reader.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"
	"internal/buildcfg"
	"internal/pkgbits"
	"strings"

	"cmd/compile/internal/base"
	"cmd/compile/internal/deadcode"
	"cmd/compile/internal/dwarfgen"
	"cmd/compile/internal/inline"
	"cmd/compile/internal/ir"
	"cmd/compile/internal/objw"
	"cmd/compile/internal/reflectdata"
	"cmd/compile/internal/staticinit"
	"cmd/compile/internal/typecheck"
	"cmd/compile/internal/types"
	"cmd/internal/obj"
	"cmd/internal/objabi"
	"cmd/internal/src"
)

// This file implements cmd/compile backend's reader for the Unified
// IR export data.

// A pkgReader reads Unified IR export data.
type pkgReader struct {
	pkgbits.PkgDecoder

	// Indices for encoded things; lazily populated as needed.
	//
	// Note: Objects (i.e., ir.Names) are lazily instantiated by
	// populating their types.Sym.Def; see objReader below.

	posBases []*src.PosBase
	pkgs     []*types.Pkg
	typs     []*types.Type

	// offset for rewriting the given (absolute!) index into the output,
	// but bitwise inverted so we can detect if we're missing the entry
	// or not.
	newindex []pkgbits.Index
}

func newPkgReader(pr pkgbits.PkgDecoder) *pkgReader {
	return &pkgReader{
		PkgDecoder: pr,

		posBases: make([]*src.PosBase, pr.NumElems(pkgbits.RelocPosBase)),
		pkgs:     make([]*types.Pkg, pr.NumElems(pkgbits.RelocPkg)),
		typs:     make([]*types.Type, pr.NumElems(pkgbits.RelocType)),

		newindex: make([]pkgbits.Index, pr.TotalElems()),
	}
}

// A pkgReaderIndex compactly identifies an index (and its
// corresponding dictionary) within a package's export data.
type pkgReaderIndex struct {
	pr        *pkgReader
	idx       pkgbits.Index
	dict      *readerDict
	methodSym *types.Sym

	synthetic func(pos src.XPos, r *reader)
}

func (pri pkgReaderIndex) asReader(k pkgbits.RelocKind, marker pkgbits.SyncMarker) *reader {
	if pri.synthetic != nil {
		return &reader{synthetic: pri.synthetic}
	}

	r := pri.pr.newReader(k, pri.idx, marker)
	r.dict = pri.dict
	r.methodSym = pri.methodSym
	return r
}

func (pr *pkgReader) newReader(k pkgbits.RelocKind, idx pkgbits.Index, marker pkgbits.SyncMarker) *reader {
	return &reader{
		Decoder: pr.NewDecoder(k, idx, marker),
		p:       pr,
	}
}

// A reader provides APIs for reading an individual element.
type reader struct {
	pkgbits.Decoder

	p *pkgReader

	dict *readerDict

	// TODO(mdempsky): The state below is all specific to reading
	// function bodies. It probably makes sense to split it out
	// separately so that it doesn't take up space in every reader
	// instance.

	curfn       *ir.Func
	locals      []*ir.Name
	closureVars []*ir.Name

	funarghack bool

	// methodSym is the name of method's name, if reading a method.
	// It's nil if reading a normal function or closure body.
	methodSym *types.Sym

	// dictParam is the .dict param, if any.
	dictParam *ir.Name

	// synthetic is a callback function to construct a synthetic
	// function body. It's used for creating the bodies of function
	// literals used to curry arguments to shaped functions.
	synthetic func(pos src.XPos, r *reader)

	// scopeVars is a stack tracking the number of variables declared in
	// the current function at the moment each open scope was opened.
	scopeVars         []int
	marker            dwarfgen.ScopeMarker
	lastCloseScopePos src.XPos

	// === details for handling inline body expansion ===

	// If we're reading in a function body because of inlining, this is
	// the call that we're inlining for.
	inlCaller    *ir.Func
	inlCall      *ir.CallExpr
	inlFunc      *ir.Func
	inlTreeIndex int
	inlPosBases  map[*src.PosBase]*src.PosBase

	// suppressInlPos tracks whether position base rewriting for
	// inlining should be suppressed. See funcLit.
	suppressInlPos int

	delayResults bool

	// Label to return to.
	retlabel *types.Sym

	// inlvars is the list of variables that the inlinee's arguments are
	// assigned to, one for each receiver and normal parameter, in order.
	inlvars ir.Nodes

	// retvars is the list of variables that the inlinee's results are
	// assigned to, one for each result parameter, in order.
	retvars ir.Nodes
}

// A readerDict represents an instantiated "compile-time dictionary,"
// used for resolving any derived types needed for instantiating a
// generic object.
//
// A compile-time dictionary can either be "shaped" or "non-shaped."
// Shaped compile-time dictionaries are only used for instantiating
// shaped type definitions and function bodies, while non-shaped
// compile-time dictionaries are used for instantiating runtime
// dictionaries.
type readerDict struct {
	shaped bool // whether this is a shaped dictionary

	// baseSym is the symbol for the object this dictionary belongs to.
	// If the object is an instantiated function or defined type, then
	// baseSym is the mangled symbol, including any type arguments.
	baseSym *types.Sym

	// For non-shaped dictionaries, shapedObj is a reference to the
	// corresponding shaped object (always a function or defined type).
	shapedObj *ir.Name

	// targs holds the implicit and explicit type arguments in use for
	// reading the current object. For example:
	//
	//	func F[T any]() {
	//		type X[U any] struct { t T; u U }
	//		var _ X[string]
	//	}
	//
	//	var _ = F[int]
	//
	// While instantiating F[int], we need to in turn instantiate
	// X[string]. [int] and [string] are explicit type arguments for F
	// and X, respectively; but [int] is also the implicit type
	// arguments for X.
	//
	// (As an analogy to function literals, explicits are the function
	// literal's formal parameters, while implicits are variables
	// captured by the function literal.)
	targs []*types.Type

	// implicits counts how many of types within targs are implicit type
	// arguments; the rest are explicit.
	implicits int

	derived      []derivedInfo // reloc index of the derived type's descriptor
	derivedTypes []*types.Type // slice of previously computed derived types

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

type readerMethodExprInfo struct {
	typeParamIdx int
	method       *types.Sym
}

func setType(n ir.Node, typ *types.Type) {
	n.SetType(typ)
	n.SetTypecheck(1)
}

func setValue(name *ir.Name, val constant.Value) {
	name.SetVal(val)
	name.Defn = nil
}

// @@@ Positions

// pos reads a position from the bitstream.
func (r *reader) pos() src.XPos {
	return base.Ctxt.PosTable.XPos(r.pos0())
}

func (r *reader) pos0() src.Pos {
	r.Sync(pkgbits.SyncPos)
	if !r.Bool() {
		return src.NoPos
	}

	posBase := r.posBase()
	line := r.Uint()
	col := r.Uint()
	return src.MakePos(posBase, line, col)
}

// posBase reads a position base from the bitstream.
func (r *reader) posBase() *src.PosBase {
	return r.inlPosBase(r.p.posBaseIdx(r.Reloc(pkgbits.RelocPosBase)))
}

// posBaseIdx returns the specified position base, reading it first if
// needed.
func (pr *pkgReader) posBaseIdx(idx pkgbits.Index) *src.PosBase {
	if b := pr.posBases[idx]; b != nil {
		return b
	}

	r := pr.newReader(pkgbits.RelocPosBase, idx, pkgbits.SyncPosBase)
	var b *src.PosBase

	absFilename := r.String()
	filename := absFilename

	// For build artifact stability, the export data format only
	// contains the "absolute" filename as returned by objabi.AbsFile.
	// However, some tests (e.g., test/run.go's asmcheck tests) expect
	// to see the full, original filename printed out. Re-expanding
	// "$GOROOT" to buildcfg.GOROOT is a close-enough approximation to
	// satisfy this.
	//
	// TODO(mdempsky): De-duplicate this logic with similar logic in
	// cmd/link/internal/ld's expandGoroot. However, this will probably
	// require being more consistent about when we use native vs UNIX
	// file paths.
	const dollarGOROOT = "$GOROOT"
	if buildcfg.GOROOT != "" && strings.HasPrefix(filename, dollarGOROOT) {
		filename = buildcfg.GOROOT + filename[len(dollarGOROOT):]
	}

	if r.Bool() {
		b = src.NewFileBase(filename, absFilename)
	} else {
		pos := r.pos0()
		line := r.Uint()
		col := r.Uint()
		b = src.NewLinePragmaBase(pos, filename, absFilename, line, col)
	}

	pr.posBases[idx] = b
	return b
}

// inlPosBase returns the inlining-adjusted src.PosBase corresponding
// to oldBase, which must be a non-inlined position. When not
// inlining, this is just oldBase.
func (r *reader) inlPosBase(oldBase *src.PosBase) *src.PosBase {
	if index := oldBase.InliningIndex(); index >= 0 {
		base.Fatalf("oldBase %v already has inlining index %v", oldBase, index)
	}

	if r.inlCall == nil || r.suppressInlPos != 0 {
		return oldBase
	}

	if newBase, ok := r.inlPosBases[oldBase]; ok {
		return newBase
	}

	newBase := src.NewInliningBase(oldBase, r.inlTreeIndex)
	r.inlPosBases[oldBase] = newBase
	return newBase
}

// inlPos returns the inlining-adjusted src.XPos corresponding to
// xpos, which must be a non-inlined position. When not inlining, this
// is just xpos.
func (r *reader) inlPos(xpos src.XPos) src.XPos {
	pos := base.Ctxt.PosTable.Pos(xpos)
	pos.SetBase(r.inlPosBase(pos.Base()))
	return base.Ctxt.PosTable.XPos(pos)
}

// @@@ Packages

// pkg reads a package reference from the bitstream.
func (r *reader) pkg() *types.Pkg {
	r.Sync(pkgbits.SyncPkg)
	return r.p.pkgIdx(r.Reloc(pkgbits.RelocPkg))
}

// pkgIdx returns the specified package from the export data, reading
// it first if needed.
func (pr *pkgReader) pkgIdx(idx pkgbits.Index) *types.Pkg {
	if pkg := pr.pkgs[idx]; pkg != nil {
		return pkg
	}

	pkg := pr.newReader(pkgbits.RelocPkg, idx, pkgbits.SyncPkgDef).doPkg()
	pr.pkgs[idx] = pkg
	return pkg
}

// doPkg reads a package definition from the bitstream.
func (r *reader) doPkg() *types.Pkg {
	path := r.String()
	switch path {
	case "":
		path = r.p.PkgPath()
	case "builtin":
		return types.BuiltinPkg
	case "unsafe":
		return types.UnsafePkg
	}

	name := r.String()

	pkg := types.NewPkg(path, "")

	if pkg.Name == "" {
		pkg.Name = name
	} else {
		base.Assertf(pkg.Name == name, "package %q has name %q, but want %q", pkg.Path, pkg.Name, name)
	}

	return pkg
}

// @@@ Types

func (r *reader) typ() *types.Type {
	return r.typWrapped(true)
}

// typWrapped is like typ, but allows suppressing generation of
// unnecessary wrappers as a compile-time optimization.
func (r *reader) typWrapped(wrapped bool) *types.Type {
	return r.p.typIdx(r.typInfo(), r.dict, wrapped)
}

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}
}

// typListIdx returns a list of the specified types, resolving derived
// types within the given dictionary.
func (pr *pkgReader) typListIdx(infos []typeInfo, dict *readerDict) []*types.Type {
	typs := make([]*types.Type, len(infos))
	for i, info := range infos {
		typs[i] = pr.typIdx(info, dict, true)
	}
	return typs
}

// typIdx returns the specified type. If info specifies a derived
// type, it's resolved within the given dictionary. If wrapped is
// true, then method wrappers will be generated, if appropriate.
func (pr *pkgReader) typIdx(info typeInfo, dict *readerDict, wrapped bool) *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)

	// For recursive type declarations involving interfaces and aliases,
	// above r.doTyp() call may have already set pr.typs[idx], so just
	// double check and return the type.
	//
	// Example:
	//
	//     type F = func(I)
	//
	//     type I interface {
	//         m(F)
	//     }
	//
	// The writer writes data types in following index order:
	//
	//     0: func(I)
	//     1: I
	//     2: interface{m(func(I))}
	//
	// The reader resolves it in following index order:
	//
	//     0 -> 1 -> 2 -> 0 -> 1
	//
	// and can divide in logically 2 steps:
	//
	//  - 0 -> 1     : first time the reader reach type I,
	//                 it creates new named type with symbol I.
	//
	//  - 2 -> 0 -> 1: the reader ends up reaching symbol I again,
	//                 now the symbol I was setup in above step, so
	//                 the reader just return the named type.
	//
	// Now, the functions called return, the pr.typs looks like below:
	//
	//  - 0 -> 1 -> 2 -> 0 : [<T> I <T>]
	//  - 0 -> 1 -> 2      : [func(I) I <T>]
	//  - 0 -> 1           : [func(I) I interface { "".m(func("".I)) }]
	//
	// The idx 1, corresponding with type I was resolved successfully
	// after r.doTyp() call.

	if prev := *where; prev != nil {
		return prev
	}

	if wrapped {
		// Only cache if we're adding wrappers, so that other callers that
		// find a cached type know it was wrapped.
		*where = typ

		r.needWrapper(typ)
	}

	if !typ.IsUntyped() {
		types.CheckSize(typ)
	}

	return typ
}

func (r *reader) doTyp() *types.Type {
	switch tag := pkgbits.CodeType(r.Code(pkgbits.SyncType)); tag {
	default:
		panic(fmt.Sprintf("unexpected type: %v", tag))

	case pkgbits.TypeBasic:
		return *basics[r.Len()]

	case pkgbits.TypeNamed:
		obj := r.obj()
		assert(obj.Op() == ir.OTYPE)
		return obj.Type()

	case pkgbits.TypeTypeParam:
		return r.dict.targs[r.Len()]

	case pkgbits.TypeArray:
		len := int64(r.Uint64())
		return types.NewArray(r.typ(), len)
	case pkgbits.TypeChan:
		dir := dirs[r.Len()]
		return types.NewChan(r.typ(), dir)
	case pkgbits.TypeMap:
		return types.NewMap(r.typ(), r.typ())
	case pkgbits.TypePointer:
		return types.NewPtr(r.typ())
	case pkgbits.TypeSignature:
		return r.signature(types.LocalPkg, 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) unionType() *types.Type {
	terms := make([]*types.Type, r.Len())
	tildes := make([]bool, len(terms))
	for i := range terms {
		tildes[i] = r.Bool()
		terms[i] = r.typ()
	}
	return types.NewUnion(terms, tildes)
}

func (r *reader) interfaceType() *types.Type {
	tpkg := types.LocalPkg // TODO(mdempsky): Remove after iexport is gone.

	nmethods, nembeddeds := r.Len(), r.Len()
	implicit := nmethods == 0 && nembeddeds == 1 && r.Bool()
	assert(!implicit) // implicit interfaces only appear in constraints

	fields := make([]*types.Field, nmethods+nembeddeds)
	methods, embeddeds := fields[:nmethods], fields[nmethods:]

	for i := range methods {
		pos := r.pos()
		pkg, sym := r.selector()
		tpkg = pkg
		mtyp := r.signature(pkg, types.FakeRecv())
		methods[i] = types.NewField(pos, sym, mtyp)
	}
	for i := range embeddeds {
		embeddeds[i] = types.NewField(src.NoXPos, nil, r.typ())
	}

	if len(fields) == 0 {
		return types.Types[types.TINTER] // empty interface
	}
	return types.NewInterface(tpkg, fields, false)
}

func (r *reader) structType() *types.Type {
	tpkg := types.LocalPkg // TODO(mdempsky): Remove after iexport is gone.
	fields := make([]*types.Field, r.Len())
	for i := range fields {
		pos := r.pos()
		pkg, sym := r.selector()
		tpkg = pkg
		ftyp := r.typ()
		tag := r.String()
		embedded := r.Bool()

		f := types.NewField(pos, sym, ftyp)
		f.Note = tag
		if embedded {
			f.Embedded = 1
		}
		fields[i] = f
	}
	return types.NewStruct(tpkg, fields)
}

func (r *reader) signature(tpkg *types.Pkg, recv *types.Field) *types.Type {
	r.Sync(pkgbits.SyncSignature)

	params := r.params(&tpkg)
	results := r.params(&tpkg)
	if r.Bool() { // variadic
		params[len(params)-1].SetIsDDD(true)
	}

	return types.NewSignature(tpkg, recv, nil, params, results)
}

func (r *reader) params(tpkg **types.Pkg) []*types.Field {
	r.Sync(pkgbits.SyncParams)
	fields := make([]*types.Field, r.Len())
	for i := range fields {
		*tpkg, fields[i] = r.param()
	}
	return fields
}

func (r *reader) param() (*types.Pkg, *types.Field) {
	r.Sync(pkgbits.SyncParam)

	pos := r.pos()
	pkg, sym := r.localIdent()
	typ := r.typ()

	return pkg, types.NewField(pos, sym, typ)
}

// @@@ Objects

// objReader maps qualified identifiers (represented as *types.Sym) to
// a pkgReader and corresponding index that can be used for reading
// that object's definition.
var objReader = map[*types.Sym]pkgReaderIndex{}

// obj reads an instantiated object reference from the bitstream.
func (r *reader) obj() ir.Node {
	return r.p.objInstIdx(r.objInfo(), r.dict, false)
}

// objInfo reads an instantiated object reference from the bitstream
// and returns the encoded reference to it, without instantiating it.
func (r *reader) objInfo() objInfo {
	r.Sync(pkgbits.SyncObject)
	assert(!r.Bool()) // TODO(mdempsky): Remove; was derived func inst.
	idx := r.Reloc(pkgbits.RelocObj)

	explicits := make([]typeInfo, r.Len())
	for i := range explicits {
		explicits[i] = r.typInfo()
	}

	return objInfo{idx, explicits}
}

// objInstIdx returns the encoded, instantiated object. If shaped is
// true, then the shaped variant of the object is returned instead.
func (pr *pkgReader) objInstIdx(info objInfo, dict *readerDict, shaped bool) ir.Node {
	explicits := pr.typListIdx(info.explicits, dict)

	var implicits []*types.Type
	if dict != nil {
		implicits = dict.targs
	}

	return pr.objIdx(info.idx, implicits, explicits, shaped)
}

// objIdx returns the specified object, instantiated with the given
// type arguments, if any. If shaped is true, then the shaped variant
// of the object is returned instead.
func (pr *pkgReader) objIdx(idx pkgbits.Index, implicits, explicits []*types.Type, shaped bool) ir.Node {
	rname := pr.newReader(pkgbits.RelocName, idx, pkgbits.SyncObject1)
	_, sym := rname.qualifiedIdent()
	tag := pkgbits.CodeObj(rname.Code(pkgbits.SyncCodeObj))

	if tag == pkgbits.ObjStub {
		assert(!sym.IsBlank())
		switch sym.Pkg {
		case types.BuiltinPkg, types.UnsafePkg:
			return sym.Def.(ir.Node)
		}
		if pri, ok := objReader[sym]; ok {
			return pri.pr.objIdx(pri.idx, nil, explicits, shaped)
		}
		base.Fatalf("unresolved stub: %v", sym)
	}

	dict := pr.objDictIdx(sym, idx, implicits, explicits, shaped)

	sym = dict.baseSym
	if !sym.IsBlank() && sym.Def != nil {
		return sym.Def.(*ir.Name)
	}

	r := pr.newReader(pkgbits.RelocObj, idx, pkgbits.SyncObject1)
	rext := pr.newReader(pkgbits.RelocObjExt, idx, pkgbits.SyncObject1)

	r.dict = dict
	rext.dict = dict

	do := func(op ir.Op, hasTParams bool) *ir.Name {
		pos := r.pos()
		setBasePos(pos)
		if hasTParams {
			r.typeParamNames()
		}

		name := ir.NewDeclNameAt(pos, op, sym)
		name.Class = ir.PEXTERN // may be overridden later
		if !sym.IsBlank() {
			if sym.Def != nil {
				base.FatalfAt(name.Pos(), "already have a definition for %v", name)
			}
			assert(sym.Def == nil)
			sym.Def = name
		}
		return name
	}

	switch tag {
	default:
		panic("unexpected object")

	case pkgbits.ObjAlias:
		name := do(ir.OTYPE, false)
		setType(name, r.typ())
		name.SetAlias(true)
		return name

	case pkgbits.ObjConst:
		name := do(ir.OLITERAL, false)
		typ := r.typ()
		val := FixValue(typ, r.Value())
		setType(name, typ)
		setValue(name, val)
		return name

	case pkgbits.ObjFunc:
		if sym.Name == "init" {
			sym = Renameinit()
		}
		name := do(ir.ONAME, true)
		setType(name, r.signature(sym.Pkg, nil))

		name.Func = ir.NewFunc(r.pos())
		name.Func.Nname = name

		if r.hasTypeParams() {
			name.Func.SetDupok(true)
			if r.dict.shaped {
				setType(name, shapeSig(name.Func, r.dict))
			} else {
				todoDicts = append(todoDicts, func() {
					r.dict.shapedObj = pr.objIdx(idx, implicits, explicits, true).(*ir.Name)
				})
			}
		}

		rext.funcExt(name, nil)
		return name

	case pkgbits.ObjType:
		name := do(ir.OTYPE, true)
		typ := types.NewNamed(name)
		setType(name, typ)
		if r.hasTypeParams() && r.dict.shaped {
			typ.SetHasShape(true)
		}

		// Important: We need to do this before SetUnderlying.
		rext.typeExt(name)

		// We need to defer CheckSize until we've called SetUnderlying to
		// handle recursive types.
		types.DeferCheckSize()
		typ.SetUnderlying(r.typWrapped(false))
		types.ResumeCheckSize()

		if r.hasTypeParams() && !r.dict.shaped {
			todoDicts = append(todoDicts, func() {
				r.dict.shapedObj = pr.objIdx(idx, implicits, explicits, true).(*ir.Name)
			})
		}

		methods := make([]*types.Field, r.Len())
		for i := range methods {
			methods[i] = r.method(rext)
		}
		if len(methods) != 0 {
			typ.Methods().Set(methods)
		}

		if !r.dict.shaped {
			r.needWrapper(typ)
		}

		return name

	case pkgbits.ObjVar:
		name := do(ir.ONAME, false)
		setType(name, r.typ())
		rext.varExt(name)
		return name
	}
}

func (dict *readerDict) mangle(sym *types.Sym) *types.Sym {
	if !dict.hasTypeParams() {
		return sym
	}

	// If sym is a locally defined generic type, we need the suffix to
	// stay at the end after mangling so that types/fmt.go can strip it
	// out again when writing the type's runtime descriptor (#54456).
	base, suffix := types.SplitVargenSuffix(sym.Name)

	var buf strings.Builder
	buf.WriteString(base)
	buf.WriteByte('[')
	for i, targ := range dict.targs {
		if i > 0 {
			if i == dict.implicits {
				buf.WriteByte(';')
			} else {
				buf.WriteByte(',')
			}
		}
		buf.WriteString(targ.LinkString())
	}
	buf.WriteByte(']')
	buf.WriteString(suffix)
	return sym.Pkg.Lookup(buf.String())
}

// shapify returns the shape type for targ.
//
// If basic is true, then the type argument is used to instantiate a
// type parameter whose constraint is a basic interface.
func shapify(targ *types.Type, basic bool) *types.Type {
	base.Assertf(targ.Kind() != types.TFORW, "%v is missing its underlying type", targ)

	// When a pointer type is used to instantiate a type parameter
	// constrained by a basic interface, we know the pointer's element
	// type can't matter to the generated code. In this case, we can use
	// an arbitrary pointer type as the shape type. (To match the
	// non-unified frontend, we use `*byte`.)
	//
	// Otherwise, we simply use the type's underlying type as its shape.
	//
	// TODO(mdempsky): It should be possible to do much more aggressive
	// shaping still; e.g., collapsing all pointer-shaped types into a
	// common type, collapsing scalars of the same size/alignment into a
	// common type, recursively shaping the element types of composite
	// types, and discarding struct field names and tags. However, we'll
	// need to start tracking how type parameters are actually used to
	// implement some of these optimizations.
	under := targ.Underlying()
	if basic && targ.IsPtr() && !targ.Elem().NotInHeap() {
		under = types.NewPtr(types.Types[types.TUINT8])
	}

	sym := types.ShapePkg.Lookup(under.LinkString())
	if sym.Def == nil {
		name := ir.NewDeclNameAt(under.Pos(), ir.OTYPE, sym)
		typ := types.NewNamed(name)
		typ.SetUnderlying(under)
		sym.Def = typed(typ, name)
	}
	res := sym.Def.Type()
	assert(res.IsShape())
	assert(res.HasShape())
	return res
}

// objDictIdx reads and returns the specified object dictionary.
func (pr *pkgReader) objDictIdx(sym *types.Sym, idx pkgbits.Index, implicits, explicits []*types.Type, shaped bool) *readerDict {
	r := pr.newReader(pkgbits.RelocObjDict, idx, pkgbits.SyncObject1)

	dict := readerDict{
		shaped: shaped,
	}

	nimplicits := r.Len()
	nexplicits := r.Len()

	if nimplicits > len(implicits) || nexplicits != len(explicits) {
		base.Fatalf("%v has %v+%v params, but instantiated with %v+%v args", sym, nimplicits, nexplicits, len(implicits), len(explicits))
	}

	dict.targs = append(implicits[:nimplicits:nimplicits], explicits...)
	dict.implicits = nimplicits

	// Within the compiler, we can just skip over the type parameters.
	for range dict.targs[dict.implicits:] {
		// Skip past bounds without actually evaluating them.
		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()}
	}

	// Runtime dictionary information; private to the compiler.

	// If any type argument is already shaped, then we're constructing a
	// shaped object, even if not explicitly requested (i.e., calling
	// objIdx with shaped==true). This can happen with instantiating
	// types that are referenced within a function body.
	for _, targ := range dict.targs {
		if targ.HasShape() {
			dict.shaped = true
			break
		}
	}

	// And if we're constructing a shaped object, then shapify all type
	// arguments.
	for i, targ := range dict.targs {
		basic := r.Bool()
		if dict.shaped {
			dict.targs[i] = shapify(targ, basic)
		}
	}

	dict.baseSym = dict.mangle(sym)

	dict.typeParamMethodExprs = make([]readerMethodExprInfo, r.Len())
	for i := range dict.typeParamMethodExprs {
		typeParamIdx := r.Len()
		_, method := r.selector()

		dict.typeParamMethodExprs[i] = readerMethodExprInfo{typeParamIdx, method}
	}

	dict.subdicts = make([]objInfo, r.Len())
	for i := range dict.subdicts {
		dict.subdicts[i] = r.objInfo()
	}

	dict.rtypes = make([]typeInfo, r.Len())
	for i := range dict.rtypes {
		dict.rtypes[i] = r.typInfo()
	}

	dict.itabs = make([]itabInfo, r.Len())
	for i := range dict.itabs {
		dict.itabs[i] = itabInfo{typ: r.typInfo(), iface: r.typInfo()}
	}

	return &dict
}

func (r *reader) typeParamNames() {
	r.Sync(pkgbits.SyncTypeParamNames)

	for range r.dict.targs[r.dict.implicits:] {
		r.pos()
		r.localIdent()
	}
}

func (r *reader) method(rext *reader) *types.Field {
	r.Sync(pkgbits.SyncMethod)
	pos := r.pos()
	pkg, sym := r.selector()
	r.typeParamNames()
	_, recv := r.param()
	typ := r.signature(pkg, recv)

	name := ir.NewNameAt(pos, ir.MethodSym(recv.Type, sym))
	setType(name, typ)

	name.Func = ir.NewFunc(r.pos())
	name.Func.Nname = name

	if r.hasTypeParams() {
		name.Func.SetDupok(true)
		if r.dict.shaped {
			typ = shapeSig(name.Func, r.dict)
			setType(name, typ)
		}
	}

	rext.funcExt(name, sym)

	meth := types.NewField(name.Func.Pos(), sym, typ)
	meth.Nname = name
	meth.SetNointerface(name.Func.Pragma&ir.Nointerface != 0)

	return meth
}

func (r *reader) qualifiedIdent() (pkg *types.Pkg, sym *types.Sym) {
	r.Sync(pkgbits.SyncSym)
	pkg = r.pkg()
	if name := r.String(); name != "" {
		sym = pkg.Lookup(name)
	}
	return
}

func (r *reader) localIdent() (pkg *types.Pkg, sym *types.Sym) {
	r.Sync(pkgbits.SyncLocalIdent)
	pkg = r.pkg()
	if name := r.String(); name != "" {
		sym = pkg.Lookup(name)
	}
	return
}

func (r *reader) selector() (origPkg *types.Pkg, sym *types.Sym) {
	r.Sync(pkgbits.SyncSelector)
	origPkg = r.pkg()
	name := r.String()
	pkg := origPkg
	if types.IsExported(name) {
		pkg = types.LocalPkg
	}