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// 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] }