DW_FORM_indirect: // (see Section 7.5.3)
  		fallthrough
  	default:
  		return fmt.Errorf("dwarf: unsupported attribute form %d / class %d", form, cls)
  	}
  	return nil
  }
  
  // PutAttrs writes the attributes for a DIE to symbol 's'.
  //
  // Note that we can (and do) add arbitrary attributes to a DIE, but
  // only the ones actually listed in the Abbrev will be written out.
  func PutAttrs(ctxt Context, s Sym, abbrev int, attr *DWAttr) {
  Outer:
  	for _, f := range abbrevs[abbrev].attr {
  		for ap := attr; ap != nil; ap = ap.Link {
  			if ap.Atr == f.attr {
  				putattr(ctxt, s, abbrev, int(f.form), int(ap.Cls), ap.Value, ap.Data)
  				continue Outer
  			}
  		}
  
  		putattr(ctxt, s, abbrev, int(f.form), 0, 0, nil)
  	}
  }
  
  // HasChildren returns true if 'die' uses an abbrev that supports children.
  func HasChildren(die *DWDie) bool {
  	return abbrevs[die.Abbrev].children != 0
  }
  
  // PutFunc writes a DIE for a function to s.
  // It also writes child DIEs for each variable in vars.
  func PutFunc(ctxt Context, s, ranges Sym, name string, external bool, startPC Sym, size int64, scopes []Scope) error {
  	Uleb128put(ctxt, s, DW_ABRV_FUNCTION)
  	putattr(ctxt, s, DW_ABRV_FUNCTION, DW_FORM_string, DW_CLS_STRING, int64(len(name)), name)
  	putattr(ctxt, s, DW_ABRV_FUNCTION, DW_FORM_addr, DW_CLS_ADDRESS, 0, startPC)
  	putattr(ctxt, s, DW_ABRV_FUNCTION, DW_FORM_addr, DW_CLS_ADDRESS, size, startPC)
  	putattr(ctxt, s, DW_ABRV_FUNCTION, DW_FORM_block1, DW_CLS_BLOCK, 1, []byte{DW_OP_call_frame_cfa})
  	var ev int64
  	if external {
  		ev = 1
  	}
  	putattr(ctxt, s, DW_ABRV_FUNCTION, DW_FORM_flag, DW_CLS_FLAG, ev, 0)
  	if len(scopes) > 0 {
  		var encbuf [20]byte
  		if putscope(ctxt, s, ranges, startPC, 0, scopes, encbuf[:0]) < int32(len(scopes)) {
  			return errors.New("multiple toplevel scopes")
  		}
  	}
  
  	Uleb128put(ctxt, s, 0)
  	return nil
  }
  
  func putscope(ctxt Context, s, ranges Sym, startPC Sym, curscope int32, scopes []Scope, encbuf []byte) int32 {
  	for _, v := range scopes[curscope].Vars {
  		putvar(ctxt, s, v, encbuf)
  	}
  	this := curscope
  	curscope++
  	for curscope < int32(len(scopes)) {
  		scope := scopes[curscope]
  		if scope.Parent != this {
  			return curscope
  		}
  
  		if len(scope.Ranges) == 1 {
  			Uleb128put(ctxt, s, DW_ABRV_LEXICAL_BLOCK_SIMPLE)
  			putattr(ctxt, s, DW_ABRV_LEXICAL_BLOCK_SIMPLE, DW_FORM_addr, DW_CLS_ADDRESS, scope.Ranges[0].Start, startPC)
  			putattr(ctxt, s, DW_ABRV_LEXICAL_BLOCK_SIMPLE, DW_FORM_addr, DW_CLS_ADDRESS, scope.Ranges[0].End, startPC)
  		} else {
  			Uleb128put(ctxt, s, DW_ABRV_LEXICAL_BLOCK_RANGES)
  			putattr(ctxt, s, DW_ABRV_LEXICAL_BLOCK_RANGES, DW_FORM_sec_offset, DW_CLS_PTR, ranges.Len(), ranges)
  
  			ctxt.AddAddress(ranges, nil, -1)
  			ctxt.AddAddress(ranges, startPC, 0)
  			for _, r := range scope.Ranges {
  				ctxt.AddAddress(ranges, nil, r.Start)
  				ctxt.AddAddress(ranges, nil, r.End)
  			}
  			ctxt.AddAddress(ranges, nil, 0)
  			ctxt.AddAddress(ranges, nil, 0)
  		}
  
  		curscope = putscope(ctxt, s, ranges, startPC, curscope, scopes, encbuf)
  
  		Uleb128put(ctxt, s, 0)
  	}
  	return curscope
  }
  
  func putvar(ctxt Context, s Sym, v *Var, encbuf []byte) {
  	n := v.Name
  
  	Uleb128put(ctxt, s, int64(v.Abbrev))
  	putattr(ctxt, s, v.Abbrev, DW_FORM_string, DW_CLS_STRING, int64(len(n)), n)
  	loc := append(encbuf[:0], DW_OP_call_frame_cfa)
  	if v.Offset != 0 {
  		loc = append(loc, DW_OP_consts)
  		loc = AppendSleb128(loc, int64(v.Offset))
  		loc = append(loc, DW_OP_plus)
  	}
  	putattr(ctxt, s, v.Abbrev, DW_FORM_block1, DW_CLS_BLOCK, int64(len(loc)), loc)
  	putattr(ctxt, s, v.Abbrev, DW_FORM_ref_addr, DW_CLS_REFERENCE, 0, v.Type)
  }
  
  // VarsByOffset attaches the methods of sort.Interface to []*Var,
  // sorting in increasing Offset.
  type VarsByOffset []*Var
  
  func (s VarsByOffset) Len() int           { return len(s) }
  func (s VarsByOffset) Less(i, j int) bool { return s[i].Offset < s[j].Offset }
  func (s VarsByOffset) Swap(i, j int)      { s[i], s[j] = s[j], s[i] }
  

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