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"
	"internal/pkgbits"

	"cmd/compile/internal/base"
	"cmd/compile/internal/ir"
	"cmd/compile/internal/syntax"
	"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.

// 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

	// Indices for previously written syntax and types2 things.

	posBasesIdx map[*syntax.PosBase]pkgbits.Index
	pkgsIdx     map[*types2.Package]pkgbits.Index
	typsIdx     map[types2.Type]pkgbits.Index
	objsIdx     map[types2.Object]pkgbits.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) *pkgWriter {
	return &pkgWriter{
		PkgEncoder: pkgbits.NewPkgEncoder(base.Debug.SyncFrames),

		m:      m,
		curpkg: pkg,
		info:   info,

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

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

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

	pkgbits.Encoder

	// 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 []*types2.TypeName

	// 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]pkgbits.Index

	// funcs lists references to generic functions that were
	// instantiated with derived types (i.e., that require
	// sub-dictionaries when called at run time).
	funcs []objInfo

	// itabs lists itabs that are needed for dynamic type assertions
	// (including type switches).
	itabs []itabInfo
}

// A derivedInfo represents a reference to an encoded generic Go type.
type derivedInfo struct {
	idx    pkgbits.Index
	needed bool
}

// 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     pkgbits.Index
	derived bool
}

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

// An itabInfo represents a reference to an encoded itab entry (i.e.,
// a non-empty interface type along with a concrete type that
// implements that interface).
//
// itabInfo is only used for
type itabInfo struct {
	typIdx pkgbits.Index // always a derived type index
	iface  typeInfo      // always a non-empty interface type
}

// 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
}

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) pkgbits.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.Sync(pkgbits.SyncPkg)
	w.Reloc(pkgbits.RelocPkg, w.p.pkgIdx(pkg))
}

// pkgIdx returns the index for the given package, adding it to the
// package export data if needed.
func (pw *pkgWriter) pkgIdx(pkg *types2.Package) pkgbits.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(0) // was package height, but not necessary anymore.

		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)

// 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 {
	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 TypeName.
			obj := types2.Universe.Lookup(typ.Name())
			assert(obj.Type() == typ)

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

	case *types2.Named:
		assert(typ.TypeParams().Len() == typ.TypeArgs().Len())

		// TODO(mdempsky): Why do we need to loop here?
		orig := typ
		for orig.TypeArgs() != nil {
			orig = orig.Origin()
		}

		w.Code(pkgbits.TypeNamed)
		w.obj(orig.Obj(), typ.TypeArgs())

	case *types2.TypeParam:
		index := func() int {
			for idx, name := range w.dict.implicits {
				if name.Type().(*types2.TypeParam) == typ {
					return idx
				}
			}

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

		w.derived = true
		w.Code(pkgbits.TypeTypeParam)
		w.Len(index)

	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:
		if typ == 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 := pkgbits.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}
}

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) {
	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) {
	explicitInfos := make([]typeInfo, explicits.Len())
	for i := range explicitInfos {
		explicitInfos[i] = w.p.typIdx(explicits.At(i), w.dict)
	}
	info := objInfo{idx: w.p.objIdx(obj), explicits: explicitInfos}

	if _, ok := obj.(*types2.Func); ok && info.anyDerived() {
		idx := -1
		for i, prev := range w.dict.funcs {
			if prev.equals(info) {
				idx = i
			}
		}
		if idx < 0 {
			idx = len(w.dict.funcs)
			w.dict.funcs = append(w.dict.funcs, info)
		}

		// TODO(mdempsky): Push up into expr; this shouldn't appear
		// outside of expression context.
		w.Sync(pkgbits.SyncObject)
		w.Bool(true)
		w.Len(idx)
		return
	}

	// TODO(mdempsky): Push up into typIdx; this shouldn't be needed
	// except while writing out types.
	if isDefinedType(obj) && obj.Pkg() == w.p.curpkg {
		decl, ok := w.p.typDecls[obj.(*types2.TypeName)]
		assert(ok)
		if len(decl.implicits) != 0 {
			w.derived = true
		}
	}

	w.Sync(pkgbits.SyncObject)
	w.Bool(false)
	w.Reloc(pkgbits.RelocObj, info.idx)

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

// objIdx returns the index for the given Object, adding it to the
// export data as needed.
func (pw *pkgWriter) objIdx(obj types2.Object) pkgbits.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]pkgbits.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:
		decl, ok := w.p.typDecls[obj]
		assert(ok)

		if obj.IsAlias() {
			w.pos(obj)
			w.typ(obj.Type())
			return pkgbits.ObjAlias
		}

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

		w.pos(obj)
		w.typeParamNames(named.TypeParams())
		wext.typeExt(obj)
		w.typExpr(decl.Type)

		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
	}
}

// typExpr writes the type represented by the given expression.
//
// TODO(mdempsky): Document how this differs from exprType.
func (w *writer) typExpr(expr syntax.Expr) {
	tv, ok := w.p.info.Types[expr]
	assert(ok)
	assert(tv.IsType())
	w.typ(tv.Type)
}

// 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)
	}

	nfuncs := len(dict.funcs)
	w.Len(nfuncs)
	for _, fn := range dict.funcs {
		w.Reloc(pkgbits.RelocObj, fn.idx)
		w.Len(len(fn.explicits))
		for _, targ := range fn.explicits {
			w.typInfo(targ)
		}
	}

	nitabs := len(dict.itabs)
	w.Len(nitabs)
	for _, itab := range dict.itabs {
		w.Len(int(itab.typIdx))
		w.typInfo(itab.iface)
	}

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

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 {
			// TODO(mdempsky): Find a better solution than embedding middle
			// dot in the symbol name; this is terrible.
			name = fmt.Sprintf("%s·%v", name, decl.gen)
		}
	}

	w.pkg(obj.Pkg())
	w.String(name)
}

// TODO(mdempsky): We should be able to omit pkg from both localIdent
// and selector, because they should always be known from context.
// However, past frustrations with this optimization in iexport make
// me a little nervous to try it again.

// localIdent writes the name of a locally declared object (i.e.,
// objects that can only be accessed by non-qualified name, within the
// context of a particular function).
func (w *writer) localIdent(obj types2.Object) {
	assert(!isGlobal(obj))
	w.Sync(pkgbits.SyncLocalIdent)
	w.pkg(obj.Pkg())
	w.String(obj.Name())
}

// selector writes the name of a field or method (i.e., objects that
// can only be accessed using selector expressions).
func (w *writer) selector(obj types2.Object) {
	w.Sync(pkgbits.SyncSelector)
	w.pkg(obj.Pkg())
	w.String(obj.Name())
}

// @@@ Compiler extensions

func (w *writer) funcExt(obj *types2.Func) {
	decl, ok := w.p.funDecls[obj]
	assert(ok)

	// TODO(mdempsky): Extend these pragma validation flags to account
	// for generics. E.g., linkname probably doesn't make sense at
	// least.

	pragma := asPragmaFlag(decl.Pragma)
	if pragma&ir.Systemstack != 0 && pragma&ir.Nosplit != 0 {
		w.p.errorf(decl, "go:nosplit and go:systemstack cannot be combined")
	}

	if decl.Body != nil {
		if pragma&ir.Noescape != 0 {
			w.p.errorf(decl, "can only use //go:noescape with external func implementations")
		}
		if (pragma&ir.UintptrKeepAlive != 0 && pragma&ir.UintptrEscapes == 0) && pragma&ir.Nosplit == 0 {
			// Stack growth can't handle uintptr arguments that may
			// be pointers (as we don't know which are pointers
			// when creating the stack map). Thus uintptrkeepalive
			// functions (and all transitive callees) must be
			// nosplit.
			//
			// N.B. uintptrescapes implies uintptrkeepalive but it
			// is OK since the arguments must escape to the heap.
			//
			// TODO(prattmic): Add recursive nosplit check of callees.
			// TODO(prattmic): Functions with no body (i.e.,
			// assembly) must also be nosplit, but we can't check
			// that here.
			w.p.errorf(decl, "go:uintptrkeepalive requires go:nosplit")
		}
	} else {
		if base.Flag.Complete || decl.Name.Value == "init" {
			// Linknamed functions are allowed to have no body. Hopefully
			// the linkname target has a body. See issue 23311.
			if _, ok := w.p.linknames[obj]; !ok {
				w.p.errorf(decl, "missing function body")
			}
		}
	}

	sig, block := obj.Type().(*types2.Signature), decl.Body
	body, closureVars := w.p.bodyIdx(sig, block, w.dict)
	assert(len(closureVars) == 0)

	w.Sync(pkgbits.SyncFuncExt)
	w.pragmaFlag(pragma)
	w.linkname(obj)
	w.Bool(false) // stub extension
	w.Reloc(pkgbits.RelocBody, body)
	w.Sync(pkgbits.SyncEOF)
}

func (w *writer) typeExt(obj *types2.TypeName) {
	decl, ok := w.p.typDecls[obj]
	assert(ok)

	w.Sync(pkgbits.SyncTypeExt)

	w.pragmaFlag(asPragmaFlag(decl.Pragma))

	// No LSym.SymIdx info yet.
	w.Int64(-1)
	w.Int64(-1)
}

func (w *writer) varExt(obj *types2.Var) {
	w.Sync(pkgbits.SyncVarExt)
	w.linkname(obj)
}

func (w *writer) linkname(obj types2.Object) {
	w.Sync(pkgbits.SyncLinkname)
	w.Int64(-1)
	w.String(w.p.linknames[obj])
}

func (w *writer) pragmaFlag(p ir.PragmaFlag) {
	w.Sync(pkgbits.SyncPragma)
	w.Int(int(p))
}

// @@@ Function bodies

// bodyIdx returns the index for the given function body (specified by
// block), adding it to the export data
func (pw *pkgWriter) bodyIdx(sig *types2.Signature, block *syntax.BlockStmt, dict *writerDict) (idx pkgbits.Index, closureVars []posVar) {
	w := pw.newWriter(pkgbits.RelocBody, pkgbits.SyncFuncBody)
	w.dict = dict

	w.funcargs(sig)
	if w.Bool(block != nil) {
		w.stmts(block.List)
		w.pos(block.Rbrace)
	}

	return w.Flush(), w.closureVars
}

func (w *writer) funcargs(sig *types2.Signature) {
	do := func(params *types2.Tuple, result bool) {
		for i := 0; i < params.Len(); i++ {
			w.funcarg(params.At(i), result)
		}
	}

	if recv := sig.Recv(); recv != nil {
		w.funcarg(recv, false)
	}
	do(sig.Params(), false)
	do(sig.Results(), true)
}

func (w *writer) funcarg(param *types2.Var, result bool) {
	if param.Name() != "" || result {
		w.addLocal(param)
	}
}

// addLocal records the declaration of a new local variable.
func (w *writer) addLocal(obj *types2.Var) {
	w.Sync(pkgbits.SyncAddLocal)
	idx := len(w.localsIdx)
	if pkgbits.EnableSync {
		w.Int(idx)
	}
	if w.localsIdx == nil {
		w.localsIdx = make(map[*types2.Var]int)
	}
	w.localsIdx[obj] = idx
}

// useLocal writes a reference to the given local or free variable
// into the bitstream.
func (w *writer) useLocal(pos syntax.Pos, obj *types2.Var) {
	w.Sync(pkgbits.SyncUseObjLocal)