[dev.typeparams] cmd/compile: add derived-type dictionaries to unified IR

This CL updates the unified IR export data serialization to explicitly
and separately record the derived types used by a declaration. The
readers currently just use this data to construct types/IR the same as
before, but eventually we can use it for emitting GC-shape
dictionaries.

Change-Id: I7d67ad9b3f1fbe69664bf19e056bc94f73507220
Reviewed-on: https://go-review.googlesource.com/c/go/+/331829
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Trust: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/noder/linker.go

@@ -134,11 +134,15 @@ func (l *linker) relocObj(pr *pkgReader, idx int) int {
 	}
 
 	w := l.pw.newEncoderRaw(relocObj)
-	bside := l.pw.newEncoderRaw(relocObjExt)
+	wext := l.pw.newEncoderRaw(relocObjExt)
-	assert(bside.idx == w.idx)
+	wdict := l.pw.newEncoderRaw(relocObjDict)
+
 	l.decls[sym] = w.idx
+	assert(wext.idx == w.idx)
+	assert(wdict.idx == w.idx)
 
 	l.relocCommon(pr, &w, relocObj, idx)
+	l.relocCommon(pr, &wdict, relocObjDict, idx)
 
 	var obj *ir.Name
 	if path == "" {
@@ -153,18 +157,18 @@ func (l *linker) relocObj(pr *pkgReader, idx int) int {
 	}
 
 	if obj != nil {
-		bside.sync(syncObject1)
+		wext.sync(syncObject1)
 		switch tag {
 		case objFunc:
-			l.relocFuncExt(&bside, obj)
+			l.relocFuncExt(&wext, obj)
 		case objType:
-			l.relocTypeExt(&bside, obj)
+			l.relocTypeExt(&wext, obj)
 		case objVar:
-			l.relocVarExt(&bside, obj)
+			l.relocVarExt(&wext, obj)
 		}
-		bside.flush()
+		wext.flush()
 	} else {
-		l.relocCommon(pr, &bside, relocObjExt, idx)
+		l.relocCommon(pr, &wext, relocObjExt, idx)
 	}
 
 	return w.idx
@@ -286,7 +290,17 @@ func (pr *pkgDecoder) peekObj(idx int) (string, string, codeObj, []int) {
 	bounds := make([]int, r.len())
 	for i := range bounds {
 		r.sync(syncType)
-		bounds[i] = r.reloc(relocType)
+		if r.bool() {
+			r.len()
+		} else {
+			r.reloc(relocType)
+		}
+
+		// TODO(mdempsky): This result now needs to include the 'derived'
+		// bool too, but none of the callers currently depend on it
+		// anyway. Either fix it to be meaningful, or just get rid of it
+		// altogether.
+		bounds[i] = -1
 	}
 
 	tag := codeObj(r.code(syncCodeObj))

src/cmd/compile/internal/noder/reader.go

@@ -56,12 +56,12 @@ func newPkgReader(pr pkgDecoder) *pkgReader {
 type pkgReaderIndex struct {
 	pr   *pkgReader
 	idx  int
-	implicits []*types.Type
+	dict *readerDict
 }
 
 func (pri pkgReaderIndex) asReader(k reloc, marker syncMarker) *reader {
 	r := pri.pr.newReader(k, pri.idx, marker)
-	r.implicits = pri.implicits
+	r.dict = pri.dict
 	return r
 }
 
@@ -77,29 +77,10 @@ type reader struct {
 
 	p *pkgReader
 
-	// 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.)
-	implicits []*types.Type
-	explicits []*types.Type
-
 	ext *reader
 
+	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
@@ -135,6 +116,35 @@ type reader struct {
 	inlvars, retvars ir.Nodes
 }
 
+type readerDict struct {
+	// 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
+
+	derivedReloc []int         // reloc index of the derived type's descriptor
+	derived      []*types.Type // slice of previously computed derived types
+}
+
 func (r *reader) setType(n ir.Node, typ *types.Type) {
 	n.SetType(typ)
 	n.SetTypecheck(1)
@@ -283,17 +293,28 @@ func (r *reader) doPkg() *types.Pkg {
 
 func (r *reader) typ() *types.Type {
 	r.sync(syncType)
-	return r.p.typIdx(r.reloc(relocType), r.implicits, r.explicits)
+	if r.bool() {
+		return r.p.typIdx(r.len(), r.dict)
+	}
+	return r.p.typIdx(r.reloc(relocType), nil)
 }
 
-func (pr *pkgReader) typIdx(idx int, implicits, explicits []*types.Type) *types.Type {
+func (pr *pkgReader) typIdx(idx int, dict *readerDict) *types.Type {
-	if typ := pr.typs[idx]; typ != nil {
+	var where **types.Type
+	if dict != nil {
+		where = &dict.derived[idx]
+		idx = dict.derivedReloc[idx]
+	} else {
+		where = &pr.typs[idx]
+	}
+
+	if typ := *where; typ != nil {
 		return typ
 	}
 
 	r := pr.newReader(relocType, idx, syncTypeIdx)
-	r.implicits = implicits
+	r.dict = dict
-	r.explicits = explicits
+
 	typ := r.doTyp()
 	assert(typ != nil)
 
@@ -336,20 +357,12 @@ func (pr *pkgReader) typIdx(idx int, implicits, explicits []*types.Type) *types.
 	//
 	// The idx 1, corresponding with type I was resolved successfully
 	// after r.doTyp() call.
-	if typ := pr.typs[idx]; typ != nil {
+
-		return typ
+	if prev := *where; prev != nil {
+		return prev
 	}
 
-	// If we have type parameters, the type might refer to them, and it
+	*where = typ
-	// wouldn't be safe to reuse those in other contexts. So we
-	// conservatively avoid caching them in that case.
-	//
-	// TODO(mdempsky): If we're clever, we should be able to still cache
-	// types by tracking which type parameters are used. However, in my
-	// attempts so far, I haven't yet succeeded in being clever enough.
-	if !r.hasTypeParams() {
-		pr.typs[idx] = typ
-	}
 
 	if !typ.IsUntyped() {
 		types.CheckSize(typ)
@@ -372,11 +385,7 @@ func (r *reader) doTyp() *types.Type {
 		return obj.Type()
 
 	case typeTypeParam:
-		idx := r.len()
+		return r.dict.targs[r.len()]
-		if idx < len(r.implicits) {
-			return r.implicits[idx]
-		}
-		return r.explicits[idx-len(r.implicits)]
 
 	case typeArray:
 		len := int64(r.uint64())
@@ -490,7 +499,12 @@ func (r *reader) obj() ir.Node {
 		explicits[i] = r.typ()
 	}
 
-	return r.p.objIdx(idx, r.implicits, explicits)
+	var implicits []*types.Type
+	if r.dict != nil {
+		implicits = r.dict.targs
+	}
+
+	return r.p.objIdx(idx, implicits, explicits)
 }
 
 func (pr *pkgReader) objIdx(idx int, implicits, explicits []*types.Type) ir.Node {
@@ -499,14 +513,11 @@ func (pr *pkgReader) objIdx(idx int, implicits, explicits []*types.Type) ir.Node
 
 	_, sym := r.qualifiedIdent()
 
-	// Middle dot indicates local defined type; see writer.sym.
+	dict := &readerDict{}
-	// TODO(mdempsky): Come up with a better way to handle this.
+	r.dict = dict
-	if strings.Contains(sym.Name, "·") {
+	r.ext.dict = dict
-		r.implicits = implicits
+
-		r.ext.implicits = implicits
+	r.typeParamBounds(sym, implicits, explicits)
-	}
-	r.explicits = explicits
-	r.ext.explicits = explicits
 
 	origSym := sym
 
@@ -515,9 +526,17 @@ func (pr *pkgReader) objIdx(idx int, implicits, explicits []*types.Type) ir.Node
 		return sym.Def.(ir.Node)
 	}
 
-	r.typeParamBounds(origSym)
 	tag := codeObj(r.code(syncCodeObj))
 
+	{
+		rdict := pr.newReader(relocObjDict, idx, syncObject1)
+		r.dict.derivedReloc = make([]int, rdict.len())
+		r.dict.derived = make([]*types.Type, len(r.dict.derivedReloc))
+		for i := range r.dict.derived {
+			r.dict.derivedReloc[i] = rdict.reloc(relocType)
+		}
+	}
+
 	do := func(op ir.Op, hasTParams bool) *ir.Name {
 		pos := r.pos()
 		if hasTParams {
@@ -542,7 +561,7 @@ func (pr *pkgReader) objIdx(idx int, implicits, explicits []*types.Type) ir.Node
 
 	case objStub:
 		if pri, ok := objReader[origSym]; ok {
-			return pri.pr.objIdx(pri.idx, pri.implicits, r.explicits)
+			return pri.pr.objIdx(pri.idx, nil, explicits)
 		}
 		if haveLegacyImports {
 			assert(!r.hasTypeParams())
@@ -621,46 +640,50 @@ func (r *reader) mangle(sym *types.Sym) *types.Sym {
 	var buf bytes.Buffer
 	buf.WriteString(sym.Name)
 	buf.WriteByte('[')
-	for i, targs := range [2][]*types.Type{r.implicits, r.explicits} {
+	for i, targ := range r.dict.targs {
-		if i > 0 && len(r.implicits) != 0 && len(r.explicits) != 0 {
+		if i > 0 {
+			if i == r.dict.implicits {
 				buf.WriteByte(';')
-		}
+			} else {
-		for j, targ := range targs {
-			if j > 0 {
 				buf.WriteByte(',')
 			}
-			// TODO(mdempsky): We need the linker to replace "" in the symbol
-			// names here.
-			buf.WriteString(targ.LinkString())
 		}
+		buf.WriteString(targ.LinkString())
 	}
 	buf.WriteByte(']')
 	return sym.Pkg.Lookup(buf.String())
 }
 
-func (r *reader) typeParamBounds(sym *types.Sym) {
+func (r *reader) typeParamBounds(sym *types.Sym, implicits, explicits []*types.Type) {
 	r.sync(syncTypeParamBounds)
 
 	nimplicits := r.len()
 	nexplicits := r.len()
 
-	if len(r.implicits) != nimplicits || len(r.explicits) != nexplicits {
+	if nimplicits > len(implicits) || nexplicits != len(explicits) {
-		base.Fatalf("%v has %v+%v params, but instantiated with %v+%v args", sym, nimplicits, nexplicits, len(r.implicits), len(r.explicits))
+		base.Fatalf("%v has %v+%v params, but instantiated with %v+%v args", sym, nimplicits, nexplicits, len(implicits), len(explicits))
 	}
 
+	r.dict.targs = append(implicits[:nimplicits:nimplicits], explicits...)
+	r.dict.implicits = nimplicits
+
 	// For stenciling, we can just skip over the type parameters.
 
-	for range r.explicits {
+	for range r.dict.targs[r.dict.implicits:] {
 		// Skip past bounds without actually evaluating them.
 		r.sync(syncType)
+		if r.bool() {
+			r.len()
+		} else {
 			r.reloc(relocType)
 		}
+	}
 }
 
 func (r *reader) typeParamNames() {
 	r.sync(syncTypeParamNames)
 
-	for range r.explicits {
+	for range r.dict.targs[r.dict.implicits:] {
 		r.pos()
 		r.localIdent()
 	}
@@ -729,7 +752,7 @@ func (r *reader) selector() (origPkg *types.Pkg, sym *types.Sym) {
 }
 
 func (r *reader) hasTypeParams() bool {
-	return len(r.implicits)+len(r.explicits) != 0
+	return r.dict != nil && len(r.dict.targs) != 0
 }
 
 // @@@ Compiler extensions
@@ -776,10 +799,10 @@ func (r *reader) funcExt(name *ir.Name) {
 				Cost:            int32(r.len()),
 				CanDelayResults: r.bool(),
 			}
-			r.addBody(name.Func, r.explicits)
+			r.addBody(name.Func)
 		}
 	} else {
-		r.addBody(name.Func, r.explicits)
+		r.addBody(name.Func)
 	}
 	r.sync(syncEOF)
 }
@@ -795,8 +818,7 @@ func (r *reader) typeExt(name *ir.Name) {
 		// type descriptor is written out as DUPOK and method wrappers are
 		// generated even for imported types.
 		var targs []*types.Type
-		targs = append(targs, r.implicits...)
+		targs = append(targs, r.dict.targs...)
-		targs = append(targs, r.explicits...)
 		typ.SetRParams(targs)
 	}
 
@@ -841,8 +863,8 @@ var bodyReader = map[*ir.Func]pkgReaderIndex{}
 // constructed.
 var todoBodies []*ir.Func
 
-func (r *reader) addBody(fn *ir.Func, implicits []*types.Type) {
+func (r *reader) addBody(fn *ir.Func) {
-	pri := pkgReaderIndex{r.p, r.reloc(relocBody), implicits}
+	pri := pkgReaderIndex{r.p, r.reloc(relocBody), r.dict}
 	bodyReader[fn] = pri
 
 	if r.curfn == nil {
@@ -1565,7 +1587,7 @@ func (r *reader) funcLit() ir.Node {
 		r.setType(cv, outer.Type())
 	}
 
-	r.addBody(fn, r.implicits)
+	r.addBody(fn)
 
 	return fn.OClosure
 }

src/cmd/compile/internal/noder/reader2.go

@@ -57,7 +57,21 @@ type reader2 struct {
 
 	p *pkgReader2
 
-	tparams []*types2.TypeName
+	dict *reader2Dict
+}
+
+type reader2Dict struct {
+	bounds []reader2TypeBound
+
+	tparams []*types2.TypeParam
+
+	derivedReloc []int
+	derived      []types2.Type
+}
+
+type reader2TypeBound struct {
+	derived  bool
+	boundIdx int
 }
 
 func (pr *pkgReader2) newReader(k reloc, idx int, marker syncMarker) *reader2 {
@@ -163,28 +177,37 @@ func (r *reader2) doPkg() *types2.Package {
 
 func (r *reader2) typ() types2.Type {
 	r.sync(syncType)
-	return r.p.typIdx(r.reloc(relocType), r.tparams)
+	if r.bool() {
+		return r.p.typIdx(r.len(), r.dict)
+	}
+	return r.p.typIdx(r.reloc(relocType), nil)
 }
 
-func (pr *pkgReader2) typIdx(idx int, tparams []*types2.TypeName) types2.Type {
+func (pr *pkgReader2) typIdx(idx int, dict *reader2Dict) types2.Type {
-	if typ := pr.typs[idx]; typ != nil {
+	var where *types2.Type
+	if dict != nil {
+		where = &dict.derived[idx]
+		idx = dict.derivedReloc[idx]
+	} else {
+		where = &pr.typs[idx]
+	}
+
+	if typ := *where; typ != nil {
 		return typ
 	}
 
 	r := pr.newReader(relocType, idx, syncTypeIdx)
-	r.tparams = tparams
+	r.dict = dict
+
 	typ := r.doTyp()
 	assert(typ != nil)
 
-	if pr.typs[idx] != nil {
+	// See comment in pkgReader.typIdx explaining how this happens.
-		// See comment in pkgReader.typIdx.
+	if prev := *where; prev != nil {
-		return pr.typs[idx]
+		return prev
-	}
-
-	if len(tparams) == 0 {
-		pr.typs[idx] = typ
 	}
 
+	*where = typ
 	return typ
 }
 
@@ -206,8 +229,7 @@ func (r *reader2) doTyp() (res types2.Type) {
 		return name.Type()
 
 	case typeTypeParam:
-		idx := r.len()
+		return r.dict.tparams[r.len()]
-		return r.tparams[idx].Type().(*types2.TypeParam)
 
 	case typeArray:
 		len := int64(r.uint64())
@@ -330,10 +352,12 @@ func (r *reader2) obj() (types2.Object, []types2.Type) {
 
 func (pr *pkgReader2) objIdx(idx int) (*types2.Package, string) {
 	r := pr.newReader(relocObj, idx, syncObject1)
+	r.dict = &reader2Dict{}
+
 	objPkg, objName := r.qualifiedIdent()
 	assert(objName != "")
 
-	bounds := r.typeParamBounds()
+	r.typeParamBounds()
 	tag := codeObj(r.code(syncCodeObj))
 
 	if tag == objStub {
@@ -341,6 +365,15 @@ func (pr *pkgReader2) objIdx(idx int) (*types2.Package, string) {
 		return objPkg, objName
 	}
 
+	{
+		rdict := r.p.newReader(relocObjDict, idx, syncObject1)
+		r.dict.derivedReloc = make([]int, rdict.len())
+		r.dict.derived = make([]types2.Type, len(r.dict.derivedReloc))
+		for i := range r.dict.derived {
+			r.dict.derivedReloc[i] = rdict.reloc(relocType)
+		}
+	}
+
 	objPkg.Scope().InsertLazy(objName, func() types2.Object {
 		switch tag {
 		default:
@@ -358,21 +391,16 @@ func (pr *pkgReader2) objIdx(idx int) (*types2.Package, string) {
 
 		case objFunc:
 			pos := r.pos()
-			r.typeParamNames(bounds)
+			tparams := r.typeParamNames()
 			sig := r.signature(nil)
-			if len(r.tparams) != 0 {
+			sig.SetTParams(tparams)
-				sig.SetTParams(r.tparams)
-			}
 			return types2.NewFunc(pos, objPkg, objName, sig)
 
 		case objType:
 			pos := r.pos()
 
 			return types2.NewTypeNameLazy(pos, objPkg, objName, func(named *types2.Named) (tparams []*types2.TypeName, underlying types2.Type, methods []*types2.Func) {
-				r.typeParamNames(bounds)
+				tparams = r.typeParamNames()
-				if len(r.tparams) != 0 {
-					tparams = r.tparams
-				}
 
 				// TODO(mdempsky): Rewrite receiver types to underlying is an
 				// Interface? The go/types importer does this (I think because
@@ -382,7 +410,7 @@ func (pr *pkgReader2) objIdx(idx int) (*types2.Package, string) {
 
 				methods = make([]*types2.Func, r.len())
 				for i := range methods {
-					methods[i] = r.method(bounds)
+					methods[i] = r.method()
 				}
 
 				return
@@ -403,51 +431,73 @@ func (r *reader2) value() (types2.Type, constant.Value) {
 	return r.typ(), r.rawValue()
 }
 
-func (r *reader2) typeParamBounds() []int {
+func (r *reader2) typeParamBounds() {
 	r.sync(syncTypeParamBounds)
 
-	// exported types never have implicit type parameters
+	if implicits := r.len(); implicits != 0 {
-	// TODO(mdempsky): Hide this from public importer.
+		base.Fatalf("unexpected object with %v implicit type parameter(s)", implicits)
-	assert(r.len() == 0)
+	}
-
+
-	bounds := make([]int, r.len())
+	r.dict.bounds = make([]reader2TypeBound, r.len())
-	for i := range bounds {
+	for i := range r.dict.bounds {
-		r.sync(syncType)
+		b := &r.dict.bounds[i]
-		bounds[i] = r.reloc(relocType)
+		r.sync(syncType)
+		b.derived = r.bool()
+		if b.derived {
+			b.boundIdx = r.len()
+		} else {
+			b.boundIdx = r.reloc(relocType)
+		}
 	}
-	return bounds
 }
 
-func (r *reader2) typeParamNames(bounds []int) {
+func (r *reader2) typeParamNames() []*types2.TypeName {
 	r.sync(syncTypeParamNames)
 
-	r.tparams = make([]*types2.TypeName, len(bounds))
+	// Note: This code assumes it only processes objects without
+	// implement type parameters. This is currently fine, because
+	// reader2 is only used to read in exported declarations, which are
+	// always package scoped.
 
-	for i := range r.tparams {
+	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.
+
+	names := make([]*types2.TypeName, len(r.dict.bounds))
+	r.dict.tparams = make([]*types2.TypeParam, len(r.dict.bounds))
+	for i := range r.dict.bounds {
 		pos := r.pos()
 		pkg, name := r.localIdent()
 
-		obj := types2.NewTypeName(pos, pkg, name, nil)
+		names[i] = types2.NewTypeName(pos, pkg, name, nil)
-		r.p.check.NewTypeParam(obj, i, nil)
+		r.dict.tparams[i] = r.p.check.NewTypeParam(names[i], i, nil)
-		r.tparams[i] = obj
 	}
 
-	for i, tparam := range r.tparams {
+	for i, bound := range r.dict.bounds {
-		bound := r.p.typIdx(bounds[i], r.tparams)
+		var dict *reader2Dict
-		tparam.Type().(*types2.TypeParam).SetBound(bound)
+		if bound.derived {
+			dict = r.dict
 		}
+		boundType := r.p.typIdx(bound.boundIdx, dict)
+		r.dict.tparams[i].SetBound(boundType)
+	}
+
+	return names
 }
 
-func (r *reader2) method(bounds []int) *types2.Func {
+func (r *reader2) method() *types2.Func {
 	r.sync(syncMethod)
 	pos := r.pos()
 	pkg, name := r.selector()
 
-	r.typeParamNames(bounds)
+	rparams := r.typeParamNames()
 	sig := r.signature(r.param())
-	if len(r.tparams) != 0 {
+	sig.SetRParams(rparams)
-		sig.SetRParams(r.tparams)
-	}
 
 	_ = r.pos() // TODO(mdempsky): Remove; this is a hacker for linker.go.
 	return types2.NewFunc(pos, pkg, name, sig)

src/cmd/compile/internal/noder/reloc.go

@@ -34,6 +34,7 @@ const (
 	relocType
 	relocObj
 	relocObjExt
+	relocObjDict
 	relocBody
 
 	numRelocs = iota

src/cmd/compile/internal/noder/unified.go

@@ -122,7 +122,7 @@ func unified(noders []*noder) {
 
 		// Instantiated generic function: add to Decls for typechecking
 		// and compilation.
-		if len(pri.implicits) != 0 && fn.OClosure == nil {
+		if pri.dict != nil && len(pri.dict.targs) != 0 && fn.OClosure == nil {
 			target.Decls = append(target.Decls, fn)
 		}
 	}

src/cmd/compile/internal/noder/writer.go

@@ -87,20 +87,27 @@ type writer struct {
 	// scope closes, and then maybe we can just use the same map for
 	// storing the TypeParams too (as their TypeName instead).
 
-	// type parameters. explicitIdx has the type parameters declared on
-	// the current object, while implicitIdx has the type parameters
-	// declared on the enclosing object (if any).
-	//
-	// TODO(mdempsky): Merge these back together, now that I've got them
-	// working.
-	implicitIdx map[*types2.TypeParam]int
-	explicitIdx map[*types2.TypeParam]int
-
 	// variables declared within this function
 	localsIdx map[*types2.Var]int
 
 	closureVars    []posObj
 	closureVarsIdx map[*types2.Var]int
+
+	dict    *writerDict
+	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 []int
+
+	// derivedIdx maps a Type to its corresponding index within the
+	// derived slice, if present.
+	derivedIdx map[types2.Type]int
 }
 
 func (pw *pkgWriter) newWriter(k reloc, marker syncMarker) *writer {
@@ -193,30 +200,39 @@ func (pw *pkgWriter) pkgIdx(pkg *types2.Package) int {
 // @@@ Types
 
 func (w *writer) typ(typ types2.Type) {
+	idx, derived := w.p.typIdx(typ, w.dict)
+
 	w.sync(syncType)
-
+	if w.bool(derived) {
-	if quirksMode() {
+		w.len(idx)
-		typ = w.p.dups.orig(typ)
+		w.derived = true
+	} else {
+		w.reloc(relocType, idx)
 	}
-
-	w.reloc(relocType, w.p.typIdx(typ, w.implicitIdx, w.explicitIdx))
 }
 
-func (pw *pkgWriter) typIdx(typ types2.Type, implicitIdx, explicitIdx map[*types2.TypeParam]int) int {
+// 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) (int, bool) {
+	if quirksMode() {
+		typ = pw.dups.orig(typ)
+	}
+
 	if idx, ok := pw.typsIdx[typ]; ok {
-		return idx
+		return idx, false
+	}
+	if dict != nil {
+		if idx, ok := dict.derivedIdx[typ]; ok {
+			return idx, true
+		}
 	}
 
 	w := pw.newWriter(relocType, syncTypeIdx)
-	w.implicitIdx = implicitIdx
+	w.dict = dict
-	w.explicitIdx = explicitIdx
 
-	pw.typsIdx[typ] = w.idx // handle cycles
-	w.doTyp(typ)
-	return w.flush()
-}
-
-func (w *writer) doTyp(typ types2.Type) {
 	switch typ := typ.(type) {
 	default:
 		base.Fatalf("unexpected type: %v (%T)", typ, typ)
@@ -251,14 +267,19 @@ func (w *writer) doTyp(typ types2.Type) {
 		w.obj(orig.Obj(), typ.TArgs())
 
 	case *types2.TypeParam:
-		w.code(typeTypeParam)
+		index := func() int {
-		if idx, ok := w.implicitIdx[typ]; ok {
+			for idx, name := range w.dict.implicits {
-			w.len(idx)
+				if name.Type().(*types2.TypeParam) == typ {
-		} else if idx, ok := w.explicitIdx[typ]; ok {
+					return idx
-			w.len(len(w.implicitIdx) + idx)
-		} else {
-			w.p.fatalf(typ.Obj(), "%v not in %v or %v", typ, w.implicitIdx, w.explicitIdx)
 				}
+			}
+
+			return len(w.dict.implicits) + typ.Index()
+		}()
+
+		w.derived = true
+		w.code(typeTypeParam)
+		w.len(index)
 
 	case *types2.Array:
 		w.code(typeArray)
@@ -300,6 +321,16 @@ func (w *writer) doTyp(typ types2.Type) {
 		w.code(typeUnion)
 		w.unionType(typ)
 	}
+
+	if w.derived {
+		idx := len(dict.derived)
+		dict.derived = append(dict.derived, w.flush())
+		dict.derivedIdx[typ] = idx
+		return idx, true
+	}
+
+	pw.typsIdx[typ] = w.idx
+	return w.flush(), false
 }
 
 func (w *writer) structType(typ *types2.Struct) {
@@ -367,13 +398,16 @@ func (w *writer) param(param *types2.Var) {
 // @@@ Objects
 
 func (w *writer) obj(obj types2.Object, explicits []types2.Type) {
-	w.sync(syncObject)
+	if isDefinedType(obj) && obj.Pkg() == w.p.curpkg {
-
+		decl, ok := w.p.typDecls[obj.(*types2.TypeName)]
-	var implicitIdx map[*types2.TypeParam]int
+		assert(ok)
-	if isDefinedType(obj) && !isGlobal(obj) {
+		if len(decl.implicits) != 0 {
-		implicitIdx = w.implicitIdx
+			w.derived = true
 		}
-	w.reloc(relocObj, w.p.objIdx(obj, implicitIdx))
+	}
+
+	w.sync(syncObject)
+	w.reloc(relocObj, w.p.objIdx(obj))
 
 	w.len(len(explicits))
 	for _, explicit := range explicits {
@@ -381,37 +415,61 @@ func (w *writer) obj(obj types2.Object, explicits []types2.Type) {
 	}
 }
 
-func (pw *pkgWriter) objIdx(obj types2.Object, implicitIdx map[*types2.TypeParam]int) int {
+func (pw *pkgWriter) objIdx(obj types2.Object) int {
 	if idx, ok := pw.globalsIdx[obj]; ok {
 		return idx
 	}
 
+	dict := &writerDict{
+		derivedIdx: make(map[types2.Type]int),
+	}
+
+	if isDefinedType(obj) && obj.Pkg() == pw.curpkg {
+		decl, ok := pw.typDecls[obj.(*types2.TypeName)]
+		assert(ok)
+		dict.implicits = decl.implicits
+	}
+
 	w := pw.newWriter(relocObj, syncObject1)
 	w.ext = pw.newWriter(relocObjExt, syncObject1)
+	wdict := pw.newWriter(relocObjDict, syncObject1)
+
+	pw.globalsIdx[obj] = w.idx // break cycles
 	assert(w.ext.idx == w.idx)
+	assert(wdict.idx == w.idx)
 
-	pw.globalsIdx[obj] = w.idx
+	w.dict = dict
+	w.ext.dict = dict
 
-	w.implicitIdx = implicitIdx
+	// Ident goes first so importer can avoid unnecessary work if
-	w.ext.implicitIdx = implicitIdx
+	// they've already resolved this object.
+	w.qualifiedIdent(obj)
+
+	w.typeParamBounds(objTypeParams(obj))
 
 	w.doObj(obj)
 
 	w.flush()
 	w.ext.flush()
 
+	// Done writing out the object description; write out the list of
+	// derived types that we found along the way.
+	//
+	// TODO(mdempsky): Record details about how derived types are
+	// actually used so reader can optimize its runtime dictionaries.
+	//
+	// TODO(mdempsky): Record details about which instantiated functions
+	// are used too.
+	wdict.len(len(dict.derived))
+	for _, typ := range dict.derived {
+		wdict.reloc(relocType, typ)
+	}
+	wdict.flush()
+
 	return w.idx
 }
 
 func (w *writer) doObj(obj types2.Object) {
-	// Ident goes first so importer can avoid unnecessary work if
-	// they've already resolved this object.
-	w.qualifiedIdent(obj)
-
-	tparams := objTypeParams(obj)
-	w.setTypeParams(tparams)
-	w.typeParamBounds(tparams)
-
 	if obj.Pkg() != w.p.curpkg {
 		w.code(objStub)
 		return
@@ -504,29 +562,12 @@ func (w *writer) value(typ types2.Type, val constant.Value) {
 	w.rawValue(val)
 }
 
-func (w *writer) setTypeParams(tparams []*types2.TypeName) {
-	if len(tparams) == 0 {
-		return
-	}
-
-	explicitIdx := make(map[*types2.TypeParam]int)
-	for _, tparam := range tparams {
-		explicitIdx[tparam.Type().(*types2.TypeParam)] = len(explicitIdx)
-	}
-
-	w.explicitIdx = explicitIdx
-	w.ext.explicitIdx = explicitIdx
-}
-
 func (w *writer) typeParamBounds(tparams []*types2.TypeName) {
 	w.sync(syncTypeParamBounds)
 
-	// TODO(mdempsky): Remove. It's useful for debugging at the moment,
+	w.len(len(w.dict.implicits))
-	// but it doesn't belong here.
-	w.len(len(w.implicitIdx))
-	w.len(len(w.explicitIdx))
-	assert(len(w.explicitIdx) == len(tparams))
 
+	w.len(len(tparams))
 	for _, tparam := range tparams {
 		w.typ(tparam.Type().(*types2.TypeParam).Bound())
 	}
@@ -546,9 +587,6 @@ func (w *writer) method(meth *types2.Func) {
 	assert(ok)
 	sig := meth.Type().(*types2.Signature)
 
-	assert(len(w.explicitIdx) == len(sig.RParams()))
-	w.setTypeParams(sig.RParams())
-
 	w.sync(syncMethod)
 	w.pos(meth)
 	w.selector(meth)
@@ -566,12 +604,15 @@ func (w *writer) qualifiedIdent(obj types2.Object) {
 	w.sync(syncSym)
 
 	name := obj.Name()
-	if isDefinedType(obj) && !isGlobal(obj) {
+	if isDefinedType(obj) && obj.Pkg() == w.p.curpkg {
-		// TODO(mdempsky): Find a better solution, this is terrible.
 		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)
@@ -630,7 +671,7 @@ func (w *writer) funcExt(obj *types2.Func) {
 	}
 
 	sig, block := obj.Type().(*types2.Signature), decl.Body
-	body, closureVars := w.p.bodyIdx(w.p.curpkg, sig, block, w.explicitIdx)
+	body, closureVars := w.p.bodyIdx(w.p.curpkg, sig, block, w.dict)
 	assert(len(closureVars) == 0)
 
 	w.sync(syncFuncExt)
@@ -672,9 +713,9 @@ func (w *writer) pragmaFlag(p ir.PragmaFlag) {
 
 // @@@ Function bodies
 
-func (pw *pkgWriter) bodyIdx(pkg *types2.Package, sig *types2.Signature, block *syntax.BlockStmt, implicitIdx map[*types2.TypeParam]int) (idx int, closureVars []posObj) {
+func (pw *pkgWriter) bodyIdx(pkg *types2.Package, sig *types2.Signature, block *syntax.BlockStmt, dict *writerDict) (idx int, closureVars []posObj) {
 	w := pw.newWriter(relocBody, syncFuncBody)
-	w.implicitIdx = implicitIdx
+	w.dict = dict
 
 	w.funcargs(sig)
 	if w.bool(block != nil) {
@@ -1238,14 +1279,13 @@ func (w *writer) funcLit(expr *syntax.FuncLit) {
 	assert(ok)
 	sig := tv.Type.(*types2.Signature)
 
+	body, closureVars := w.p.bodyIdx(w.p.curpkg, sig, expr.Body, w.dict)
+
 	w.sync(syncFuncLit)
 	w.pos(expr)
 	w.pos(expr.Type) // for QuirksMode
 	w.signature(sig)
 
-	block := expr.Body
-	body, closureVars := w.p.bodyIdx(w.p.curpkg, sig, block, w.implicitIdx)
-
 	w.len(len(closureVars))
 	for _, cv := range closureVars {
 		w.pos(cv.pos)
@@ -1297,6 +1337,9 @@ func (w *writer) op(op ir.Op) {
 type typeDeclGen struct {
 	*syntax.TypeDecl
 	gen int
+
+	// Implicit type parameters in scope at this type declaration.
+	implicits []*types2.TypeName
 }
 
 type fileImports struct {
@@ -1308,6 +1351,19 @@ type declCollector struct {
 	typegen    *int
 	file       *fileImports
 	withinFunc bool
+	implicits  []*types2.TypeName
+}
+
+func (c *declCollector) withTParams(obj types2.Object) *declCollector {
+	tparams := objTypeParams(obj)
+	if len(tparams) == 0 {
+		return c
+	}
+
+	copy := *c
+	copy.implicits = copy.implicits[:len(copy.implicits):len(copy.implicits)]
+	copy.implicits = append(copy.implicits, objTypeParams(obj)...)
+	return &copy
 }
 
 func (c *declCollector) Visit(n syntax.Node) syntax.Visitor {
@@ -1336,9 +1392,11 @@ func (c *declCollector) Visit(n syntax.Node) syntax.Visitor {
 		obj := pw.info.Defs[n.Name].(*types2.Func)
 		pw.funDecls[obj] = n
 
+		return c.withTParams(obj)
+
 	case *syntax.TypeDecl:
 		obj := pw.info.Defs[n.Name].(*types2.TypeName)
-		d := typeDeclGen{TypeDecl: n}
+		d := typeDeclGen{TypeDecl: n, implicits: c.implicits}
 
 		if n.Alias {
 			pw.checkPragmas(n.Pragma, 0, false)
@@ -1346,7 +1404,7 @@ func (c *declCollector) Visit(n syntax.Node) syntax.Visitor {
 			pw.checkPragmas(n.Pragma, typePragmas, false)
 
 			// Assign a unique ID to function-scoped defined types.
-			if !isGlobal(obj) {
+			if c.withinFunc {
 				*c.typegen++
 				d.gen = *c.typegen
 			}
@@ -1354,6 +1412,12 @@ func (c *declCollector) Visit(n syntax.Node) syntax.Visitor {
 
 		pw.typDecls[obj] = d
 
+		// TODO(mdempsky): Omit? Not strictly necessary; only matters for
+		// type declarations within function literals within parameterized
+		// type declarations, but types2 the function literals will be
+		// constant folded away.
+		return c.withTParams(obj)
+
 	case *syntax.VarDecl:
 		pw.checkPragmas(n.Pragma, 0, true)
 
@@ -1510,8 +1574,11 @@ func (w *writer) pkgDecl(decl syntax.Decl) {
 			break // skip generic type decls
 		}
 
-		name := w.p.info.Defs[decl.Name].(*types2.TypeName)
+		if decl.Name.Value == "_" {
+			break // skip blank type decls
+		}
 
+		name := w.p.info.Defs[decl.Name].(*types2.TypeName)
 		// Skip type declarations for interfaces that are only usable as
 		// type parameter bounds.
 		if iface, ok := name.Type().Underlying().(*types2.Interface); ok && iface.IsConstraint() {
@@ -1671,7 +1738,11 @@ func fieldIndex(info *types2.Info, str *types2.Struct, key *syntax.Name) int {
 func objTypeParams(obj types2.Object) []*types2.TypeName {
 	switch obj := obj.(type) {
 	case *types2.Func:
-		return obj.Type().(*types2.Signature).TParams()
+		sig := obj.Type().(*types2.Signature)
+		if sig.Recv() != nil {
+			return sig.RParams()
+		}
+		return sig.TParams()
 	case *types2.TypeName:
 		if !obj.IsAlias() {
 			return obj.Type().(*types2.Named).TParams()