diff --git a/go/gcexportdata/gcexportdata.go b/go/gcexportdata/gcexportdata.go
index ddc276cfbcb28..2ed25a750248e 100644
--- a/go/gcexportdata/gcexportdata.go
+++ b/go/gcexportdata/gcexportdata.go
@@ -116,13 +116,29 @@ func Read(in io.Reader, fset *token.FileSet, imports map[string]*types.Package,
 	// The indexed export format starts with an 'i'; the older
 	// binary export format starts with a 'c', 'd', or 'v'
 	// (from "version"). Select appropriate importer.
-	if len(data) > 0 && data[0] == 'i' {
-		_, pkg, err := gcimporter.IImportData(fset, imports, data[1:], path)
-		return pkg, err
-	}
+	if len(data) > 0 {
+		switch data[0] {
+		case 'i':
+			_, pkg, err := gcimporter.IImportData(fset, imports, data[1:], path)
+			return pkg, err
+
+		case 'v', 'c', 'd':
+			_, pkg, err := gcimporter.BImportData(fset, imports, data, path)
+			return pkg, err
 
-	_, pkg, err := gcimporter.BImportData(fset, imports, data, path)
-	return pkg, err
+		case 'u':
+			_, pkg, err := gcimporter.UImportData(fset, imports, data[1:], path)
+			return pkg, err
+
+		default:
+			l := len(data)
+			if l > 10 {
+				l = 10
+			}
+			return nil, fmt.Errorf("unexpected export data with prefix %q for path %s", string(data[:l]), path)
+		}
+	}
+	return nil, fmt.Errorf("empty export data for %s", path)
 }
 
 // Write writes encoded type information for the specified package to out.
diff --git a/go/internal/gcimporter/gcimporter.go b/go/internal/gcimporter/gcimporter.go
index 493bfa03b0f62..e96c39600d16b 100644
--- a/go/internal/gcimporter/gcimporter.go
+++ b/go/internal/gcimporter/gcimporter.go
@@ -181,8 +181,9 @@ func Import(packages map[string]*types.Package, path, srcDir string, lookup func
 	defer rc.Close()
 
 	var hdr string
+	var size int64
 	buf := bufio.NewReader(rc)
-	if hdr, _, err = FindExportData(buf); err != nil {
+	if hdr, size, err = FindExportData(buf); err != nil {
 		return
 	}
 
@@ -210,10 +211,27 @@ func Import(packages map[string]*types.Package, path, srcDir string, lookup func
 		// The indexed export format starts with an 'i'; the older
 		// binary export format starts with a 'c', 'd', or 'v'
 		// (from "version"). Select appropriate importer.
-		if len(data) > 0 && data[0] == 'i' {
-			_, pkg, err = IImportData(fset, packages, data[1:], id)
-		} else {
-			_, pkg, err = BImportData(fset, packages, data, id)
+		if len(data) > 0 {
+			switch data[0] {
+			case 'i':
+				_, pkg, err := IImportData(fset, packages, data[1:], id)
+				return pkg, err
+
+			case 'v', 'c', 'd':
+				_, pkg, err := BImportData(fset, packages, data, id)
+				return pkg, err
+
+			case 'u':
+				_, pkg, err := UImportData(fset, packages, data[1:size], id)
+				return pkg, err
+
+			default:
+				l := len(data)
+				if l > 10 {
+					l = 10
+				}
+				return nil, fmt.Errorf("unexpected export data with prefix %q for path %s", string(data[:l]), id)
+			}
 		}
 
 	default:
diff --git a/go/internal/gcimporter/gcimporter_test.go b/go/internal/gcimporter/gcimporter_test.go
index 4e992af76b3f7..66c09269d88f6 100644
--- a/go/internal/gcimporter/gcimporter_test.go
+++ b/go/internal/gcimporter/gcimporter_test.go
@@ -45,6 +45,10 @@ func needsCompiler(t *testing.T, compiler string) {
 // compile runs the compiler on filename, with dirname as the working directory,
 // and writes the output file to outdirname.
 func compile(t *testing.T, dirname, filename, outdirname string) string {
+	return compilePkg(t, dirname, filename, outdirname, "p")
+}
+
+func compilePkg(t *testing.T, dirname, filename, outdirname, pkg string) string {
 	testenv.NeedsGoBuild(t)
 
 	// filename must end with ".go"
@@ -53,12 +57,12 @@ func compile(t *testing.T, dirname, filename, outdirname string) string {
 	}
 	basename := filepath.Base(filename)
 	outname := filepath.Join(outdirname, basename[:len(basename)-2]+"o")
-	cmd := exec.Command("go", "tool", "compile", "-p=p", "-o", outname, filename)
+	cmd := exec.Command("go", "tool", "compile", "-p="+pkg, "-o", outname, filename)
 	cmd.Dir = dirname
 	out, err := cmd.CombinedOutput()
 	if err != nil {
 		t.Logf("%s", out)
-		t.Fatalf("go tool compile %s failed: %s", filename, err)
+		t.Fatalf("(cd %v && %v) failed: %s", cmd.Dir, cmd, err)
 	}
 	return outname
 }
@@ -140,7 +144,11 @@ func TestImportTestdata(t *testing.T) {
 		// For now, we just test the presence of a few packages
 		// that we know are there for sure.
 		got := fmt.Sprint(pkg.Imports())
-		for _, want := range []string{"go/ast", "go/token"} {
+		wants := []string{"go/ast", "go/token"}
+		if unifiedIR {
+			wants = []string{"go/ast"}
+		}
+		for _, want := range wants {
 			if !strings.Contains(got, want) {
 				t.Errorf(`Package("exports").Imports() = %s, does not contain %s`, got, want)
 			}
@@ -364,6 +372,14 @@ func verifyInterfaceMethodRecvs(t *testing.T, named *types.Named, level int) {
 		return // not an interface
 	}
 
+	// The unified IR importer always sets interface method receiver
+	// parameters to point to the Interface type, rather than the Named.
+	// See #49906.
+	var want types.Type = named
+	if unifiedIR {
+		want = iface
+	}
+
 	// check explicitly declared methods
 	for i := 0; i < iface.NumExplicitMethods(); i++ {
 		m := iface.ExplicitMethod(i)
@@ -372,8 +388,8 @@ func verifyInterfaceMethodRecvs(t *testing.T, named *types.Named, level int) {
 			t.Errorf("%s: missing receiver type", m)
 			continue
 		}
-		if recv.Type() != named {
-			t.Errorf("%s: got recv type %s; want %s", m, recv.Type(), named)
+		if recv.Type() != want {
+			t.Errorf("%s: got recv type %s; want %s", m, recv.Type(), want)
 		}
 	}
 
@@ -451,7 +467,7 @@ func TestIssue13566(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	compile(t, "testdata", "a.go", testoutdir)
+	compilePkg(t, "testdata", "a.go", testoutdir, apkg(testoutdir))
 	compile(t, testoutdir, bpath, testoutdir)
 
 	// import must succeed (test for issue at hand)
@@ -611,13 +627,22 @@ func TestIssue51836(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	compile(t, dir, "a.go", testoutdir)
+	compilePkg(t, dir, "a.go", testoutdir, apkg(testoutdir))
 	compile(t, testoutdir, bpath, testoutdir)
 
 	// import must succeed (test for issue at hand)
 	_ = importPkg(t, "./testdata/aa", tmpdir)
 }
 
+// apkg returns the package "a" prefixed by (as a package) testoutdir
+func apkg(testoutdir string) string {
+	apkg := testoutdir + "/a"
+	if os.PathSeparator != '/' {
+		apkg = strings.ReplaceAll(apkg, string(os.PathSeparator), "/")
+	}
+	return apkg
+}
+
 func importPkg(t *testing.T, path, srcDir string) *types.Package {
 	pkg, err := Import(make(map[string]*types.Package), path, srcDir, nil)
 	if err != nil {
diff --git a/go/internal/gcimporter/unified_no.go b/go/internal/gcimporter/unified_no.go
new file mode 100644
index 0000000000000..286bf445483d8
--- /dev/null
+++ b/go/internal/gcimporter/unified_no.go
@@ -0,0 +1,10 @@
+// Copyright 2022 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.
+
+//go:build !(go1.18 && goexperiment.unified)
+// +build !go1.18 !goexperiment.unified
+
+package gcimporter
+
+const unifiedIR = false
diff --git a/go/internal/gcimporter/unified_yes.go b/go/internal/gcimporter/unified_yes.go
new file mode 100644
index 0000000000000..b5d69ffbe682d
--- /dev/null
+++ b/go/internal/gcimporter/unified_yes.go
@@ -0,0 +1,10 @@
+// Copyright 2022 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.
+
+//go:build go1.18 && goexperiment.unified
+// +build go1.18,goexperiment.unified
+
+package gcimporter
+
+const unifiedIR = true
diff --git a/go/internal/gcimporter/ureader_no.go b/go/internal/gcimporter/ureader_no.go
new file mode 100644
index 0000000000000..8eb20729c2ad5
--- /dev/null
+++ b/go/internal/gcimporter/ureader_no.go
@@ -0,0 +1,19 @@
+// Copyright 2022 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.
+
+//go:build !go1.18
+// +build !go1.18
+
+package gcimporter
+
+import (
+	"fmt"
+	"go/token"
+	"go/types"
+)
+
+func UImportData(fset *token.FileSet, imports map[string]*types.Package, data []byte, path string) (_ int, pkg *types.Package, err error) {
+	err = fmt.Errorf("go/tools compiled with a Go version earlier than 1.18 cannot read unified IR export data")
+	return
+}
diff --git a/go/internal/gcimporter/ureader_yes.go b/go/internal/gcimporter/ureader_yes.go
new file mode 100644
index 0000000000000..3c1a4375435a7
--- /dev/null
+++ b/go/internal/gcimporter/ureader_yes.go
@@ -0,0 +1,612 @@
+// Copyright 2021 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.
+
+// Derived from go/internal/gcimporter/ureader.go
+
+//go:build go1.18
+// +build go1.18
+
+package gcimporter
+
+import (
+	"go/token"
+	"go/types"
+	"strings"
+
+	"golang.org/x/tools/go/internal/pkgbits"
+)
+
+// A pkgReader holds the shared state for reading a unified IR package
+// description.
+type pkgReader struct {
+	pkgbits.PkgDecoder
+
+	fake fakeFileSet
+
+	ctxt    *types.Context
+	imports map[string]*types.Package // previously imported packages, indexed by path
+
+	// lazily initialized arrays corresponding to the unified IR
+	// PosBase, Pkg, and Type sections, respectively.
+	posBases []string // position bases (i.e., file names)
+	pkgs     []*types.Package
+	typs     []types.Type
+
+	// laterFns holds functions that need to be invoked at the end of
+	// import reading.
+	laterFns []func()
+}
+
+// later adds a function to be invoked at the end of import reading.
+func (pr *pkgReader) later(fn func()) {
+	pr.laterFns = append(pr.laterFns, fn)
+}
+
+// See cmd/compile/internal/noder.derivedInfo.
+type derivedInfo struct {
+	idx    pkgbits.Index
+	needed bool
+}
+
+// See cmd/compile/internal/noder.typeInfo.
+type typeInfo struct {
+	idx     pkgbits.Index
+	derived bool
+}
+
+func UImportData(fset *token.FileSet, imports map[string]*types.Package, data []byte, path string) (_ int, pkg *types.Package, err error) {
+	s := string(data)
+	s = s[:strings.LastIndex(s, "\n$$\n")]
+	input := pkgbits.NewPkgDecoder(path, s)
+	pkg = readUnifiedPackage(fset, nil, imports, input)
+	return
+}
+
+// readUnifiedPackage reads a package description from the given
+// unified IR export data decoder.
+func readUnifiedPackage(fset *token.FileSet, ctxt *types.Context, imports map[string]*types.Package, input pkgbits.PkgDecoder) *types.Package {
+	pr := pkgReader{
+		PkgDecoder: input,
+
+		fake: fakeFileSet{
+			fset:  fset,
+			files: make(map[string]*fileInfo),
+		},
+
+		ctxt:    ctxt,
+		imports: imports,
+
+		posBases: make([]string, input.NumElems(pkgbits.RelocPosBase)),
+		pkgs:     make([]*types.Package, input.NumElems(pkgbits.RelocPkg)),
+		typs:     make([]types.Type, input.NumElems(pkgbits.RelocType)),
+	}
+	defer pr.fake.setLines()
+
+	r := pr.newReader(pkgbits.RelocMeta, pkgbits.PublicRootIdx, pkgbits.SyncPublic)
+	pkg := r.pkg()
+	r.Bool() // has init
+
+	for i, n := 0, r.Len(); i < n; i++ {
+		// As if r.obj(), but avoiding the Scope.Lookup call,
+		// to avoid eager loading of imports.
+		r.Sync(pkgbits.SyncObject)
+		assert(!r.Bool())
+		r.p.objIdx(r.Reloc(pkgbits.RelocObj))
+		assert(r.Len() == 0)
+	}
+
+	r.Sync(pkgbits.SyncEOF)
+
+	for _, fn := range pr.laterFns {
+		fn()
+	}
+
+	pkg.MarkComplete()
+	return pkg
+}
+
+// A reader holds the state for reading a single unified IR element
+// within a package.
+type reader struct {
+	pkgbits.Decoder
+
+	p *pkgReader
+
+	dict *readerDict
+}
+
+// A readerDict holds the state for type parameters that parameterize
+// the current unified IR element.
+type readerDict struct {
+	// bounds is a slice of typeInfos corresponding to the underlying
+	// bounds of the element's type parameters.
+	bounds []typeInfo
+
+	// tparams is a slice of the constructed TypeParams for the element.
+	tparams []*types.TypeParam
+
+	// devived is a slice of types derived from tparams, which may be
+	// instantiated while reading the current element.
+	derived      []derivedInfo
+	derivedTypes []types.Type // lazily instantiated from derived
+}
+
+func (pr *pkgReader) newReader(k pkgbits.RelocKind, idx pkgbits.Index, marker pkgbits.SyncMarker) *reader {
+	return &reader{
+		Decoder: pr.NewDecoder(k, idx, marker),
+		p:       pr,
+	}
+}
+
+// @@@ Positions
+
+func (r *reader) pos() token.Pos {
+	r.Sync(pkgbits.SyncPos)
+	if !r.Bool() {
+		return token.NoPos
+	}
+
+	// TODO(mdempsky): Delta encoding.
+	posBase := r.posBase()
+	line := r.Uint()
+	col := r.Uint()
+	return r.p.fake.pos(posBase, int(line), int(col))
+}
+
+func (r *reader) posBase() string {
+	return r.p.posBaseIdx(r.Reloc(pkgbits.RelocPosBase))
+}
+
+func (pr *pkgReader) posBaseIdx(idx pkgbits.Index) string {
+	if b := pr.posBases[idx]; b != "" {
+		return b
+	}
+
+	r := pr.newReader(pkgbits.RelocPosBase, idx, pkgbits.SyncPosBase)
+
+	// Within types2, position bases have a lot more details (e.g.,
+	// keeping track of where //line directives appeared exactly).
+	//
+	// For go/types, we just track the file name.
+
+	filename := r.String()
+
+	if r.Bool() { // file base
+		// Was: "b = token.NewTrimmedFileBase(filename, true)"
+	} else { // line base
+		pos := r.pos()
+		line := r.Uint()
+		col := r.Uint()
+
+		// Was: "b = token.NewLineBase(pos, filename, true, line, col)"
+		_, _, _ = pos, line, col
+	}
+
+	b := filename
+	pr.posBases[idx] = b
+	return b
+}
+
+// @@@ Packages
+
+func (r *reader) pkg() *types.Package {
+	r.Sync(pkgbits.SyncPkg)
+	return r.p.pkgIdx(r.Reloc(pkgbits.RelocPkg))
+}
+
+func (pr *pkgReader) pkgIdx(idx pkgbits.Index) *types.Package {
+	// TODO(mdempsky): Consider using some non-nil pointer to indicate
+	// the universe scope, so we don't need to keep re-reading it.
+	if pkg := pr.pkgs[idx]; pkg != nil {
+		return pkg
+	}
+
+	pkg := pr.newReader(pkgbits.RelocPkg, idx, pkgbits.SyncPkgDef).doPkg()
+	pr.pkgs[idx] = pkg
+	return pkg
+}
+
+func (r *reader) doPkg() *types.Package {
+	path := r.String()
+	switch path {
+	case "":
+		path = r.p.PkgPath()
+	case "builtin":
+		return nil // universe
+	case "unsafe":
+		return types.Unsafe
+	}
+
+	if pkg := r.p.imports[path]; pkg != nil {
+		return pkg
+	}
+
+	name := r.String()
+
+	pkg := types.NewPackage(path, name)
+	r.p.imports[path] = pkg
+
+	imports := make([]*types.Package, r.Len())
+	for i := range imports {
+		imports[i] = r.pkg()
+	}
+	pkg.SetImports(imports)
+
+	return pkg
+}
+
+// @@@ Types
+
+func (r *reader) typ() types.Type {
+	return r.p.typIdx(r.typInfo(), r.dict)
+}
+
+func (r *reader) typInfo() typeInfo {
+	r.Sync(pkgbits.SyncType)
+	if r.Bool() {
+		return typeInfo{idx: pkgbits.Index(r.Len()), derived: true}
+	}
+	return typeInfo{idx: r.Reloc(pkgbits.RelocType), derived: false}
+}
+
+func (pr *pkgReader) typIdx(info typeInfo, dict *readerDict) types.Type {
+	idx := info.idx
+	var where *types.Type
+	if info.derived {
+		where = &dict.derivedTypes[idx]
+		idx = dict.derived[idx].idx
+	} else {
+		where = &pr.typs[idx]
+	}
+
+	if typ := *where; typ != nil {
+		return typ
+	}
+
+	r := pr.newReader(pkgbits.RelocType, idx, pkgbits.SyncTypeIdx)
+	r.dict = dict
+
+	typ := r.doTyp()
+	assert(typ != nil)
+
+	// See comment in pkgReader.typIdx explaining how this happens.
+	if prev := *where; prev != nil {
+		return prev
+	}
+
+	*where = typ
+	return typ
+}
+
+func (r *reader) doTyp() (res types.Type) {
+	switch tag := pkgbits.CodeType(r.Code(pkgbits.SyncType)); tag {
+	default:
+		errorf("unhandled type tag: %v", tag)
+		panic("unreachable")
+
+	case pkgbits.TypeBasic:
+		return types.Typ[r.Len()]
+
+	case pkgbits.TypeNamed:
+		obj, targs := r.obj()
+		name := obj.(*types.TypeName)
+		if len(targs) != 0 {
+			t, _ := types.Instantiate(r.p.ctxt, name.Type(), targs, false)
+			return t
+		}
+		return name.Type()
+
+	case pkgbits.TypeTypeParam:
+		return r.dict.tparams[r.Len()]
+
+	case pkgbits.TypeArray:
+		len := int64(r.Uint64())
+		return types.NewArray(r.typ(), len)
+	case pkgbits.TypeChan:
+		dir := types.ChanDir(r.Len())
+		return types.NewChan(dir, r.typ())
+	case pkgbits.TypeMap:
+		return types.NewMap(r.typ(), r.typ())
+	case pkgbits.TypePointer:
+		return types.NewPointer(r.typ())
+	case pkgbits.TypeSignature:
+		return r.signature(nil, nil, nil)
+	case pkgbits.TypeSlice:
+		return types.NewSlice(r.typ())
+	case pkgbits.TypeStruct:
+		return r.structType()
+	case pkgbits.TypeInterface:
+		return r.interfaceType()
+	case pkgbits.TypeUnion:
+		return r.unionType()
+	}
+}
+
+func (r *reader) structType() *types.Struct {
+	fields := make([]*types.Var, r.Len())
+	var tags []string
+	for i := range fields {
+		pos := r.pos()
+		pkg, name := r.selector()
+		ftyp := r.typ()
+		tag := r.String()
+		embedded := r.Bool()
+
+		fields[i] = types.NewField(pos, pkg, name, ftyp, embedded)
+		if tag != "" {
+			for len(tags) < i {
+				tags = append(tags, "")
+			}
+			tags = append(tags, tag)
+		}
+	}
+	return types.NewStruct(fields, tags)
+}
+
+func (r *reader) unionType() *types.Union {
+	terms := make([]*types.Term, r.Len())
+	for i := range terms {
+		terms[i] = types.NewTerm(r.Bool(), r.typ())
+	}
+	return types.NewUnion(terms)
+}
+
+func (r *reader) interfaceType() *types.Interface {
+	methods := make([]*types.Func, r.Len())
+	embeddeds := make([]types.Type, r.Len())
+	implicit := len(methods) == 0 && len(embeddeds) == 1 && r.Bool()
+
+	for i := range methods {
+		pos := r.pos()
+		pkg, name := r.selector()
+		mtyp := r.signature(nil, nil, nil)
+		methods[i] = types.NewFunc(pos, pkg, name, mtyp)
+	}
+
+	for i := range embeddeds {
+		embeddeds[i] = r.typ()
+	}
+
+	iface := types.NewInterfaceType(methods, embeddeds)
+	if implicit {
+		iface.MarkImplicit()
+	}
+	return iface
+}
+
+func (r *reader) signature(recv *types.Var, rtparams, tparams []*types.TypeParam) *types.Signature {
+	r.Sync(pkgbits.SyncSignature)
+
+	params := r.params()
+	results := r.params()
+	variadic := r.Bool()
+
+	return types.NewSignatureType(recv, rtparams, tparams, params, results, variadic)
+}
+
+func (r *reader) params() *types.Tuple {
+	r.Sync(pkgbits.SyncParams)
+
+	params := make([]*types.Var, r.Len())
+	for i := range params {
+		params[i] = r.param()
+	}
+
+	return types.NewTuple(params...)
+}
+
+func (r *reader) param() *types.Var {
+	r.Sync(pkgbits.SyncParam)
+
+	pos := r.pos()
+	pkg, name := r.localIdent()
+	typ := r.typ()
+
+	return types.NewParam(pos, pkg, name, typ)
+}
+
+// @@@ Objects
+
+func (r *reader) obj() (types.Object, []types.Type) {
+	r.Sync(pkgbits.SyncObject)
+
+	assert(!r.Bool())
+
+	pkg, name := r.p.objIdx(r.Reloc(pkgbits.RelocObj))
+	obj := pkgScope(pkg).Lookup(name)
+
+	targs := make([]types.Type, r.Len())
+	for i := range targs {
+		targs[i] = r.typ()
+	}
+
+	return obj, targs
+}
+
+func (pr *pkgReader) objIdx(idx pkgbits.Index) (*types.Package, string) {
+	rname := pr.newReader(pkgbits.RelocName, idx, pkgbits.SyncObject1)
+
+	objPkg, objName := rname.qualifiedIdent()
+	assert(objName != "")
+
+	tag := pkgbits.CodeObj(rname.Code(pkgbits.SyncCodeObj))
+
+	if tag == pkgbits.ObjStub {
+		assert(objPkg == nil || objPkg == types.Unsafe)
+		return objPkg, objName
+	}
+
+	if objPkg.Scope().Lookup(objName) == nil {
+		dict := pr.objDictIdx(idx)
+
+		r := pr.newReader(pkgbits.RelocObj, idx, pkgbits.SyncObject1)
+		r.dict = dict
+
+		declare := func(obj types.Object) {
+			objPkg.Scope().Insert(obj)
+		}
+
+		switch tag {
+		default:
+			panic("weird")
+
+		case pkgbits.ObjAlias:
+			pos := r.pos()
+			typ := r.typ()
+			declare(types.NewTypeName(pos, objPkg, objName, typ))
+
+		case pkgbits.ObjConst:
+			pos := r.pos()
+			typ := r.typ()
+			val := r.Value()
+			declare(types.NewConst(pos, objPkg, objName, typ, val))
+
+		case pkgbits.ObjFunc:
+			pos := r.pos()
+			tparams := r.typeParamNames()
+			sig := r.signature(nil, nil, tparams)
+			declare(types.NewFunc(pos, objPkg, objName, sig))
+
+		case pkgbits.ObjType:
+			pos := r.pos()
+
+			obj := types.NewTypeName(pos, objPkg, objName, nil)
+			named := types.NewNamed(obj, nil, nil)
+			declare(obj)
+
+			named.SetTypeParams(r.typeParamNames())
+
+			// TODO(mdempsky): Rewrite receiver types to underlying is an
+			// Interface? The go/types importer does this (I think because
+			// unit tests expected that), but cmd/compile doesn't care
+			// about it, so maybe we can avoid worrying about that here.
+			rhs := r.typ()
+			r.p.later(func() {
+				underlying := rhs.Underlying()
+				named.SetUnderlying(underlying)
+			})
+
+			for i, n := 0, r.Len(); i < n; i++ {
+				named.AddMethod(r.method())
+			}
+
+		case pkgbits.ObjVar:
+			pos := r.pos()
+			typ := r.typ()
+			declare(types.NewVar(pos, objPkg, objName, typ))
+		}
+	}
+
+	return objPkg, objName
+}
+
+func (pr *pkgReader) objDictIdx(idx pkgbits.Index) *readerDict {
+	r := pr.newReader(pkgbits.RelocObjDict, idx, pkgbits.SyncObject1)
+
+	var dict readerDict
+
+	if implicits := r.Len(); implicits != 0 {
+		errorf("unexpected object with %v implicit type parameter(s)", implicits)
+	}
+
+	dict.bounds = make([]typeInfo, r.Len())
+	for i := range dict.bounds {
+		dict.bounds[i] = r.typInfo()
+	}
+
+	dict.derived = make([]derivedInfo, r.Len())
+	dict.derivedTypes = make([]types.Type, len(dict.derived))
+	for i := range dict.derived {
+		dict.derived[i] = derivedInfo{r.Reloc(pkgbits.RelocType), r.Bool()}
+	}
+
+	// function references follow, but reader doesn't need those
+
+	return &dict
+}
+
+func (r *reader) typeParamNames() []*types.TypeParam {
+	r.Sync(pkgbits.SyncTypeParamNames)
+
+	// Note: This code assumes it only processes objects without
+	// implement type parameters. This is currently fine, because
+	// reader is only used to read in exported declarations, which are
+	// always package scoped.
+
+	if len(r.dict.bounds) == 0 {
+		return nil
+	}
+
+	// Careful: Type parameter lists may have cycles. To allow for this,
+	// we construct the type parameter list in two passes: first we
+	// create all the TypeNames and TypeParams, then we construct and
+	// set the bound type.
+
+	r.dict.tparams = make([]*types.TypeParam, len(r.dict.bounds))
+	for i := range r.dict.bounds {
+		pos := r.pos()
+		pkg, name := r.localIdent()
+
+		tname := types.NewTypeName(pos, pkg, name, nil)
+		r.dict.tparams[i] = types.NewTypeParam(tname, nil)
+	}
+
+	typs := make([]types.Type, len(r.dict.bounds))
+	for i, bound := range r.dict.bounds {
+		typs[i] = r.p.typIdx(bound, r.dict)
+	}
+
+	// TODO(mdempsky): This is subtle, elaborate further.
+	//
+	// We have to save tparams outside of the closure, because
+	// typeParamNames() can be called multiple times with the same
+	// dictionary instance.
+	//
+	// Also, this needs to happen later to make sure SetUnderlying has
+	// been called.
+	//
+	// TODO(mdempsky): Is it safe to have a single "later" slice or do
+	// we need to have multiple passes? See comments on CL 386002 and
+	// go.dev/issue/52104.
+	tparams := r.dict.tparams
+	r.p.later(func() {
+		for i, typ := range typs {
+			tparams[i].SetConstraint(typ)
+		}
+	})
+
+	return r.dict.tparams
+}
+
+func (r *reader) method() *types.Func {
+	r.Sync(pkgbits.SyncMethod)
+	pos := r.pos()
+	pkg, name := r.selector()
+
+	rparams := r.typeParamNames()
+	sig := r.signature(r.param(), rparams, nil)
+
+	_ = r.pos() // TODO(mdempsky): Remove; this is a hacker for linker.go.
+	return types.NewFunc(pos, pkg, name, sig)
+}
+
+func (r *reader) qualifiedIdent() (*types.Package, string) { return r.ident(pkgbits.SyncSym) }
+func (r *reader) localIdent() (*types.Package, string)     { return r.ident(pkgbits.SyncLocalIdent) }
+func (r *reader) selector() (*types.Package, string)       { return r.ident(pkgbits.SyncSelector) }
+
+func (r *reader) ident(marker pkgbits.SyncMarker) (*types.Package, string) {
+	r.Sync(marker)
+	return r.pkg(), r.String()
+}
+
+// pkgScope returns pkg.Scope().
+// If pkg is nil, it returns types.Universe instead.
+//
+// TODO(mdempsky): Remove after x/tools can depend on Go 1.19.
+func pkgScope(pkg *types.Package) *types.Scope {
+	if pkg != nil {
+		return pkg.Scope()
+	}
+	return types.Universe
+}
diff --git a/go/internal/pkgbits/codes.go b/go/internal/pkgbits/codes.go
new file mode 100644
index 0000000000000..f0cabde96eba9
--- /dev/null
+++ b/go/internal/pkgbits/codes.go
@@ -0,0 +1,77 @@
+// Copyright 2021 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 pkgbits
+
+// A Code is an enum value that can be encoded into bitstreams.
+//
+// Code types are preferable for enum types, because they allow
+// Decoder to detect desyncs.
+type Code interface {
+	// Marker returns the SyncMarker for the Code's dynamic type.
+	Marker() SyncMarker
+
+	// Value returns the Code's ordinal value.
+	Value() int
+}
+
+// A CodeVal distinguishes among go/constant.Value encodings.
+type CodeVal int
+
+func (c CodeVal) Marker() SyncMarker { return SyncVal }
+func (c CodeVal) Value() int         { return int(c) }
+
+// Note: These values are public and cannot be changed without
+// updating the go/types importers.
+
+const (
+	ValBool CodeVal = iota
+	ValString
+	ValInt64
+	ValBigInt
+	ValBigRat
+	ValBigFloat
+)
+
+// A CodeType distinguishes among go/types.Type encodings.
+type CodeType int
+
+func (c CodeType) Marker() SyncMarker { return SyncType }
+func (c CodeType) Value() int         { return int(c) }
+
+// Note: These values are public and cannot be changed without
+// updating the go/types importers.
+
+const (
+	TypeBasic CodeType = iota
+	TypeNamed
+	TypePointer
+	TypeSlice
+	TypeArray
+	TypeChan
+	TypeMap
+	TypeSignature
+	TypeStruct
+	TypeInterface
+	TypeUnion
+	TypeTypeParam
+)
+
+// A CodeObj distinguishes among go/types.Object encodings.
+type CodeObj int
+
+func (c CodeObj) Marker() SyncMarker { return SyncCodeObj }
+func (c CodeObj) Value() int         { return int(c) }
+
+// Note: These values are public and cannot be changed without
+// updating the go/types importers.
+
+const (
+	ObjAlias CodeObj = iota
+	ObjConst
+	ObjType
+	ObjFunc
+	ObjVar
+	ObjStub
+)
diff --git a/go/internal/pkgbits/decoder.go b/go/internal/pkgbits/decoder.go
new file mode 100644
index 0000000000000..2bc793668ec90
--- /dev/null
+++ b/go/internal/pkgbits/decoder.go
@@ -0,0 +1,433 @@
+// Copyright 2021 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 pkgbits
+
+import (
+	"encoding/binary"
+	"fmt"
+	"go/constant"
+	"go/token"
+	"math/big"
+	"os"
+	"runtime"
+	"strings"
+)
+
+// A PkgDecoder provides methods for decoding a package's Unified IR
+// export data.
+type PkgDecoder struct {
+	// version is the file format version.
+	version uint32
+
+	// sync indicates whether the file uses sync markers.
+	sync bool
+
+	// pkgPath is the package path for the package to be decoded.
+	//
+	// TODO(mdempsky): Remove; unneeded since CL 391014.
+	pkgPath string
+
+	// elemData is the full data payload of the encoded package.
+	// Elements are densely and contiguously packed together.
+	//
+	// The last 8 bytes of elemData are the package fingerprint.
+	elemData string
+
+	// elemEnds stores the byte-offset end positions of element
+	// bitstreams within elemData.
+	//
+	// For example, element I's bitstream data starts at elemEnds[I-1]
+	// (or 0, if I==0) and ends at elemEnds[I].
+	//
+	// Note: elemEnds is indexed by absolute indices, not
+	// section-relative indices.
+	elemEnds []uint32
+
+	// elemEndsEnds stores the index-offset end positions of relocation
+	// sections within elemEnds.
+	//
+	// For example, section K's end positions start at elemEndsEnds[K-1]
+	// (or 0, if K==0) and end at elemEndsEnds[K].
+	elemEndsEnds [numRelocs]uint32
+}
+
+// PkgPath returns the package path for the package
+//
+// TODO(mdempsky): Remove; unneeded since CL 391014.
+func (pr *PkgDecoder) PkgPath() string { return pr.pkgPath }
+
+// SyncMarkers reports whether pr uses sync markers.
+func (pr *PkgDecoder) SyncMarkers() bool { return pr.sync }
+
+// NewPkgDecoder returns a PkgDecoder initialized to read the Unified
+// IR export data from input. pkgPath is the package path for the
+// compilation unit that produced the export data.
+//
+// TODO(mdempsky): Remove pkgPath parameter; unneeded since CL 391014.
+func NewPkgDecoder(pkgPath, input string) PkgDecoder {
+	pr := PkgDecoder{
+		pkgPath: pkgPath,
+	}
+
+	// TODO(mdempsky): Implement direct indexing of input string to
+	// avoid copying the position information.
+
+	r := strings.NewReader(input)
+
+	assert(binary.Read(r, binary.LittleEndian, &pr.version) == nil)
+
+	switch pr.version {
+	default:
+		panic(fmt.Errorf("unsupported version: %v", pr.version))
+	case 0:
+		// no flags
+	case 1:
+		var flags uint32
+		assert(binary.Read(r, binary.LittleEndian, &flags) == nil)
+		pr.sync = flags&flagSyncMarkers != 0
+	}
+
+	assert(binary.Read(r, binary.LittleEndian, pr.elemEndsEnds[:]) == nil)
+
+	pr.elemEnds = make([]uint32, pr.elemEndsEnds[len(pr.elemEndsEnds)-1])
+	assert(binary.Read(r, binary.LittleEndian, pr.elemEnds[:]) == nil)
+
+	pos, err := r.Seek(0, os.SEEK_CUR)
+	assert(err == nil)
+
+	pr.elemData = input[pos:]
+	assert(len(pr.elemData)-8 == int(pr.elemEnds[len(pr.elemEnds)-1]))
+
+	return pr
+}
+
+// NumElems returns the number of elements in section k.
+func (pr *PkgDecoder) NumElems(k RelocKind) int {
+	count := int(pr.elemEndsEnds[k])
+	if k > 0 {
+		count -= int(pr.elemEndsEnds[k-1])
+	}
+	return count
+}
+
+// TotalElems returns the total number of elements across all sections.
+func (pr *PkgDecoder) TotalElems() int {
+	return len(pr.elemEnds)
+}
+
+// Fingerprint returns the package fingerprint.
+func (pr *PkgDecoder) Fingerprint() [8]byte {
+	var fp [8]byte
+	copy(fp[:], pr.elemData[len(pr.elemData)-8:])
+	return fp
+}
+
+// AbsIdx returns the absolute index for the given (section, index)
+// pair.
+func (pr *PkgDecoder) AbsIdx(k RelocKind, idx Index) int {
+	absIdx := int(idx)
+	if k > 0 {
+		absIdx += int(pr.elemEndsEnds[k-1])
+	}
+	if absIdx >= int(pr.elemEndsEnds[k]) {
+		errorf("%v:%v is out of bounds; %v", k, idx, pr.elemEndsEnds)
+	}
+	return absIdx
+}
+
+// DataIdx returns the raw element bitstream for the given (section,
+// index) pair.
+func (pr *PkgDecoder) DataIdx(k RelocKind, idx Index) string {
+	absIdx := pr.AbsIdx(k, idx)
+
+	var start uint32
+	if absIdx > 0 {
+		start = pr.elemEnds[absIdx-1]
+	}
+	end := pr.elemEnds[absIdx]
+
+	return pr.elemData[start:end]
+}
+
+// StringIdx returns the string value for the given string index.
+func (pr *PkgDecoder) StringIdx(idx Index) string {
+	return pr.DataIdx(RelocString, idx)
+}
+
+// NewDecoder returns a Decoder for the given (section, index) pair,
+// and decodes the given SyncMarker from the element bitstream.
+func (pr *PkgDecoder) NewDecoder(k RelocKind, idx Index, marker SyncMarker) Decoder {
+	r := pr.NewDecoderRaw(k, idx)
+	r.Sync(marker)
+	return r
+}
+
+// NewDecoderRaw returns a Decoder for the given (section, index) pair.
+//
+// Most callers should use NewDecoder instead.
+func (pr *PkgDecoder) NewDecoderRaw(k RelocKind, idx Index) Decoder {
+	r := Decoder{
+		common: pr,
+		k:      k,
+		Idx:    idx,
+	}
+
+	// TODO(mdempsky) r.data.Reset(...) after #44505 is resolved.
+	r.Data = *strings.NewReader(pr.DataIdx(k, idx))
+
+	r.Sync(SyncRelocs)
+	r.Relocs = make([]RelocEnt, r.Len())
+	for i := range r.Relocs {
+		r.Sync(SyncReloc)
+		r.Relocs[i] = RelocEnt{RelocKind(r.Len()), Index(r.Len())}
+	}
+
+	return r
+}
+
+// A Decoder provides methods for decoding an individual element's
+// bitstream data.
+type Decoder struct {
+	common *PkgDecoder
+
+	Relocs []RelocEnt
+	Data   strings.Reader
+
+	k   RelocKind
+	Idx Index
+}
+
+func (r *Decoder) checkErr(err error) {
+	if err != nil {
+		errorf("unexpected decoding error: %w", err)
+	}
+}
+
+func (r *Decoder) rawUvarint() uint64 {
+	x, err := binary.ReadUvarint(&r.Data)
+	r.checkErr(err)
+	return x
+}
+
+func (r *Decoder) rawVarint() int64 {
+	ux := r.rawUvarint()
+
+	// Zig-zag decode.
+	x := int64(ux >> 1)
+	if ux&1 != 0 {
+		x = ^x
+	}
+	return x
+}
+
+func (r *Decoder) rawReloc(k RelocKind, idx int) Index {
+	e := r.Relocs[idx]
+	assert(e.Kind == k)
+	return e.Idx
+}
+
+// Sync decodes a sync marker from the element bitstream and asserts
+// that it matches the expected marker.
+//
+// If r.common.sync is false, then Sync is a no-op.
+func (r *Decoder) Sync(mWant SyncMarker) {
+	if !r.common.sync {
+		return
+	}
+
+	pos, _ := r.Data.Seek(0, os.SEEK_CUR) // TODO(mdempsky): io.SeekCurrent after #44505 is resolved
+	mHave := SyncMarker(r.rawUvarint())
+	writerPCs := make([]int, r.rawUvarint())
+	for i := range writerPCs {
+		writerPCs[i] = int(r.rawUvarint())
+	}
+
+	if mHave == mWant {
+		return
+	}
+
+	// There's some tension here between printing:
+	//
+	// (1) full file paths that tools can recognize (e.g., so emacs
+	//     hyperlinks the "file:line" text for easy navigation), or
+	//
+	// (2) short file paths that are easier for humans to read (e.g., by
+	//     omitting redundant or irrelevant details, so it's easier to
+	//     focus on the useful bits that remain).
+	//
+	// The current formatting favors the former, as it seems more
+	// helpful in practice. But perhaps the formatting could be improved
+	// to better address both concerns. For example, use relative file
+	// paths if they would be shorter, or rewrite file paths to contain
+	// "$GOROOT" (like objabi.AbsFile does) if tools can be taught how
+	// to reliably expand that again.
+
+	fmt.Printf("export data desync: package %q, section %v, index %v, offset %v\n", r.common.pkgPath, r.k, r.Idx, pos)
+
+	fmt.Printf("\nfound %v, written at:\n", mHave)
+	if len(writerPCs) == 0 {
+		fmt.Printf("\t[stack trace unavailable; recompile package %q with -d=syncframes]\n", r.common.pkgPath)
+	}
+	for _, pc := range writerPCs {
+		fmt.Printf("\t%s\n", r.common.StringIdx(r.rawReloc(RelocString, pc)))
+	}
+
+	fmt.Printf("\nexpected %v, reading at:\n", mWant)
+	var readerPCs [32]uintptr // TODO(mdempsky): Dynamically size?
+	n := runtime.Callers(2, readerPCs[:])
+	for _, pc := range fmtFrames(readerPCs[:n]...) {
+		fmt.Printf("\t%s\n", pc)
+	}
+
+	// We already printed a stack trace for the reader, so now we can
+	// simply exit. Printing a second one with panic or base.Fatalf
+	// would just be noise.
+	os.Exit(1)
+}
+
+// Bool decodes and returns a bool value from the element bitstream.
+func (r *Decoder) Bool() bool {
+	r.Sync(SyncBool)
+	x, err := r.Data.ReadByte()
+	r.checkErr(err)
+	assert(x < 2)
+	return x != 0
+}
+
+// Int64 decodes and returns an int64 value from the element bitstream.
+func (r *Decoder) Int64() int64 {
+	r.Sync(SyncInt64)
+	return r.rawVarint()
+}
+
+// Int64 decodes and returns a uint64 value from the element bitstream.
+func (r *Decoder) Uint64() uint64 {
+	r.Sync(SyncUint64)
+	return r.rawUvarint()
+}
+
+// Len decodes and returns a non-negative int value from the element bitstream.
+func (r *Decoder) Len() int { x := r.Uint64(); v := int(x); assert(uint64(v) == x); return v }
+
+// Int decodes and returns an int value from the element bitstream.
+func (r *Decoder) Int() int { x := r.Int64(); v := int(x); assert(int64(v) == x); return v }
+
+// Uint decodes and returns a uint value from the element bitstream.
+func (r *Decoder) Uint() uint { x := r.Uint64(); v := uint(x); assert(uint64(v) == x); return v }
+
+// Code decodes a Code value from the element bitstream and returns
+// its ordinal value. It's the caller's responsibility to convert the
+// result to an appropriate Code type.
+//
+// TODO(mdempsky): Ideally this method would have signature "Code[T
+// Code] T" instead, but we don't allow generic methods and the
+// compiler can't depend on generics yet anyway.
+func (r *Decoder) Code(mark SyncMarker) int {
+	r.Sync(mark)
+	return r.Len()
+}
+
+// Reloc decodes a relocation of expected section k from the element
+// bitstream and returns an index to the referenced element.
+func (r *Decoder) Reloc(k RelocKind) Index {
+	r.Sync(SyncUseReloc)
+	return r.rawReloc(k, r.Len())
+}
+
+// String decodes and returns a string value from the element
+// bitstream.
+func (r *Decoder) String() string {
+	r.Sync(SyncString)
+	return r.common.StringIdx(r.Reloc(RelocString))
+}
+
+// Strings decodes and returns a variable-length slice of strings from
+// the element bitstream.
+func (r *Decoder) Strings() []string {
+	res := make([]string, r.Len())
+	for i := range res {
+		res[i] = r.String()
+	}
+	return res
+}
+
+// Value decodes and returns a constant.Value from the element
+// bitstream.
+func (r *Decoder) Value() constant.Value {
+	r.Sync(SyncValue)
+	isComplex := r.Bool()
+	val := r.scalar()
+	if isComplex {
+		val = constant.BinaryOp(val, token.ADD, constant.MakeImag(r.scalar()))
+	}
+	return val
+}
+
+func (r *Decoder) scalar() constant.Value {
+	switch tag := CodeVal(r.Code(SyncVal)); tag {
+	default:
+		panic(fmt.Errorf("unexpected scalar tag: %v", tag))
+
+	case ValBool:
+		return constant.MakeBool(r.Bool())
+	case ValString:
+		return constant.MakeString(r.String())
+	case ValInt64:
+		return constant.MakeInt64(r.Int64())
+	case ValBigInt:
+		return constant.Make(r.bigInt())
+	case ValBigRat:
+		num := r.bigInt()
+		denom := r.bigInt()
+		return constant.Make(new(big.Rat).SetFrac(num, denom))
+	case ValBigFloat:
+		return constant.Make(r.bigFloat())
+	}
+}
+
+func (r *Decoder) bigInt() *big.Int {
+	v := new(big.Int).SetBytes([]byte(r.String()))
+	if r.Bool() {
+		v.Neg(v)
+	}
+	return v
+}
+
+func (r *Decoder) bigFloat() *big.Float {
+	v := new(big.Float).SetPrec(512)
+	assert(v.UnmarshalText([]byte(r.String())) == nil)
+	return v
+}
+
+// @@@ Helpers
+
+// TODO(mdempsky): These should probably be removed. I think they're a
+// smell that the export data format is not yet quite right.
+
+// PeekPkgPath returns the package path for the specified package
+// index.
+func (pr *PkgDecoder) PeekPkgPath(idx Index) string {
+	r := pr.NewDecoder(RelocPkg, idx, SyncPkgDef)
+	path := r.String()
+	if path == "" {
+		path = pr.pkgPath
+	}
+	return path
+}
+
+// PeekObj returns the package path, object name, and CodeObj for the
+// specified object index.
+func (pr *PkgDecoder) PeekObj(idx Index) (string, string, CodeObj) {
+	r := pr.NewDecoder(RelocName, idx, SyncObject1)
+	r.Sync(SyncSym)
+	r.Sync(SyncPkg)
+	path := pr.PeekPkgPath(r.Reloc(RelocPkg))
+	name := r.String()
+	assert(name != "")
+
+	tag := CodeObj(r.Code(SyncCodeObj))
+
+	return path, name, tag
+}
diff --git a/go/internal/pkgbits/doc.go b/go/internal/pkgbits/doc.go
new file mode 100644
index 0000000000000..c8a2796b5e4cb
--- /dev/null
+++ b/go/internal/pkgbits/doc.go
@@ -0,0 +1,32 @@
+// Copyright 2022 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 pkgbits implements low-level coding abstractions for
+// Unified IR's export data format.
+//
+// At a low-level, a package is a collection of bitstream elements.
+// Each element has a "kind" and a dense, non-negative index.
+// Elements can be randomly accessed given their kind and index.
+//
+// Individual elements are sequences of variable-length values (e.g.,
+// integers, booleans, strings, go/constant values, cross-references
+// to other elements). Package pkgbits provides APIs for encoding and
+// decoding these low-level values, but the details of mapping
+// higher-level Go constructs into elements is left to higher-level
+// abstractions.
+//
+// Elements may cross-reference each other with "relocations." For
+// example, an element representing a pointer type has a relocation
+// referring to the element type.
+//
+// Go constructs may be composed as a constellation of multiple
+// elements. For example, a declared function may have one element to
+// describe the object (e.g., its name, type, position), and a
+// separate element to describe its function body. This allows readers
+// some flexibility in efficiently seeking or re-reading data (e.g.,
+// inlining requires re-reading the function body for each inlined
+// call, without needing to re-read the object-level details).
+//
+// This is a copy of internal/pkgbits in the Go implementation.
+package pkgbits
diff --git a/go/internal/pkgbits/encoder.go b/go/internal/pkgbits/encoder.go
new file mode 100644
index 0000000000000..c50c838caaecd
--- /dev/null
+++ b/go/internal/pkgbits/encoder.go
@@ -0,0 +1,379 @@
+// Copyright 2021 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 pkgbits
+
+import (
+	"bytes"
+	"crypto/md5"
+	"encoding/binary"
+	"go/constant"
+	"io"
+	"math/big"
+	"runtime"
+)
+
+// currentVersion is the current version number.
+//
+//   - v0: initial prototype
+//
+//   - v1: adds the flags uint32 word
+const currentVersion uint32 = 1
+
+// A PkgEncoder provides methods for encoding a package's Unified IR
+// export data.
+type PkgEncoder struct {
+	// elems holds the bitstream for previously encoded elements.
+	elems [numRelocs][]string
+
+	// stringsIdx maps previously encoded strings to their index within
+	// the RelocString section, to allow deduplication. That is,
+	// elems[RelocString][stringsIdx[s]] == s (if present).
+	stringsIdx map[string]Index
+
+	// syncFrames is the number of frames to write at each sync
+	// marker. A negative value means sync markers are omitted.
+	syncFrames int
+}
+
+// SyncMarkers reports whether pw uses sync markers.
+func (pw *PkgEncoder) SyncMarkers() bool { return pw.syncFrames >= 0 }
+
+// NewPkgEncoder returns an initialized PkgEncoder.
+//
+// syncFrames is the number of caller frames that should be serialized
+// at Sync points. Serializing additional frames results in larger
+// export data files, but can help diagnosing desync errors in
+// higher-level Unified IR reader/writer code. If syncFrames is
+// negative, then sync markers are omitted entirely.
+func NewPkgEncoder(syncFrames int) PkgEncoder {
+	return PkgEncoder{
+		stringsIdx: make(map[string]Index),
+		syncFrames: syncFrames,
+	}
+}
+
+// DumpTo writes the package's encoded data to out0 and returns the
+// package fingerprint.
+func (pw *PkgEncoder) DumpTo(out0 io.Writer) (fingerprint [8]byte) {
+	h := md5.New()
+	out := io.MultiWriter(out0, h)
+
+	writeUint32 := func(x uint32) {
+		assert(binary.Write(out, binary.LittleEndian, x) == nil)
+	}
+
+	writeUint32(currentVersion)
+
+	var flags uint32
+	if pw.SyncMarkers() {
+		flags |= flagSyncMarkers
+	}
+	writeUint32(flags)
+
+	// Write elemEndsEnds.
+	var sum uint32
+	for _, elems := range &pw.elems {
+		sum += uint32(len(elems))
+		writeUint32(sum)
+	}
+
+	// Write elemEnds.
+	sum = 0
+	for _, elems := range &pw.elems {
+		for _, elem := range elems {
+			sum += uint32(len(elem))
+			writeUint32(sum)
+		}
+	}
+
+	// Write elemData.
+	for _, elems := range &pw.elems {
+		for _, elem := range elems {
+			_, err := io.WriteString(out, elem)
+			assert(err == nil)
+		}
+	}
+
+	// Write fingerprint.
+	copy(fingerprint[:], h.Sum(nil))
+	_, err := out0.Write(fingerprint[:])
+	assert(err == nil)
+
+	return
+}
+
+// StringIdx adds a string value to the strings section, if not
+// already present, and returns its index.
+func (pw *PkgEncoder) StringIdx(s string) Index {
+	if idx, ok := pw.stringsIdx[s]; ok {
+		assert(pw.elems[RelocString][idx] == s)
+		return idx
+	}
+
+	idx := Index(len(pw.elems[RelocString]))
+	pw.elems[RelocString] = append(pw.elems[RelocString], s)
+	pw.stringsIdx[s] = idx
+	return idx
+}
+
+// NewEncoder returns an Encoder for a new element within the given
+// section, and encodes the given SyncMarker as the start of the
+// element bitstream.
+func (pw *PkgEncoder) NewEncoder(k RelocKind, marker SyncMarker) Encoder {
+	e := pw.NewEncoderRaw(k)
+	e.Sync(marker)
+	return e
+}
+
+// NewEncoderRaw returns an Encoder for a new element within the given
+// section.
+//
+// Most callers should use NewEncoder instead.
+func (pw *PkgEncoder) NewEncoderRaw(k RelocKind) Encoder {
+	idx := Index(len(pw.elems[k]))
+	pw.elems[k] = append(pw.elems[k], "") // placeholder
+
+	return Encoder{
+		p:   pw,
+		k:   k,
+		Idx: idx,
+	}
+}
+
+// An Encoder provides methods for encoding an individual element's
+// bitstream data.
+type Encoder struct {
+	p *PkgEncoder
+
+	Relocs []RelocEnt
+	Data   bytes.Buffer // accumulated element bitstream data
+
+	encodingRelocHeader bool
+
+	k   RelocKind
+	Idx Index // index within relocation section
+}
+
+// Flush finalizes the element's bitstream and returns its Index.
+func (w *Encoder) Flush() Index {
+	var sb bytes.Buffer // TODO(mdempsky): strings.Builder after #44505 is resolved
+
+	// Backup the data so we write the relocations at the front.
+	var tmp bytes.Buffer
+	io.Copy(&tmp, &w.Data)
+
+	// TODO(mdempsky): Consider writing these out separately so they're
+	// easier to strip, along with function bodies, so that we can prune
+	// down to just the data that's relevant to go/types.
+	if w.encodingRelocHeader {
+		panic("encodingRelocHeader already true; recursive flush?")
+	}
+	w.encodingRelocHeader = true
+	w.Sync(SyncRelocs)
+	w.Len(len(w.Relocs))
+	for _, rEnt := range w.Relocs {
+		w.Sync(SyncReloc)
+		w.Len(int(rEnt.Kind))
+		w.Len(int(rEnt.Idx))
+	}
+
+	io.Copy(&sb, &w.Data)
+	io.Copy(&sb, &tmp)
+	w.p.elems[w.k][w.Idx] = sb.String()
+
+	return w.Idx
+}
+
+func (w *Encoder) checkErr(err error) {
+	if err != nil {
+		errorf("unexpected encoding error: %v", err)
+	}
+}
+
+func (w *Encoder) rawUvarint(x uint64) {
+	var buf [binary.MaxVarintLen64]byte
+	n := binary.PutUvarint(buf[:], x)
+	_, err := w.Data.Write(buf[:n])
+	w.checkErr(err)
+}
+
+func (w *Encoder) rawVarint(x int64) {
+	// Zig-zag encode.
+	ux := uint64(x) << 1
+	if x < 0 {
+		ux = ^ux
+	}
+
+	w.rawUvarint(ux)
+}
+
+func (w *Encoder) rawReloc(r RelocKind, idx Index) int {
+	// TODO(mdempsky): Use map for lookup; this takes quadratic time.
+	for i, rEnt := range w.Relocs {
+		if rEnt.Kind == r && rEnt.Idx == idx {
+			return i
+		}
+	}
+
+	i := len(w.Relocs)
+	w.Relocs = append(w.Relocs, RelocEnt{r, idx})
+	return i
+}
+
+func (w *Encoder) Sync(m SyncMarker) {
+	if !w.p.SyncMarkers() {
+		return
+	}
+
+	// Writing out stack frame string references requires working
+	// relocations, but writing out the relocations themselves involves
+	// sync markers. To prevent infinite recursion, we simply trim the
+	// stack frame for sync markers within the relocation header.
+	var frames []string
+	if !w.encodingRelocHeader && w.p.syncFrames > 0 {
+		pcs := make([]uintptr, w.p.syncFrames)
+		n := runtime.Callers(2, pcs)
+		frames = fmtFrames(pcs[:n]...)
+	}
+
+	// TODO(mdempsky): Save space by writing out stack frames as a
+	// linked list so we can share common stack frames.
+	w.rawUvarint(uint64(m))
+	w.rawUvarint(uint64(len(frames)))
+	for _, frame := range frames {
+		w.rawUvarint(uint64(w.rawReloc(RelocString, w.p.StringIdx(frame))))
+	}
+}
+
+// Bool encodes and writes a bool value into the element bitstream,
+// and then returns the bool value.
+//
+// For simple, 2-alternative encodings, the idiomatic way to call Bool
+// is something like:
+//
+//	if w.Bool(x != 0) {
+//		// alternative #1
+//	} else {
+//		// alternative #2
+//	}
+//
+// For multi-alternative encodings, use Code instead.
+func (w *Encoder) Bool(b bool) bool {
+	w.Sync(SyncBool)
+	var x byte
+	if b {
+		x = 1
+	}
+	err := w.Data.WriteByte(x)
+	w.checkErr(err)
+	return b
+}
+
+// Int64 encodes and writes an int64 value into the element bitstream.
+func (w *Encoder) Int64(x int64) {
+	w.Sync(SyncInt64)
+	w.rawVarint(x)
+}
+
+// Uint64 encodes and writes a uint64 value into the element bitstream.
+func (w *Encoder) Uint64(x uint64) {
+	w.Sync(SyncUint64)
+	w.rawUvarint(x)
+}
+
+// Len encodes and writes a non-negative int value into the element bitstream.
+func (w *Encoder) Len(x int) { assert(x >= 0); w.Uint64(uint64(x)) }
+
+// Int encodes and writes an int value into the element bitstream.
+func (w *Encoder) Int(x int) { w.Int64(int64(x)) }
+
+// Len encodes and writes a uint value into the element bitstream.
+func (w *Encoder) Uint(x uint) { w.Uint64(uint64(x)) }
+
+// Reloc encodes and writes a relocation for the given (section,
+// index) pair into the element bitstream.
+//
+// Note: Only the index is formally written into the element
+// bitstream, so bitstream decoders must know from context which
+// section an encoded relocation refers to.
+func (w *Encoder) Reloc(r RelocKind, idx Index) {
+	w.Sync(SyncUseReloc)
+	w.Len(w.rawReloc(r, idx))
+}
+
+// Code encodes and writes a Code value into the element bitstream.
+func (w *Encoder) Code(c Code) {
+	w.Sync(c.Marker())
+	w.Len(c.Value())
+}
+
+// String encodes and writes a string value into the element
+// bitstream.
+//
+// Internally, strings are deduplicated by adding them to the strings
+// section (if not already present), and then writing a relocation
+// into the element bitstream.
+func (w *Encoder) String(s string) {
+	w.Sync(SyncString)
+	w.Reloc(RelocString, w.p.StringIdx(s))
+}
+
+// Strings encodes and writes a variable-length slice of strings into
+// the element bitstream.
+func (w *Encoder) Strings(ss []string) {
+	w.Len(len(ss))
+	for _, s := range ss {
+		w.String(s)
+	}
+}
+
+// Value encodes and writes a constant.Value into the element
+// bitstream.
+func (w *Encoder) Value(val constant.Value) {
+	w.Sync(SyncValue)
+	if w.Bool(val.Kind() == constant.Complex) {
+		w.scalar(constant.Real(val))
+		w.scalar(constant.Imag(val))
+	} else {
+		w.scalar(val)
+	}
+}
+
+func (w *Encoder) scalar(val constant.Value) {
+	switch v := constant.Val(val).(type) {
+	default:
+		errorf("unhandled %v (%v)", val, val.Kind())
+	case bool:
+		w.Code(ValBool)
+		w.Bool(v)
+	case string:
+		w.Code(ValString)
+		w.String(v)
+	case int64:
+		w.Code(ValInt64)
+		w.Int64(v)
+	case *big.Int:
+		w.Code(ValBigInt)
+		w.bigInt(v)
+	case *big.Rat:
+		w.Code(ValBigRat)
+		w.bigInt(v.Num())
+		w.bigInt(v.Denom())
+	case *big.Float:
+		w.Code(ValBigFloat)
+		w.bigFloat(v)
+	}
+}
+
+func (w *Encoder) bigInt(v *big.Int) {
+	b := v.Bytes()
+	w.String(string(b)) // TODO: More efficient encoding.
+	w.Bool(v.Sign() < 0)
+}
+
+func (w *Encoder) bigFloat(v *big.Float) {
+	b := v.Append(nil, 'p', -1)
+	w.String(string(b)) // TODO: More efficient encoding.
+}
diff --git a/go/internal/pkgbits/flags.go b/go/internal/pkgbits/flags.go
new file mode 100644
index 0000000000000..654222745facd
--- /dev/null
+++ b/go/internal/pkgbits/flags.go
@@ -0,0 +1,9 @@
+// Copyright 2022 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 pkgbits
+
+const (
+	flagSyncMarkers = 1 << iota // file format contains sync markers
+)
diff --git a/go/internal/pkgbits/frames_go1.go b/go/internal/pkgbits/frames_go1.go
new file mode 100644
index 0000000000000..5294f6a63edd7
--- /dev/null
+++ b/go/internal/pkgbits/frames_go1.go
@@ -0,0 +1,21 @@
+// Copyright 2021 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.
+
+//go:build !go1.7
+// +build !go1.7
+
+// TODO(mdempsky): Remove after #44505 is resolved
+
+package pkgbits
+
+import "runtime"
+
+func walkFrames(pcs []uintptr, visit frameVisitor) {
+	for _, pc := range pcs {
+		fn := runtime.FuncForPC(pc)
+		file, line := fn.FileLine(pc)
+
+		visit(file, line, fn.Name(), pc-fn.Entry())
+	}
+}
diff --git a/go/internal/pkgbits/frames_go17.go b/go/internal/pkgbits/frames_go17.go
new file mode 100644
index 0000000000000..2324ae7adfe20
--- /dev/null
+++ b/go/internal/pkgbits/frames_go17.go
@@ -0,0 +1,28 @@
+// Copyright 2021 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.
+
+//go:build go1.7
+// +build go1.7
+
+package pkgbits
+
+import "runtime"
+
+// walkFrames calls visit for each call frame represented by pcs.
+//
+// pcs should be a slice of PCs, as returned by runtime.Callers.
+func walkFrames(pcs []uintptr, visit frameVisitor) {
+	if len(pcs) == 0 {
+		return
+	}
+
+	frames := runtime.CallersFrames(pcs)
+	for {
+		frame, more := frames.Next()
+		visit(frame.File, frame.Line, frame.Function, frame.PC-frame.Entry)
+		if !more {
+			return
+		}
+	}
+}
diff --git a/go/internal/pkgbits/reloc.go b/go/internal/pkgbits/reloc.go
new file mode 100644
index 0000000000000..7a8f04ab3fc66
--- /dev/null
+++ b/go/internal/pkgbits/reloc.go
@@ -0,0 +1,42 @@
+// Copyright 2021 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 pkgbits
+
+// A RelocKind indicates a particular section within a unified IR export.
+type RelocKind int
+
+// An Index represents a bitstream element index within a particular
+// section.
+type Index int
+
+// A relocEnt (relocation entry) is an entry in an element's local
+// reference table.
+//
+// TODO(mdempsky): Rename this too.
+type RelocEnt struct {
+	Kind RelocKind
+	Idx  Index
+}
+
+// Reserved indices within the meta relocation section.
+const (
+	PublicRootIdx  Index = 0
+	PrivateRootIdx Index = 1
+)
+
+const (
+	RelocString RelocKind = iota
+	RelocMeta
+	RelocPosBase
+	RelocPkg
+	RelocName
+	RelocType
+	RelocObj
+	RelocObjExt
+	RelocObjDict
+	RelocBody
+
+	numRelocs = iota
+)
diff --git a/go/internal/pkgbits/support.go b/go/internal/pkgbits/support.go
new file mode 100644
index 0000000000000..ad26d3b28cae1
--- /dev/null
+++ b/go/internal/pkgbits/support.go
@@ -0,0 +1,17 @@
+// Copyright 2022 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 pkgbits
+
+import "fmt"
+
+func assert(b bool) {
+	if !b {
+		panic("assertion failed")
+	}
+}
+
+func errorf(format string, args ...interface{}) {
+	panic(fmt.Errorf(format, args...))
+}
diff --git a/go/internal/pkgbits/sync.go b/go/internal/pkgbits/sync.go
new file mode 100644
index 0000000000000..5bd51ef717007
--- /dev/null
+++ b/go/internal/pkgbits/sync.go
@@ -0,0 +1,113 @@
+// Copyright 2021 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 pkgbits
+
+import (
+	"fmt"
+	"strings"
+)
+
+// fmtFrames formats a backtrace for reporting reader/writer desyncs.
+func fmtFrames(pcs ...uintptr) []string {
+	res := make([]string, 0, len(pcs))
+	walkFrames(pcs, func(file string, line int, name string, offset uintptr) {
+		// Trim package from function name. It's just redundant noise.
+		name = strings.TrimPrefix(name, "cmd/compile/internal/noder.")
+
+		res = append(res, fmt.Sprintf("%s:%v: %s +0x%v", file, line, name, offset))
+	})
+	return res
+}
+
+type frameVisitor func(file string, line int, name string, offset uintptr)
+
+// SyncMarker is an enum type that represents markers that may be
+// written to export data to ensure the reader and writer stay
+// synchronized.
+type SyncMarker int
+
+//go:generate stringer -type=SyncMarker -trimprefix=Sync
+
+const (
+	_ SyncMarker = iota
+
+	// Public markers (known to go/types importers).
+
+	// Low-level coding markers.
+	SyncEOF
+	SyncBool
+	SyncInt64
+	SyncUint64
+	SyncString
+	SyncValue
+	SyncVal
+	SyncRelocs
+	SyncReloc
+	SyncUseReloc
+
+	// Higher-level object and type markers.
+	SyncPublic
+	SyncPos
+	SyncPosBase
+	SyncObject
+	SyncObject1
+	SyncPkg
+	SyncPkgDef
+	SyncMethod
+	SyncType
+	SyncTypeIdx
+	SyncTypeParamNames
+	SyncSignature
+	SyncParams
+	SyncParam
+	SyncCodeObj
+	SyncSym
+	SyncLocalIdent
+	SyncSelector
+
+	// Private markers (only known to cmd/compile).
+	SyncPrivate
+
+	SyncFuncExt
+	SyncVarExt
+	SyncTypeExt
+	SyncPragma
+
+	SyncExprList
+	SyncExprs
+	SyncExpr
+	SyncExprType
+	SyncAssign
+	SyncOp
+	SyncFuncLit
+	SyncCompLit
+
+	SyncDecl
+	SyncFuncBody
+	SyncOpenScope
+	SyncCloseScope
+	SyncCloseAnotherScope
+	SyncDeclNames
+	SyncDeclName
+
+	SyncStmts
+	SyncBlockStmt
+	SyncIfStmt
+	SyncForStmt
+	SyncSwitchStmt
+	SyncRangeStmt
+	SyncCaseClause
+	SyncCommClause
+	SyncSelectStmt
+	SyncDecls
+	SyncLabeledStmt
+	SyncUseObjLocal
+	SyncAddLocal
+	SyncLinkname
+	SyncStmt1
+	SyncStmtsEnd
+	SyncLabel
+	SyncOptLabel
+)
diff --git a/go/internal/pkgbits/syncmarker_string.go b/go/internal/pkgbits/syncmarker_string.go
new file mode 100644
index 0000000000000..4a5b0ca5f2ffc
--- /dev/null
+++ b/go/internal/pkgbits/syncmarker_string.go
@@ -0,0 +1,89 @@
+// Code generated by "stringer -type=SyncMarker -trimprefix=Sync"; DO NOT EDIT.
+
+package pkgbits
+
+import "strconv"
+
+func _() {
+	// An "invalid array index" compiler error signifies that the constant values have changed.
+	// Re-run the stringer command to generate them again.
+	var x [1]struct{}
+	_ = x[SyncEOF-1]
+	_ = x[SyncBool-2]
+	_ = x[SyncInt64-3]
+	_ = x[SyncUint64-4]
+	_ = x[SyncString-5]
+	_ = x[SyncValue-6]
+	_ = x[SyncVal-7]
+	_ = x[SyncRelocs-8]
+	_ = x[SyncReloc-9]
+	_ = x[SyncUseReloc-10]
+	_ = x[SyncPublic-11]
+	_ = x[SyncPos-12]
+	_ = x[SyncPosBase-13]
+	_ = x[SyncObject-14]
+	_ = x[SyncObject1-15]
+	_ = x[SyncPkg-16]
+	_ = x[SyncPkgDef-17]
+	_ = x[SyncMethod-18]
+	_ = x[SyncType-19]
+	_ = x[SyncTypeIdx-20]
+	_ = x[SyncTypeParamNames-21]
+	_ = x[SyncSignature-22]
+	_ = x[SyncParams-23]
+	_ = x[SyncParam-24]
+	_ = x[SyncCodeObj-25]
+	_ = x[SyncSym-26]
+	_ = x[SyncLocalIdent-27]
+	_ = x[SyncSelector-28]
+	_ = x[SyncPrivate-29]
+	_ = x[SyncFuncExt-30]
+	_ = x[SyncVarExt-31]
+	_ = x[SyncTypeExt-32]
+	_ = x[SyncPragma-33]
+	_ = x[SyncExprList-34]
+	_ = x[SyncExprs-35]
+	_ = x[SyncExpr-36]
+	_ = x[SyncExprType-37]
+	_ = x[SyncAssign-38]
+	_ = x[SyncOp-39]
+	_ = x[SyncFuncLit-40]
+	_ = x[SyncCompLit-41]
+	_ = x[SyncDecl-42]
+	_ = x[SyncFuncBody-43]
+	_ = x[SyncOpenScope-44]
+	_ = x[SyncCloseScope-45]
+	_ = x[SyncCloseAnotherScope-46]
+	_ = x[SyncDeclNames-47]
+	_ = x[SyncDeclName-48]
+	_ = x[SyncStmts-49]
+	_ = x[SyncBlockStmt-50]
+	_ = x[SyncIfStmt-51]
+	_ = x[SyncForStmt-52]
+	_ = x[SyncSwitchStmt-53]
+	_ = x[SyncRangeStmt-54]
+	_ = x[SyncCaseClause-55]
+	_ = x[SyncCommClause-56]
+	_ = x[SyncSelectStmt-57]
+	_ = x[SyncDecls-58]
+	_ = x[SyncLabeledStmt-59]
+	_ = x[SyncUseObjLocal-60]
+	_ = x[SyncAddLocal-61]
+	_ = x[SyncLinkname-62]
+	_ = x[SyncStmt1-63]
+	_ = x[SyncStmtsEnd-64]
+	_ = x[SyncLabel-65]
+	_ = x[SyncOptLabel-66]
+}
+
+const _SyncMarker_name = "EOFBoolInt64Uint64StringValueValRelocsRelocUseRelocPublicPosPosBaseObjectObject1PkgPkgDefMethodTypeTypeIdxTypeParamNamesSignatureParamsParamCodeObjSymLocalIdentSelectorPrivateFuncExtVarExtTypeExtPragmaExprListExprsExprExprTypeAssignOpFuncLitCompLitDeclFuncBodyOpenScopeCloseScopeCloseAnotherScopeDeclNamesDeclNameStmtsBlockStmtIfStmtForStmtSwitchStmtRangeStmtCaseClauseCommClauseSelectStmtDeclsLabeledStmtUseObjLocalAddLocalLinknameStmt1StmtsEndLabelOptLabel"
+
+var _SyncMarker_index = [...]uint16{0, 3, 7, 12, 18, 24, 29, 32, 38, 43, 51, 57, 60, 67, 73, 80, 83, 89, 95, 99, 106, 120, 129, 135, 140, 147, 150, 160, 168, 175, 182, 188, 195, 201, 209, 214, 218, 226, 232, 234, 241, 248, 252, 260, 269, 279, 296, 305, 313, 318, 327, 333, 340, 350, 359, 369, 379, 389, 394, 405, 416, 424, 432, 437, 445, 450, 458}
+
+func (i SyncMarker) String() string {
+	i -= 1
+	if i < 0 || i >= SyncMarker(len(_SyncMarker_index)-1) {
+		return "SyncMarker(" + strconv.FormatInt(int64(i+1), 10) + ")"
+	}
+	return _SyncMarker_name[_SyncMarker_index[i]:_SyncMarker_index[i+1]]
+}