go/internal/gcimporter/iexport.go - tools - Git at Google

 // Copyright 2019 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.

 // Indexed binary package export.
 // This file was derived from $GOROOT/src/cmd/compile/internal/gc/iexport.go;
 // see that file for specification of the format.

 package gcimporter

 import (
 	"bytes"
 	"encoding/binary"
 	"go/ast"
 	"go/constant"
 	"go/token"
 	"go/types"
 	"io"
 	"math/big"
 	"reflect"
 	"sort"
 )

 // Current indexed export format version. Increase with each format change.
 // 0: Go1.11 encoding
 const iexportVersion = 0

 // IExportData returns the binary export data for pkg.
 //
 // If no file set is provided, position info will be missing.
 // The package path of the top-level package will not be recorded,
 // so that calls to IImportData can override with a provided package path.
 func IExportData(fset *token.FileSet, pkg *types.Package) (b []byte, err error) {
 	defer func() {
 		if e := recover(); e != nil {
 			if ierr, ok := e.(internalError); ok {
 				err = ierr
 				return
 			}
 			// Not an internal error; panic again.
 			panic(e)
 		}
 	}()

 	p := iexporter{
 		out:         bytes.NewBuffer(nil),
 		fset:        fset,
 		allPkgs:     map[*types.Package]bool{},
 		stringIndex: map[string]uint64{},
 		declIndex:   map[types.Object]uint64{},
 		typIndex:    map[types.Type]uint64{},
 		localpkg:    pkg,
 	}

 	for i, pt := range predeclared() {
 		p.typIndex[pt] = uint64(i)
 	}
 	if len(p.typIndex) > predeclReserved {
 		panic(internalErrorf("too many predeclared types: %d > %d", len(p.typIndex), predeclReserved))
 	}

 	// Initialize work queue with exported declarations.
 	scope := pkg.Scope()
 	for _, name := range scope.Names() {
 		if ast.IsExported(name) {
 			p.pushDecl(scope.Lookup(name))
 		}
 	}

 	// Loop until no more work.
 	for !p.declTodo.empty() {
 		p.doDecl(p.declTodo.popHead())
 	}

 	// Append indices to data0 section.
 	dataLen := uint64(p.data0.Len())
 	w := p.newWriter()
 	w.writeIndex(p.declIndex)
 	w.flush()

 	// Assemble header.
 	var hdr intWriter
 	hdr.WriteByte('i')
 	hdr.uint64(iexportVersion)
 	hdr.uint64(uint64(p.strings.Len()))
 	hdr.uint64(dataLen)

 	// Flush output.
 	io.Copy(p.out, &hdr)
 	io.Copy(p.out, &p.strings)
 	io.Copy(p.out, &p.data0)

 	return p.out.Bytes(), nil
 }

 // writeIndex writes out an object index. mainIndex indicates whether
 // we're writing out the main index, which is also read by
 // non-compiler tools and includes a complete package description
 // (i.e., name and height).
 func (w *exportWriter) writeIndex(index map[types.Object]uint64) {
 	// Build a map from packages to objects from that package.
 	pkgObjs := map[*types.Package][]types.Object{}

 	// For the main index, make sure to include every package that
 	// we reference, even if we're not exporting (or reexporting)
 	// any symbols from it.
 	pkgObjs[w.p.localpkg] = nil
 	for pkg := range w.p.allPkgs {
 		pkgObjs[pkg] = nil
 	}

 	for obj := range index {
 		pkgObjs[obj.Pkg()] = append(pkgObjs[obj.Pkg()], obj)
 	}

 	var pkgs []*types.Package
 	for pkg, objs := range pkgObjs {
 		pkgs = append(pkgs, pkg)

 		sort.Slice(objs, func(i, j int) bool {
 			return objs[i].Name() < objs[j].Name()
 		})
 	}

 	sort.Slice(pkgs, func(i, j int) bool {
 		return w.exportPath(pkgs[i]) < w.exportPath(pkgs[j])
 	})

 	w.uint64(uint64(len(pkgs)))
 	for _, pkg := range pkgs {
 		w.string(w.exportPath(pkg))
 		w.string(pkg.Name())
 		w.uint64(uint64(0)) // package height is not needed for go/types

 		objs := pkgObjs[pkg]
 		w.uint64(uint64(len(objs)))
 		for _, obj := range objs {
 			w.string(obj.Name())
 			w.uint64(index[obj])
 		}
 	}
 }

 type iexporter struct {
 	fset *token.FileSet
 	out  *bytes.Buffer

 	localpkg *types.Package

 	// allPkgs tracks all packages that have been referenced by
 	// the export data, so we can ensure to include them in the
 	// main index.
 	allPkgs map[*types.Package]bool

 	declTodo objQueue

 	strings     intWriter
 	stringIndex map[string]uint64

 	data0     intWriter
 	declIndex map[types.Object]uint64
 	typIndex  map[types.Type]uint64
 }

 // stringOff returns the offset of s within the string section.
 // If not already present, it's added to the end.
 func (p *iexporter) stringOff(s string) uint64 {
 	off, ok := p.stringIndex[s]
 	if !ok {
 		off = uint64(p.strings.Len())
 		p.stringIndex[s] = off

 		p.strings.uint64(uint64(len(s)))
 		p.strings.WriteString(s)
 	}
 	return off
 }

 // pushDecl adds n to the declaration work queue, if not already present.
 func (p *iexporter) pushDecl(obj types.Object) {
 	// Package unsafe is known to the compiler and predeclared.
 	assert(obj.Pkg() != types.Unsafe)

 	if _, ok := p.declIndex[obj]; ok {
 		return
 	}

 	p.declIndex[obj] = ^uint64(0) // mark n present in work queue
 	p.declTodo.pushTail(obj)
 }

 // exportWriter handles writing out individual data section chunks.
 type exportWriter struct {
 	p *iexporter

 	data     intWriter
 	currPkg  *types.Package
 	prevFile string
 	prevLine int64
 }

 func (w *exportWriter) exportPath(pkg *types.Package) string {
 	if pkg == w.p.localpkg {
 		return ""
 	}
 	return pkg.Path()
 }

 func (p *iexporter) doDecl(obj types.Object) {
 	w := p.newWriter()
 	w.setPkg(obj.Pkg(), false)

 	switch obj := obj.(type) {
 	case *types.Var:
 		w.tag('V')
 		w.pos(obj.Pos())
 		w.typ(obj.Type(), obj.Pkg())

 	case *types.Func:
 		sig, _ := obj.Type().(*types.Signature)
 		if sig.Recv() != nil {
 			panic(internalErrorf("unexpected method: %v", sig))
 		}
 		w.tag('F')
 		w.pos(obj.Pos())
 		w.signature(sig)

 	case *types.Const:
 		w.tag('C')
 		w.pos(obj.Pos())
 		w.value(obj.Type(), obj.Val())

 	case *types.TypeName:
 		if obj.IsAlias() {
 			w.tag('A')
 			w.pos(obj.Pos())
 			w.typ(obj.Type(), obj.Pkg())
 			break
 		}

 		// Defined type.
 		w.tag('T')
 		w.pos(obj.Pos())

 		underlying := obj.Type().Underlying()
 		w.typ(underlying, obj.Pkg())

 		t := obj.Type()
 		if types.IsInterface(t) {
 			break
 		}

 		named, ok := t.(*types.Named)
 		if !ok {
 			panic(internalErrorf("%s is not a defined type", t))
 		}

 		n := named.NumMethods()
 		w.uint64(uint64(n))
 		for i := 0; i < n; i++ {
 			m := named.Method(i)
 			w.pos(m.Pos())
 			w.string(m.Name())
 			sig, _ := m.Type().(*types.Signature)
 			w.param(sig.Recv())
 			w.signature(sig)
 		}

 	default:
 		panic(internalErrorf("unexpected object: %v", obj))
 	}

 	p.declIndex[obj] = w.flush()
 }

 func (w *exportWriter) tag(tag byte) {
 	w.data.WriteByte(tag)
 }

 func (w *exportWriter) pos(pos token.Pos) {
 	if w.p.fset == nil {
 		w.int64(0)
 		return
 	}

 	p := w.p.fset.Position(pos)
 	file := p.Filename
 	line := int64(p.Line)

 	// When file is the same as the last position (common case),
 	// we can save a few bytes by delta encoding just the line
 	// number.
 	//
 	// Note: Because data objects may be read out of order (or not
 	// at all), we can only apply delta encoding within a single
 	// object. This is handled implicitly by tracking prevFile and
 	// prevLine as fields of exportWriter.

 	if file == w.prevFile {
 		delta := line - w.prevLine
 		w.int64(delta)
 		if delta == deltaNewFile {
 			w.int64(-1)
 		}
 	} else {
 		w.int64(deltaNewFile)
 		w.int64(line) // line >= 0
 		w.string(file)
 		w.prevFile = file
 	}
 	w.prevLine = line
 }

 func (w *exportWriter) pkg(pkg *types.Package) {
 	// Ensure any referenced packages are declared in the main index.
 	w.p.allPkgs[pkg] = true

 	w.string(w.exportPath(pkg))
 }

 func (w *exportWriter) qualifiedIdent(obj types.Object) {
 	// Ensure any referenced declarations are written out too.
 	w.p.pushDecl(obj)

 	w.string(obj.Name())
 	w.pkg(obj.Pkg())
 }

 func (w *exportWriter) typ(t types.Type, pkg *types.Package) {
 	w.data.uint64(w.p.typOff(t, pkg))
 }

 func (p *iexporter) newWriter() *exportWriter {
 	return &exportWriter{p: p}
 }

 func (w *exportWriter) flush() uint64 {
 	off := uint64(w.p.data0.Len())
 	io.Copy(&w.p.data0, &w.data)
 	return off
 }

 func (p *iexporter) typOff(t types.Type, pkg *types.Package) uint64 {
 	off, ok := p.typIndex[t]
 	if !ok {
 		w := p.newWriter()
 		w.doTyp(t, pkg)
 		off = predeclReserved + w.flush()
 		p.typIndex[t] = off
 	}
 	return off
 }

 func (w *exportWriter) startType(k itag) {
 	w.data.uint64(uint64(k))
 }

 func (w *exportWriter) doTyp(t types.Type, pkg *types.Package) {
 	switch t := t.(type) {
 	case *types.Named:
 		w.startType(definedType)
 		w.qualifiedIdent(t.Obj())

 	case *types.Pointer:
 		w.startType(pointerType)
 		w.typ(t.Elem(), pkg)

 	case *types.Slice:
 		w.startType(sliceType)
 		w.typ(t.Elem(), pkg)

 	case *types.Array:
 		w.startType(arrayType)
 		w.uint64(uint64(t.Len()))
 		w.typ(t.Elem(), pkg)

 	case *types.Chan:
 		w.startType(chanType)
 		// 1 RecvOnly; 2 SendOnly; 3 SendRecv
 		var dir uint64
 		switch t.Dir() {
 		case types.RecvOnly:
 			dir = 1
 		case types.SendOnly:
 			dir = 2
 		case types.SendRecv:
 			dir = 3
 		}
 		w.uint64(dir)
 		w.typ(t.Elem(), pkg)

 	case *types.Map:
 		w.startType(mapType)
 		w.typ(t.Key(), pkg)
 		w.typ(t.Elem(), pkg)

 	case *types.Signature:
 		w.startType(signatureType)
 		w.setPkg(pkg, true)
 		w.signature(t)

 	case *types.Struct:
 		w.startType(structType)
 		w.setPkg(pkg, true)

 		n := t.NumFields()
 		w.uint64(uint64(n))
 		for i := 0; i < n; i++ {
 			f := t.Field(i)
 			w.pos(f.Pos())
 			w.string(f.Name())
 			w.typ(f.Type(), pkg)
 			w.bool(f.Anonymous())
 			w.string(t.Tag(i)) // note (or tag)
 		}

 	case *types.Interface:
 		w.startType(interfaceType)
 		w.setPkg(pkg, true)

 		n := t.NumEmbeddeds()
 		w.uint64(uint64(n))
 		for i := 0; i < n; i++ {
 			f := t.Embedded(i)
 			w.pos(f.Obj().Pos())
 			w.typ(f.Obj().Type(), f.Obj().Pkg())
 		}

 		n = t.NumExplicitMethods()
 		w.uint64(uint64(n))
 		for i := 0; i < n; i++ {
 			m := t.ExplicitMethod(i)
 			w.pos(m.Pos())
 			w.string(m.Name())
 			sig, _ := m.Type().(*types.Signature)
 			w.signature(sig)
 		}

 	default:
 		panic(internalErrorf("unexpected type: %v, %v", t, reflect.TypeOf(t)))
 	}
 }

 func (w *exportWriter) setPkg(pkg *types.Package, write bool) {
 	if write {
 		w.pkg(pkg)
 	}

 	w.currPkg = pkg
 }

 func (w *exportWriter) signature(sig *types.Signature) {
 	w.paramList(sig.Params())
 	w.paramList(sig.Results())
 	if sig.Params().Len() > 0 {
 		w.bool(sig.Variadic())
 	}
 }

 func (w *exportWriter) paramList(tup *types.Tuple) {
 	n := tup.Len()
 	w.uint64(uint64(n))
 	for i := 0; i < n; i++ {
 		w.param(tup.At(i))
 	}
 }

 func (w *exportWriter) param(obj types.Object) {
 	w.pos(obj.Pos())
 	w.localIdent(obj)
 	w.typ(obj.Type(), obj.Pkg())
 }

 func (w *exportWriter) value(typ types.Type, v constant.Value) {
 	w.typ(typ, nil)

 	switch v.Kind() {
 	case constant.Bool:
 		w.bool(constant.BoolVal(v))
 	case constant.Int:
 		var i big.Int
 		if i64, exact := constant.Int64Val(v); exact {
 			i.SetInt64(i64)
 		} else if ui64, exact := constant.Uint64Val(v); exact {
 			i.SetUint64(ui64)
 		} else {
 			i.SetString(v.ExactString(), 10)
 		}
 		w.mpint(&i, typ)
 	case constant.Float:
 		f := constantToFloat(v)
 		w.mpfloat(f, typ)
 	case constant.Complex:
 		w.mpfloat(constantToFloat(constant.Real(v)), typ)
 		w.mpfloat(constantToFloat(constant.Imag(v)), typ)
 	case constant.String:
 		w.string(constant.StringVal(v))
 	case constant.Unknown:
 		// package contains type errors
 	default:
 		panic(internalErrorf("unexpected value %v (%T)", v, v))
 	}
 }

 // constantToFloat converts a constant.Value with kind constant.Float to a
 // big.Float.
 func constantToFloat(x constant.Value) *big.Float {
 	assert(x.Kind() == constant.Float)
 	// Use the same floating-point precision (512) as cmd/compile
 	// (see Mpprec in cmd/compile/internal/gc/mpfloat.go).
 	const mpprec = 512
 	var f big.Float
 	f.SetPrec(mpprec)
 	if v, exact := constant.Float64Val(x); exact {
 		// float64
 		f.SetFloat64(v)
 	} else if num, denom := constant.Num(x), constant.Denom(x); num.Kind() == constant.Int {
 		// TODO(gri): add big.Rat accessor to constant.Value.
 		n := valueToRat(num)
 		d := valueToRat(denom)
 		f.SetRat(n.Quo(n, d))
 	} else {
 		// Value too large to represent as a fraction => inaccessible.
 		// TODO(gri): add big.Float accessor to constant.Value.
 		_, ok := f.SetString(x.ExactString())
 		assert(ok)
 	}
 	return &f
 }

 // mpint exports a multi-precision integer.
 //
 // For unsigned types, small values are written out as a single
 // byte. Larger values are written out as a length-prefixed big-endian
 // byte string, where the length prefix is encoded as its complement.
 // For example, bytes 0, 1, and 2 directly represent the integer
 // values 0, 1, and 2; while bytes 255, 254, and 253 indicate a 1-,
 // 2-, and 3-byte big-endian string follow.
 //
 // Encoding for signed types use the same general approach as for
 // unsigned types, except small values use zig-zag encoding and the
 // bottom bit of length prefix byte for large values is reserved as a
 // sign bit.
 //
 // The exact boundary between small and large encodings varies
 // according to the maximum number of bytes needed to encode a value
 // of type typ. As a special case, 8-bit types are always encoded as a
 // single byte.
 //
 // TODO(mdempsky): Is this level of complexity really worthwhile?
 func (w *exportWriter) mpint(x *big.Int, typ types.Type) {
 	basic, ok := typ.Underlying().(*types.Basic)
 	if !ok {
 		panic(internalErrorf("unexpected type %v (%T)", typ.Underlying(), typ.Underlying()))
 	}

 	signed, maxBytes := intSize(basic)

 	negative := x.Sign() < 0
 	if !signed && negative {
 		panic(internalErrorf("negative unsigned integer; type %v, value %v", typ, x))
 	}

 	b := x.Bytes()
 	if len(b) > 0 && b[0] == 0 {
 		panic(internalErrorf("leading zeros"))
 	}
 	if uint(len(b)) > maxBytes {
 		panic(internalErrorf("bad mpint length: %d > %d (type %v, value %v)", len(b), maxBytes, typ, x))
 	}

 	maxSmall := 256 - maxBytes
 	if signed {
 		maxSmall = 256 - 2*maxBytes
 	}
 	if maxBytes == 1 {
 		maxSmall = 256
 	}

 	// Check if x can use small value encoding.
 	if len(b) <= 1 {
 		var ux uint
 		if len(b) == 1 {
 			ux = uint(b[0])
 		}
 		if signed {
 			ux <<= 1
 			if negative {
 				ux--
 			}
 		}
 		if ux < maxSmall {
 			w.data.WriteByte(byte(ux))
 			return
 		}
 	}

 	n := 256 - uint(len(b))
 	if signed {
 		n = 256 - 2*uint(len(b))
 		if negative {
 			n |= 1
 		}
 	}
 	if n < maxSmall || n >= 256 {
 		panic(internalErrorf("encoding mistake: %d, %v, %v => %d", len(b), signed, negative, n))
 	}

 	w.data.WriteByte(byte(n))
 	w.data.Write(b)
 }

 // mpfloat exports a multi-precision floating point number.
 //
 // The number's value is decomposed into mantissa × 2**exponent, where
 // mantissa is an integer. The value is written out as mantissa (as a
 // multi-precision integer) and then the exponent, except exponent is
 // omitted if mantissa is zero.
 func (w *exportWriter) mpfloat(f *big.Float, typ types.Type) {
 	if f.IsInf() {
 		panic("infinite constant")
 	}

 	// Break into f = mant × 2**exp, with 0.5 <= mant < 1.
 	var mant big.Float
 	exp := int64(f.MantExp(&mant))

 	// Scale so that mant is an integer.
 	prec := mant.MinPrec()
 	mant.SetMantExp(&mant, int(prec))
 	exp -= int64(prec)

 	manti, acc := mant.Int(nil)
 	if acc != big.Exact {
 		panic(internalErrorf("mantissa scaling failed for %f (%s)", f, acc))
 	}
 	w.mpint(manti, typ)
 	if manti.Sign() != 0 {
 		w.int64(exp)
 	}
 }

 func (w *exportWriter) bool(b bool) bool {
 	var x uint64
 	if b {
 		x = 1
 	}
 	w.uint64(x)
 	return b
 }

 func (w *exportWriter) int64(x int64)   { w.data.int64(x) }
 func (w *exportWriter) uint64(x uint64) { w.data.uint64(x) }
 func (w *exportWriter) string(s string) { w.uint64(w.p.stringOff(s)) }

 func (w *exportWriter) localIdent(obj types.Object) {
 	// Anonymous parameters.
 	if obj == nil {
 		w.string("")
 		return
 	}

 	name := obj.Name()
 	if name == "_" {
 		w.string("_")
 		return
 	}

 	w.string(name)
 }

 type intWriter struct {
 	bytes.Buffer
 }

 func (w *intWriter) int64(x int64) {
 	var buf [binary.MaxVarintLen64]byte
 	n := binary.PutVarint(buf[:], x)
 	w.Write(buf[:n])
 }

 func (w *intWriter) uint64(x uint64) {
 	var buf [binary.MaxVarintLen64]byte
 	n := binary.PutUvarint(buf[:], x)
 	w.Write(buf[:n])
 }

 func assert(cond bool) {
 	if !cond {
 		panic("internal error: assertion failed")
 	}
 }

 // The below is copied from go/src/cmd/compile/internal/gc/syntax.go.

 // objQueue is a FIFO queue of types.Object. The zero value of objQueue is
 // a ready-to-use empty queue.
 type objQueue struct {
 	ring       []types.Object
 	head, tail int
 }

 // empty returns true if q contains no Nodes.
 func (q *objQueue) empty() bool {
 	return q.head == q.tail
 }

 // pushTail appends n to the tail of the queue.
 func (q *objQueue) pushTail(obj types.Object) {
 	if len(q.ring) == 0 {
 		q.ring = make([]types.Object, 16)
 	} else if q.head+len(q.ring) == q.tail {
 		// Grow the ring.
 		nring := make([]types.Object, len(q.ring)*2)
 		// Copy the old elements.
 		part := q.ring[q.head%len(q.ring):]
 		if q.tail-q.head <= len(part) {
 			part = part[:q.tail-q.head]
 			copy(nring, part)
 		} else {
 			pos := copy(nring, part)
 			copy(nring[pos:], q.ring[:q.tail%len(q.ring)])
 		}
 		q.ring, q.head, q.tail = nring, 0, q.tail-q.head
 	}

 	q.ring[q.tail%len(q.ring)] = obj
 	q.tail++
 }

 // popHead pops a node from the head of the queue. It panics if q is empty.
 func (q *objQueue) popHead() types.Object {
 	if q.empty() {
 		panic("dequeue empty")
 	}
 	obj := q.ring[q.head%len(q.ring)]
 	q.head++
 	return obj
 }
	// Copyright 2019 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.

	// Indexed binary package export.
	// This file was derived from $GOROOT/src/cmd/compile/internal/gc/iexport.go;
	// see that file for specification of the format.

	package gcimporter

	import (
	"bytes"
	"encoding/binary"
	"go/ast"
	"go/constant"
	"go/token"
	"go/types"
	"io"
	"math/big"
	"reflect"
	"sort"
	)

	// Current indexed export format version. Increase with each format change.
	// 0: Go1.11 encoding
	const iexportVersion = 0

	// IExportData returns the binary export data for pkg.
	//
	// If no file set is provided, position info will be missing.
	// The package path of the top-level package will not be recorded,
	// so that calls to IImportData can override with a provided package path.
	func IExportData(fset token.FileSet, pkg types.Package) (b []byte, err error) {
	defer func() {
	if e := recover(); e != nil {
	if ierr, ok := e.(internalError); ok {
	err = ierr
	return
	}
	// Not an internal error; panic again.
	panic(e)
	}
	}()

	p := iexporter{
	out: bytes.NewBuffer(nil),
	fset: fset,
	allPkgs: map[*types.Package]bool{},
	stringIndex: map[string]uint64{},
	declIndex: map[types.Object]uint64{},
	typIndex: map[types.Type]uint64{},
	localpkg: pkg,
	}

	for i, pt := range predeclared() {
	p.typIndex[pt] = uint64(i)
	}
	if len(p.typIndex) > predeclReserved {
	panic(internalErrorf("too many predeclared types: %d > %d", len(p.typIndex), predeclReserved))
	}

	// Initialize work queue with exported declarations.
	scope := pkg.Scope()
	for _, name := range scope.Names() {
	if ast.IsExported(name) {
	p.pushDecl(scope.Lookup(name))
	}
	}

	// Loop until no more work.
	for !p.declTodo.empty() {
	p.doDecl(p.declTodo.popHead())
	}

	// Append indices to data0 section.
	dataLen := uint64(p.data0.Len())
	w := p.newWriter()
	w.writeIndex(p.declIndex)
	w.flush()

	// Assemble header.
	var hdr intWriter
	hdr.WriteByte('i')
	hdr.uint64(iexportVersion)
	hdr.uint64(uint64(p.strings.Len()))
	hdr.uint64(dataLen)

	// Flush output.
	io.Copy(p.out, &hdr)
	io.Copy(p.out, &p.strings)
	io.Copy(p.out, &p.data0)

	return p.out.Bytes(), nil
	}

	// writeIndex writes out an object index. mainIndex indicates whether
	// we're writing out the main index, which is also read by
	// non-compiler tools and includes a complete package description
	// (i.e., name and height).
	func (w *exportWriter) writeIndex(index map[types.Object]uint64) {
	// Build a map from packages to objects from that package.
	pkgObjs := map[*types.Package][]types.Object{}

	// For the main index, make sure to include every package that
	// we reference, even if we're not exporting (or reexporting)
	// any symbols from it.
	pkgObjs[w.p.localpkg] = nil
	for pkg := range w.p.allPkgs {
	pkgObjs[pkg] = nil
	}

	for obj := range index {
	pkgObjs[obj.Pkg()] = append(pkgObjs[obj.Pkg()], obj)
	}

	var pkgs []*types.Package
	for pkg, objs := range pkgObjs {
	pkgs = append(pkgs, pkg)

	sort.Slice(objs, func(i, j int) bool {
	return objs[i].Name() < objs[j].Name()
	})
	}

	sort.Slice(pkgs, func(i, j int) bool {
	return w.exportPath(pkgs[i]) < w.exportPath(pkgs[j])
	})

	w.uint64(uint64(len(pkgs)))
	for _, pkg := range pkgs {
	w.string(w.exportPath(pkg))
	w.string(pkg.Name())
	w.uint64(uint64(0)) // package height is not needed for go/types

	objs := pkgObjs[pkg]
	w.uint64(uint64(len(objs)))
	for _, obj := range objs {
	w.string(obj.Name())
	w.uint64(index[obj])
	}
	}
	}

	type iexporter struct {
	fset *token.FileSet
	out *bytes.Buffer

	localpkg *types.Package

	// allPkgs tracks all packages that have been referenced by
	// the export data, so we can ensure to include them in the
	// main index.
	allPkgs map[*types.Package]bool

	declTodo objQueue

	strings intWriter
	stringIndex map[string]uint64

	data0 intWriter
	declIndex map[types.Object]uint64
	typIndex map[types.Type]uint64
	}

	// stringOff returns the offset of s within the string section.
	// If not already present, it's added to the end.
	func (p *iexporter) stringOff(s string) uint64 {
	off, ok := p.stringIndex[s]
	if !ok {
	off = uint64(p.strings.Len())
	p.stringIndex[s] = off

	p.strings.uint64(uint64(len(s)))
	p.strings.WriteString(s)
	}
	return off
	}

	// pushDecl adds n to the declaration work queue, if not already present.
	func (p *iexporter) pushDecl(obj types.Object) {
	// Package unsafe is known to the compiler and predeclared.
	assert(obj.Pkg() != types.Unsafe)

	if _, ok := p.declIndex[obj]; ok {
	return
	}

	p.declIndex[obj] = ^uint64(0) // mark n present in work queue
	p.declTodo.pushTail(obj)
	}

	// exportWriter handles writing out individual data section chunks.
	type exportWriter struct {
	p *iexporter

	data intWriter
	currPkg *types.Package
	prevFile string
	prevLine int64
	}

	func (w exportWriter) exportPath(pkg types.Package) string {
	if pkg == w.p.localpkg {
	return ""
	}
	return pkg.Path()
	}

	func (p *iexporter) doDecl(obj types.Object) {
	w := p.newWriter()
	w.setPkg(obj.Pkg(), false)

	switch obj := obj.(type) {
	case *types.Var:
	w.tag('V')
	w.pos(obj.Pos())
	w.typ(obj.Type(), obj.Pkg())

	case *types.Func:
	sig, _ := obj.Type().(*types.Signature)
	if sig.Recv() != nil {
	panic(internalErrorf("unexpected method: %v", sig))
	}
	w.tag('F')
	w.pos(obj.Pos())
	w.signature(sig)

	case *types.Const:
	w.tag('C')
	w.pos(obj.Pos())
	w.value(obj.Type(), obj.Val())

	case *types.TypeName:
	if obj.IsAlias() {
	w.tag('A')
	w.pos(obj.Pos())
	w.typ(obj.Type(), obj.Pkg())
	break
	}

	// Defined type.
	w.tag('T')
	w.pos(obj.Pos())

	underlying := obj.Type().Underlying()
	w.typ(underlying, obj.Pkg())

	t := obj.Type()
	if types.IsInterface(t) {
	break
	}

	named, ok := t.(*types.Named)
	if !ok {
	panic(internalErrorf("%s is not a defined type", t))
	}

	n := named.NumMethods()
	w.uint64(uint64(n))
	for i := 0; i < n; i++ {
	m := named.Method(i)
	w.pos(m.Pos())
	w.string(m.Name())
	sig, _ := m.Type().(*types.Signature)
	w.param(sig.Recv())
	w.signature(sig)
	}

	default:
	panic(internalErrorf("unexpected object: %v", obj))
	}

	p.declIndex[obj] = w.flush()
	}

	func (w *exportWriter) tag(tag byte) {
	w.data.WriteByte(tag)
	}

	func (w *exportWriter) pos(pos token.Pos) {
	if w.p.fset == nil {
	w.int64(0)
	return
	}

	p := w.p.fset.Position(pos)
	file := p.Filename
	line := int64(p.Line)

	// When file is the same as the last position (common case),
	// we can save a few bytes by delta encoding just the line
	// number.
	//
	// Note: Because data objects may be read out of order (or not
	// at all), we can only apply delta encoding within a single
	// object. This is handled implicitly by tracking prevFile and
	// prevLine as fields of exportWriter.

	if file == w.prevFile {
	delta := line - w.prevLine
	w.int64(delta)
	if delta == deltaNewFile {
	w.int64(-1)
	}
	} else {
	w.int64(deltaNewFile)
	w.int64(line) // line >= 0
	w.string(file)
	w.prevFile = file
	}
	w.prevLine = line
	}

	func (w exportWriter) pkg(pkg types.Package) {
	// Ensure any referenced packages are declared in the main index.
	w.p.allPkgs[pkg] = true

	w.string(w.exportPath(pkg))
	}

	func (w *exportWriter) qualifiedIdent(obj types.Object) {
	// Ensure any referenced declarations are written out too.
	w.p.pushDecl(obj)

	w.string(obj.Name())
	w.pkg(obj.Pkg())
	}

	func (w exportWriter) typ(t types.Type, pkg types.Package) {
	w.data.uint64(w.p.typOff(t, pkg))
	}

	func (p iexporter) newWriter() exportWriter {
	return &exportWriter{p: p}
	}

	func (w *exportWriter) flush() uint64 {
	off := uint64(w.p.data0.Len())
	io.Copy(&w.p.data0, &w.data)
	return off
	}

	func (p iexporter) typOff(t types.Type, pkg types.Package) uint64 {
	off, ok := p.typIndex[t]
	if !ok {
	w := p.newWriter()
	w.doTyp(t, pkg)
	off = predeclReserved + w.flush()
	p.typIndex[t] = off
	}
	return off
	}

	func (w *exportWriter) startType(k itag) {
	w.data.uint64(uint64(k))
	}

	func (w exportWriter) doTyp(t types.Type, pkg types.Package) {
	switch t := t.(type) {
	case *types.Named:
	w.startType(definedType)
	w.qualifiedIdent(t.Obj())

	case *types.Pointer:
	w.startType(pointerType)
	w.typ(t.Elem(), pkg)

	case *types.Slice:
	w.startType(sliceType)
	w.typ(t.Elem(), pkg)

	case *types.Array:
	w.startType(arrayType)
	w.uint64(uint64(t.Len()))
	w.typ(t.Elem(), pkg)

	case *types.Chan:
	w.startType(chanType)
	// 1 RecvOnly; 2 SendOnly; 3 SendRecv
	var dir uint64
	switch t.Dir() {
	case types.RecvOnly:
	dir = 1
	case types.SendOnly:
	dir = 2
	case types.SendRecv:
	dir = 3
	}
	w.uint64(dir)
	w.typ(t.Elem(), pkg)

	case *types.Map:
	w.startType(mapType)
	w.typ(t.Key(), pkg)
	w.typ(t.Elem(), pkg)

	case *types.Signature:
	w.startType(signatureType)
	w.setPkg(pkg, true)
	w.signature(t)

	case *types.Struct:
	w.startType(structType)
	w.setPkg(pkg, true)

	n := t.NumFields()
	w.uint64(uint64(n))
	for i := 0; i < n; i++ {
	f := t.Field(i)
	w.pos(f.Pos())
	w.string(f.Name())
	w.typ(f.Type(), pkg)
	w.bool(f.Anonymous())
	w.string(t.Tag(i)) // note (or tag)
	}

	case *types.Interface:
	w.startType(interfaceType)
	w.setPkg(pkg, true)

	n := t.NumEmbeddeds()
	w.uint64(uint64(n))
	for i := 0; i < n; i++ {
	f := t.Embedded(i)
	w.pos(f.Obj().Pos())
	w.typ(f.Obj().Type(), f.Obj().Pkg())
	}

	n = t.NumExplicitMethods()
	w.uint64(uint64(n))
	for i := 0; i < n; i++ {
	m := t.ExplicitMethod(i)
	w.pos(m.Pos())
	w.string(m.Name())
	sig, _ := m.Type().(*types.Signature)
	w.signature(sig)
	}

	default:
	panic(internalErrorf("unexpected type: %v, %v", t, reflect.TypeOf(t)))
	}
	}

	func (w exportWriter) setPkg(pkg types.Package, write bool) {
	if write {
	w.pkg(pkg)
	}

	w.currPkg = pkg
	}

	func (w exportWriter) signature(sig types.Signature) {
	w.paramList(sig.Params())
	w.paramList(sig.Results())
	if sig.Params().Len() > 0 {
	w.bool(sig.Variadic())
	}
	}

	func (w exportWriter) paramList(tup types.Tuple) {
	n := tup.Len()
	w.uint64(uint64(n))
	for i := 0; i < n; i++ {
	w.param(tup.At(i))
	}
	}

	func (w *exportWriter) param(obj types.Object) {
	w.pos(obj.Pos())
	w.localIdent(obj)
	w.typ(obj.Type(), obj.Pkg())
	}

	func (w *exportWriter) value(typ types.Type, v constant.Value) {
	w.typ(typ, nil)

	switch v.Kind() {
	case constant.Bool:
	w.bool(constant.BoolVal(v))
	case constant.Int:
	var i big.Int
	if i64, exact := constant.Int64Val(v); exact {
	i.SetInt64(i64)
	} else if ui64, exact := constant.Uint64Val(v); exact {
	i.SetUint64(ui64)
	} else {
	i.SetString(v.ExactString(), 10)
	}
	w.mpint(&i, typ)
	case constant.Float:
	f := constantToFloat(v)
	w.mpfloat(f, typ)
	case constant.Complex:
	w.mpfloat(constantToFloat(constant.Real(v)), typ)
	w.mpfloat(constantToFloat(constant.Imag(v)), typ)
	case constant.String:
	w.string(constant.StringVal(v))
	case constant.Unknown:
	// package contains type errors
	default:
	panic(internalErrorf("unexpected value %v (%T)", v, v))
	}
	}

	// constantToFloat converts a constant.Value with kind constant.Float to a
	// big.Float.
	func constantToFloat(x constant.Value) *big.Float {
	assert(x.Kind() == constant.Float)
	// Use the same floating-point precision (512) as cmd/compile
	// (see Mpprec in cmd/compile/internal/gc/mpfloat.go).
	const mpprec = 512
	var f big.Float
	f.SetPrec(mpprec)
	if v, exact := constant.Float64Val(x); exact {
	// float64
	f.SetFloat64(v)
	} else if num, denom := constant.Num(x), constant.Denom(x); num.Kind() == constant.Int {
	// TODO(gri): add big.Rat accessor to constant.Value.
	n := valueToRat(num)
	d := valueToRat(denom)
	f.SetRat(n.Quo(n, d))
	} else {
	// Value too large to represent as a fraction => inaccessible.
	// TODO(gri): add big.Float accessor to constant.Value.
	_, ok := f.SetString(x.ExactString())
	assert(ok)
	}
	return &f
	}

	// mpint exports a multi-precision integer.
	//
	// For unsigned types, small values are written out as a single
	// byte. Larger values are written out as a length-prefixed big-endian
	// byte string, where the length prefix is encoded as its complement.
	// For example, bytes 0, 1, and 2 directly represent the integer
	// values 0, 1, and 2; while bytes 255, 254, and 253 indicate a 1-,
	// 2-, and 3-byte big-endian string follow.
	//
	// Encoding for signed types use the same general approach as for
	// unsigned types, except small values use zig-zag encoding and the
	// bottom bit of length prefix byte for large values is reserved as a
	// sign bit.
	//
	// The exact boundary between small and large encodings varies
	// according to the maximum number of bytes needed to encode a value
	// of type typ. As a special case, 8-bit types are always encoded as a
	// single byte.
	//
	// TODO(mdempsky): Is this level of complexity really worthwhile?
	func (w exportWriter) mpint(x big.Int, typ types.Type) {
	basic, ok := typ.Underlying().(*types.Basic)
	if !ok {
	panic(internalErrorf("unexpected type %v (%T)", typ.Underlying(), typ.Underlying()))
	}

	signed, maxBytes := intSize(basic)

	negative := x.Sign() < 0
	if !signed && negative {
	panic(internalErrorf("negative unsigned integer; type %v, value %v", typ, x))
	}

	b := x.Bytes()
	if len(b) > 0 && b[0] == 0 {
	panic(internalErrorf("leading zeros"))
	}
	if uint(len(b)) > maxBytes {
	panic(internalErrorf("bad mpint length: %d > %d (type %v, value %v)", len(b), maxBytes, typ, x))
	}

	maxSmall := 256 - maxBytes
	if signed {
	maxSmall = 256 - 2*maxBytes
	}
	if maxBytes == 1 {
	maxSmall = 256
	}

	// Check if x can use small value encoding.
	if len(b) <= 1 {
	var ux uint
	if len(b) == 1 {
	ux = uint(b[0])
	}
	if signed {
	ux <<= 1
	if negative {
	ux--
	}
	}
	if ux < maxSmall {
	w.data.WriteByte(byte(ux))
	return
	}
	}

	n := 256 - uint(len(b))
	if signed {
	n = 256 - 2*uint(len(b))
	if negative {
	n \|= 1
	}
	}
	if n < maxSmall \|\| n >= 256 {
	panic(internalErrorf("encoding mistake: %d, %v, %v => %d", len(b), signed, negative, n))
	}

	w.data.WriteByte(byte(n))
	w.data.Write(b)
	}

	// mpfloat exports a multi-precision floating point number.
	//
	// The number's value is decomposed into mantissa × 2**exponent, where
	// mantissa is an integer. The value is written out as mantissa (as a
	// multi-precision integer) and then the exponent, except exponent is
	// omitted if mantissa is zero.
	func (w exportWriter) mpfloat(f big.Float, typ types.Type) {
	if f.IsInf() {
	panic("infinite constant")
	}

	// Break into f = mant × 2**exp, with 0.5 <= mant < 1.
	var mant big.Float
	exp := int64(f.MantExp(&mant))

	// Scale so that mant is an integer.
	prec := mant.MinPrec()
	mant.SetMantExp(&mant, int(prec))
	exp -= int64(prec)

	manti, acc := mant.Int(nil)
	if acc != big.Exact {
	panic(internalErrorf("mantissa scaling failed for %f (%s)", f, acc))
	}
	w.mpint(manti, typ)
	if manti.Sign() != 0 {
	w.int64(exp)
	}
	}

	func (w *exportWriter) bool(b bool) bool {
	var x uint64
	if b {
	x = 1
	}
	w.uint64(x)
	return b
	}

	func (w *exportWriter) int64(x int64) { w.data.int64(x) }
	func (w *exportWriter) uint64(x uint64) { w.data.uint64(x) }
	func (w *exportWriter) string(s string) { w.uint64(w.p.stringOff(s)) }

	func (w *exportWriter) localIdent(obj types.Object) {
	// Anonymous parameters.
	if obj == nil {
	w.string("")
	return
	}

	name := obj.Name()
	if name == "_" {
	w.string("_")
	return
	}

	w.string(name)
	}

	type intWriter struct {
	bytes.Buffer
	}

	func (w *intWriter) int64(x int64) {
	var buf [binary.MaxVarintLen64]byte
	n := binary.PutVarint(buf[:], x)
	w.Write(buf[:n])
	}

	func (w *intWriter) uint64(x uint64) {
	var buf [binary.MaxVarintLen64]byte
	n := binary.PutUvarint(buf[:], x)
	w.Write(buf[:n])
	}

	func assert(cond bool) {
	if !cond {
	panic("internal error: assertion failed")
	}
	}

	// The below is copied from go/src/cmd/compile/internal/gc/syntax.go.

	// objQueue is a FIFO queue of types.Object. The zero value of objQueue is
	// a ready-to-use empty queue.
	type objQueue struct {
	ring []types.Object
	head, tail int
	}

	// empty returns true if q contains no Nodes.
	func (q *objQueue) empty() bool {
	return q.head == q.tail
	}

	// pushTail appends n to the tail of the queue.
	func (q *objQueue) pushTail(obj types.Object) {
	if len(q.ring) == 0 {
	q.ring = make([]types.Object, 16)
	} else if q.head+len(q.ring) == q.tail {
	// Grow the ring.
	nring := make([]types.Object, len(q.ring)*2)
	// Copy the old elements.
	part := q.ring[q.head%len(q.ring):]
	if q.tail-q.head <= len(part) {
	part = part[:q.tail-q.head]
	copy(nring, part)
	} else {
	pos := copy(nring, part)
	copy(nring[pos:], q.ring[:q.tail%len(q.ring)])
	}
	q.ring, q.head, q.tail = nring, 0, q.tail-q.head
	}

	q.ring[q.tail%len(q.ring)] = obj
	q.tail++
	}

	// popHead pops a node from the head of the queue. It panics if q is empty.
	func (q *objQueue) popHead() types.Object {
	if q.empty() {
	panic("dequeue empty")
	}
	obj := q.ring[q.head%len(q.ring)]
	q.head++
	return obj
	}