go.tools/go/importer: fine-tuning of export format, test against std lib

(TBR adonovan)

- provide version string rather than version number (more flexible)
- match naming of tag constants with documentation (forth-coming)
- more formats for constant values (1-byte booleans, short 64bit
  values, support for floating point numbers +123p+45)
- more regular format for signatures: always print with type tag;
  this permits (future) sharing of identical signatures and possibly
  large export data size reduction
- test export and import of all type-checked std library packages
- compare import of all std library packages against corresponding
  import of gc-generated export data and verify that the packages
  match 100% (except for floating point values which are exact when
  coming from go/types, and possibly inexact when coming from gc)

On average, export data generated by the importer is ~28% of the size
of the gc-generated export data for corresponding packages of the std
library:

        package 		size	gc size

        archive/tar		2945	12030	24%
        archive/zip		4122	15796	26%
        bufio			1776	6171	28%
        bytes			2540	8006	31%
        compress/bzip2		146	419	34%
        compress/flate		2305	6356	36%
        compress/gzip		4262	17917	23%
        compress/lzw		336	832	40%
        compress/zlib		2503	9814	25%
        container/heap		255	527	48%
        container/list		653	2971	21%
        container/ring		246	707	34%
        crypto			368	1015	36%
        crypto/aes		219	499	43%
        crypto/cipher		809	1875	43%
        crypto/des		254	636	39%
        crypto/dsa		3324	15609	21%
        crypto/ecdsa		3833	18567	20%
        crypto/elliptic		3801	17417	21%
        crypto/hmac		205	459	44%
        crypto/md5		206	413	49%
        crypto/rand		3070	15047	20%
        crypto/rc4		180	443	40%
        crypto/rsa		4205	17941	23%
        crypto/sha1		208	414	50%
        crypto/sha256		272	555	49%
        crypto/sha512		274	555	49%
        crypto/subtle		247	1157	21%
        crypto/tls		13769	48330	28%
        crypto/x509		10716	40556	26%
        crypto/x509/pkix	5670	24616	23%
        database/sql		4227	15957	26%
        database/sql/driver	1239	2806	44%
        debug/dwarf		5165	12066	42%
        debug/elf		14205	36401	39%
        debug/goobj		992	2495	39%
        debug/gosym		1808	4798	37%
        debug/macho		4560	13173	34%
        debug/pe		3694	10530	35%
        encoding		261	387	67%
        encoding/ascii85	368	1541	23%
        encoding/asn1		479	1661	28%
        encoding/base32		608	2636	23%
        encoding/base64		610	2636	23%
        encoding/binary		1006	3591	28%
        encoding/csv		2287	8405	27%
        encoding/gob		9161	30565	29%
        encoding/hex		384	1331	28%
        encoding/json		6564	23334	28%
        encoding/pem		208	529	39%
        encoding/xml		7964	27220	29%
        errors			52	277	18%
        expvar			955	2978	32%
        flag			2475	18028	13%
        fmt			1141	3141	36%
        go/ast			11595	41019	28%
        go/build		3736	13920	26%
        go/doc			2647	9523	27%
        go/format		1310	5010	26%
        go/parser		4346	18525	23%
        go/printer		1737	6442	26%
        go/scanner		2360	8598	27%
        go/token		2290	7599	30%
        hash			215	595	36%
        hash/adler32		217	391	55%
        hash/crc32		399	1475	27%
        hash/crc64		324	1327	24%
        hash/fnv		338	3964	8%
        html			80	253	31%
        html/template		7981	37471	21%
        image			4270	18894	22%
        image/color		845	3154	26%
        image/color/palette	121	319	37%
        image/draw		947	4807	19%
        image/gif		1468	6611	22%
        image/jpeg		971	4534	21%
        image/png		880	4432	19%
        index/suffixarray	3702	13960	26%
        io			2049	6117	33%
        io/ioutil		2845	12060	23%
        log			2416	10241	23%
        log/syslog		3102	12208	25%
        math			2361	6008	39%
        math/big		3648	12250	29%
        math/cmplx		473	1542	30%
        math/rand		786	2002	39%
        mime			180	595	30%
        mime/multipart		2761	9338	29%
        net			10593	34352	30%
        net/http		24542	84065	29%
        net/http/cgi		12859	50970	25%
        net/http/cookiejar	2845	12429	22%
        net/http/fcgi		12331	49111	25%
        net/http/httptest	18953	75243	25%
        net/http/httputil	13994	54386	25%
        net/http/pprof		12153	49039	24%
        net/mail		1803	8599	20%
        net/rpc			18503	69044	26%
        net/rpc/jsonrpc		1063	2737	38%
        net/smtp		11775	46821	25%
        net/textproto		2684	8678	30%
        net/url			767	2886	26%
        os			5712	20293	28%
        os/exec			4705	18350	25%
        os/signal		125	330	37%
        os/user			234	720	32%
        path			231	770	30%
        path/filepath		2236	9721	23%
        reflect			4518	13467	33%
        regexp			3608	11167	32%
        regexp/syntax		2184	5157	42%
        runtime			1563	3407	45%
        runtime/cgo		36	66	54%
        runtime/debug		1714	8010	21%
        runtime/pprof		485	1358	35%
        runtime/race		38	43	88%
        sort			748	2658	28%
        strconv			949	2573	36%
        strings			1846	6220	29%
        sync			712	1970	36%
        sync/atomic		1089	2506	43%
        syscall			38975	79396	49%
        testing			3734	13626	27%
        testing/iotest		332	2378	13%
        testing/quick		4411	16434	26%
        text/scanner		1821	5349	34%
        text/tabwriter		1593	4937	32%
        text/template		6882	33329	20%
        text/template/parse	3946	12760	30%
        time			3031	9703	31%
        unicode			3148	8676	36%
        unicode/utf16		163	760	21%
        unicode/utf8		475	1122	42%

        total			463641	1644936	28%

R=adonovan
TBR=adonovan
CC=golang-codereviews
https://golang.org/cl/37250044

go/importer/export.go

@@ -23,30 +23,40 @@ const (
 
 const (
 	magic   = "\n$$ exports $$\n"
-	version = 0
+	version = "v0"
 )
 
 // Object and type tags. Must be < 0.
 const (
+	// Packages
+	packageTag = -(iota + 1)
+
 	// Objects
-	_Package = -(iota + 1)
-	_Const
-	_Type
-	_Var
-	_Func
+	constTag
+	typeTag
+	varTag
+	funcTag
 
 	// Types
-	_Basic
-	_Array
-	_Slice
-	_Struct
-	_Pointer
-	_Tuple
-	_Signature
-	_Interface
-	_Map
-	_Chan
-	_Named
+	basicTag
+	arrayTag
+	sliceTag
+	structTag
+	pointerTag
+	signatureTag
+	interfaceTag
+	mapTag
+	chanTag
+	namedTag
+
+	// Values
+	intTag
+	floatTag
+	fractionTag
+	complexTag
+	stringTag
+	falseTag
+	trueTag
 )
 
 // ExportData serializes the interface (exported package objects)
@@ -72,7 +82,7 @@ func ExportData(pkg *types.Package) []byte {
 		defer p.tracef("\n")
 	}
 
-	p.int(version)
+	p.string(version)
 
 	p.pkg(pkg)
 
@@ -117,7 +127,7 @@ func (p *exporter) pkg(pkg *types.Package) {
 	p.pkgIndex[pkg] = len(p.pkgIndex)
 
 	// otherwise, write the package tag (< 0) and package data
-	p.int(_Package)
+	p.int(packageTag)
 	p.string(pkg.Name())
 	p.string(pkg.Path())
 }
@@ -130,67 +140,98 @@ func (p *exporter) obj(obj types.Object) {
 
 	switch obj := obj.(type) {
 	case *types.Const:
-		p.int(_Const)
+		p.int(constTag)
 		p.string(obj.Name())
 		p.typ(obj.Type())
-		p.val(obj.Val())
+		p.value(obj.Val())
 	case *types.TypeName:
-		p.int(_Type)
+		p.int(typeTag)
 		// name is written by corresponding named type
 		p.typ(obj.Type().(*types.Named))
 	case *types.Var:
-		p.int(_Var)
+		p.int(varTag)
 		p.string(obj.Name())
 		p.typ(obj.Type())
 	case *types.Func:
-		p.int(_Func)
+		p.int(funcTag)
 		p.string(obj.Name())
-		p.signature(obj.Type().(*types.Signature))
+		p.typ(obj.Type())
 	default:
 		panic(fmt.Sprintf("unexpected object type %T", obj))
 	}
 }
 
-func (p *exporter) val(x exact.Value) {
+func (p *exporter) value(x exact.Value) {
 	if trace {
 		p.tracef("value { ")
 		defer p.tracef("} ")
 	}
 
-	kind := x.Kind()
-	p.int(int(kind))
-	switch kind {
+	switch kind := x.Kind(); kind {
 	case exact.Bool:
-		p.bool(exact.BoolVal(x))
+		tag := falseTag
+		if exact.BoolVal(x) {
+			tag = trueTag
+		}
+		p.int(tag)
 	case exact.String:
+		p.int(stringTag)
 		p.string(exact.StringVal(x))
 	case exact.Int:
-		p.intVal(x)
+		if i, ok := exact.Int64Val(x); ok {
+			p.int(intTag)
+			p.int64(i)
+			return
+		}
+		p.int(floatTag)
+		p.float(x)
 	case exact.Float:
-		p.floatVal(x)
+		p.int(fractionTag)
+		p.fraction(x)
 	case exact.Complex:
-		p.floatVal(exact.Real(x))
-		p.floatVal(exact.Imag(x))
+		p.int(complexTag)
+		p.fraction(exact.Real(x))
+		p.fraction(exact.Imag(x))
 	default:
 		panic(fmt.Sprintf("unexpected value kind %d", kind))
 	}
 }
 
-func (p *exporter) intVal(x exact.Value) {
+func (p *exporter) float(x exact.Value) {
 	sign := exact.Sign(x)
 	p.int(sign)
-	if sign != 0 {
-		p.bytes(exact.Bytes(x))
-	}
+	if sign == 0 {
+		return
 	}
 
-func (p *exporter) floatVal(x exact.Value) {
-	p.intVal(exact.Num(x))
-	if exact.Sign(x) != 0 {
-		// TODO(gri): For gc-generated constants, the denominator is
-		// often a large power of two. Use a more compact representation.
-		p.bytes(exact.Bytes(exact.Denom(x)))
+	p.ufloat(x)
 }
+
+func (p *exporter) fraction(x exact.Value) {
+	sign := exact.Sign(x)
+	p.int(sign)
+	if sign == 0 {
+		return
+	}
+
+	p.ufloat(exact.Num(x))
+	p.ufloat(exact.Denom(x))
+}
+
+func (p *exporter) ufloat(x exact.Value) {
+	mant := exact.Bytes(x)
+	exp8 := -1
+	for i, b := range mant {
+		if b != 0 {
+			exp8 = i
+			break
+		}
+	}
+	if exp8 < 0 {
+		panic(fmt.Sprintf("%s has no mantissa", x))
+	}
+	p.int(exp8 * 8)
+	p.bytes(mant[exp8:])
 }
 
 func (p *exporter) typ(typ types.Type) {
@@ -215,20 +256,20 @@ func (p *exporter) typ(typ types.Type) {
 		// This permits faithful reconstruction of the alias type (i.e.,
 		// keeping the name). If we decide to eliminate the distinction
 		// between the alias types, this code can go.
-		p.int(_Basic)
+		p.int(basicTag)
 		p.string(t.Name())
 
 	case *types.Array:
-		p.int(_Array)
-		p.typ(t.Elem())
+		p.int(arrayTag)
 		p.int64(t.Len())
+		p.typ(t.Elem())
 
 	case *types.Slice:
-		p.int(_Slice)
+		p.int(sliceTag)
 		p.typ(t.Elem())
 
 	case *types.Struct:
-		p.int(_Struct)
+		p.int(structTag)
 		n := t.NumFields()
 		p.int(n)
 		for i := 0; i < n; i++ {
@@ -237,35 +278,44 @@ func (p *exporter) typ(typ types.Type) {
 		}
 
 	case *types.Pointer:
-		p.int(_Pointer)
+		p.int(pointerTag)
 		p.typ(t.Elem())
 
 	case *types.Signature:
-		p.int(_Signature)
+		p.int(signatureTag)
 		p.signature(t)
 
 	case *types.Interface:
-		p.int(_Interface)
-		n := t.NumMethods()
+		p.int(interfaceTag)
+
+		// write methods
+		n := t.NumExplicitMethods()
 		p.int(n)
 		for i := 0; i < n; i++ {
-			m := t.Method(i)
+			m := t.ExplicitMethod(i)
 			p.qualifiedName(m.Pkg(), m.Name())
-			p.signature(m.Type().(*types.Signature))
+			p.typ(m.Type())
+		}
+
+		// write embedded interfaces
+		m := t.NumEmbeddeds()
+		p.int(m)
+		for i := 0; i < m; i++ {
+			p.typ(t.Embedded(i))
 		}
 
 	case *types.Map:
-		p.int(_Map)
+		p.int(mapTag)
 		p.typ(t.Key())
 		p.typ(t.Elem())
 
 	case *types.Chan:
-		p.int(_Chan)
+		p.int(chanTag)
 		p.int(int(t.Dir()))
 		p.typ(t.Elem())
 
 	case *types.Named:
-		p.int(_Named)
+		p.int(namedTag)
 
 		// write type object
 		obj := t.Obj()
@@ -281,7 +331,7 @@ func (p *exporter) typ(typ types.Type) {
 		for i := 0; i < n; i++ {
 			m := t.Method(i)
 			p.string(m.Name())
-			p.signature(m.Type().(*types.Signature))
+			p.typ(m.Type())
 		}
 
 	default:
@@ -302,7 +352,7 @@ func (p *exporter) field(f *types.Var) {
 
 func (p *exporter) qualifiedName(pkg *types.Package, name string) {
 	p.string(name)
-	// exported names don't write package
+	// exported names don't need package
 	if !exported(name) {
 		if pkg == nil {
 			panic(fmt.Sprintf("nil package for unexported qualified name %s", name))

go/importer/import.go

@@ -42,7 +42,7 @@ func ImportData(imports map[string]*types.Package, data []byte) (*types.Package,
 	}
 	p.typList = append(p.typList, types.Universe.Lookup("error").Type())
 
-	if v := p.int(); v != version {
+	if v := p.string(); v != version {
 		return nil, fmt.Errorf("unknown version: got %d; want %d", v, version)
 	}
 
@@ -85,7 +85,7 @@ func (p *importer) pkg() *types.Package {
 	}
 
 	// otherwise, i is the package tag (< 0)
-	if i != _Package {
+	if i != packageTag {
 		panic(fmt.Sprintf("unexpected package tag %d", i))
 	}
 
@@ -107,16 +107,16 @@ func (p *importer) pkg() *types.Package {
 func (p *importer) obj(pkg *types.Package) {
 	var obj types.Object
 	switch tag := p.int(); tag {
-	case _Const:
-		obj = types.NewConst(token.NoPos, pkg, p.string(), p.typ(), p.val())
-	case _Type:
+	case constTag:
+		obj = types.NewConst(token.NoPos, pkg, p.string(), p.typ(), p.value())
+	case typeTag:
 		// type object is added to scope via respective named type
 		_ = p.typ().(*types.Named)
 		return
-	case _Var:
+	case varTag:
 		obj = types.NewVar(token.NoPos, pkg, p.string(), p.typ())
-	case _Func:
-		obj = types.NewFunc(token.NoPos, pkg, p.string(), p.signature())
+	case funcTag:
+		obj = types.NewFunc(token.NoPos, pkg, p.string(), p.typ().(*types.Signature))
 	default:
 		panic(fmt.Sprintf("unexpected object tag %d", tag))
 	}
@@ -126,43 +126,64 @@ func (p *importer) obj(pkg *types.Package) {
 	}
 }
 
-func (p *importer) val() exact.Value {
+func (p *importer) value() exact.Value {
 	switch kind := exact.Kind(p.int()); kind {
-	case exact.Bool:
-		return exact.MakeBool(p.bool())
-	case exact.String:
+	case falseTag:
+		return exact.MakeBool(false)
+	case trueTag:
+		return exact.MakeBool(true)
+	case stringTag:
 		return exact.MakeString(p.string())
-	case exact.Int:
-		return p.intVal()
-	case exact.Float:
-		return p.floatVal()
-	case exact.Complex:
-		re := p.floatVal()
-		im := p.floatVal()
+	case intTag:
+		return exact.MakeInt64(p.int64())
+	case floatTag:
+		return p.float()
+	case fractionTag:
+		return p.fraction()
+	case complexTag:
+		re := p.fraction()
+		im := p.fraction()
 		return exact.BinaryOp(re, token.ADD, exact.MakeImag(im))
 	default:
 		panic(fmt.Sprintf("unexpected value kind %d", kind))
 	}
 }
 
-func (p *importer) intVal() exact.Value {
+func (p *importer) float() exact.Value {
 	sign := p.int()
-	var bytes []byte
-	if sign != 0 {
-		bytes = p.bytes()
+	if sign == 0 {
+		return exact.MakeInt64(0)
 	}
-	x := exact.MakeFromBytes(bytes)
+
+	x := p.ufloat()
 	if sign < 0 {
 		x = exact.UnaryOp(token.SUB, x, 0)
 	}
 	return x
 }
 
-func (p *importer) floatVal() exact.Value {
-	x := p.intVal()
-	if exact.Sign(x) != 0 {
-		y := exact.MakeFromBytes(p.bytes())
-		x = exact.BinaryOp(x, token.QUO, y)
+func (p *importer) fraction() exact.Value {
+	sign := p.int()
+	if sign == 0 {
+		return exact.MakeInt64(0)
+	}
+
+	x := exact.BinaryOp(p.ufloat(), token.QUO, p.ufloat())
+	if sign < 0 {
+		x = exact.UnaryOp(token.SUB, x, 0)
+	}
+	return x
+}
+
+func (p *importer) ufloat() exact.Value {
+	exp := p.int()
+	x := exact.MakeFromBytes(p.bytes())
+	switch {
+	case exp < 0:
+		d := exact.Shift(exact.MakeInt64(1), token.SHL, uint(-exp))
+		x = exact.BinaryOp(x, token.QUO, d)
+	case exp > 0:
+		x = exact.Shift(x, token.SHL, uint(exp))
 	}
 	return x
 }
@@ -180,26 +201,27 @@ func (p *importer) typ() types.Type {
 
 	// otherwise, i is the type tag (< 0)
 	switch i {
-	case _Basic:
+	case basicTag:
 		t := types.Universe.Lookup(p.string()).(*types.TypeName).Type().(*types.Basic)
 		p.record(t)
 		return t
 
-	case _Array:
+	case arrayTag:
 		t := new(types.Array)
 		p.record(t)
 
-		*t = *types.NewArray(p.typ(), p.int64())
+		n := p.int64()
+		*t = *types.NewArray(p.typ(), n)
 		return t
 
-	case _Slice:
+	case sliceTag:
 		t := new(types.Slice)
 		p.record(t)
 
 		*t = *types.NewSlice(p.typ())
 		return t
 
-	case _Struct:
+	case structTag:
 		t := new(types.Struct)
 		p.record(t)
 
@@ -213,47 +235,52 @@ func (p *importer) typ() types.Type {
 		*t = *types.NewStruct(fields, tags)
 		return t
 
-	case _Pointer:
+	case pointerTag:
 		t := new(types.Pointer)
 		p.record(t)
 
 		*t = *types.NewPointer(p.typ())
 		return t
 
-	case _Signature:
+	case signatureTag:
 		t := new(types.Signature)
 		p.record(t)
 
 		*t = *p.signature()
 		return t
 
-	case _Interface:
+	case interfaceTag:
 		t := new(types.Interface)
 		p.record(t)
 
 		methods := make([]*types.Func, p.int())
 		for i := range methods {
 			pkg, name := p.qualifiedName()
-			methods[i] = types.NewFunc(token.NoPos, pkg, name, p.signature())
+			methods[i] = types.NewFunc(token.NoPos, pkg, name, p.typ().(*types.Signature))
 		}
-		*t = *types.NewInterface(methods, nil)
+
+		embeddeds := make([]*types.Named, p.int())
+		for i := range embeddeds {
+			embeddeds[i] = p.typ().(*types.Named)
+		}
+		*t = *types.NewInterface(methods, embeddeds)
 		return t
 
-	case _Map:
+	case mapTag:
 		t := new(types.Map)
 		p.record(t)
 
 		*t = *types.NewMap(p.typ(), p.typ())
 		return t
 
-	case _Chan:
+	case chanTag:
 		t := new(types.Chan)
 		p.record(t)
 
 		*t = *types.NewChan(types.ChanDir(p.int()), p.typ())
 		return t
 
-	case _Named:
+	case namedTag:
 		// import type object
 		name := p.string()
 		pkg := p.pkg()
@@ -277,7 +304,7 @@ func (p *importer) typ() types.Type {
 		// read associated methods
 		n := p.int()
 		for i := 0; i < n; i++ {
-			t.AddMethod(types.NewFunc(token.NoPos, pkg, p.string(), p.signature()))
+			t.AddMethod(types.NewFunc(token.NoPos, pkg, p.string(), p.typ().(*types.Signature)))
 		}
 
 		return t

go/importer/import_test.go

@@ -5,19 +5,27 @@
 package importer
 
 import (
+	"bufio"
 	"bytes"
+	"flag"
 	"fmt"
 	"go/ast"
 	"go/build"
 	"go/parser"
 	"go/token"
+	"os"
 	"path/filepath"
+	"runtime"
+	"sort"
 	"testing"
+	"time"
 
-	_ "code.google.com/p/go.tools/go/gcimporter"
+	"code.google.com/p/go.tools/go/gcimporter"
 	"code.google.com/p/go.tools/go/types"
 )
 
+var verbose = flag.Bool("importer.v", false, "verbose mode")
+
 var tests = []string{
 	`package p`,
 
@@ -84,34 +92,60 @@ func TestImportSrc(t *testing.T) {
 			t.Errorf("typecheck failed: %s", err)
 			continue
 		}
-		testExportImport(t, pkg)
+		testExportImport(t, pkg, "")
 	}
 }
 
-// TODO(gri) expand to std library
-var libs = []string{
-	"../exact",
-	"../gcimporter",
-	"../importer",
-	"../types",
-	"../types/typemap",
-}
+func TestImportStdLib(t *testing.T) {
+	start := time.Now()
 
-func TestImportLib(t *testing.T) {
-	// TODO(gri) Enable once we can run these tests on builders.
-	return
-	for _, lib := range libs {
-		pkg, err := pkgFor(lib)
+	libs, err := stdLibs()
 	if err != nil {
+		t.Fatalf("could not compute list of std libraries: %s", err)
+	}
+
+	// make sure printed go/types types and gc-imported types
+	// can be compared reasonably well
+	types.GcCompatibilityMode = true
+
+	var totSize, totGcsize, n int
+	for _, lib := range libs {
+		// limit run time for short tests
+		if testing.Short() && time.Since(start) >= 750*time.Millisecond {
+			return
+		}
+
+		pkg, err := pkgForPath(lib)
+		switch err := err.(type) {
+		case nil:
+			// ok
+		case *build.NoGoError:
+			// no Go files - ignore
+			continue
+		default:
 			t.Errorf("typecheck failed: %s", err)
 			continue
 		}
-		testExportImport(t, pkg)
+
+		size, gcsize := testExportImport(t, pkg, lib)
+		if *verbose {
+			fmt.Printf("%s\t%d\t%d\t%d%%\n", lib, size, gcsize, int(float64(size)*100/float64(gcsize)))
 		}
+		totSize += size
+		totGcsize += gcsize
+		n++
 	}
 
-func testExportImport(t *testing.T, pkg0 *types.Package) {
+	if *verbose {
+		fmt.Printf("\n%d\t%d\t%d%%\n", totSize, totGcsize, int(float64(totSize)*100/float64(totGcsize)))
+	}
+
+	types.GcCompatibilityMode = false
+}
+
+func testExportImport(t *testing.T, pkg0 *types.Package, path string) (size, gcsize int) {
 	data := ExportData(pkg0)
+	size = len(data)
 
 	imports := make(map[string]*types.Package)
 	pkg1, err := ImportData(imports, data)
@@ -123,8 +157,30 @@ func testExportImport(t *testing.T, pkg0 *types.Package) {
 	s0 := pkgString(pkg0)
 	s1 := pkgString(pkg1)
 	if s1 != s0 {
-		t.Errorf("package %s: \ngot:\n%s\nwant:\n%s\n", pkg0.Name(), s1, s0)
+		t.Errorf("package %s: \nimport got:\n%s\nwant:\n%s\n", pkg0.Name(), s1, s0)
 	}
+
+	// If we have a standard library, compare also against the gcimported package.
+	if path == "" {
+		return // not std library
+	}
+
+	gcdata, err := gcExportData(path)
+	gcsize = len(gcdata)
+
+	imports = make(map[string]*types.Package)
+	pkg2, err := gcImportData(imports, gcdata, path)
+	if err != nil {
+		t.Errorf("package %s: gcimport failed: %s", pkg0.Name(), err)
+		return
+	}
+
+	s2 := pkgString(pkg2)
+	if s2 != s0 {
+		t.Errorf("package %s: \ngcimport got:\n%s\nwant:\n%s\n", pkg0.Name(), s2, s0)
+	}
+
+	return
 }
 
 func pkgForSource(src string) (*types.Package, error) {
@@ -139,11 +195,11 @@ func pkgForSource(src string) (*types.Package, error) {
 	return types.Check("import-test", fset, []*ast.File{f})
 }
 
-func pkgFor(path string) (*types.Package, error) {
+func pkgForPath(path string) (*types.Package, error) {
 	// collect filenames
 	ctxt := build.Default
-	pkginfo, err := ctxt.ImportDir(path, 0)
-	if _, nogo := err.(*build.NoGoError); err != nil && !nogo {
+	pkginfo, err := ctxt.Import(path, "", 0)
+	if err != nil {
 		return nil, err
 	}
 	filenames := append(pkginfo.GoFiles, pkginfo.CgoFiles...)
@@ -153,16 +209,19 @@ func pkgFor(path string) (*types.Package, error) {
 	files := make([]*ast.File, len(filenames))
 	for i, filename := range filenames {
 		var err error
-		files[i], err = parser.ParseFile(fset, filepath.Join(path, filename), nil, 0)
+		files[i], err = parser.ParseFile(fset, filepath.Join(pkginfo.Dir, filename), nil, 0)
 		if err != nil {
 			return nil, err
 		}
 	}
 
 	// typecheck files
-	return types.Check(path, fset, files)
+	// (we only care about exports and thus can ignore function bodies)
+	conf := types.Config{IgnoreFuncBodies: true, FakeImportC: true}
+	return conf.Check(path, fset, files, nil)
 }
 
+// pkgString returns a string representation of a package's exported interface.
 func pkgString(pkg *types.Package) string {
 	var buf bytes.Buffer
 
@@ -176,16 +235,35 @@ func pkgString(pkg *types.Package) string {
 
 			switch obj := obj.(type) {
 			case *types.Const:
+				// For now only print constant values if they are not float
+				// or complex. This permits comparing go/types results with
+				// gc-generated gcimported package interfaces.
+				info := obj.Type().Underlying().(*types.Basic).Info()
+				if info&types.IsFloat == 0 && info&types.IsComplex == 0 {
 					fmt.Fprintf(&buf, " = %s", obj.Val())
+				}
 
 			case *types.TypeName:
+				// Print associated methods.
 				// Basic types (e.g., unsafe.Pointer) have *types.Basic
 				// type rather than *types.Named; so we need to check.
 				if typ, _ := obj.Type().(*types.Named); typ != nil {
 					if n := typ.NumMethods(); n > 0 {
-						buf.WriteString("\nmethods (\n")
+						// Sort methods by name so that we get the
+						// same order independent of whether the
+						// methods got imported or coming directly
+						// for the source.
+						// TODO(gri) This should probably be done
+						// in go/types.
+						list := make([]*types.Func, n)
 						for i := 0; i < n; i++ {
-							fmt.Fprintf(&buf, "\t%s\n", typ.Method(i))
+							list[i] = typ.Method(i)
+						}
+						sort.Sort(byName(list))
+
+						buf.WriteString("\nmethods (\n")
+						for _, m := range list {
+							fmt.Fprintf(&buf, "\t%s\n", m)
 						}
 						buf.WriteString(")")
 					}
@@ -197,3 +275,78 @@ func pkgString(pkg *types.Package) string {
 
 	return buf.String()
 }
+
+var stdLibRoot = filepath.Join(runtime.GOROOT(), "src", "pkg") + string(filepath.Separator)
+
+// The following std libraries are excluded from the stdLibs list.
+var excluded = map[string]bool{
+	"builtin": true, // contains type declarations with cycles
+	"unsafe":  true, // contains fake declarations
+}
+
+// stdLibs returns the list if standard library package paths.
+func stdLibs() (list []string, err error) {
+	err = filepath.Walk(stdLibRoot, func(path string, info os.FileInfo, err error) error {
+		if err == nil && info.IsDir() {
+			if info.Name() == "testdata" {
+				return filepath.SkipDir
+			}
+			pkgPath := path[len(stdLibRoot):] // remove stdLibRoot
+			if len(pkgPath) > 0 && !excluded[pkgPath] {
+				list = append(list, pkgPath)
+			}
+		}
+		return nil
+	})
+	return
+}
+
+type byName []*types.Func
+
+func (a byName) Len() int           { return len(a) }
+func (a byName) Swap(i, j int)      { a[i], a[j] = a[j], a[i] }
+func (a byName) Less(i, j int) bool { return a[i].Name() < a[j].Name() }
+
+// gcExportData returns the gc-generated export data for the given path.
+// It is based on a trimmed-down version of gcimporter.Import which does
+// not do the actual import, does not handle package unsafe, and assumes
+// that path is a correct standard library package path (no canonicalization,
+// or handling of local import paths).
+func gcExportData(path string) ([]byte, error) {
+	filename, id := gcimporter.FindPkg(path, "")
+	if filename == "" {
+		return nil, fmt.Errorf("can't find import: %s", path)
+	}
+	if id != path {
+		panic("path should be canonicalized")
+	}
+
+	f, err := os.Open(filename)
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+
+	buf := bufio.NewReader(f)
+	if err = gcimporter.FindExportData(buf); err != nil {
+		return nil, err
+	}
+
+	var data []byte
+	for {
+		line, err := buf.ReadBytes('\n')
+		if err != nil {
+			return nil, err
+		}
+		data = append(data, line...)
+		// export data ends in "$$\n"
+		if len(line) == 3 && line[0] == '$' && line[1] == '$' {
+			return data, nil
+		}
+	}
+}
+
+func gcImportData(imports map[string]*types.Package, data []byte, path string) (*types.Package, error) {
+	filename := fmt.Sprintf("<filename for %s>", path) // so we have a decent error message if necessary
+	return gcimporter.ImportData(imports, filename, path, bufio.NewReader(bytes.NewBuffer(data)))
+}