pclntab.go

Documentation: debug/gosym

     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  	ver120
    28  )
    29  
    30  // A LineTable is a data structure mapping program counters to line numbers.
    31  //
    32  // In Go 1.1 and earlier, each function (represented by a [Func]) had its own LineTable,
    33  // and the line number corresponded to a numbering of all source lines in the
    34  // program, across all files. That absolute line number would then have to be
    35  // converted separately to a file name and line number within the file.
    36  //
    37  // In Go 1.2, the format of the data changed so that there is a single LineTable
    38  // for the entire program, shared by all Funcs, and there are no absolute line
    39  // numbers, just line numbers within specific files.
    40  //
    41  // For the most part, LineTable's methods should be treated as an internal
    42  // detail of the package; callers should use the methods on [Table] instead.
    43  type LineTable struct {
    44  	Data []byte
    45  	PC   uint64
    46  	Line int
    47  
    48  	// This mutex is used to keep parsing of pclntab synchronous.
    49  	mu sync.Mutex
    50  
    51  	// Contains the version of the pclntab section.
    52  	version version
    53  
    54  	// Go 1.2/1.16/1.18 state
    55  	binary      binary.ByteOrder
    56  	quantum     uint32
    57  	ptrsize     uint32
    58  	textStart   uint64 // address of runtime.text symbol (1.18+)
    59  	funcnametab []byte
    60  	cutab       []byte
    61  	funcdata    []byte
    62  	functab     []byte
    63  	nfunctab    uint32
    64  	filetab     []byte
    65  	pctab       []byte // points to the pctables.
    66  	nfiletab    uint32
    67  	funcNames   map[uint32]string // cache the function names
    68  	strings     map[uint32]string // interned substrings of Data, keyed by offset
    69  	// fileMap varies depending on the version of the object file.
    70  	// For ver12, it maps the name to the index in the file table.
    71  	// For ver116, it maps the name to the offset in filetab.
    72  	fileMap map[string]uint32
    73  }
    74  
    75  // NOTE(rsc): This is wrong for GOARCH=arm, which uses a quantum of 4,
    76  // but we have no idea whether we're using arm or not. This only
    77  // matters in the old (pre-Go 1.2) symbol table format, so it's not worth
    78  // fixing.
    79  const oldQuantum = 1
    80  
    81  func (t *LineTable) parse(targetPC uint64, targetLine int) (b []byte, pc uint64, line int) {
    82  	// The PC/line table can be thought of as a sequence of
    83  	//  <pc update>* <line update>
    84  	// batches. Each update batch results in a (pc, line) pair,
    85  	// where line applies to every PC from pc up to but not
    86  	// including the pc of the next pair.
    87  	//
    88  	// Here we process each update individually, which simplifies
    89  	// the code, but makes the corner cases more confusing.
    90  	b, pc, line = t.Data, t.PC, t.Line
    91  	for pc <= targetPC && line != targetLine && len(b) > 0 {
    92  		code := b[0]
    93  		b = b[1:]
    94  		switch {
    95  		case code == 0:
    96  			if len(b) < 4 {
    97  				b = b[0:0]
    98  				break
    99  			}
   100  			val := binary.BigEndian.Uint32(b)
   101  			b = b[4:]
   102  			line += int(val)
   103  		case code <= 64:
   104  			line += int(code)
   105  		case code <= 128:
   106  			line -= int(code - 64)
   107  		default:
   108  			pc += oldQuantum * uint64(code-128)
   109  			continue
   110  		}
   111  		pc += oldQuantum
   112  	}
   113  	return b, pc, line
   114  }
   115  
   116  func (t *LineTable) slice(pc uint64) *LineTable {
   117  	data, pc, line := t.parse(pc, -1)
   118  	return &LineTable{Data: data, PC: pc, Line: line}
   119  }
   120  
   121  // PCToLine returns the line number for the given program counter.
   122  //
   123  // Deprecated: Use Table's PCToLine method instead.
   124  func (t *LineTable) PCToLine(pc uint64) int {
   125  	if t.isGo12() {
   126  		return t.go12PCToLine(pc)
   127  	}
   128  	_, _, line := t.parse(pc, -1)
   129  	return line
   130  }
   131  
   132  // LineToPC returns the program counter for the given line number,
   133  // considering only program counters before maxpc.
   134  //
   135  // Deprecated: Use Table's LineToPC method instead.
   136  func (t *LineTable) LineToPC(line int, maxpc uint64) uint64 {
   137  	if t.isGo12() {
   138  		return 0
   139  	}
   140  	_, pc, line1 := t.parse(maxpc, line)
   141  	if line1 != line {
   142  		return 0
   143  	}
   144  	// Subtract quantum from PC to account for post-line increment
   145  	return pc - oldQuantum
   146  }
   147  
   148  // NewLineTable returns a new PC/line table
   149  // corresponding to the encoded data.
   150  // Text must be the start address of the
   151  // corresponding text segment.
   152  func NewLineTable(data []byte, text uint64) *LineTable {
   153  	return &LineTable{Data: data, PC: text, Line: 0, funcNames: make(map[uint32]string), strings: make(map[uint32]string)}
   154  }
   155  
   156  // Go 1.2 symbol table format.
   157  // See golang.org/s/go12symtab.
   158  //
   159  // A general note about the methods here: rather than try to avoid
   160  // index out of bounds errors, we trust Go to detect them, and then
   161  // we recover from the panics and treat them as indicative of a malformed
   162  // or incomplete table.
   163  //
   164  // The methods called by symtab.go, which begin with "go12" prefixes,
   165  // are expected to have that recovery logic.
   166  
   167  // isGo12 reports whether this is a Go 1.2 (or later) symbol table.
   168  func (t *LineTable) isGo12() bool {
   169  	t.parsePclnTab()
   170  	return t.version >= ver12
   171  }
   172  
   173  const (
   174  	go12magic  = 0xfffffffb
   175  	go116magic = 0xfffffffa
   176  	go118magic = 0xfffffff0
   177  	go120magic = 0xfffffff1
   178  )
   179  
   180  // uintptr returns the pointer-sized value encoded at b.
   181  // The pointer size is dictated by the table being read.
   182  func (t *LineTable) uintptr(b []byte) uint64 {
   183  	if t.ptrsize == 4 {
   184  		return uint64(t.binary.Uint32(b))
   185  	}
   186  	return t.binary.Uint64(b)
   187  }
   188  
   189  // parsePclnTab parses the pclntab, setting the version.
   190  func (t *LineTable) parsePclnTab() {
   191  	t.mu.Lock()
   192  	defer t.mu.Unlock()
   193  	if t.version != verUnknown {
   194  		return
   195  	}
   196  
   197  	// Note that during this function, setting the version is the last thing we do.
   198  	// If we set the version too early, and parsing failed (likely as a panic on
   199  	// slice lookups), we'd have a mistaken version.
   200  	//
   201  	// Error paths through this code will default the version to 1.1.
   202  	t.version = ver11
   203  
   204  	if !disableRecover {
   205  		defer func() {
   206  			// If we panic parsing, assume it's a Go 1.1 pclntab.
   207  			recover()
   208  		}()
   209  	}
   210  
   211  	// Check header: 4-byte magic, two zeros, pc quantum, pointer size.
   212  	if len(t.Data) < 16 || t.Data[4] != 0 || t.Data[5] != 0 ||
   213  		(t.Data[6] != 1 && t.Data[6] != 2 && t.Data[6] != 4) || // pc quantum
   214  		(t.Data[7] != 4 && t.Data[7] != 8) { // pointer size
   215  		return
   216  	}
   217  
   218  	var possibleVersion version
   219  	leMagic := binary.LittleEndian.Uint32(t.Data)
   220  	beMagic := binary.BigEndian.Uint32(t.Data)
   221  	switch {
   222  	case leMagic == go12magic:
   223  		t.binary, possibleVersion = binary.LittleEndian, ver12
   224  	case beMagic == go12magic:
   225  		t.binary, possibleVersion = binary.BigEndian, ver12
   226  	case leMagic == go116magic:
   227  		t.binary, possibleVersion = binary.LittleEndian, ver116
   228  	case beMagic == go116magic:
   229  		t.binary, possibleVersion = binary.BigEndian, ver116
   230  	case leMagic == go118magic:
   231  		t.binary, possibleVersion = binary.LittleEndian, ver118
   232  	case beMagic == go118magic:
   233  		t.binary, possibleVersion = binary.BigEndian, ver118
   234  	case leMagic == go120magic:
   235  		t.binary, possibleVersion = binary.LittleEndian, ver120
   236  	case beMagic == go120magic:
   237  		t.binary, possibleVersion = binary.BigEndian, ver120
   238  	default:
   239  		return
   240  	}
   241  	t.version = possibleVersion
   242  
   243  	// quantum and ptrSize are the same between 1.2, 1.16, and 1.18
   244  	t.quantum = uint32(t.Data[6])
   245  	t.ptrsize = uint32(t.Data[7])
   246  
   247  	offset := func(word uint32) uint64 {
   248  		return t.uintptr(t.Data[8+word*t.ptrsize:])
   249  	}
   250  	data := func(word uint32) []byte {
   251  		return t.Data[offset(word):]
   252  	}
   253  
   254  	switch possibleVersion {
   255  	case ver118, ver120:
   256  		t.nfunctab = uint32(offset(0))
   257  		t.nfiletab = uint32(offset(1))
   258  		t.textStart = t.PC // use the start PC instead of reading from the table, which may be unrelocated
   259  		t.funcnametab = data(3)
   260  		t.cutab = data(4)
   261  		t.filetab = data(5)
   262  		t.pctab = data(6)
   263  		t.funcdata = data(7)
   264  		t.functab = data(7)
   265  		functabsize := (int(t.nfunctab)*2 + 1) * t.functabFieldSize()
   266  		t.functab = t.functab[:functabsize]
   267  	case ver116:
   268  		t.nfunctab = uint32(offset(0))
   269  		t.nfiletab = uint32(offset(1))
   270  		t.funcnametab = data(2)
   271  		t.cutab = data(3)
   272  		t.filetab = data(4)
   273  		t.pctab = data(5)
   274  		t.funcdata = data(6)
   275  		t.functab = data(6)
   276  		functabsize := (int(t.nfunctab)*2 + 1) * t.functabFieldSize()
   277  		t.functab = t.functab[:functabsize]
   278  	case ver12:
   279  		t.nfunctab = uint32(t.uintptr(t.Data[8:]))
   280  		t.funcdata = t.Data
   281  		t.funcnametab = t.Data
   282  		t.functab = t.Data[8+t.ptrsize:]
   283  		t.pctab = t.Data
   284  		functabsize := (int(t.nfunctab)*2 + 1) * t.functabFieldSize()
   285  		fileoff := t.binary.Uint32(t.functab[functabsize:])
   286  		t.functab = t.functab[:functabsize]
   287  		t.filetab = t.Data[fileoff:]
   288  		t.nfiletab = t.binary.Uint32(t.filetab)
   289  		t.filetab = t.filetab[:t.nfiletab*4]
   290  	default:
   291  		panic("unreachable")
   292  	}
   293  }
   294  
   295  // go12Funcs returns a slice of Funcs derived from the Go 1.2+ pcln table.
   296  func (t *LineTable) go12Funcs() []Func {
   297  	// Assume it is malformed and return nil on error.
   298  	if !disableRecover {
   299  		defer func() {
   300  			recover()
   301  		}()
   302  	}
   303  
   304  	ft := t.funcTab()
   305  	funcs := make([]Func, ft.Count())
   306  	syms := make([]Sym, len(funcs))
   307  	for i := range funcs {
   308  		f := &funcs[i]
   309  		f.Entry = ft.pc(i)
   310  		f.End = ft.pc(i + 1)
   311  		info := t.funcData(uint32(i))
   312  		f.LineTable = t
   313  		f.FrameSize = int(info.deferreturn())
   314  		syms[i] = Sym{
   315  			Value:     f.Entry,
   316  			Type:      'T',
   317  			Name:      t.funcName(info.nameOff()),
   318  			GoType:    0,
   319  			Func:      f,
   320  			goVersion: t.version,
   321  		}
   322  		f.Sym = &syms[i]
   323  	}
   324  	return funcs
   325  }
   326  
   327  // findFunc returns the funcData corresponding to the given program counter.
   328  func (t *LineTable) findFunc(pc uint64) funcData {
   329  	ft := t.funcTab()
   330  	if pc < ft.pc(0) || pc >= ft.pc(ft.Count()) {
   331  		return funcData{}
   332  	}
   333  	idx := sort.Search(int(t.nfunctab), func(i int) bool {
   334  		return ft.pc(i) > pc
   335  	})
   336  	idx--
   337  	return t.funcData(uint32(idx))
   338  }
   339  
   340  // readvarint reads, removes, and returns a varint from *pp.
   341  func (t *LineTable) readvarint(pp *[]byte) uint32 {
   342  	var v, shift uint32
   343  	p := *pp
   344  	for shift = 0; ; shift += 7 {
   345  		b := p[0]
   346  		p = p[1:]
   347  		v |= (uint32(b) & 0x7F) << shift
   348  		if b&0x80 == 0 {
   349  			break
   350  		}
   351  	}
   352  	*pp = p
   353  	return v
   354  }
   355  
   356  // funcName returns the name of the function found at off.
   357  func (t *LineTable) funcName(off uint32) string {
   358  	if s, ok := t.funcNames[off]; ok {
   359  		return s
   360  	}
   361  	i := bytes.IndexByte(t.funcnametab[off:], 0)
   362  	s := string(t.funcnametab[off : off+uint32(i)])
   363  	t.funcNames[off] = s
   364  	return s
   365  }
   366  
   367  // stringFrom returns a Go string found at off from a position.
   368  func (t *LineTable) stringFrom(arr []byte, off uint32) string {
   369  	if s, ok := t.strings[off]; ok {
   370  		return s
   371  	}
   372  	i := bytes.IndexByte(arr[off:], 0)
   373  	s := string(arr[off : off+uint32(i)])
   374  	t.strings[off] = s
   375  	return s
   376  }
   377  
   378  // string returns a Go string found at off.
   379  func (t *LineTable) string(off uint32) string {
   380  	return t.stringFrom(t.funcdata, off)
   381  }
   382  
   383  // functabFieldSize returns the size in bytes of a single functab field.
   384  func (t *LineTable) functabFieldSize() int {
   385  	if t.version >= ver118 {
   386  		return 4
   387  	}
   388  	return int(t.ptrsize)
   389  }
   390  
   391  // funcTab returns t's funcTab.
   392  func (t *LineTable) funcTab() funcTab {
   393  	return funcTab{LineTable: t, sz: t.functabFieldSize()}
   394  }
   395  
   396  // funcTab is memory corresponding to a slice of functab structs, followed by an invalid PC.
   397  // A functab struct is a PC and a func offset.
   398  type funcTab struct {
   399  	*LineTable
   400  	sz int // cached result of t.functabFieldSize
   401  }
   402  
   403  // Count returns the number of func entries in f.
   404  func (f funcTab) Count() int {
   405  	return int(f.nfunctab)
   406  }
   407  
   408  // pc returns the PC of the i'th func in f.
   409  func (f funcTab) pc(i int) uint64 {
   410  	u := f.uint(f.functab[2*i*f.sz:])
   411  	if f.version >= ver118 {
   412  		u += f.textStart
   413  	}
   414  	return u
   415  }
   416  
   417  // funcOff returns the funcdata offset of the i'th func in f.
   418  func (f funcTab) funcOff(i int) uint64 {
   419  	return f.uint(f.functab[(2*i+1)*f.sz:])
   420  }
   421  
   422  // uint returns the uint stored at b.
   423  func (f funcTab) uint(b []byte) uint64 {
   424  	if f.sz == 4 {
   425  		return uint64(f.binary.Uint32(b))
   426  	}
   427  	return f.binary.Uint64(b)
   428  }
   429  
   430  // funcData is memory corresponding to an _func struct.
   431  type funcData struct {
   432  	t    *LineTable // LineTable this data is a part of
   433  	data []byte     // raw memory for the function
   434  }
   435  
   436  // funcData returns the ith funcData in t.functab.
   437  func (t *LineTable) funcData(i uint32) funcData {
   438  	data := t.funcdata[t.funcTab().funcOff(int(i)):]
   439  	return funcData{t: t, data: data}
   440  }
   441  
   442  // IsZero reports whether f is the zero value.
   443  func (f funcData) IsZero() bool {
   444  	return f.t == nil && f.data == nil
   445  }
   446  
   447  // entryPC returns the func's entry PC.
   448  func (f *funcData) entryPC() uint64 {
   449  	// In Go 1.18, the first field of _func changed
   450  	// from a uintptr entry PC to a uint32 entry offset.
   451  	if f.t.version >= ver118 {
   452  		// TODO: support multiple text sections.
   453  		// See runtime/symtab.go:(*moduledata).textAddr.
   454  		return uint64(f.t.binary.Uint32(f.data)) + f.t.textStart
   455  	}
   456  	return f.t.uintptr(f.data)
   457  }
   458  
   459  func (f funcData) nameOff() uint32     { return f.field(1) }
   460  func (f funcData) deferreturn() uint32 { return f.field(3) }
   461  func (f funcData) pcfile() uint32      { return f.field(5) }
   462  func (f funcData) pcln() uint32        { return f.field(6) }
   463  func (f funcData) cuOffset() uint32    { return f.field(8) }
   464  
   465  // field returns the nth field of the _func struct.
   466  // It panics if n == 0 or n > 9; for n == 0, call f.entryPC.
   467  // Most callers should use a named field accessor (just above).
   468  func (f funcData) field(n uint32) uint32 {
   469  	if n == 0 || n > 9 {
   470  		panic("bad funcdata field")
   471  	}
   472  	// In Go 1.18, the first field of _func changed
   473  	// from a uintptr entry PC to a uint32 entry offset.
   474  	sz0 := f.t.ptrsize
   475  	if f.t.version >= ver118 {
   476  		sz0 = 4
   477  	}
   478  	off := sz0 + (n-1)*4 // subsequent fields are 4 bytes each
   479  	data := f.data[off:]
   480  	return f.t.binary.Uint32(data)
   481  }
   482  
   483  // step advances to the next pc, value pair in the encoded table.
   484  func (t *LineTable) step(p *[]byte, pc *uint64, val *int32, first bool) bool {
   485  	uvdelta := t.readvarint(p)
   486  	if uvdelta == 0 && !first {
   487  		return false
   488  	}
   489  	if uvdelta&1 != 0 {
   490  		uvdelta = ^(uvdelta >> 1)
   491  	} else {
   492  		uvdelta >>= 1
   493  	}
   494  	vdelta := int32(uvdelta)
   495  	pcdelta := t.readvarint(p) * t.quantum
   496  	*pc += uint64(pcdelta)
   497  	*val += vdelta
   498  	return true
   499  }
   500  
   501  // pcvalue reports the value associated with the target pc.
   502  // off is the offset to the beginning of the pc-value table,
   503  // and entry is the start PC for the corresponding function.
   504  func (t *LineTable) pcvalue(off uint32, entry, targetpc uint64) int32 {
   505  	p := t.pctab[off:]
   506  
   507  	val := int32(-1)
   508  	pc := entry
   509  	for t.step(&p, &pc, &val, pc == entry) {
   510  		if targetpc < pc {
   511  			return val
   512  		}
   513  	}
   514  	return -1
   515  }
   516  
   517  // findFileLine scans one function in the binary looking for a
   518  // program counter in the given file on the given line.
   519  // It does so by running the pc-value tables mapping program counter
   520  // to file number. Since most functions come from a single file, these
   521  // are usually short and quick to scan. If a file match is found, then the
   522  // code goes to the expense of looking for a simultaneous line number match.
   523  func (t *LineTable) findFileLine(entry uint64, filetab, linetab uint32, filenum, line int32, cutab []byte) uint64 {
   524  	if filetab == 0 || linetab == 0 {
   525  		return 0
   526  	}
   527  
   528  	fp := t.pctab[filetab:]
   529  	fl := t.pctab[linetab:]
   530  	fileVal := int32(-1)
   531  	filePC := entry
   532  	lineVal := int32(-1)
   533  	linePC := entry
   534  	fileStartPC := filePC
   535  	for t.step(&fp, &filePC, &fileVal, filePC == entry) {
   536  		fileIndex := fileVal
   537  		if t.version == ver116 || t.version == ver118 || t.version == ver120 {
   538  			fileIndex = int32(t.binary.Uint32(cutab[fileVal*4:]))
   539  		}
   540  		if fileIndex == filenum && fileStartPC < filePC {
   541  			// fileIndex is in effect starting at fileStartPC up to
   542  			// but not including filePC, and it's the file we want.
   543  			// Run the PC table looking for a matching line number
   544  			// or until we reach filePC.
   545  			lineStartPC := linePC
   546  			for linePC < filePC && t.step(&fl, &linePC, &lineVal, linePC == entry) {
   547  				// lineVal is in effect until linePC, and lineStartPC < filePC.
   548  				if lineVal == line {
   549  					if fileStartPC <= lineStartPC {
   550  						return lineStartPC
   551  					}
   552  					if fileStartPC < linePC {
   553  						return fileStartPC
   554  					}
   555  				}
   556  				lineStartPC = linePC
   557  			}
   558  		}
   559  		fileStartPC = filePC
   560  	}
   561  	return 0
   562  }
   563  
   564  // go12PCToLine maps program counter to line number for the Go 1.2+ pcln table.
   565  func (t *LineTable) go12PCToLine(pc uint64) (line int) {
   566  	defer func() {
   567  		if !disableRecover && recover() != nil {
   568  			line = -1
   569  		}
   570  	}()
   571  
   572  	f := t.findFunc(pc)
   573  	if f.IsZero() {
   574  		return -1
   575  	}
   576  	entry := f.entryPC()
   577  	linetab := f.pcln()
   578  	return int(t.pcvalue(linetab, entry, pc))
   579  }
   580  
   581  // go12PCToFile maps program counter to file name for the Go 1.2+ pcln table.
   582  func (t *LineTable) go12PCToFile(pc uint64) (file string) {
   583  	defer func() {
   584  		if !disableRecover && recover() != nil {
   585  			file = ""
   586  		}
   587  	}()
   588  
   589  	f := t.findFunc(pc)
   590  	if f.IsZero() {
   591  		return ""
   592  	}
   593  	entry := f.entryPC()
   594  	filetab := f.pcfile()
   595  	fno := t.pcvalue(filetab, entry, pc)
   596  	if t.version == ver12 {
   597  		if fno <= 0 {
   598  			return ""
   599  		}
   600  		return t.string(t.binary.Uint32(t.filetab[4*fno:]))
   601  	}
   602  	// Go ≥ 1.16
   603  	if fno < 0 { // 0 is valid for ≥ 1.16
   604  		return ""
   605  	}
   606  	cuoff := f.cuOffset()
   607  	if fnoff := t.binary.Uint32(t.cutab[(cuoff+uint32(fno))*4:]); fnoff != ^uint32(0) {
   608  		return t.stringFrom(t.filetab, fnoff)
   609  	}
   610  	return ""
   611  }
   612  
   613  // go12LineToPC maps a (file, line) pair to a program counter for the Go 1.2+ pcln table.
   614  func (t *LineTable) go12LineToPC(file string, line int) (pc uint64) {
   615  	defer func() {
   616  		if !disableRecover && recover() != nil {
   617  			pc = 0
   618  		}
   619  	}()
   620  
   621  	t.initFileMap()
   622  	filenum, ok := t.fileMap[file]
   623  	if !ok {
   624  		return 0
   625  	}
   626  
   627  	// Scan all functions.
   628  	// If this turns out to be a bottleneck, we could build a map[int32][]int32
   629  	// mapping file number to a list of functions with code from that file.
   630  	var cutab []byte
   631  	for i := uint32(0); i < t.nfunctab; i++ {
   632  		f := t.funcData(i)
   633  		entry := f.entryPC()
   634  		filetab := f.pcfile()
   635  		linetab := f.pcln()
   636  		if t.version == ver116 || t.version == ver118 || t.version == ver120 {
   637  			if f.cuOffset() == ^uint32(0) {
   638  				// skip functions without compilation unit (not real function, or linker generated)
   639  				continue
   640  			}
   641  			cutab = t.cutab[f.cuOffset()*4:]
   642  		}
   643  		pc := t.findFileLine(entry, filetab, linetab, int32(filenum), int32(line), cutab)
   644  		if pc != 0 {
   645  			return pc
   646  		}
   647  	}
   648  	return 0
   649  }
   650  
   651  // initFileMap initializes the map from file name to file number.
   652  func (t *LineTable) initFileMap() {
   653  	t.mu.Lock()
   654  	defer t.mu.Unlock()
   655  
   656  	if t.fileMap != nil {
   657  		return
   658  	}
   659  	m := make(map[string]uint32)
   660  
   661  	if t.version == ver12 {
   662  		for i := uint32(1); i < t.nfiletab; i++ {
   663  			s := t.string(t.binary.Uint32(t.filetab[4*i:]))
   664  			m[s] = i
   665  		}
   666  	} else {
   667  		var pos uint32
   668  		for i := uint32(0); i < t.nfiletab; i++ {
   669  			s := t.stringFrom(t.filetab, pos)
   670  			m[s] = pos
   671  			pos += uint32(len(s) + 1)
   672  		}
   673  	}
   674  	t.fileMap = m
   675  }
   676  
   677  // go12MapFiles adds to m a key for every file in the Go 1.2 LineTable.
   678  // Every key maps to obj. That's not a very interesting map, but it provides
   679  // a way for callers to obtain the list of files in the program.
   680  func (t *LineTable) go12MapFiles(m map[string]*Obj, obj *Obj) {
   681  	if !disableRecover {
   682  		defer func() {
   683  			recover()
   684  		}()
   685  	}
   686  
   687  	t.initFileMap()
   688  	for file := range t.fileMap {
   689  		m[file] = obj
   690  	}
   691  }
   692  
   693  // disableRecover causes this package not to swallow panics.
   694  // This is useful when making changes.
   695  const disableRecover = false
   696
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