pclntab.go

     1  // Copyright 2009 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  /*
     6   * Line tables
     7   */
     8  
     9  package gosym
    10  
    11  import (
    12  	"bytes"
    13  	"encoding/binary"
    14  	"sort"
    15  	"sync"
    16  )
    17  
    18  // version of the pclntab
    19  type version int
    20  
    21  const (
    22  	verUnknown version = iota
    23  	ver11
    24  	ver12
    25  	ver116
    26  	ver118
    27  )
    28  
    29  // A LineTable is a data structure mapping program counters to line numbers.
    30  //
    31  // In Go 1.1 and earlier, each function (represented by a Func) had its own LineTable,
    32  // and the line number corresponded to a numbering of all source lines in the
    33  // program, across all files. That absolute line number would then have to be
    34  // converted separately to a file name and line number within the file.
    35  //
    36  // In Go 1.2, the format of the data changed so that there is a single LineTable
    37  // for the entire program, shared by all Funcs, and there are no absolute line
    38  // numbers, just line numbers within specific files.
    39  //
    40  // For the most part, LineTable's methods should be treated as an internal
    41  // detail of the package; callers should use the methods on Table instead.
    42  type LineTable struct {
    43  	Data []byte
    44  	PC   uint64
    45  	Line int
    46  
    47  	// This mutex is used to keep parsing of pclntab synchronous.
    48  	mu sync.Mutex
    49  
    50  	// Contains the version of the pclntab section.
    51  	version version
    52  
    53  	// Go 1.2/1.16/1.18 state
    54  	binary      binary.ByteOrder
    55  	quantum     uint32
    56  	ptrsize     uint32
    57  	textStart   uint64 // address of runtime.text symbol (1.18+)
    58  	funcnametab []byte
    59  	cutab       []byte
    60  	funcdata    []byte
    61  	functab     []byte
    62  	nfunctab    uint32
    63  	filetab     []byte
    64  	pctab       []byte // points to the pctables.
    65  	nfiletab    uint32
    66  	funcNames   map[uint32]string // cache the function names
    67  	strings     map[uint32]string // interned substrings of Data, keyed by offset
    68  	// fileMap varies depending on the version of the object file.
    69  	// For ver12, it maps the name to the index in the file table.
    70  	// For ver116, it maps the name to the offset in filetab.
    71  	fileMap map[string]uint32
    72  }
    73  
    74  // NOTE(rsc): This is wrong for GOARCH=arm, which uses a quantum of 4,
    75  // but we have no idea whether we're using arm or not. This only
    76  // matters in the old (pre-Go 1.2) symbol table format, so it's not worth
    77  // fixing.
    78  const oldQuantum = 1
    79  
    80  func (t *LineTable) parse(targetPC uint64, targetLine int) (b []byte, pc uint64, line int) {
    81  	// The PC/line table can be thought of as a sequence of
    82  	//  <pc update>* <line update>
    83  	// batches. Each update batch results in a (pc, line) pair,
    84  	// where line applies to every PC from pc up to but not
    85  	// including the pc of the next pair.
    86  	//
    87  	// Here we process each update individually, which simplifies
    88  	// the code, but makes the corner cases more confusing.
    89  	b, pc, line = t.Data, t.PC, t.Line
    90  	for pc <= targetPC && line != targetLine && len(b) > 0 {
    91  		code := b[0]
    92  		b = b[1:]
    93  		switch {
    94  		case code == 0:
    95  			if len(b) < 4 {
    96  				b = b[0:0]
    97  				break
    98  			}
    99  			val := binary.BigEndian.Uint32(b)
   100  			b = b[4:]
   101  			line += int(val)
   102  		case code <= 64:
   103  			line += int(code)
   104  		case code <= 128:
   105  			line -= int(code - 64)
   106  		default:
   107  			pc += oldQuantum * uint64(code-128)
   108  			continue
   109  		}
   110  		pc += oldQuantum
   111  	}
   112  	return b, pc, line
   113  }
   114  
   115  func (t *LineTable) slice(pc uint64) *LineTable {
   116  	data, pc, line := t.parse(pc, -1)
   117  	return &LineTable{Data: data, PC: pc, Line: line}
   118  }
   119  
   120  // PCToLine returns the line number for the given program counter.
   121  //
   122  // Deprecated: Use Table's PCToLine method instead.
   123  func (t *LineTable) PCToLine(pc uint64) int {
   124  	if t.isGo12() {
   125  		return t.go12PCToLine(pc)
   126  	}
   127  	_, _, line := t.parse(pc, -1)
   128  	return line
   129  }
   130  
   131  // LineToPC returns the program counter for the given line number,
   132  // considering only program counters before maxpc.
   133  //
   134  // Deprecated: Use Table's LineToPC method instead.
   135  func (t *LineTable) LineToPC(line int, maxpc uint64) uint64 {
   136  	if t.isGo12() {
   137  		return 0
   138  	}
   139  	_, pc, line1 := t.parse(maxpc, line)
   140  	if line1 != line {
   141  		return 0
   142  	}
   143  	// Subtract quantum from PC to account for post-line increment
   144  	return pc - oldQuantum
   145  }
   146  
   147  // NewLineTable returns a new PC/line table
   148  // corresponding to the encoded data.
   149  // Text must be the start address of the
   150  // corresponding text segment.
   151  func NewLineTable(data []byte, text uint64) *LineTable {
   152  	return &LineTable{Data: data, PC: text, Line: 0, funcNames: make(map[uint32]string), strings: make(map[uint32]string)}
   153  }
   154  
   155  // Go 1.2 symbol table format.
   156  // See golang.org/s/go12symtab.
   157  //
   158  // A general note about the methods here: rather than try to avoid
   159  // index out of bounds errors, we trust Go to detect them, and then
   160  // we recover from the panics and treat them as indicative of a malformed
   161  // or incomplete table.
   162  //
   163  // The methods called by symtab.go, which begin with "go12" prefixes,
   164  // are expected to have that recovery logic.
   165  
   166  // isGo12 reports whether this is a Go 1.2 (or later) symbol table.
   167  func (t *LineTable) isGo12() bool {
   168  	t.parsePclnTab()
   169  	return t.version >= ver12
   170  }
   171  
   172  const (
   173  	go12magic  = 0xfffffffb
   174  	go116magic = 0xfffffffa
   175  	go118magic = 0xfffffff0
   176  )
   177  
   178  // uintptr returns the pointer-sized value encoded at b.
   179  // The pointer size is dictated by the table being read.
   180  func (t *LineTable) uintptr(b []byte) uint64 {
   181  	if t.ptrsize == 4 {
   182  		return uint64(t.binary.Uint32(b))
   183  	}
   184  	return t.binary.Uint64(b)
   185  }
   186  
   187  // parsePclnTab parses the pclntab, setting the version.
   188  func (t *LineTable) parsePclnTab() {
   189  	t.mu.Lock()
   190  	defer t.mu.Unlock()
   191  	if t.version != verUnknown {
   192  		return
   193  	}
   194  
   195  	// Note that during this function, setting the version is the last thing we do.
   196  	// If we set the version too early, and parsing failed (likely as a panic on
   197  	// slice lookups), we'd have a mistaken version.
   198  	//
   199  	// Error paths through this code will default the version to 1.1.
   200  	t.version = ver11
   201  
   202  	if !disableRecover {
   203  		defer func() {
   204  			// If we panic parsing, assume it's a Go 1.1 pclntab.
   205  			recover()
   206  		}()
   207  	}
   208  
   209  	// Check header: 4-byte magic, two zeros, pc quantum, pointer size.
   210  	if len(t.Data) < 16 || t.Data[4] != 0 || t.Data[5] != 0 ||
   211  		(t.Data[6] != 1 && t.Data[6] != 2 && t.Data[6] != 4) || // pc quantum
   212  		(t.Data[7] != 4 && t.Data[7] != 8) { // pointer size
   213  		return
   214  	}
   215  
   216  	var possibleVersion version
   217  	leMagic := binary.LittleEndian.Uint32(t.Data)
   218  	beMagic := binary.BigEndian.Uint32(t.Data)
   219  	switch {
   220  	case leMagic == go12magic:
   221  		t.binary, possibleVersion = binary.LittleEndian, ver12
   222  	case beMagic == go12magic:
   223  		t.binary, possibleVersion = binary.BigEndian, ver12
   224  	case leMagic == go116magic:
   225  		t.binary, possibleVersion = binary.LittleEndian, ver116
   226  	case beMagic == go116magic:
   227  		t.binary, possibleVersion = binary.BigEndian, ver116
   228  	case leMagic == go118magic:
   229  		t.binary, possibleVersion = binary.LittleEndian, ver118
   230  	case beMagic == go118magic:
   231  		t.binary, possibleVersion = binary.BigEndian, ver118
   232  	default:
   233  		return
   234  	}
   235  	t.version = possibleVersion
   236  
   237  	// quantum and ptrSize are the same between 1.2, 1.16, and 1.18
   238  	t.quantum = uint32(t.Data[6])
   239  	t.ptrsize = uint32(t.Data[7])
   240  
   241  	offset := func(word uint32) uint64 {
   242  		return t.uintptr(t.Data[8+word*t.ptrsize:])
   243  	}
   244  	data := func(word uint32) []byte {
   245  		return t.Data[offset(word):]
   246  	}
   247  
   248  	switch possibleVersion {
   249  	case ver118:
   250  		t.nfunctab = uint32(offset(0))
   251  		t.nfiletab = uint32(offset(1))
   252  		t.textStart = t.PC // use the start PC instead of reading from the table, which may be unrelocated
   253  		t.funcnametab = data(3)
   254  		t.cutab = data(4)
   255  		t.filetab = data(5)
   256  		t.pctab = data(6)
   257  		t.funcdata = data(7)
   258  		t.functab = data(7)
   259  		functabsize := (int(t.nfunctab)*2 + 1) * t.functabFieldSize()
   260  		t.functab = t.functab[:functabsize]
   261  	case ver116:
   262  		t.nfunctab = uint32(offset(0))
   263  		t.nfiletab = uint32(offset(1))
   264  		t.funcnametab = data(2)
   265  		t.cutab = data(3)
   266  		t.filetab = data(4)
   267  		t.pctab = data(5)
   268  		t.funcdata = data(6)
   269  		t.functab = data(6)
   270  		functabsize := (int(t.nfunctab)*2 + 1) * t.functabFieldSize()
   271  		t.functab = t.functab[:functabsize]
   272  	case ver12:
   273  		t.nfunctab = uint32(t.uintptr(t.Data[8:]))
   274  		t.funcdata = t.Data
   275  		t.funcnametab = t.Data
   276  		t.functab = t.Data[8+t.ptrsize:]
   277  		t.pctab = t.Data
   278  		functabsize := (int(t.nfunctab)*2 + 1) * t.functabFieldSize()
   279  		fileoff := t.binary.Uint32(t.functab[functabsize:])
   280  		t.functab = t.functab[:functabsize]
   281  		t.filetab = t.Data[fileoff:]
   282  		t.nfiletab = t.binary.Uint32(t.filetab)
   283  		t.filetab = t.filetab[:t.nfiletab*4]
   284  	default:
   285  		panic("unreachable")
   286  	}
   287  }
   288  
   289  // go12Funcs returns a slice of Funcs derived from the Go 1.2+ pcln table.
   290  func (t *LineTable) go12Funcs() []Func {
   291  	// Assume it is malformed and return nil on error.
   292  	if !disableRecover {
   293  		defer func() {
   294  			recover()
   295  		}()
   296  	}
   297  
   298  	ft := t.funcTab()
   299  	funcs := make([]Func, ft.Count())
   300  	syms := make([]Sym, len(funcs))
   301  	for i := range funcs {
   302  		f := &funcs[i]
   303  		f.Entry = ft.pc(i)
   304  		f.End = ft.pc(i + 1)
   305  		info := t.funcData(uint32(i))
   306  		f.LineTable = t
   307  		f.FrameSize = int(info.deferreturn())
   308  		syms[i] = Sym{
   309  			Value:  f.Entry,
   310  			Type:   'T',
   311  			Name:   t.funcName(info.nameoff()),
   312  			GoType: 0,
   313  			Func:   f,
   314  		}
   315  		f.Sym = &syms[i]
   316  	}
   317  	return funcs
   318  }
   319  
   320  // findFunc returns the funcData corresponding to the given program counter.
   321  func (t *LineTable) findFunc(pc uint64) funcData {
   322  	ft := t.funcTab()
   323  	if pc < ft.pc(0) || pc >= ft.pc(ft.Count()) {
   324  		return funcData{}
   325  	}
   326  	idx := sort.Search(int(t.nfunctab), func(i int) bool {
   327  		return ft.pc(i) > pc
   328  	})
   329  	idx--
   330  	return t.funcData(uint32(idx))
   331  }
   332  
   333  // readvarint reads, removes, and returns a varint from *pp.
   334  func (t *LineTable) readvarint(pp *[]byte) uint32 {
   335  	var v, shift uint32
   336  	p := *pp
   337  	for shift = 0; ; shift += 7 {
   338  		b := p[0]
   339  		p = p[1:]
   340  		v |= (uint32(b) & 0x7F) << shift
   341  		if b&0x80 == 0 {
   342  			break
   343  		}
   344  	}
   345  	*pp = p
   346  	return v
   347  }
   348  
   349  // funcName returns the name of the function found at off.
   350  func (t *LineTable) funcName(off uint32) string {
   351  	if s, ok := t.funcNames[off]; ok {
   352  		return s
   353  	}
   354  	i := bytes.IndexByte(t.funcnametab[off:], 0)
   355  	s := string(t.funcnametab[off : off+uint32(i)])
   356  	t.funcNames[off] = s
   357  	return s
   358  }
   359  
   360  // stringFrom returns a Go string found at off from a position.
   361  func (t *LineTable) stringFrom(arr []byte, off uint32) string {
   362  	if s, ok := t.strings[off]; ok {
   363  		return s
   364  	}
   365  	i := bytes.IndexByte(arr[off:], 0)
   366  	s := string(arr[off : off+uint32(i)])
   367  	t.strings[off] = s
   368  	return s
   369  }
   370  
   371  // string returns a Go string found at off.
   372  func (t *LineTable) string(off uint32) string {
   373  	return t.stringFrom(t.funcdata, off)
   374  }
   375  
   376  // functabFieldSize returns the size in bytes of a single functab field.
   377  func (t *LineTable) functabFieldSize() int {
   378  	if t.version >= ver118 {
   379  		return 4
   380  	}
   381  	return int(t.ptrsize)
   382  }
   383  
   384  // funcTab returns t's funcTab.
   385  func (t *LineTable) funcTab() funcTab {
   386  	return funcTab{LineTable: t, sz: t.functabFieldSize()}
   387  }
   388  
   389  // funcTab is memory corresponding to a slice of functab structs, followed by an invalid PC.
   390  // A functab struct is a PC and a func offset.
   391  type funcTab struct {
   392  	*LineTable
   393  	sz int // cached result of t.functabFieldSize
   394  }
   395  
   396  // Count returns the number of func entries in f.
   397  func (f funcTab) Count() int {
   398  	return int(f.nfunctab)
   399  }
   400  
   401  // pc returns the PC of the i'th func in f.
   402  func (f funcTab) pc(i int) uint64 {
   403  	u := f.uint(f.functab[2*i*f.sz:])
   404  	if f.version >= ver118 {
   405  		u += f.textStart
   406  	}
   407  	return u
   408  }
   409  
   410  // funcOff returns the funcdata offset of the i'th func in f.
   411  func (f funcTab) funcOff(i int) uint64 {
   412  	return f.uint(f.functab[(2*i+1)*f.sz:])
   413  }
   414  
   415  // uint returns the uint stored at b.
   416  func (f funcTab) uint(b []byte) uint64 {
   417  	if f.sz == 4 {
   418  		return uint64(f.binary.Uint32(b))
   419  	}
   420  	return f.binary.Uint64(b)
   421  }
   422  
   423  // funcData is memory corresponding to an _func struct.
   424  type funcData struct {
   425  	t    *LineTable // LineTable this data is a part of
   426  	data []byte     // raw memory for the function
   427  }
   428  
   429  // funcData returns the ith funcData in t.functab.
   430  func (t *LineTable) funcData(i uint32) funcData {
   431  	data := t.funcdata[t.funcTab().funcOff(int(i)):]
   432  	return funcData{t: t, data: data}
   433  }
   434  
   435  // IsZero reports whether f is the zero value.
   436  func (f funcData) IsZero() bool {
   437  	return f.t == nil && f.data == nil
   438  }
   439  
   440  // entryPC returns the func's entry PC.
   441  func (f *funcData) entryPC() uint64 {
   442  	// In Go 1.18, the first field of _func changed
   443  	// from a uintptr entry PC to a uint32 entry offset.
   444  	if f.t.version >= ver118 {
   445  		// TODO: support multiple text sections.
   446  		// See runtime/symtab.go:(*moduledata).textAddr.
   447  		return uint64(f.t.binary.Uint32(f.data)) + f.t.textStart
   448  	}
   449  	return f.t.uintptr(f.data)
   450  }
   451  
   452  func (f funcData) nameoff() uint32     { return f.field(1) }
   453  func (f funcData) deferreturn() uint32 { return f.field(3) }
   454  func (f funcData) pcfile() uint32      { return f.field(5) }
   455  func (f funcData) pcln() uint32        { return f.field(6) }
   456  func (f funcData) cuOffset() uint32    { return f.field(8) }
   457  
   458  // field returns the nth field of the _func struct.
   459  // It panics if n == 0 or n > 9; for n == 0, call f.entryPC.
   460  // Most callers should use a named field accessor (just above).
   461  func (f funcData) field(n uint32) uint32 {
   462  	if n == 0 || n > 9 {
   463  		panic("bad funcdata field")
   464  	}
   465  	// In Go 1.18, the first field of _func changed
   466  	// from a uintptr entry PC to a uint32 entry offset.
   467  	sz0 := f.t.ptrsize
   468  	if f.t.version >= ver118 {
   469  		sz0 = 4
   470  	}
   471  	off := sz0 + (n-1)*4 // subsequent fields are 4 bytes each
   472  	data := f.data[off:]
   473  	return f.t.binary.Uint32(data)
   474  }
   475  
   476  // step advances to the next pc, value pair in the encoded table.
   477  func (t *LineTable) step(p *[]byte, pc *uint64, val *int32, first bool) bool {
   478  	uvdelta := t.readvarint(p)
   479  	if uvdelta == 0 && !first {
   480  		return false
   481  	}
   482  	if uvdelta&1 != 0 {
   483  		uvdelta = ^(uvdelta >> 1)
   484  	} else {
   485  		uvdelta >>= 1
   486  	}
   487  	vdelta := int32(uvdelta)
   488  	pcdelta := t.readvarint(p) * t.quantum
   489  	*pc += uint64(pcdelta)
   490  	*val += vdelta
   491  	return true
   492  }
   493  
   494  // pcvalue reports the value associated with the target pc.
   495  // off is the offset to the beginning of the pc-value table,
   496  // and entry is the start PC for the corresponding function.
   497  func (t *LineTable) pcvalue(off uint32, entry, targetpc uint64) int32 {
   498  	p := t.pctab[off:]
   499  
   500  	val := int32(-1)
   501  	pc := entry
   502  	for t.step(&p, &pc, &val, pc == entry) {
   503  		if targetpc < pc {
   504  			return val
   505  		}
   506  	}
   507  	return -1
   508  }
   509  
   510  // findFileLine scans one function in the binary looking for a
   511  // program counter in the given file on the given line.
   512  // It does so by running the pc-value tables mapping program counter
   513  // to file number. Since most functions come from a single file, these
   514  // are usually short and quick to scan. If a file match is found, then the
   515  // code goes to the expense of looking for a simultaneous line number match.
   516  func (t *LineTable) findFileLine(entry uint64, filetab, linetab uint32, filenum, line int32, cutab []byte) uint64 {
   517  	if filetab == 0 || linetab == 0 {
   518  		return 0
   519  	}
   520  
   521  	fp := t.pctab[filetab:]
   522  	fl := t.pctab[linetab:]
   523  	fileVal := int32(-1)
   524  	filePC := entry
   525  	lineVal := int32(-1)
   526  	linePC := entry
   527  	fileStartPC := filePC
   528  	for t.step(&fp, &filePC, &fileVal, filePC == entry) {
   529  		fileIndex := fileVal
   530  		if t.version == ver116 || t.version == ver118 {
   531  			fileIndex = int32(t.binary.Uint32(cutab[fileVal*4:]))
   532  		}
   533  		if fileIndex == filenum && fileStartPC < filePC {
   534  			// fileIndex is in effect starting at fileStartPC up to
   535  			// but not including filePC, and it's the file we want.
   536  			// Run the PC table looking for a matching line number
   537  			// or until we reach filePC.
   538  			lineStartPC := linePC
   539  			for linePC < filePC && t.step(&fl, &linePC, &lineVal, linePC == entry) {
   540  				// lineVal is in effect until linePC, and lineStartPC < filePC.
   541  				if lineVal == line {
   542  					if fileStartPC <= lineStartPC {
   543  						return lineStartPC
   544  					}
   545  					if fileStartPC < linePC {
   546  						return fileStartPC
   547  					}
   548  				}
   549  				lineStartPC = linePC
   550  			}
   551  		}
   552  		fileStartPC = filePC
   553  	}
   554  	return 0
   555  }
   556  
   557  // go12PCToLine maps program counter to line number for the Go 1.2+ pcln table.
   558  func (t *LineTable) go12PCToLine(pc uint64) (line int) {
   559  	defer func() {
   560  		if !disableRecover && recover() != nil {
   561  			line = -1
   562  		}
   563  	}()
   564  
   565  	f := t.findFunc(pc)
   566  	if f.IsZero() {
   567  		return -1
   568  	}
   569  	entry := f.entryPC()
   570  	linetab := f.pcln()
   571  	return int(t.pcvalue(linetab, entry, pc))
   572  }
   573  
   574  // go12PCToFile maps program counter to file name for the Go 1.2+ pcln table.
   575  func (t *LineTable) go12PCToFile(pc uint64) (file string) {
   576  	defer func() {
   577  		if !disableRecover && recover() != nil {
   578  			file = ""
   579  		}
   580  	}()
   581  
   582  	f := t.findFunc(pc)
   583  	if f.IsZero() {
   584  		return ""
   585  	}
   586  	entry := f.entryPC()
   587  	filetab := f.pcfile()
   588  	fno := t.pcvalue(filetab, entry, pc)
   589  	if t.version == ver12 {
   590  		if fno <= 0 {
   591  			return ""
   592  		}
   593  		return t.string(t.binary.Uint32(t.filetab[4*fno:]))
   594  	}
   595  	// Go ≥ 1.16
   596  	if fno < 0 { // 0 is valid for ≥ 1.16
   597  		return ""
   598  	}
   599  	cuoff := f.cuOffset()
   600  	if fnoff := t.binary.Uint32(t.cutab[(cuoff+uint32(fno))*4:]); fnoff != ^uint32(0) {
   601  		return t.stringFrom(t.filetab, fnoff)
   602  	}
   603  	return ""
   604  }
   605  
   606  // go12LineToPC maps a (file, line) pair to a program counter for the Go 1.2+ pcln table.
   607  func (t *LineTable) go12LineToPC(file string, line int) (pc uint64) {
   608  	defer func() {
   609  		if !disableRecover && recover() != nil {
   610  			pc = 0
   611  		}
   612  	}()
   613  
   614  	t.initFileMap()
   615  	filenum, ok := t.fileMap[file]
   616  	if !ok {
   617  		return 0
   618  	}
   619  
   620  	// Scan all functions.
   621  	// If this turns out to be a bottleneck, we could build a map[int32][]int32
   622  	// mapping file number to a list of functions with code from that file.
   623  	var cutab []byte
   624  	for i := uint32(0); i < t.nfunctab; i++ {
   625  		f := t.funcData(i)
   626  		entry := f.entryPC()
   627  		filetab := f.pcfile()
   628  		linetab := f.pcln()
   629  		if t.version == ver116 || t.version == ver118 {
   630  			cutab = t.cutab[f.cuOffset()*4:]
   631  		}
   632  		pc := t.findFileLine(entry, filetab, linetab, int32(filenum), int32(line), cutab)
   633  		if pc != 0 {
   634  			return pc
   635  		}
   636  	}
   637  	return 0
   638  }
   639  
   640  // initFileMap initializes the map from file name to file number.
   641  func (t *LineTable) initFileMap() {
   642  	t.mu.Lock()
   643  	defer t.mu.Unlock()
   644  
   645  	if t.fileMap != nil {
   646  		return
   647  	}
   648  	m := make(map[string]uint32)
   649  
   650  	if t.version == ver12 {
   651  		for i := uint32(1); i < t.nfiletab; i++ {
   652  			s := t.string(t.binary.Uint32(t.filetab[4*i:]))
   653  			m[s] = i
   654  		}
   655  	} else {
   656  		var pos uint32
   657  		for i := uint32(0); i < t.nfiletab; i++ {
   658  			s := t.stringFrom(t.filetab, pos)
   659  			m[s] = pos
   660  			pos += uint32(len(s) + 1)
   661  		}
   662  	}
   663  	t.fileMap = m
   664  }
   665  
   666  // go12MapFiles adds to m a key for every file in the Go 1.2 LineTable.
   667  // Every key maps to obj. That's not a very interesting map, but it provides
   668  // a way for callers to obtain the list of files in the program.
   669  func (t *LineTable) go12MapFiles(m map[string]*Obj, obj *Obj) {
   670  	if !disableRecover {
   671  		defer func() {
   672  			recover()
   673  		}()
   674  	}
   675  
   676  	t.initFileMap()
   677  	for file := range t.fileMap {
   678  		m[file] = obj
   679  	}
   680  }
   681  
   682  // disableRecover causes this package not to swallow panics.
   683  // This is useful when making changes.
   684  const disableRecover = false
   685
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