decoder.go

Documentation: internal/pkgbits

     1  // Copyright 2021 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  package pkgbits
     6  
     7  import (
     8  	"encoding/binary"
     9  	"errors"
    10  	"fmt"
    11  	"go/constant"
    12  	"go/token"
    13  	"io"
    14  	"math/big"
    15  	"os"
    16  	"runtime"
    17  	"strings"
    18  )
    19  
    20  // A PkgDecoder provides methods for decoding a package's Unified IR
    21  // export data.
    22  type PkgDecoder struct {
    23  	// version is the file format version.
    24  	version uint32
    25  
    26  	// sync indicates whether the file uses sync markers.
    27  	sync bool
    28  
    29  	// pkgPath is the package path for the package to be decoded.
    30  	//
    31  	// TODO(mdempsky): Remove; unneeded since CL 391014.
    32  	pkgPath string
    33  
    34  	// elemData is the full data payload of the encoded package.
    35  	// Elements are densely and contiguously packed together.
    36  	//
    37  	// The last 8 bytes of elemData are the package fingerprint.
    38  	elemData string
    39  
    40  	// elemEnds stores the byte-offset end positions of element
    41  	// bitstreams within elemData.
    42  	//
    43  	// For example, element I's bitstream data starts at elemEnds[I-1]
    44  	// (or 0, if I==0) and ends at elemEnds[I].
    45  	//
    46  	// Note: elemEnds is indexed by absolute indices, not
    47  	// section-relative indices.
    48  	elemEnds []uint32
    49  
    50  	// elemEndsEnds stores the index-offset end positions of relocation
    51  	// sections within elemEnds.
    52  	//
    53  	// For example, section K's end positions start at elemEndsEnds[K-1]
    54  	// (or 0, if K==0) and end at elemEndsEnds[K].
    55  	elemEndsEnds [numRelocs]uint32
    56  
    57  	scratchRelocEnt []RelocEnt
    58  }
    59  
    60  // PkgPath returns the package path for the package
    61  //
    62  // TODO(mdempsky): Remove; unneeded since CL 391014.
    63  func (pr *PkgDecoder) PkgPath() string { return pr.pkgPath }
    64  
    65  // SyncMarkers reports whether pr uses sync markers.
    66  func (pr *PkgDecoder) SyncMarkers() bool { return pr.sync }
    67  
    68  // NewPkgDecoder returns a PkgDecoder initialized to read the Unified
    69  // IR export data from input. pkgPath is the package path for the
    70  // compilation unit that produced the export data.
    71  //
    72  // TODO(mdempsky): Remove pkgPath parameter; unneeded since CL 391014.
    73  func NewPkgDecoder(pkgPath, input string) PkgDecoder {
    74  	pr := PkgDecoder{
    75  		pkgPath: pkgPath,
    76  	}
    77  
    78  	// TODO(mdempsky): Implement direct indexing of input string to
    79  	// avoid copying the position information.
    80  
    81  	r := strings.NewReader(input)
    82  
    83  	assert(binary.Read(r, binary.LittleEndian, &pr.version) == nil)
    84  
    85  	switch pr.version {
    86  	default:
    87  		panic(fmt.Errorf("unsupported version: %v", pr.version))
    88  	case 0:
    89  		// no flags
    90  	case 1:
    91  		var flags uint32
    92  		assert(binary.Read(r, binary.LittleEndian, &flags) == nil)
    93  		pr.sync = flags&flagSyncMarkers != 0
    94  	}
    95  
    96  	assert(binary.Read(r, binary.LittleEndian, pr.elemEndsEnds[:]) == nil)
    97  
    98  	pr.elemEnds = make([]uint32, pr.elemEndsEnds[len(pr.elemEndsEnds)-1])
    99  	assert(binary.Read(r, binary.LittleEndian, pr.elemEnds[:]) == nil)
   100  
   101  	pos, err := r.Seek(0, io.SeekCurrent)
   102  	assert(err == nil)
   103  
   104  	pr.elemData = input[pos:]
   105  	assert(len(pr.elemData)-8 == int(pr.elemEnds[len(pr.elemEnds)-1]))
   106  
   107  	return pr
   108  }
   109  
   110  // NumElems returns the number of elements in section k.
   111  func (pr *PkgDecoder) NumElems(k RelocKind) int {
   112  	count := int(pr.elemEndsEnds[k])
   113  	if k > 0 {
   114  		count -= int(pr.elemEndsEnds[k-1])
   115  	}
   116  	return count
   117  }
   118  
   119  // TotalElems returns the total number of elements across all sections.
   120  func (pr *PkgDecoder) TotalElems() int {
   121  	return len(pr.elemEnds)
   122  }
   123  
   124  // Fingerprint returns the package fingerprint.
   125  func (pr *PkgDecoder) Fingerprint() [8]byte {
   126  	var fp [8]byte
   127  	copy(fp[:], pr.elemData[len(pr.elemData)-8:])
   128  	return fp
   129  }
   130  
   131  // AbsIdx returns the absolute index for the given (section, index)
   132  // pair.
   133  func (pr *PkgDecoder) AbsIdx(k RelocKind, idx Index) int {
   134  	absIdx := int(idx)
   135  	if k > 0 {
   136  		absIdx += int(pr.elemEndsEnds[k-1])
   137  	}
   138  	if absIdx >= int(pr.elemEndsEnds[k]) {
   139  		errorf("%v:%v is out of bounds; %v", k, idx, pr.elemEndsEnds)
   140  	}
   141  	return absIdx
   142  }
   143  
   144  // DataIdx returns the raw element bitstream for the given (section,
   145  // index) pair.
   146  func (pr *PkgDecoder) DataIdx(k RelocKind, idx Index) string {
   147  	absIdx := pr.AbsIdx(k, idx)
   148  
   149  	var start uint32
   150  	if absIdx > 0 {
   151  		start = pr.elemEnds[absIdx-1]
   152  	}
   153  	end := pr.elemEnds[absIdx]
   154  
   155  	return pr.elemData[start:end]
   156  }
   157  
   158  // StringIdx returns the string value for the given string index.
   159  func (pr *PkgDecoder) StringIdx(idx Index) string {
   160  	return pr.DataIdx(RelocString, idx)
   161  }
   162  
   163  // NewDecoder returns a Decoder for the given (section, index) pair,
   164  // and decodes the given SyncMarker from the element bitstream.
   165  func (pr *PkgDecoder) NewDecoder(k RelocKind, idx Index, marker SyncMarker) Decoder {
   166  	r := pr.NewDecoderRaw(k, idx)
   167  	r.Sync(marker)
   168  	return r
   169  }
   170  
   171  // TempDecoder returns a Decoder for the given (section, index) pair,
   172  // and decodes the given SyncMarker from the element bitstream.
   173  // If possible the Decoder should be RetireDecoder'd when it is no longer
   174  // needed, this will avoid heap allocations.
   175  func (pr *PkgDecoder) TempDecoder(k RelocKind, idx Index, marker SyncMarker) Decoder {
   176  	r := pr.TempDecoderRaw(k, idx)
   177  	r.Sync(marker)
   178  	return r
   179  }
   180  
   181  func (pr *PkgDecoder) RetireDecoder(d *Decoder) {
   182  	pr.scratchRelocEnt = d.Relocs
   183  	d.Relocs = nil
   184  }
   185  
   186  // NewDecoderRaw returns a Decoder for the given (section, index) pair.
   187  //
   188  // Most callers should use NewDecoder instead.
   189  func (pr *PkgDecoder) NewDecoderRaw(k RelocKind, idx Index) Decoder {
   190  	r := Decoder{
   191  		common: pr,
   192  		k:      k,
   193  		Idx:    idx,
   194  	}
   195  
   196  	r.Data.Reset(pr.DataIdx(k, idx))
   197  	r.Sync(SyncRelocs)
   198  	r.Relocs = make([]RelocEnt, r.Len())
   199  	for i := range r.Relocs {
   200  		r.Sync(SyncReloc)
   201  		r.Relocs[i] = RelocEnt{RelocKind(r.Len()), Index(r.Len())}
   202  	}
   203  
   204  	return r
   205  }
   206  
   207  func (pr *PkgDecoder) TempDecoderRaw(k RelocKind, idx Index) Decoder {
   208  	r := Decoder{
   209  		common: pr,
   210  		k:      k,
   211  		Idx:    idx,
   212  	}
   213  
   214  	r.Data.Reset(pr.DataIdx(k, idx))
   215  	r.Sync(SyncRelocs)
   216  	l := r.Len()
   217  	if cap(pr.scratchRelocEnt) >= l {
   218  		r.Relocs = pr.scratchRelocEnt[:l]
   219  		pr.scratchRelocEnt = nil
   220  	} else {
   221  		r.Relocs = make([]RelocEnt, l)
   222  	}
   223  	for i := range r.Relocs {
   224  		r.Sync(SyncReloc)
   225  		r.Relocs[i] = RelocEnt{RelocKind(r.Len()), Index(r.Len())}
   226  	}
   227  
   228  	return r
   229  }
   230  
   231  // A Decoder provides methods for decoding an individual element's
   232  // bitstream data.
   233  type Decoder struct {
   234  	common *PkgDecoder
   235  
   236  	Relocs []RelocEnt
   237  	Data   strings.Reader
   238  
   239  	k   RelocKind
   240  	Idx Index
   241  }
   242  
   243  func (r *Decoder) checkErr(err error) {
   244  	if err != nil {
   245  		errorf("unexpected decoding error: %w", err)
   246  	}
   247  }
   248  
   249  func (r *Decoder) rawUvarint() uint64 {
   250  	x, err := readUvarint(&r.Data)
   251  	r.checkErr(err)
   252  	return x
   253  }
   254  
   255  // readUvarint is a type-specialized copy of encoding/binary.ReadUvarint.
   256  // This avoids the interface conversion and thus has better escape properties,
   257  // which flows up the stack.
   258  func readUvarint(r *strings.Reader) (uint64, error) {
   259  	var x uint64
   260  	var s uint
   261  	for i := 0; i < binary.MaxVarintLen64; i++ {
   262  		b, err := r.ReadByte()
   263  		if err != nil {
   264  			if i > 0 && err == io.EOF {
   265  				err = io.ErrUnexpectedEOF
   266  			}
   267  			return x, err
   268  		}
   269  		if b < 0x80 {
   270  			if i == binary.MaxVarintLen64-1 && b > 1 {
   271  				return x, overflow
   272  			}
   273  			return x | uint64(b)<<s, nil
   274  		}
   275  		x |= uint64(b&0x7f) << s
   276  		s += 7
   277  	}
   278  	return x, overflow
   279  }
   280  
   281  var overflow = errors.New("pkgbits: readUvarint overflows a 64-bit integer")
   282  
   283  func (r *Decoder) rawVarint() int64 {
   284  	ux := r.rawUvarint()
   285  
   286  	// Zig-zag decode.
   287  	x := int64(ux >> 1)
   288  	if ux&1 != 0 {
   289  		x = ^x
   290  	}
   291  	return x
   292  }
   293  
   294  func (r *Decoder) rawReloc(k RelocKind, idx int) Index {
   295  	e := r.Relocs[idx]
   296  	assert(e.Kind == k)
   297  	return e.Idx
   298  }
   299  
   300  // Sync decodes a sync marker from the element bitstream and asserts
   301  // that it matches the expected marker.
   302  //
   303  // If EnableSync is false, then Sync is a no-op.
   304  func (r *Decoder) Sync(mWant SyncMarker) {
   305  	if !r.common.sync {
   306  		return
   307  	}
   308  
   309  	pos, _ := r.Data.Seek(0, io.SeekCurrent)
   310  	mHave := SyncMarker(r.rawUvarint())
   311  	writerPCs := make([]int, r.rawUvarint())
   312  	for i := range writerPCs {
   313  		writerPCs[i] = int(r.rawUvarint())
   314  	}
   315  
   316  	if mHave == mWant {
   317  		return
   318  	}
   319  
   320  	// There's some tension here between printing:
   321  	//
   322  	// (1) full file paths that tools can recognize (e.g., so emacs
   323  	//     hyperlinks the "file:line" text for easy navigation), or
   324  	//
   325  	// (2) short file paths that are easier for humans to read (e.g., by
   326  	//     omitting redundant or irrelevant details, so it's easier to
   327  	//     focus on the useful bits that remain).
   328  	//
   329  	// The current formatting favors the former, as it seems more
   330  	// helpful in practice. But perhaps the formatting could be improved
   331  	// to better address both concerns. For example, use relative file
   332  	// paths if they would be shorter, or rewrite file paths to contain
   333  	// "$GOROOT" (like objabi.AbsFile does) if tools can be taught how
   334  	// to reliably expand that again.
   335  
   336  	fmt.Printf("export data desync: package %q, section %v, index %v, offset %v\n", r.common.pkgPath, r.k, r.Idx, pos)
   337  
   338  	fmt.Printf("\nfound %v, written at:\n", mHave)
   339  	if len(writerPCs) == 0 {
   340  		fmt.Printf("\t[stack trace unavailable; recompile package %q with -d=syncframes]\n", r.common.pkgPath)
   341  	}
   342  	for _, pc := range writerPCs {
   343  		fmt.Printf("\t%s\n", r.common.StringIdx(r.rawReloc(RelocString, pc)))
   344  	}
   345  
   346  	fmt.Printf("\nexpected %v, reading at:\n", mWant)
   347  	var readerPCs [32]uintptr // TODO(mdempsky): Dynamically size?
   348  	n := runtime.Callers(2, readerPCs[:])
   349  	for _, pc := range fmtFrames(readerPCs[:n]...) {
   350  		fmt.Printf("\t%s\n", pc)
   351  	}
   352  
   353  	// We already printed a stack trace for the reader, so now we can
   354  	// simply exit. Printing a second one with panic or base.Fatalf
   355  	// would just be noise.
   356  	os.Exit(1)
   357  }
   358  
   359  // Bool decodes and returns a bool value from the element bitstream.
   360  func (r *Decoder) Bool() bool {
   361  	r.Sync(SyncBool)
   362  	x, err := r.Data.ReadByte()
   363  	r.checkErr(err)
   364  	assert(x < 2)
   365  	return x != 0
   366  }
   367  
   368  // Int64 decodes and returns an int64 value from the element bitstream.
   369  func (r *Decoder) Int64() int64 {
   370  	r.Sync(SyncInt64)
   371  	return r.rawVarint()
   372  }
   373  
   374  // Int64 decodes and returns a uint64 value from the element bitstream.
   375  func (r *Decoder) Uint64() uint64 {
   376  	r.Sync(SyncUint64)
   377  	return r.rawUvarint()
   378  }
   379  
   380  // Len decodes and returns a non-negative int value from the element bitstream.
   381  func (r *Decoder) Len() int { x := r.Uint64(); v := int(x); assert(uint64(v) == x); return v }
   382  
   383  // Int decodes and returns an int value from the element bitstream.
   384  func (r *Decoder) Int() int { x := r.Int64(); v := int(x); assert(int64(v) == x); return v }
   385  
   386  // Uint decodes and returns a uint value from the element bitstream.
   387  func (r *Decoder) Uint() uint { x := r.Uint64(); v := uint(x); assert(uint64(v) == x); return v }
   388  
   389  // Code decodes a Code value from the element bitstream and returns
   390  // its ordinal value. It's the caller's responsibility to convert the
   391  // result to an appropriate Code type.
   392  //
   393  // TODO(mdempsky): Ideally this method would have signature "Code[T
   394  // Code] T" instead, but we don't allow generic methods and the
   395  // compiler can't depend on generics yet anyway.
   396  func (r *Decoder) Code(mark SyncMarker) int {
   397  	r.Sync(mark)
   398  	return r.Len()
   399  }
   400  
   401  // Reloc decodes a relocation of expected section k from the element
   402  // bitstream and returns an index to the referenced element.
   403  func (r *Decoder) Reloc(k RelocKind) Index {
   404  	r.Sync(SyncUseReloc)
   405  	return r.rawReloc(k, r.Len())
   406  }
   407  
   408  // String decodes and returns a string value from the element
   409  // bitstream.
   410  func (r *Decoder) String() string {
   411  	r.Sync(SyncString)
   412  	return r.common.StringIdx(r.Reloc(RelocString))
   413  }
   414  
   415  // Strings decodes and returns a variable-length slice of strings from
   416  // the element bitstream.
   417  func (r *Decoder) Strings() []string {
   418  	res := make([]string, r.Len())
   419  	for i := range res {
   420  		res[i] = r.String()
   421  	}
   422  	return res
   423  }
   424  
   425  // Value decodes and returns a constant.Value from the element
   426  // bitstream.
   427  func (r *Decoder) Value() constant.Value {
   428  	r.Sync(SyncValue)
   429  	isComplex := r.Bool()
   430  	val := r.scalar()
   431  	if isComplex {
   432  		val = constant.BinaryOp(val, token.ADD, constant.MakeImag(r.scalar()))
   433  	}
   434  	return val
   435  }
   436  
   437  func (r *Decoder) scalar() constant.Value {
   438  	switch tag := CodeVal(r.Code(SyncVal)); tag {
   439  	default:
   440  		panic(fmt.Errorf("unexpected scalar tag: %v", tag))
   441  
   442  	case ValBool:
   443  		return constant.MakeBool(r.Bool())
   444  	case ValString:
   445  		return constant.MakeString(r.String())
   446  	case ValInt64:
   447  		return constant.MakeInt64(r.Int64())
   448  	case ValBigInt:
   449  		return constant.Make(r.bigInt())
   450  	case ValBigRat:
   451  		num := r.bigInt()
   452  		denom := r.bigInt()
   453  		return constant.Make(new(big.Rat).SetFrac(num, denom))
   454  	case ValBigFloat:
   455  		return constant.Make(r.bigFloat())
   456  	}
   457  }
   458  
   459  func (r *Decoder) bigInt() *big.Int {
   460  	v := new(big.Int).SetBytes([]byte(r.String()))
   461  	if r.Bool() {
   462  		v.Neg(v)
   463  	}
   464  	return v
   465  }
   466  
   467  func (r *Decoder) bigFloat() *big.Float {
   468  	v := new(big.Float).SetPrec(512)
   469  	assert(v.UnmarshalText([]byte(r.String())) == nil)
   470  	return v
   471  }
   472  
   473  // @@@ Helpers
   474  
   475  // TODO(mdempsky): These should probably be removed. I think they're a
   476  // smell that the export data format is not yet quite right.
   477  
   478  // PeekPkgPath returns the package path for the specified package
   479  // index.
   480  func (pr *PkgDecoder) PeekPkgPath(idx Index) string {
   481  	var path string
   482  	{
   483  		r := pr.TempDecoder(RelocPkg, idx, SyncPkgDef)
   484  		path = r.String()
   485  		pr.RetireDecoder(&r)
   486  	}
   487  	if path == "" {
   488  		path = pr.pkgPath
   489  	}
   490  	return path
   491  }
   492  
   493  // PeekObj returns the package path, object name, and CodeObj for the
   494  // specified object index.
   495  func (pr *PkgDecoder) PeekObj(idx Index) (string, string, CodeObj) {
   496  	var ridx Index
   497  	var name string
   498  	var rcode int
   499  	{
   500  		r := pr.TempDecoder(RelocName, idx, SyncObject1)
   501  		r.Sync(SyncSym)
   502  		r.Sync(SyncPkg)
   503  		ridx = r.Reloc(RelocPkg)
   504  		name = r.String()
   505  		rcode = r.Code(SyncCodeObj)
   506  		pr.RetireDecoder(&r)
   507  	}
   508  
   509  	path := pr.PeekPkgPath(ridx)
   510  	assert(name != "")
   511  
   512  	tag := CodeObj(rcode)
   513  
   514  	return path, name, tag
   515  }
   516
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