src/debug/dwarf/line.go - go - Git at Google

 // Copyright 2015 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

 import (
 	"errors"
 	"fmt"
 	"io"
 	"path"
 	"strings"
 )

 // A LineReader reads a sequence of LineEntry structures from a DWARF
 // "line" section for a single compilation unit. LineEntries occur in
 // order of increasing PC and each LineEntry gives metadata for the
 // instructions from that LineEntry's PC to just before the next
 // LineEntry's PC. The last entry will have its EndSequence field set.
 type LineReader struct {
 	buf buf

 	// Original .debug_line section data. Used by Seek.
 	section []byte

 	// Header information
 	version              uint16
 	minInstructionLength int
 	maxOpsPerInstruction int
 	defaultIsStmt        bool
 	lineBase             int
 	lineRange            int
 	opcodeBase           int
 	opcodeLengths        []int
 	directories          []string
 	fileEntries          []*LineFile

 	programOffset Offset // section offset of line number program
 	endOffset     Offset // section offset of byte following program

 	initialFileEntries int // initial length of fileEntries

 	// Current line number program state machine registers
 	state     LineEntry // public state
 	fileIndex int       // private state
 }

 // A LineEntry is a row in a DWARF line table.
 type LineEntry struct {
 	// Address is the program-counter value of a machine
 	// instruction generated by the compiler. This LineEntry
 	// applies to each instruction from Address to just before the
 	// Address of the next LineEntry.
 	Address uint64

 	// OpIndex is the index of an operation within a VLIW
 	// instruction. The index of the first operation is 0. For
 	// non-VLIW architectures, it will always be 0. Address and
 	// OpIndex together form an operation pointer that can
 	// reference any individual operation within the instruction
 	// stream.
 	OpIndex int

 	// File is the source file corresponding to these
 	// instructions.
 	File *LineFile

 	// Line is the source code line number corresponding to these
 	// instructions. Lines are numbered beginning at 1. It may be
 	// 0 if these instructions cannot be attributed to any source
 	// line.
 	Line int

 	// Column is the column number within the source line of these
 	// instructions. Columns are numbered beginning at 1. It may
 	// be 0 to indicate the "left edge" of the line.
 	Column int

 	// IsStmt indicates that Address is a recommended breakpoint
 	// location, such as the beginning of a line, statement, or a
 	// distinct subpart of a statement.
 	IsStmt bool

 	// BasicBlock indicates that Address is the beginning of a
 	// basic block.
 	BasicBlock bool

 	// PrologueEnd indicates that Address is one (of possibly
 	// many) PCs where execution should be suspended for a
 	// breakpoint on entry to the containing function.
 	//
 	// Added in DWARF 3.
 	PrologueEnd bool

 	// EpilogueBegin indicates that Address is one (of possibly
 	// many) PCs where execution should be suspended for a
 	// breakpoint on exit from this function.
 	//
 	// Added in DWARF 3.
 	EpilogueBegin bool

 	// ISA is the instruction set architecture for these
 	// instructions. Possible ISA values should be defined by the
 	// applicable ABI specification.
 	//
 	// Added in DWARF 3.
 	ISA int

 	// Discriminator is an arbitrary integer indicating the block
 	// to which these instructions belong. It serves to
 	// distinguish among multiple blocks that may all have with
 	// the same source file, line, and column. Where only one
 	// block exists for a given source position, it should be 0.
 	//
 	// Added in DWARF 3.
 	Discriminator int

 	// EndSequence indicates that Address is the first byte after
 	// the end of a sequence of target machine instructions. If it
 	// is set, only this and the Address field are meaningful. A
 	// line number table may contain information for multiple
 	// potentially disjoint instruction sequences. The last entry
 	// in a line table should always have EndSequence set.
 	EndSequence bool
 }

 // A LineFile is a source file referenced by a DWARF line table entry.
 type LineFile struct {
 	Name   string
 	Mtime  uint64 // Implementation defined modification time, or 0 if unknown
 	Length int    // File length, or 0 if unknown
 }

 // LineReader returns a new reader for the line table of compilation
 // unit cu, which must be an Entry with tag TagCompileUnit.
 //
 // If this compilation unit has no line table, it returns nil, nil.
 func (d *Data) LineReader(cu *Entry) (*LineReader, error) {
 	if d.line == nil {
 		// No line tables available.
 		return nil, nil
 	}

 	// Get line table information from cu.
 	off, ok := cu.Val(AttrStmtList).(int64)
 	if !ok {
 		// cu has no line table.
 		return nil, nil
 	}
 	if off > int64(len(d.line)) {
 		return nil, errors.New("AttrStmtList value out of range")
 	}
 	// AttrCompDir is optional if all file names are absolute. Use
 	// the empty string if it's not present.
 	compDir, _ := cu.Val(AttrCompDir).(string)

 	// Create the LineReader.
 	u := &d.unit[d.offsetToUnit(cu.Offset)]
 	buf := makeBuf(d, u, "line", Offset(off), d.line[off:])
 	// The compilation directory is implicitly directories[0].
 	r := LineReader{buf: buf, section: d.line, directories: []string{compDir}}

 	// Read the header.
 	if err := r.readHeader(); err != nil {
 		return nil, err
 	}

 	// Initialize line reader state.
 	r.Reset()

 	return &r, nil
 }

 // readHeader reads the line number program header from r.buf and sets
 // all of the header fields in r.
 func (r *LineReader) readHeader() error {
 	buf := &r.buf

 	// Read basic header fields [DWARF2 6.2.4].
 	hdrOffset := buf.off
 	unitLength, dwarf64 := buf.unitLength()
 	r.endOffset = buf.off + unitLength
 	if r.endOffset > buf.off+Offset(len(buf.data)) {
 		return DecodeError{"line", hdrOffset, fmt.Sprintf("line table end %d exceeds section size %d", r.endOffset, buf.off+Offset(len(buf.data)))}
 	}
 	r.version = buf.uint16()
 	if buf.err == nil && (r.version < 2 || r.version > 4) {
 		// DWARF goes to all this effort to make new opcodes
 		// backward-compatible, and then adds fields right in
 		// the middle of the header in new versions, so we're
 		// picky about only supporting known line table
 		// versions.
 		return DecodeError{"line", hdrOffset, fmt.Sprintf("unknown line table version %d", r.version)}
 	}
 	var headerLength Offset
 	if dwarf64 {
 		headerLength = Offset(buf.uint64())
 	} else {
 		headerLength = Offset(buf.uint32())
 	}
 	r.programOffset = buf.off + headerLength
 	r.minInstructionLength = int(buf.uint8())
 	if r.version >= 4 {
 		// [DWARF4 6.2.4]
 		r.maxOpsPerInstruction = int(buf.uint8())
 	} else {
 		r.maxOpsPerInstruction = 1
 	}
 	r.defaultIsStmt = buf.uint8() != 0
 	r.lineBase = int(int8(buf.uint8()))
 	r.lineRange = int(buf.uint8())

 	// Validate header.
 	if buf.err != nil {
 		return buf.err
 	}
 	if r.maxOpsPerInstruction == 0 {
 		return DecodeError{"line", hdrOffset, "invalid maximum operations per instruction: 0"}
 	}
 	if r.lineRange == 0 {
 		return DecodeError{"line", hdrOffset, "invalid line range: 0"}
 	}

 	// Read standard opcode length table. This table starts with opcode 1.
 	r.opcodeBase = int(buf.uint8())
 	r.opcodeLengths = make([]int, r.opcodeBase)
 	for i := 1; i < r.opcodeBase; i++ {
 		r.opcodeLengths[i] = int(buf.uint8())
 	}

 	// Validate opcode lengths.
 	if buf.err != nil {
 		return buf.err
 	}
 	for i, length := range r.opcodeLengths {
 		if known, ok := knownOpcodeLengths[i]; ok && known != length {
 			return DecodeError{"line", hdrOffset, fmt.Sprintf("opcode %d expected to have length %d, but has length %d", i, known, length)}
 		}
 	}

 	// Read include directories table. The caller already set
 	// directories[0] to the compilation directory.
 	for {
 		directory := buf.string()
 		if buf.err != nil {
 			return buf.err
 		}
 		if len(directory) == 0 {
 			break
 		}
 		if !pathIsAbs(directory) {
 			// Relative paths are implicitly relative to
 			// the compilation directory.
 			directory = pathJoin(r.directories[0], directory)
 		}
 		r.directories = append(r.directories, directory)
 	}

 	// Read file name list. File numbering starts with 1, so leave
 	// the first entry nil.
 	r.fileEntries = make([]*LineFile, 1)
 	for {
 		if done, err := r.readFileEntry(); err != nil {
 			return err
 		} else if done {
 			break
 		}
 	}
 	r.initialFileEntries = len(r.fileEntries)

 	return buf.err
 }

 // readFileEntry reads a file entry from either the header or a
 // DW_LNE_define_file extended opcode and adds it to r.fileEntries. A
 // true return value indicates that there are no more entries to read.
 func (r *LineReader) readFileEntry() (bool, error) {
 	name := r.buf.string()
 	if r.buf.err != nil {
 		return false, r.buf.err
 	}
 	if len(name) == 0 {
 		return true, nil
 	}
 	off := r.buf.off
 	dirIndex := int(r.buf.uint())
 	if !pathIsAbs(name) {
 		if dirIndex >= len(r.directories) {
 			return false, DecodeError{"line", off, "directory index too large"}
 		}
 		name = pathJoin(r.directories[dirIndex], name)
 	}
 	mtime := r.buf.uint()
 	length := int(r.buf.uint())

 	r.fileEntries = append(r.fileEntries, &LineFile{name, mtime, length})
 	return false, nil
 }

 // updateFile updates r.state.File after r.fileIndex has
 // changed or r.fileEntries has changed.
 func (r *LineReader) updateFile() {
 	if r.fileIndex < len(r.fileEntries) {
 		r.state.File = r.fileEntries[r.fileIndex]
 	} else {
 		r.state.File = nil
 	}
 }

 // Next sets *entry to the next row in this line table and moves to
 // the next row. If there are no more entries and the line table is
 // properly terminated, it returns io.EOF.
 //
 // Rows are always in order of increasing entry.Address, but
 // entry.Line may go forward or backward.
 func (r *LineReader) Next(entry *LineEntry) error {
 	if r.buf.err != nil {
 		return r.buf.err
 	}

 	// Execute opcodes until we reach an opcode that emits a line
 	// table entry.
 	for {
 		if len(r.buf.data) == 0 {
 			return io.EOF
 		}
 		emit := r.step(entry)
 		if r.buf.err != nil {
 			return r.buf.err
 		}
 		if emit {
 			return nil
 		}
 	}
 }

 // knownOpcodeLengths gives the opcode lengths (in varint arguments)
 // of known standard opcodes.
 var knownOpcodeLengths = map[int]int{
 	lnsCopy:             0,
 	lnsAdvancePC:        1,
 	lnsAdvanceLine:      1,
 	lnsSetFile:          1,
 	lnsNegateStmt:       0,
 	lnsSetBasicBlock:    0,
 	lnsConstAddPC:       0,
 	lnsSetPrologueEnd:   0,
 	lnsSetEpilogueBegin: 0,
 	lnsSetISA:           1,
 	// lnsFixedAdvancePC takes a uint8 rather than a varint; it's
 	// unclear what length the header is supposed to claim, so
 	// ignore it.
 }

 // step processes the next opcode and updates r.state. If the opcode
 // emits a row in the line table, this updates *entry and returns
 // true.
 func (r *LineReader) step(entry *LineEntry) bool {
 	opcode := int(r.buf.uint8())

 	if opcode >= r.opcodeBase {
 		// Special opcode [DWARF2 6.2.5.1, DWARF4 6.2.5.1]
 		adjustedOpcode := opcode - r.opcodeBase
 		r.advancePC(adjustedOpcode / r.lineRange)
 		lineDelta := r.lineBase + adjustedOpcode%r.lineRange
 		r.state.Line += lineDelta
 		goto emit
 	}

 	switch opcode {
 	case 0:
 		// Extended opcode [DWARF2 6.2.5.3]
 		length := Offset(r.buf.uint())
 		startOff := r.buf.off
 		opcode := r.buf.uint8()

 		switch opcode {
 		case lneEndSequence:
 			r.state.EndSequence = true
 			*entry = r.state
 			r.resetState()

 		case lneSetAddress:
 			r.state.Address = r.buf.addr()

 		case lneDefineFile:
 			if done, err := r.readFileEntry(); err != nil {
 				r.buf.err = err
 				return false
 			} else if done {
 				r.buf.err = DecodeError{"line", startOff, "malformed DW_LNE_define_file operation"}
 				return false
 			}
 			r.updateFile()

 		case lneSetDiscriminator:
 			// [DWARF4 6.2.5.3]
 			r.state.Discriminator = int(r.buf.uint())
 		}

 		r.buf.skip(int(startOff + length - r.buf.off))

 		if opcode == lneEndSequence {
 			return true
 		}

 	// Standard opcodes [DWARF2 6.2.5.2]
 	case lnsCopy:
 		goto emit

 	case lnsAdvancePC:
 		r.advancePC(int(r.buf.uint()))

 	case lnsAdvanceLine:
 		r.state.Line += int(r.buf.int())

 	case lnsSetFile:
 		r.fileIndex = int(r.buf.uint())
 		r.updateFile()

 	case lnsSetColumn:
 		r.state.Column = int(r.buf.uint())

 	case lnsNegateStmt:
 		r.state.IsStmt = !r.state.IsStmt

 	case lnsSetBasicBlock:
 		r.state.BasicBlock = true

 	case lnsConstAddPC:
 		r.advancePC((255 - r.opcodeBase) / r.lineRange)

 	case lnsFixedAdvancePC:
 		r.state.Address += uint64(r.buf.uint16())

 	// DWARF3 standard opcodes [DWARF3 6.2.5.2]
 	case lnsSetPrologueEnd:
 		r.state.PrologueEnd = true

 	case lnsSetEpilogueBegin:
 		r.state.EpilogueBegin = true

 	case lnsSetISA:
 		r.state.ISA = int(r.buf.uint())

 	default:
 		// Unhandled standard opcode. Skip the number of
 		// arguments that the prologue says this opcode has.
 		for i := 0; i < r.opcodeLengths[opcode]; i++ {
 			r.buf.uint()
 		}
 	}
 	return false

 emit:
 	*entry = r.state
 	r.state.BasicBlock = false
 	r.state.PrologueEnd = false
 	r.state.EpilogueBegin = false
 	r.state.Discriminator = 0
 	return true
 }

 // advancePC advances "operation pointer" (the combination of Address
 // and OpIndex) in r.state by opAdvance steps.
 func (r *LineReader) advancePC(opAdvance int) {
 	opIndex := r.state.OpIndex + opAdvance
 	r.state.Address += uint64(r.minInstructionLength * (opIndex / r.maxOpsPerInstruction))
 	r.state.OpIndex = opIndex % r.maxOpsPerInstruction
 }

 // A LineReaderPos represents a position in a line table.
 type LineReaderPos struct {
 	// off is the current offset in the DWARF line section.
 	off Offset
 	// numFileEntries is the length of fileEntries.
 	numFileEntries int
 	// state and fileIndex are the statement machine state at
 	// offset off.
 	state     LineEntry
 	fileIndex int
 }

 // Tell returns the current position in the line table.
 func (r *LineReader) Tell() LineReaderPos {
 	return LineReaderPos{r.buf.off, len(r.fileEntries), r.state, r.fileIndex}
 }

 // Seek restores the line table reader to a position returned by Tell.
 //
 // The argument pos must have been returned by a call to Tell on this
 // line table.
 func (r *LineReader) Seek(pos LineReaderPos) {
 	r.buf.off = pos.off
 	r.buf.data = r.section[r.buf.off:r.endOffset]
 	r.fileEntries = r.fileEntries[:pos.numFileEntries]
 	r.state = pos.state
 	r.fileIndex = pos.fileIndex
 }

 // Reset repositions the line table reader at the beginning of the
 // line table.
 func (r *LineReader) Reset() {
 	// Reset buffer to the line number program offset.
 	r.buf.off = r.programOffset
 	r.buf.data = r.section[r.buf.off:r.endOffset]

 	// Reset file entries list.
 	r.fileEntries = r.fileEntries[:r.initialFileEntries]

 	// Reset line number program state.
 	r.resetState()
 }

 // resetState resets r.state to its default values
 func (r *LineReader) resetState() {
 	// Reset the state machine registers to the defaults given in
 	// [DWARF4 6.2.2].
 	r.state = LineEntry{
 		Address:       0,
 		OpIndex:       0,
 		File:          nil,
 		Line:          1,
 		Column:        0,
 		IsStmt:        r.defaultIsStmt,
 		BasicBlock:    false,
 		PrologueEnd:   false,
 		EpilogueBegin: false,
 		ISA:           0,
 		Discriminator: 0,
 	}
 	r.fileIndex = 1
 	r.updateFile()
 }

 // ErrUnknownPC is the error returned by LineReader.ScanPC when the
 // seek PC is not covered by any entry in the line table.
 var ErrUnknownPC = errors.New("ErrUnknownPC")

 // SeekPC sets *entry to the LineEntry that includes pc and positions
 // the reader on the next entry in the line table. If necessary, this
 // will seek backwards to find pc.
 //
 // If pc is not covered by any entry in this line table, SeekPC
 // returns ErrUnknownPC. In this case, *entry and the final seek
 // position are unspecified.
 //
 // Note that DWARF line tables only permit sequential, forward scans.
 // Hence, in the worst case, this takes time linear in the size of the
 // line table. If the caller wishes to do repeated fast PC lookups, it
 // should build an appropriate index of the line table.
 func (r *LineReader) SeekPC(pc uint64, entry *LineEntry) error {
 	if err := r.Next(entry); err != nil {
 		return err
 	}
 	if entry.Address > pc {
 		// We're too far. Start at the beginning of the table.
 		r.Reset()
 		if err := r.Next(entry); err != nil {
 			return err
 		}
 		if entry.Address > pc {
 			// The whole table starts after pc.
 			r.Reset()
 			return ErrUnknownPC
 		}
 	}

 	// Scan until we pass pc, then back up one.
 	for {
 		var next LineEntry
 		pos := r.Tell()
 		if err := r.Next(&next); err != nil {
 			if err == io.EOF {
 				return ErrUnknownPC
 			}
 			return err
 		}
 		if next.Address > pc {
 			if entry.EndSequence {
 				// pc is in a hole in the table.
 				return ErrUnknownPC
 			}
 			// entry is the desired entry. Back up the
 			// cursor to "next" and return success.
 			r.Seek(pos)
 			return nil
 		}
 		*entry = next
 	}
 }

 // pathIsAbs reports whether path is an absolute path (or "full path
 // name" in DWARF parlance). This is in "whatever form makes sense for
 // the host system", so this accepts both UNIX-style and DOS-style
 // absolute paths. We avoid the filepath package because we want this
 // to behave the same regardless of our host system and because we
 // don't know what system the paths came from.
 func pathIsAbs(path string) bool {
 	_, path = splitDrive(path)
 	return len(path) > 0 && (path[0] == '/' || path[0] == '\\')
 }

 // pathJoin joins dirname and filename. filename must be relative.
 // DWARF paths can be UNIX-style or DOS-style, so this handles both.
 func pathJoin(dirname, filename string) string {
 	if len(dirname) == 0 {
 		return filename
 	}
 	// dirname should be absolute, which means we can determine
 	// whether it's a DOS path reasonably reliably by looking for
 	// a drive letter or UNC path.
 	drive, dirname := splitDrive(dirname)
 	if drive == "" {
 		// UNIX-style path.
 		return path.Join(dirname, filename)
 	}
 	// DOS-style path.
 	drive2, filename := splitDrive(filename)
 	if drive2 != "" {
 		if strings.ToLower(drive) != strings.ToLower(drive2) {
 			// Different drives. There's not much we can
 			// do here, so just ignore the directory.
 			return drive2 + filename
 		}
 		// Drives are the same. Ignore drive on filename.
 	}
 	if !(strings.HasSuffix(dirname, "/") || strings.HasSuffix(dirname, `\`)) && dirname != "" {
 		dirname += `\`
 	}
 	return drive + dirname + filename
 }

 // splitDrive splits the DOS drive letter or UNC share point from
 // path, if any. path == drive + rest
 func splitDrive(path string) (drive, rest string) {
 	if len(path) >= 2 && path[1] == ':' {
 		if c := path[0]; 'a' <= c && c <= 'z' || 'A' <= c && c <= 'Z' {
 			return path[:2], path[2:]
 		}
 	}
 	if len(path) > 3 && (path[0] == '\\' || path[0] == '/') && (path[1] == '\\' || path[1] == '/') {
 		// Normalize the path so we can search for just \ below.
 		npath := strings.Replace(path, "/", `\`, -1)
 		// Get the host part, which must be non-empty.
 		slash1 := strings.IndexByte(npath[2:], '\\') + 2
 		if slash1 > 2 {
 			// Get the mount-point part, which must be non-empty.
 			slash2 := strings.IndexByte(npath[slash1+1:], '\\') + slash1 + 1
 			if slash2 > slash1 {
 				return path[:slash2], path[slash2:]
 			}
 		}
 	}
 	return "", path
 }
	// Copyright 2015 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

	import (
	"errors"
	"fmt"
	"io"
	"path"
	"strings"
	)

	// A LineReader reads a sequence of LineEntry structures from a DWARF
	// "line" section for a single compilation unit. LineEntries occur in
	// order of increasing PC and each LineEntry gives metadata for the
	// instructions from that LineEntry's PC to just before the next
	// LineEntry's PC. The last entry will have its EndSequence field set.
	type LineReader struct {
	buf buf

	// Original .debug_line section data. Used by Seek.
	section []byte

	// Header information
	version uint16
	minInstructionLength int
	maxOpsPerInstruction int
	defaultIsStmt bool
	lineBase int
	lineRange int
	opcodeBase int
	opcodeLengths []int
	directories []string
	fileEntries []*LineFile

	programOffset Offset // section offset of line number program
	endOffset Offset // section offset of byte following program

	initialFileEntries int // initial length of fileEntries

	// Current line number program state machine registers
	state LineEntry // public state
	fileIndex int // private state
	}

	// A LineEntry is a row in a DWARF line table.
	type LineEntry struct {
	// Address is the program-counter value of a machine
	// instruction generated by the compiler. This LineEntry
	// applies to each instruction from Address to just before the
	// Address of the next LineEntry.
	Address uint64

	// OpIndex is the index of an operation within a VLIW
	// instruction. The index of the first operation is 0. For
	// non-VLIW architectures, it will always be 0. Address and
	// OpIndex together form an operation pointer that can
	// reference any individual operation within the instruction
	// stream.
	OpIndex int

	// File is the source file corresponding to these
	// instructions.
	File *LineFile

	// Line is the source code line number corresponding to these
	// instructions. Lines are numbered beginning at 1. It may be
	// 0 if these instructions cannot be attributed to any source
	// line.
	Line int

	// Column is the column number within the source line of these
	// instructions. Columns are numbered beginning at 1. It may
	// be 0 to indicate the "left edge" of the line.
	Column int

	// IsStmt indicates that Address is a recommended breakpoint
	// location, such as the beginning of a line, statement, or a
	// distinct subpart of a statement.
	IsStmt bool

	// BasicBlock indicates that Address is the beginning of a
	// basic block.
	BasicBlock bool

	// PrologueEnd indicates that Address is one (of possibly
	// many) PCs where execution should be suspended for a
	// breakpoint on entry to the containing function.
	//
	// Added in DWARF 3.
	PrologueEnd bool

	// EpilogueBegin indicates that Address is one (of possibly
	// many) PCs where execution should be suspended for a
	// breakpoint on exit from this function.
	//
	// Added in DWARF 3.
	EpilogueBegin bool

	// ISA is the instruction set architecture for these
	// instructions. Possible ISA values should be defined by the
	// applicable ABI specification.
	//
	// Added in DWARF 3.
	ISA int

	// Discriminator is an arbitrary integer indicating the block
	// to which these instructions belong. It serves to
	// distinguish among multiple blocks that may all have with
	// the same source file, line, and column. Where only one
	// block exists for a given source position, it should be 0.
	//
	// Added in DWARF 3.
	Discriminator int

	// EndSequence indicates that Address is the first byte after
	// the end of a sequence of target machine instructions. If it
	// is set, only this and the Address field are meaningful. A
	// line number table may contain information for multiple
	// potentially disjoint instruction sequences. The last entry
	// in a line table should always have EndSequence set.
	EndSequence bool
	}

	// A LineFile is a source file referenced by a DWARF line table entry.
	type LineFile struct {
	Name string
	Mtime uint64 // Implementation defined modification time, or 0 if unknown
	Length int // File length, or 0 if unknown
	}

	// LineReader returns a new reader for the line table of compilation
	// unit cu, which must be an Entry with tag TagCompileUnit.
	//
	// If this compilation unit has no line table, it returns nil, nil.
	func (d Data) LineReader(cu Entry) (*LineReader, error) {
	if d.line == nil {
	// No line tables available.
	return nil, nil
	}

	// Get line table information from cu.
	off, ok := cu.Val(AttrStmtList).(int64)
	if !ok {
	// cu has no line table.
	return nil, nil
	}
	if off > int64(len(d.line)) {
	return nil, errors.New("AttrStmtList value out of range")
	}
	// AttrCompDir is optional if all file names are absolute. Use
	// the empty string if it's not present.
	compDir, _ := cu.Val(AttrCompDir).(string)

	// Create the LineReader.
	u := &d.unit[d.offsetToUnit(cu.Offset)]
	buf := makeBuf(d, u, "line", Offset(off), d.line[off:])
	// The compilation directory is implicitly directories[0].
	r := LineReader{buf: buf, section: d.line, directories: []string{compDir}}

	// Read the header.
	if err := r.readHeader(); err != nil {
	return nil, err
	}

	// Initialize line reader state.
	r.Reset()

	return &r, nil
	}

	// readHeader reads the line number program header from r.buf and sets
	// all of the header fields in r.
	func (r *LineReader) readHeader() error {
	buf := &r.buf

	// Read basic header fields [DWARF2 6.2.4].
	hdrOffset := buf.off
	unitLength, dwarf64 := buf.unitLength()
	r.endOffset = buf.off + unitLength
	if r.endOffset > buf.off+Offset(len(buf.data)) {
	return DecodeError{"line", hdrOffset, fmt.Sprintf("line table end %d exceeds section size %d", r.endOffset, buf.off+Offset(len(buf.data)))}
	}
	r.version = buf.uint16()
	if buf.err == nil && (r.version < 2 \|\| r.version > 4) {
	// DWARF goes to all this effort to make new opcodes
	// backward-compatible, and then adds fields right in
	// the middle of the header in new versions, so we're
	// picky about only supporting known line table
	// versions.
	return DecodeError{"line", hdrOffset, fmt.Sprintf("unknown line table version %d", r.version)}
	}
	var headerLength Offset
	if dwarf64 {
	headerLength = Offset(buf.uint64())
	} else {
	headerLength = Offset(buf.uint32())
	}
	r.programOffset = buf.off + headerLength
	r.minInstructionLength = int(buf.uint8())
	if r.version >= 4 {
	// [DWARF4 6.2.4]
	r.maxOpsPerInstruction = int(buf.uint8())
	} else {
	r.maxOpsPerInstruction = 1
	}
	r.defaultIsStmt = buf.uint8() != 0
	r.lineBase = int(int8(buf.uint8()))
	r.lineRange = int(buf.uint8())

	// Validate header.
	if buf.err != nil {
	return buf.err
	}
	if r.maxOpsPerInstruction == 0 {
	return DecodeError{"line", hdrOffset, "invalid maximum operations per instruction: 0"}
	}
	if r.lineRange == 0 {
	return DecodeError{"line", hdrOffset, "invalid line range: 0"}
	}

	// Read standard opcode length table. This table starts with opcode 1.
	r.opcodeBase = int(buf.uint8())
	r.opcodeLengths = make([]int, r.opcodeBase)
	for i := 1; i < r.opcodeBase; i++ {
	r.opcodeLengths[i] = int(buf.uint8())
	}

	// Validate opcode lengths.
	if buf.err != nil {
	return buf.err
	}
	for i, length := range r.opcodeLengths {
	if known, ok := knownOpcodeLengths[i]; ok && known != length {
	return DecodeError{"line", hdrOffset, fmt.Sprintf("opcode %d expected to have length %d, but has length %d", i, known, length)}
	}
	}

	// Read include directories table. The caller already set
	// directories[0] to the compilation directory.
	for {
	directory := buf.string()
	if buf.err != nil {
	return buf.err
	}
	if len(directory) == 0 {
	break
	}
	if !pathIsAbs(directory) {
	// Relative paths are implicitly relative to
	// the compilation directory.
	directory = pathJoin(r.directories[0], directory)
	}
	r.directories = append(r.directories, directory)
	}

	// Read file name list. File numbering starts with 1, so leave
	// the first entry nil.
	r.fileEntries = make([]*LineFile, 1)
	for {
	if done, err := r.readFileEntry(); err != nil {
	return err
	} else if done {
	break
	}
	}
	r.initialFileEntries = len(r.fileEntries)

	return buf.err
	}

	// readFileEntry reads a file entry from either the header or a
	// DW_LNE_define_file extended opcode and adds it to r.fileEntries. A
	// true return value indicates that there are no more entries to read.
	func (r *LineReader) readFileEntry() (bool, error) {
	name := r.buf.string()
	if r.buf.err != nil {
	return false, r.buf.err
	}
	if len(name) == 0 {
	return true, nil
	}
	off := r.buf.off
	dirIndex := int(r.buf.uint())
	if !pathIsAbs(name) {
	if dirIndex >= len(r.directories) {
	return false, DecodeError{"line", off, "directory index too large"}
	}
	name = pathJoin(r.directories[dirIndex], name)
	}
	mtime := r.buf.uint()
	length := int(r.buf.uint())

	r.fileEntries = append(r.fileEntries, &LineFile{name, mtime, length})
	return false, nil
	}

	// updateFile updates r.state.File after r.fileIndex has
	// changed or r.fileEntries has changed.
	func (r *LineReader) updateFile() {
	if r.fileIndex < len(r.fileEntries) {
	r.state.File = r.fileEntries[r.fileIndex]
	} else {
	r.state.File = nil
	}
	}

	// Next sets *entry to the next row in this line table and moves to
	// the next row. If there are no more entries and the line table is
	// properly terminated, it returns io.EOF.
	//
	// Rows are always in order of increasing entry.Address, but
	// entry.Line may go forward or backward.
	func (r LineReader) Next(entry LineEntry) error {
	if r.buf.err != nil {
	return r.buf.err
	}

	// Execute opcodes until we reach an opcode that emits a line
	// table entry.
	for {
	if len(r.buf.data) == 0 {
	return io.EOF
	}
	emit := r.step(entry)
	if r.buf.err != nil {
	return r.buf.err
	}
	if emit {
	return nil
	}
	}
	}

	// knownOpcodeLengths gives the opcode lengths (in varint arguments)
	// of known standard opcodes.
	var knownOpcodeLengths = map[int]int{
	lnsCopy: 0,
	lnsAdvancePC: 1,
	lnsAdvanceLine: 1,
	lnsSetFile: 1,
	lnsNegateStmt: 0,
	lnsSetBasicBlock: 0,
	lnsConstAddPC: 0,
	lnsSetPrologueEnd: 0,
	lnsSetEpilogueBegin: 0,
	lnsSetISA: 1,
	// lnsFixedAdvancePC takes a uint8 rather than a varint; it's
	// unclear what length the header is supposed to claim, so
	// ignore it.
	}

	// step processes the next opcode and updates r.state. If the opcode
	// emits a row in the line table, this updates *entry and returns
	// true.
	func (r LineReader) step(entry LineEntry) bool {
	opcode := int(r.buf.uint8())

	if opcode >= r.opcodeBase {
	// Special opcode [DWARF2 6.2.5.1, DWARF4 6.2.5.1]
	adjustedOpcode := opcode - r.opcodeBase
	r.advancePC(adjustedOpcode / r.lineRange)
	lineDelta := r.lineBase + adjustedOpcode%r.lineRange
	r.state.Line += lineDelta
	goto emit
	}

	switch opcode {
	case 0:
	// Extended opcode [DWARF2 6.2.5.3]
	length := Offset(r.buf.uint())
	startOff := r.buf.off
	opcode := r.buf.uint8()

	switch opcode {
	case lneEndSequence:
	r.state.EndSequence = true
	*entry = r.state
	r.resetState()

	case lneSetAddress:
	r.state.Address = r.buf.addr()

	case lneDefineFile:
	if done, err := r.readFileEntry(); err != nil {
	r.buf.err = err
	return false
	} else if done {
	r.buf.err = DecodeError{"line", startOff, "malformed DW_LNE_define_file operation"}
	return false
	}
	r.updateFile()

	case lneSetDiscriminator:
	// [DWARF4 6.2.5.3]
	r.state.Discriminator = int(r.buf.uint())
	}

	r.buf.skip(int(startOff + length - r.buf.off))

	if opcode == lneEndSequence {
	return true
	}

	// Standard opcodes [DWARF2 6.2.5.2]
	case lnsCopy:
	goto emit

	case lnsAdvancePC:
	r.advancePC(int(r.buf.uint()))

	case lnsAdvanceLine:
	r.state.Line += int(r.buf.int())

	case lnsSetFile:
	r.fileIndex = int(r.buf.uint())
	r.updateFile()

	case lnsSetColumn:
	r.state.Column = int(r.buf.uint())

	case lnsNegateStmt:
	r.state.IsStmt = !r.state.IsStmt

	case lnsSetBasicBlock:
	r.state.BasicBlock = true

	case lnsConstAddPC:
	r.advancePC((255 - r.opcodeBase) / r.lineRange)

	case lnsFixedAdvancePC:
	r.state.Address += uint64(r.buf.uint16())

	// DWARF3 standard opcodes [DWARF3 6.2.5.2]
	case lnsSetPrologueEnd:
	r.state.PrologueEnd = true

	case lnsSetEpilogueBegin:
	r.state.EpilogueBegin = true

	case lnsSetISA:
	r.state.ISA = int(r.buf.uint())

	default:
	// Unhandled standard opcode. Skip the number of
	// arguments that the prologue says this opcode has.
	for i := 0; i < r.opcodeLengths[opcode]; i++ {
	r.buf.uint()
	}
	}
	return false

	emit:
	*entry = r.state
	r.state.BasicBlock = false
	r.state.PrologueEnd = false
	r.state.EpilogueBegin = false
	r.state.Discriminator = 0
	return true
	}

	// advancePC advances "operation pointer" (the combination of Address
	// and OpIndex) in r.state by opAdvance steps.
	func (r *LineReader) advancePC(opAdvance int) {
	opIndex := r.state.OpIndex + opAdvance
	r.state.Address += uint64(r.minInstructionLength * (opIndex / r.maxOpsPerInstruction))
	r.state.OpIndex = opIndex % r.maxOpsPerInstruction
	}

	// A LineReaderPos represents a position in a line table.
	type LineReaderPos struct {
	// off is the current offset in the DWARF line section.
	off Offset
	// numFileEntries is the length of fileEntries.
	numFileEntries int
	// state and fileIndex are the statement machine state at
	// offset off.
	state LineEntry
	fileIndex int
	}

	// Tell returns the current position in the line table.
	func (r *LineReader) Tell() LineReaderPos {
	return LineReaderPos{r.buf.off, len(r.fileEntries), r.state, r.fileIndex}
	}

	// Seek restores the line table reader to a position returned by Tell.
	//
	// The argument pos must have been returned by a call to Tell on this
	// line table.
	func (r *LineReader) Seek(pos LineReaderPos) {
	r.buf.off = pos.off
	r.buf.data = r.section[r.buf.off:r.endOffset]
	r.fileEntries = r.fileEntries[:pos.numFileEntries]
	r.state = pos.state
	r.fileIndex = pos.fileIndex
	}

	// Reset repositions the line table reader at the beginning of the
	// line table.
	func (r *LineReader) Reset() {
	// Reset buffer to the line number program offset.
	r.buf.off = r.programOffset
	r.buf.data = r.section[r.buf.off:r.endOffset]

	// Reset file entries list.
	r.fileEntries = r.fileEntries[:r.initialFileEntries]

	// Reset line number program state.
	r.resetState()
	}

	// resetState resets r.state to its default values
	func (r *LineReader) resetState() {
	// Reset the state machine registers to the defaults given in
	// [DWARF4 6.2.2].
	r.state = LineEntry{
	Address: 0,
	OpIndex: 0,
	File: nil,
	Line: 1,
	Column: 0,
	IsStmt: r.defaultIsStmt,
	BasicBlock: false,
	PrologueEnd: false,
	EpilogueBegin: false,
	ISA: 0,
	Discriminator: 0,
	}
	r.fileIndex = 1
	r.updateFile()
	}

	// ErrUnknownPC is the error returned by LineReader.ScanPC when the
	// seek PC is not covered by any entry in the line table.
	var ErrUnknownPC = errors.New("ErrUnknownPC")

	// SeekPC sets *entry to the LineEntry that includes pc and positions
	// the reader on the next entry in the line table. If necessary, this
	// will seek backwards to find pc.
	//
	// If pc is not covered by any entry in this line table, SeekPC
	// returns ErrUnknownPC. In this case, *entry and the final seek
	// position are unspecified.
	//
	// Note that DWARF line tables only permit sequential, forward scans.
	// Hence, in the worst case, this takes time linear in the size of the
	// line table. If the caller wishes to do repeated fast PC lookups, it
	// should build an appropriate index of the line table.
	func (r LineReader) SeekPC(pc uint64, entry LineEntry) error {
	if err := r.Next(entry); err != nil {
	return err
	}
	if entry.Address > pc {
	// We're too far. Start at the beginning of the table.
	r.Reset()
	if err := r.Next(entry); err != nil {
	return err
	}
	if entry.Address > pc {
	// The whole table starts after pc.
	r.Reset()
	return ErrUnknownPC
	}
	}

	// Scan until we pass pc, then back up one.
	for {
	var next LineEntry
	pos := r.Tell()
	if err := r.Next(&next); err != nil {
	if err == io.EOF {
	return ErrUnknownPC
	}
	return err
	}
	if next.Address > pc {
	if entry.EndSequence {
	// pc is in a hole in the table.
	return ErrUnknownPC
	}
	// entry is the desired entry. Back up the
	// cursor to "next" and return success.
	r.Seek(pos)
	return nil
	}
	*entry = next
	}
	}

	// pathIsAbs reports whether path is an absolute path (or "full path
	// name" in DWARF parlance). This is in "whatever form makes sense for
	// the host system", so this accepts both UNIX-style and DOS-style
	// absolute paths. We avoid the filepath package because we want this
	// to behave the same regardless of our host system and because we
	// don't know what system the paths came from.
	func pathIsAbs(path string) bool {
	_, path = splitDrive(path)
	return len(path) > 0 && (path[0] == '/' \|\| path[0] == '\\')
	}

	// pathJoin joins dirname and filename. filename must be relative.
	// DWARF paths can be UNIX-style or DOS-style, so this handles both.
	func pathJoin(dirname, filename string) string {
	if len(dirname) == 0 {
	return filename
	}
	// dirname should be absolute, which means we can determine
	// whether it's a DOS path reasonably reliably by looking for
	// a drive letter or UNC path.
	drive, dirname := splitDrive(dirname)
	if drive == "" {
	// UNIX-style path.
	return path.Join(dirname, filename)
	}
	// DOS-style path.
	drive2, filename := splitDrive(filename)
	if drive2 != "" {
	if strings.ToLower(drive) != strings.ToLower(drive2) {
	// Different drives. There's not much we can
	// do here, so just ignore the directory.
	return drive2 + filename
	}
	// Drives are the same. Ignore drive on filename.
	}
	if !(strings.HasSuffix(dirname, "/") \|\| strings.HasSuffix(dirname, `\`)) && dirname != "" {
	dirname += `\`
	}
	return drive + dirname + filename
	}

	// splitDrive splits the DOS drive letter or UNC share point from
	// path, if any. path == drive + rest
	func splitDrive(path string) (drive, rest string) {
	if len(path) >= 2 && path[1] == ':' {
	if c := path[0]; 'a' <= c && c <= 'z' \|\| 'A' <= c && c <= 'Z' {
	return path[:2], path[2:]
	}
	}
	if len(path) > 3 && (path[0] == '\\' \|\| path[0] == '/') && (path[1] == '\\' \|\| path[1] == '/') {
	// Normalize the path so we can search for just \ below.
	npath := strings.Replace(path, "/", `\`, -1)
	// Get the host part, which must be non-empty.
	slash1 := strings.IndexByte(npath[2:], '\\') + 2
	if slash1 > 2 {
	// Get the mount-point part, which must be non-empty.
	slash2 := strings.IndexByte(npath[slash1+1:], '\\') + slash1 + 1
	if slash2 > slash1 {
	return path[:slash2], path[slash2:]
	}
	}
	}
	return "", path
	}