writer.go

// 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 noder

import (
	"fmt"
	"go/constant"
	"go/token"
	"go/version"
	"internal/buildcfg"
	"internal/pkgbits"
	"os"
	"strings"

	"cmd/compile/internal/base"
	"cmd/compile/internal/ir"
	"cmd/compile/internal/syntax"
	"cmd/compile/internal/types"
	"cmd/compile/internal/types2"
)

// This file implements the Unified IR package writer and defines the
// Unified IR export data format.
//
// Low-level coding details (e.g., byte-encoding of individual
// primitive values, or handling element bitstreams and
// cross-references) are handled by internal/pkgbits, so here we only
// concern ourselves with higher-level worries like mapping Go
// language constructs into elements.

// There are two central types in the writing process: the "writer"
// type handles writing out individual elements, while the "pkgWriter"
// type keeps track of which elements have already been created.
//
// For each sort of "thing" (e.g., position, package, object, type)
// that can be written into the export data, there are generally
// several methods that work together:
//
// - writer.thing handles writing out a *use* of a thing, which often
//   means writing a relocation to that thing's encoded index.
//
// - pkgWriter.thingIdx handles reserving an index for a thing, and
//   writing out any elements needed for the thing.
//
// - writer.doThing handles writing out the *definition* of a thing,
//   which in general is a mix of low-level coding primitives (e.g.,
//   ints and strings) or uses of other things.
//
// A design goal of Unified IR is to have a single, canonical writer
// implementation, but multiple reader implementations each tailored
// to their respective needs. For example, within cmd/compile's own
// backend, inlining is implemented largely by just re-running the
// function body reading code.

// TODO(mdempsky): Add an importer for Unified IR to the x/tools repo,
// and better document the file format boundary between public and
// private data.

type index = pkgbits.Index

func assert(p bool) { base.Assert(p) }

// A pkgWriter constructs Unified IR export data from the results of
// running the types2 type checker on a Go compilation unit.
type pkgWriter struct {
	pkgbits.PkgEncoder

	m                     posMap
	curpkg                *types2.Package
	info                  *types2.Info
	rangeFuncBodyClosures map[*syntax.FuncLit]bool // non-public information, e.g., which functions are closures range function bodies?

	// Indices for previously written syntax and types2 things.

	posBasesIdx map[*syntax.PosBase]index
	pkgsIdx     map[*types2.Package]index
	typsIdx     map[types2.Type]index
	objsIdx     map[types2.Object]index

	// Maps from types2.Objects back to their syntax.Decl.

	funDecls map[*types2.Func]*syntax.FuncDecl
	typDecls map[*types2.TypeName]typeDeclGen

	// linknames maps package-scope objects to their linker symbol name,
	// if specified by a //go:linkname directive.
	linknames map[types2.Object]string

	// cgoPragmas accumulates any //go:cgo_* pragmas that need to be
	// passed through to cmd/link.
	cgoPragmas [][]string
}

// newPkgWriter returns an initialized pkgWriter for the specified
// package.
func newPkgWriter(m posMap, pkg *types2.Package, info *types2.Info, otherInfo map[*syntax.FuncLit]bool) *pkgWriter {
	return &pkgWriter{
		PkgEncoder: pkgbits.NewPkgEncoder(base.Debug.SyncFrames),

		m:                     m,
		curpkg:                pkg,
		info:                  info,
		rangeFuncBodyClosures: otherInfo,

		pkgsIdx: make(map[*types2.Package]index),
		objsIdx: make(map[types2.Object]index),
		typsIdx: make(map[types2.Type]index),

		posBasesIdx: make(map[*syntax.PosBase]index),

		funDecls: make(map[*types2.Func]*syntax.FuncDecl),
		typDecls: make(map[*types2.TypeName]typeDeclGen),

		linknames: make(map[types2.Object]string),
	}
}

// errorf reports a user error about thing p.
func (pw *pkgWriter) errorf(p poser, msg string, args ...interface{}) {
	base.ErrorfAt(pw.m.pos(p), 0, msg, args...)
}

// fatalf reports an internal compiler error about thing p.
func (pw *pkgWriter) fatalf(p poser, msg string, args ...interface{}) {
	base.FatalfAt(pw.m.pos(p), msg, args...)
}

// unexpected reports a fatal error about a thing of unexpected
// dynamic type.
func (pw *pkgWriter) unexpected(what string, p poser) {
	pw.fatalf(p, "unexpected %s: %v (%T)", what, p, p)
}

func (pw *pkgWriter) typeAndValue(x syntax.Expr) syntax.TypeAndValue {
	tv, ok := pw.maybeTypeAndValue(x)
	if !ok {
		pw.fatalf(x, "missing Types entry: %v", syntax.String(x))
	}
	return tv
}

func (pw *pkgWriter) maybeTypeAndValue(x syntax.Expr) (syntax.TypeAndValue, bool) {
	tv := x.GetTypeInfo()

	// If x is a generic function whose type arguments are inferred
	// from assignment context, then we need to find its inferred type
	// in Info.Instances instead.
	if name, ok := x.(*syntax.Name); ok {
		if inst, ok := pw.info.Instances[name]; ok {
			tv.Type = inst.Type
		}
	}

	return tv, tv.Type != nil
}

// typeOf returns the Type of the given value expression.
func (pw *pkgWriter) typeOf(expr syntax.Expr) types2.Type {
	tv := pw.typeAndValue(expr)
	if !tv.IsValue() {
		pw.fatalf(expr, "expected value: %v", syntax.String(expr))
	}
	return tv.Type
}

// A writer provides APIs for writing out an individual element.
type writer struct {
	p *pkgWriter

	pkgbits.Encoder

	// sig holds the signature for the current function body, if any.
	sig *types2.Signature

	// TODO(mdempsky): We should be able to prune localsIdx whenever a
	// scope closes, and then maybe we can just use the same map for
	// storing the TypeParams too (as their TypeName instead).

	// localsIdx tracks any local variables declared within this
	// function body. It's unused for writing out non-body things.
	localsIdx map[*types2.Var]int

	// closureVars tracks any free variables that are referenced by this
	// function body. It's unused for writing out non-body things.
	closureVars    []posVar
	closureVarsIdx map[*types2.Var]int // index of previously seen free variables

	dict *writerDict

	// derived tracks whether the type being written out references any
	// type parameters. It's unused for writing non-type things.
	derived bool
}

// A writerDict tracks types and objects that are used by a declaration.
type writerDict struct {
	// implicits is a slice of type parameters from the enclosing
	// declarations.
	implicits []*types2.TypeParam

	// derived is a slice of type indices for computing derived types
	// (i.e., types that depend on the declaration's type parameters).
	derived []derivedInfo

	// derivedIdx maps a Type to its corresponding index within the
	// derived slice, if present.
	derivedIdx map[types2.Type]index

	// These slices correspond to entries in the runtime dictionary.
	typeParamMethodExprs []writerMethodExprInfo
	subdicts             []objInfo
	rtypes               []typeInfo
	itabs                []itabInfo
}

type itabInfo struct {
	typ   typeInfo
	iface typeInfo
}

// typeParamIndex returns the index of the given type parameter within
// the dictionary. This may differ from typ.Index() when there are
// implicit type parameters due to defined types declared within a
// generic function or method.
func (dict *writerDict) typeParamIndex(typ *types2.TypeParam) int {
	for idx, implicit := range dict.implicits {
		if implicit == typ {
			return idx
		}
	}

	return len(dict.implicits) + typ.Index()
}

// A derivedInfo represents a reference to an encoded generic Go type.
type derivedInfo struct {
	idx    index
	needed bool // TODO(mdempsky): Remove.
}

// A typeInfo represents a reference to an encoded Go type.
//
// If derived is true, then the typeInfo represents a generic Go type
// that contains type parameters. In this case, idx is an index into
// the readerDict.derived{,Types} arrays.
//
// Otherwise, the typeInfo represents a non-generic Go type, and idx
// is an index into the reader.typs array instead.
type typeInfo struct {
	idx     index
	derived bool
}

// An objInfo represents a reference to an encoded, instantiated (if
// applicable) Go object.
type objInfo struct {
	idx       index      // index for the generic function declaration
	explicits []typeInfo // info for the type arguments
}

// A selectorInfo represents a reference to an encoded field or method
// name (i.e., objects that can only be accessed using selector
// expressions).
type selectorInfo struct {
	pkgIdx  index
	nameIdx index
}

// anyDerived reports whether any of info's explicit type arguments
// are derived types.
func (info objInfo) anyDerived() bool {
	for _, explicit := range info.explicits {
		if explicit.derived {
			return true
		}
	}
	return false
}

// equals reports whether info and other represent the same Go object
// (i.e., same base object and identical type arguments, if any).
func (info objInfo) equals(other objInfo) bool {
	if info.idx != other.idx {
		return false
	}
	assert(len(info.explicits) == len(other.explicits))
	for i, targ := range info.explicits {
		if targ != other.explicits[i] {
			return false
		}
	}
	return true
}

type writerMethodExprInfo struct {
	typeParamIdx int
	methodInfo   selectorInfo
}

// typeParamMethodExprIdx returns the index where the given encoded
// method expression function pointer appears within this dictionary's
// type parameters method expressions section, adding it if necessary.
func (dict *writerDict) typeParamMethodExprIdx(typeParamIdx int, methodInfo selectorInfo) int {
	newInfo := writerMethodExprInfo{typeParamIdx, methodInfo}

	for idx, oldInfo := range dict.typeParamMethodExprs {
		if oldInfo == newInfo {
			return idx
		}
	}

	idx := len(dict.typeParamMethodExprs)
	dict.typeParamMethodExprs = append(dict.typeParamMethodExprs, newInfo)
	return idx
}

// subdictIdx returns the index where the given encoded object's
// runtime dictionary appears within this dictionary's subdictionary
// section, adding it if necessary.
func (dict *writerDict) subdictIdx(newInfo objInfo) int {
	for idx, oldInfo := range dict.subdicts {
		if oldInfo.equals(newInfo) {
			return idx
		}
	}

	idx := len(dict.subdicts)
	dict.subdicts = append(dict.subdicts, newInfo)
	return idx
}

// rtypeIdx returns the index where the given encoded type's
// *runtime._type value appears within this dictionary's rtypes
// section, adding it if necessary.
func (dict *writerDict) rtypeIdx(newInfo typeInfo) int {
	for idx, oldInfo := range dict.rtypes {
		if oldInfo == newInfo {
			return idx
		}
	}

	idx := len(dict.rtypes)
	dict.rtypes = append(dict.rtypes, newInfo)
	return idx
}

// itabIdx returns the index where the given encoded type pair's
// *runtime.itab value appears within this dictionary's itabs section,
// adding it if necessary.
func (dict *writerDict) itabIdx(typInfo, ifaceInfo typeInfo) int {
	newInfo := itabInfo{typInfo, ifaceInfo}

	for idx, oldInfo := range dict.itabs {
		if oldInfo == newInfo {
			return idx
		}
	}

	idx := len(dict.itabs)
	dict.itabs = append(dict.itabs, newInfo)
	return idx
}

func (pw *pkgWriter) newWriter(k pkgbits.RelocKind, marker pkgbits.SyncMarker) *writer {
	return &writer{
		Encoder: pw.NewEncoder(k, marker),
		p:       pw,
	}
}

// @@@ Positions

// pos writes the position of p into the element bitstream.
func (w *writer) pos(p poser) {
	w.Sync(pkgbits.SyncPos)
	pos := p.Pos()

	// TODO(mdempsky): Track down the remaining cases here and fix them.
	if !w.Bool(pos.IsKnown()) {
		return
	}

	// TODO(mdempsky): Delta encoding.
	w.posBase(pos.Base())
	w.Uint(pos.Line())
	w.Uint(pos.Col())
}

// posBase writes a reference to the given PosBase into the element
// bitstream.
func (w *writer) posBase(b *syntax.PosBase) {
	w.Reloc(pkgbits.RelocPosBase, w.p.posBaseIdx(b))
}

// posBaseIdx returns the index for the given PosBase.
func (pw *pkgWriter) posBaseIdx(b *syntax.PosBase) index {
	if idx, ok := pw.posBasesIdx[b]; ok {
		return idx
	}

	w := pw.newWriter(pkgbits.RelocPosBase, pkgbits.SyncPosBase)
	w.p.posBasesIdx[b] = w.Idx

	w.String(trimFilename(b))

	if !w.Bool(b.IsFileBase()) {
		w.pos(b)
		w.Uint(b.Line())
		w.Uint(b.Col())
	}

	return w.Flush()
}

// @@@ Packages

// pkg writes a use of the given Package into the element bitstream.
func (w *writer) pkg(pkg *types2.Package) {
	w.pkgRef(w.p.pkgIdx(pkg))
}

func (w *writer) pkgRef(idx index) {
	w.Sync(pkgbits.SyncPkg)
	w.Reloc(pkgbits.RelocPkg, idx)
}

// pkgIdx returns the index for the given package, adding it to the
// package export data if needed.
func (pw *pkgWriter) pkgIdx(pkg *types2.Package) index {
	if idx, ok := pw.pkgsIdx[pkg]; ok {
		return idx
	}

	w := pw.newWriter(pkgbits.RelocPkg, pkgbits.SyncPkgDef)
	pw.pkgsIdx[pkg] = w.Idx

	// The universe and package unsafe need to be handled specially by
	// importers anyway, so we serialize them using just their package
	// path. This ensures that readers don't confuse them for
	// user-defined packages.
	switch pkg {
	case nil: // universe
		w.String("builtin") // same package path used by godoc
	case types2.Unsafe:
		w.String("unsafe")
	default:
		// TODO(mdempsky): Write out pkg.Path() for curpkg too.
		var path string
		if pkg != w.p.curpkg {
			path = pkg.Path()
		}
		base.Assertf(path != "builtin" && path != "unsafe", "unexpected path for user-defined package: %q", path)
		w.String(path)
		w.String(pkg.Name())

		w.Len(len(pkg.Imports()))
		for _, imp := range pkg.Imports() {
			w.pkg(imp)
		}
	}

	return w.Flush()
}

// @@@ Types

var (
	anyTypeName        = types2.Universe.Lookup("any").(*types2.TypeName)
	comparableTypeName = types2.Universe.Lookup("comparable").(*types2.TypeName)
	runeTypeName       = types2.Universe.Lookup("rune").(*types2.TypeName)
)

// typ writes a use of the given type into the bitstream.
func (w *writer) typ(typ types2.Type) {
	w.typInfo(w.p.typIdx(typ, w.dict))
}

// typInfo writes a use of the given type (specified as a typeInfo
// instead) into the bitstream.
func (w *writer) typInfo(info typeInfo) {
	w.Sync(pkgbits.SyncType)
	if w.Bool(info.derived) {
		w.Len(int(info.idx))
		w.derived = true
	} else {
		w.Reloc(pkgbits.RelocType, info.idx)
	}
}

// typIdx returns the index where the export data description of type
// can be read back in. If no such index exists yet, it's created.
//
// typIdx also reports whether typ is a derived type; that is, whether
// its identity depends on type parameters.
func (pw *pkgWriter) typIdx(typ types2.Type, dict *writerDict) typeInfo {
	// Strip non-global aliases, because they only appear in inline
	// bodies anyway. Otherwise, they can cause types.Sym collisions
	// (e.g., "main.C" for both of the local type aliases in
	// test/fixedbugs/issue50190.go).
	for {
		if alias, ok := typ.(*types2.Alias); ok && !isGlobal(alias.Obj()) {
			typ = alias.Rhs()
		} else {
			break
		}
	}

	if idx, ok := pw.typsIdx[typ]; ok {
		return typeInfo{idx: idx, derived: false}
	}
	if dict != nil {
		if idx, ok := dict.derivedIdx[typ]; ok {
			return typeInfo{idx: idx, derived: true}
		}
	}

	w := pw.newWriter(pkgbits.RelocType, pkgbits.SyncTypeIdx)
	w.dict = dict

	switch typ := typ.(type) {
	default:
		base.Fatalf("unexpected type: %v (%T)", typ, typ)

	case *types2.Basic:
		switch kind := typ.Kind(); {
		case kind == types2.Invalid:
			base.Fatalf("unexpected types2.Invalid")

		case types2.Typ[kind] == typ:
			w.Code(pkgbits.TypeBasic)
			w.Len(int(kind))

		default:
			// Handle "byte" and "rune" as references to their TypeNames.
			obj := types2.Universe.Lookup(typ.Name()).(*types2.TypeName)
			assert(obj.Type() == typ)

			w.Code(pkgbits.TypeNamed)
			w.namedType(obj, nil)
		}

	case *types2.Named:
		w.Code(pkgbits.TypeNamed)
		w.namedType(splitNamed(typ))

	case *types2.Alias:
		w.Code(pkgbits.TypeNamed)
		w.namedType(splitAlias(typ))

	case *types2.TypeParam:
		w.derived = true
		w.Code(pkgbits.TypeTypeParam)
		w.Len(w.dict.typeParamIndex(typ))

	case *types2.Array:
		w.Code(pkgbits.TypeArray)
		w.Uint64(uint64(typ.Len()))
		w.typ(typ.Elem())

	case *types2.Chan:
		w.Code(pkgbits.TypeChan)
		w.Len(int(typ.Dir()))
		w.typ(typ.Elem())

	case *types2.Map:
		w.Code(pkgbits.TypeMap)
		w.typ(typ.Key())
		w.typ(typ.Elem())

	case *types2.Pointer:
		w.Code(pkgbits.TypePointer)
		w.typ(typ.Elem())

	case *types2.Signature:
		base.Assertf(typ.TypeParams() == nil, "unexpected type params: %v", typ)
		w.Code(pkgbits.TypeSignature)
		w.signature(typ)

	case *types2.Slice:
		w.Code(pkgbits.TypeSlice)
		w.typ(typ.Elem())

	case *types2.Struct:
		w.Code(pkgbits.TypeStruct)
		w.structType(typ)

	case *types2.Interface:
		// Handle "any" as reference to its TypeName.
		// The underlying "any" interface is canonical, so this logic handles both
		// GODEBUG=gotypesalias=1 (when any is represented as a types2.Alias), and
		// gotypesalias=0.
		if types2.Unalias(typ) == types2.Unalias(anyTypeName.Type()) {
			w.Code(pkgbits.TypeNamed)
			w.obj(anyTypeName, nil)
			break
		}

		w.Code(pkgbits.TypeInterface)
		w.interfaceType(typ)

	case *types2.Union:
		w.Code(pkgbits.TypeUnion)
		w.unionType(typ)
	}

	if w.derived {
		idx := index(len(dict.derived))
		dict.derived = append(dict.derived, derivedInfo{idx: w.Flush()})
		dict.derivedIdx[typ] = idx
		return typeInfo{idx: idx, derived: true}
	}

	pw.typsIdx[typ] = w.Idx
	return typeInfo{idx: w.Flush(), derived: false}
}

// namedType writes a use of the given named type into the bitstream.
func (w *writer) namedType(obj *types2.TypeName, targs *types2.TypeList) {
	// Named types that are declared within a generic function (and
	// thus have implicit type parameters) are always derived types.
	if w.p.hasImplicitTypeParams(obj) {
		w.derived = true
	}

	w.obj(obj, targs)
}

func (w *writer) structType(typ *types2.Struct) {
	w.Len(typ.NumFields())
	for i := 0; i < typ.NumFields(); i++ {
		f := typ.Field(i)
		w.pos(f)
		w.selector(f)
		w.typ(f.Type())
		w.String(typ.Tag(i))
		w.Bool(f.Embedded())
	}
}

func (w *writer) unionType(typ *types2.Union) {
	w.Len(typ.Len())
	for i := 0; i < typ.Len(); i++ {
		t := typ.Term(i)
		w.Bool(t.Tilde())
		w.typ(t.Type())
	}
}

func (w *writer) interfaceType(typ *types2.Interface) {
	// If typ has no embedded types but it's not a basic interface, then
	// the natural description we write out below will fail to
	// reconstruct it.
	if typ.NumEmbeddeds() == 0 && !typ.IsMethodSet() {
		// Currently, this can only happen for the underlying Interface of
		// "comparable", which is needed to handle type declarations like
		// "type C comparable".
		assert(typ == comparableTypeName.Type().(*types2.Named).Underlying())

		// Export as "interface{ comparable }".
		w.Len(0)                         // NumExplicitMethods
		w.Len(1)                         // NumEmbeddeds
		w.Bool(false)                    // IsImplicit
		w.typ(comparableTypeName.Type()) // EmbeddedType(0)
		return
	}

	w.Len(typ.NumExplicitMethods())
	w.Len(typ.NumEmbeddeds())

	if typ.NumExplicitMethods() == 0 && typ.NumEmbeddeds() == 1 {
		w.Bool(typ.IsImplicit())
	} else {
		// Implicit interfaces always have 0 explicit methods and 1
		// embedded type, so we skip writing out the implicit flag
		// otherwise as a space optimization.
		assert(!typ.IsImplicit())
	}

	for i := 0; i < typ.NumExplicitMethods(); i++ {
		m := typ.ExplicitMethod(i)
		sig := m.Type().(*types2.Signature)
		assert(sig.TypeParams() == nil)

		w.pos(m)
		w.selector(m)
		w.signature(sig)
	}

	for i := 0; i < typ.NumEmbeddeds(); i++ {
		w.typ(typ.EmbeddedType(i))
	}
}

func (w *writer) signature(sig *types2.Signature) {
	w.Sync(pkgbits.SyncSignature)
	w.params(sig.Params())
	w.params(sig.Results())
	w.Bool(sig.Variadic())
}

func (w *writer) params(typ *types2.Tuple) {
	w.Sync(pkgbits.SyncParams)
	w.Len(typ.Len())
	for i := 0; i < typ.Len(); i++ {
		w.param(typ.At(i))
	}
}

func (w *writer) param(param *types2.Var) {
	w.Sync(pkgbits.SyncParam)
	w.pos(param)
	w.localIdent(param)
	w.typ(param.Type())
}

// @@@ Objects

// obj writes a use of the given object into the bitstream.
//
// If obj is a generic object, then explicits are the explicit type
// arguments used to instantiate it (i.e., used to substitute the
// object's own declared type parameters).
func (w *writer) obj(obj types2.Object, explicits *types2.TypeList) {
	w.objInfo(w.p.objInstIdx(obj, explicits, w.dict))
}

// objInfo writes a use of the given encoded object into the
// bitstream.
func (w *writer) objInfo(info objInfo) {
	w.Sync(pkgbits.SyncObject)
	if w.Version().Has(pkgbits.DerivedFuncInstance) {
		w.Bool(false)
	}
	w.Reloc(pkgbits.RelocObj, info.idx)

	w.Len(len(info.explicits))
	for _, info := range info.explicits {
		w.typInfo(info)
	}
}

// objInstIdx returns the indices for an object and a corresponding
// list of type arguments used to instantiate it, adding them to the
// export data as needed.
func (pw *pkgWriter) objInstIdx(obj types2.Object, explicits *types2.TypeList, dict *writerDict) objInfo {
	explicitInfos := make([]typeInfo, explicits.Len())
	for i := range explicitInfos {
		explicitInfos[i] = pw.typIdx(explicits.At(i), dict)
	}
	return objInfo{idx: pw.objIdx(obj), explicits: explicitInfos}
}

// objIdx returns the index for the given Object, adding it to the
// export data as needed.
func (pw *pkgWriter) objIdx(obj types2.Object) index {
	// TODO(mdempsky): Validate that obj is a global object (or a local
	// defined type, which we hoist to global scope anyway).

	if idx, ok := pw.objsIdx[obj]; ok {
		return idx
	}

	dict := &writerDict{
		derivedIdx: make(map[types2.Type]index),
	}

	if isDefinedType(obj) && obj.Pkg() == pw.curpkg {
		decl, ok := pw.typDecls[obj.(*types2.TypeName)]
		assert(ok)
		dict.implicits = decl.implicits
	}

	// We encode objects into 4 elements across different sections, all
	// sharing the same index:
	//
	// - RelocName has just the object's qualified name (i.e.,
	//   Object.Pkg and Object.Name) and the CodeObj indicating what
	//   specific type of Object it is (Var, Func, etc).
	//
	// - RelocObj has the remaining public details about the object,
	//   relevant to go/types importers.
	//
	// - RelocObjExt has additional private details about the object,
	//   which are only relevant to cmd/compile itself. This is
	//   separated from RelocObj so that go/types importers are
	//   unaffected by internal compiler changes.
	//
	// - RelocObjDict has public details about the object's type
	//   parameters and derived type's used by the object. This is
	//   separated to facilitate the eventual introduction of
	//   shape-based stenciling.
	//
	// TODO(mdempsky): Re-evaluate whether RelocName still makes sense
	// to keep separate from RelocObj.

	w := pw.newWriter(pkgbits.RelocObj, pkgbits.SyncObject1)
	wext := pw.newWriter(pkgbits.RelocObjExt, pkgbits.SyncObject1)
	wname := pw.newWriter(pkgbits.RelocName, pkgbits.SyncObject1)
	wdict := pw.newWriter(pkgbits.RelocObjDict, pkgbits.SyncObject1)

	pw.objsIdx[obj] = w.Idx // break cycles
	assert(wext.Idx == w.Idx)
	assert(wname.Idx == w.Idx)
	assert(wdict.Idx == w.Idx)

	w.dict = dict
	wext.dict = dict

	code := w.doObj(wext, obj)
	w.Flush()
	wext.Flush()

	wname.qualifiedIdent(obj)
	wname.Code(code)
	wname.Flush()

	wdict.objDict(obj, w.dict)
	wdict.Flush()

	return w.Idx
}

// doObj writes the RelocObj definition for obj to w, and the
// RelocObjExt definition to wext.
func (w *writer) doObj(wext *writer, obj types2.Object) pkgbits.CodeObj {
	if obj.Pkg() != w.p.curpkg {
		return pkgbits.ObjStub
	}

	switch obj := obj.(type) {
	default:
		w.p.unexpected("object", obj)
		panic("unreachable")

	case *types2.Const:
		w.pos(obj)
		w.typ(obj.Type())
		w.Value(obj.Val())
		return pkgbits.ObjConst

	case *types2.Func:
		decl, ok := w.p.funDecls[obj]
		assert(ok)
		sig := obj.Type().(*types2.Signature)

		w.pos(obj)
		w.typeParamNames(sig.TypeParams())
		w.signature(sig)
		w.pos(decl)
		wext.funcExt(obj)
		return pkgbits.ObjFunc

	case *types2.TypeName:
		if obj.IsAlias() {
			w.pos(obj)
			t := obj.Type()
			if alias, ok := t.(*types2.Alias); ok { // materialized alias
				t = alias.Rhs()
			}
			w.typ(t)
			return pkgbits.ObjAlias
		}

		named := obj.Type().(*types2.Named)
		assert(named.TypeArgs() == nil)

		w.pos(obj)
		w.typeParamNames(named.TypeParams())
		wext.typeExt(obj)
		w.typ(named.Underlying())

		w.Len(named.NumMethods())
		for i := 0; i < named.NumMethods(); i++ {
			w.method(wext, named.Method(i))
		}

		return pkgbits.ObjType

	case *types2.Var:
		w.pos(obj)
		w.typ(obj.Type())
		wext.varExt(obj)
		return pkgbits.ObjVar
	}
}

// objDict writes the dictionary needed for reading the given object.
func (w *writer) objDict(obj types2.Object, dict *writerDict) {
	// TODO(mdempsky): Split objDict into multiple entries? reader.go
	// doesn't care about the type parameter bounds, and reader2.go
	// doesn't care about referenced functions.

	w.dict = dict // TODO(mdempsky): This is a bit sketchy.

	w.Len(len(dict.implicits))

	tparams := objTypeParams(obj)
	ntparams := tparams.Len()
	w.Len(ntparams)
	for i := 0; i < ntparams; i++ {
		w.typ(tparams.At(i).Constraint())
	}

	nderived := len(dict.derived)
	w.Len(nderived)
	for _, typ := range dict.derived {
		w.Reloc(pkgbits.RelocType, typ.idx)
		w.Bool(typ.needed)
	}

	// Write runtime dictionary information.
	//
	// N.B., the go/types importer reads up to the section, but doesn't
	// read any further, so it's safe to change. (See TODO above.)

	// For each type parameter, write out whether the constraint is a
	// basic interface. This is used to determine how aggressively we
	// can shape corresponding type arguments.
	//
	// This is somewhat redundant with writing out the full type
	// parameter constraints above, but the compiler currently skips
	// over those. Also, we don't care about the *declared* constraints,
	// but how the type parameters are actually *used*. E.g., if a type
	// parameter is constrained to `int | uint` but then never used in
	// arithmetic/conversions/etc, we could shape those together.
	for _, implicit := range dict.implicits {
		w.Bool(implicit.Underlying().(*types2.Interface).IsMethodSet())
	}
	for i := 0; i < ntparams; i++ {
		tparam := tparams.At(i)
		w.Bool(tparam.Underlying().(*types2.Interface).IsMethodSet())
	}

	w.Len(len(dict.typeParamMethodExprs))
	for _, info := range dict.typeParamMethodExprs {
		w.Len(info.typeParamIdx)
		w.selectorInfo(info.methodInfo)
	}

	w.Len(len(dict.subdicts))
	for _, info := range dict.subdicts {
		w.objInfo(info)
	}

	w.Len(len(dict.rtypes))
	for _, info := range dict.rtypes {
		w.typInfo(info)
	}

	w.Len(len(dict.itabs))
	for _, info := range dict.itabs {
		w.typInfo(info.typ)
		w.typInfo(info.iface)
	}

	assert(len(dict.derived) == nderived)
}

func (w *writer) typeParamNames(tparams *types2.TypeParamList) {
	w.Sync(pkgbits.SyncTypeParamNames)

	ntparams := tparams.Len()
	for i := 0; i < ntparams; i++ {
		tparam := tparams.At(i).Obj()
		w.pos(tparam)
		w.localIdent(tparam)
	}
}

func (w *writer) method(wext *writer, meth *types2.Func) {
	decl, ok := w.p.funDecls[meth]
	assert(ok)
	sig := meth.Type().(*types2.Signature)

	w.Sync(pkgbits.SyncMethod)
	w.pos(meth)
	w.selector(meth)
	w.typeParamNames(sig.RecvTypeParams())
	w.param(sig.Recv())
	w.signature(sig)

	w.pos(decl) // XXX: Hack to workaround linker limitations.
	wext.funcExt(meth)
}

// qualifiedIdent writes out the name of an object declared at package
// scope. (For now, it's also used to refer to local defined types.)
func (w *writer) qualifiedIdent(obj types2.Object) {
	w.Sync(pkgbits.SyncSym)

	name := obj.Name()
	if isDefinedType(obj) && obj.Pkg() == w.p.curpkg {
		decl, ok := w.p.typDecls[obj.(*types2.TypeName)]
		assert(ok)
		if decl.gen != 0 {
			// For local defined types, we embed a scope-disambiguation
			// number directly into their name. types.SplitVargenSuffix then
			// knows to look for this.