cmd/compile, runtime: new static name encoding

Create a byte encoding designed for static Go names.

It is intended to be a compact representation of a name
and optional tag data that can be turned into a Go string
without allocating, and describes whether or not it is
exported without unicode table.

The encoding is described in reflect/type.go:

// The first byte is a bit field containing:
//
//	1<<0 the name is exported
//	1<<1 tag data follows the name
//	1<<2 pkgPath *string follow the name and tag
//
// The next two bytes are the data length:
//
//	 l := uint16(data[1])<<8 | uint16(data[2])
//
// Bytes [3:3+l] are the string data.
//
// If tag data follows then bytes 3+l and 3+l+1 are the tag length,
// with the data following.
//
// If the import path follows, then ptrSize bytes at the end of
// the data form a *string. The import path is only set for concrete
// methods that are defined in a different package than their type.

Shrinks binary sizes:

	cmd/go: 164KB (1.6%)
	jujud:  1.0MB (1.5%)

For #6853.

Change-Id: I46b6591015b17936a443c9efb5009de8dfe8b609
Reviewed-on: https://go-review.googlesource.com/20968
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/compile/internal/gc/reflect.go

@@ -53,8 +53,8 @@ const (
 	MAXVALSIZE = 128
 )
 
-func structfieldSize() int       { return 5 * Widthptr } // Sizeof(runtime.structfield{})
-func imethodSize() int           { return 3 * Widthptr } // Sizeof(runtime.imethod{})
+func structfieldSize() int       { return 3 * Widthptr } // Sizeof(runtime.structfield{})
+func imethodSize() int           { return 2 * Widthptr } // Sizeof(runtime.imethod{})
 func uncommonSize(t *Type) int { // Sizeof(runtime.uncommontype{})
 	if t.Sym == nil && len(methods(t)) == 0 {
 		return 0
@@ -441,8 +441,12 @@ func dimportpath(p *Pkg) {
 }
 
 func dgopkgpath(s *Sym, ot int, pkg *Pkg) int {
+	return dgopkgpathLSym(Linksym(s), ot, pkg)
+}
+
+func dgopkgpathLSym(s *obj.LSym, ot int, pkg *Pkg) int {
 	if pkg == nil {
-		return dgostringptr(s, ot, "")
+		return duintxxLSym(s, ot, 0, Widthptr)
 	}
 
 	if pkg == localpkg && myimportpath == "" {
@@ -451,39 +455,117 @@ func dgopkgpath(s *Sym, ot int, pkg *Pkg) int {
 		// go.importpath.""., which the linker will rewrite using the correct import path.
 		// Every package that imports this one directly defines the symbol.
 		// See also https://groups.google.com/forum/#!topic/golang-dev/myb9s53HxGQ.
-		ns := Pkglookup("importpath.\"\".", mkpkg("go"))
-		return dsymptr(s, ot, ns, 0)
+		ns := obj.Linklookup(Ctxt, `go.importpath."".`, 0)
+		return dsymptrLSym(s, ot, ns, 0)
 	}
 
 	dimportpath(pkg)
-	return dsymptr(s, ot, pkg.Pathsym, 0)
+	return dsymptrLSym(s, ot, Linksym(pkg.Pathsym), 0)
+}
+
+// isExportedField reports whether a struct field is exported.
+func isExportedField(ft *Field) bool {
+	if ft.Sym != nil && ft.Embedded == 0 {
+		return exportname(ft.Sym.Name)
+	} else {
+		if ft.Type.Sym != nil &&
+			(ft.Type.Sym.Pkg == builtinpkg || !exportname(ft.Type.Sym.Name)) {
+			return false
+		} else {
+			return true
+		}
+	}
+}
+
+// dnameField dumps a reflect.name for a struct field.
+func dnameField(s *Sym, ot int, ft *Field) int {
+	var name, tag string
+	if ft.Sym != nil && ft.Embedded == 0 {
+		name = ft.Sym.Name
+	}
+	if ft.Note != nil {
+		tag = *ft.Note
+	}
+	return dname(s, ot, name, tag, nil, isExportedField(ft))
+}
+
+var dnameCount int
+
+// dname dumps a reflect.name for a struct field or method.
+func dname(s *Sym, ot int, name, tag string, pkg *Pkg, exported bool) int {
+	if len(name) > 1<<16-1 {
+		Fatalf("name too long: %s", name)
+	}
+	if len(tag) > 1<<16-1 {
+		Fatalf("tag too long: %s", tag)
+	}
+
+	// Encode name and tag. See reflect/type.go for details.
+	var bits byte
+	l := 1 + 2 + len(name)
+	if exported {
+		bits |= 1 << 0
+	}
+	if len(tag) > 0 {
+		l += 2 + len(tag)
+		bits |= 1 << 1
+	}
+	if pkg != nil {
+		bits |= 1 << 2
+	}
+	b := make([]byte, l)
+	b[0] = bits
+	b[1] = uint8(len(name) >> 8)
+	b[2] = uint8(len(name))
+	copy(b[3:], name)
+	if len(tag) > 0 {
+		tb := b[3+len(name):]
+		tb[0] = uint8(len(tag) >> 8)
+		tb[1] = uint8(len(tag))
+		copy(tb[2:], tag)
+	}
+
+	// Very few names require a pkgPath *string (only those
+	// defined in a different package than their type). So if
+	// there is no pkgPath, we treat the name contents as string
+	// data that duplicates across packages.
+	var bsym *obj.LSym
+	if pkg == nil {
+		_, bsym = stringsym(string(b))
+	} else {
+		bsymname := fmt.Sprintf(`go.string."".methodname.%d`, dnameCount)
+		dnameCount++
+		bsym = obj.Linklookup(Ctxt, bsymname, 0)
+		bsym.P = b
+		boff := len(b)
+		boff = int(Rnd(int64(boff), int64(Widthptr)))
+		boff = dgopkgpathLSym(bsym, boff, pkg)
+		ggloblLSym(bsym, int32(boff), obj.RODATA|obj.LOCAL)
+	}
+
+	ot = dsymptrLSym(Linksym(s), ot, bsym, 0)
+
+	return ot
 }
 
 // dextratype dumps the fields of a runtime.uncommontype.
 // dataAdd is the offset in bytes after the header where the
 // backing array of the []method field is written (by dextratypeData).
-func dextratype(sym *Sym, off int, t *Type, dataAdd int) int {
+func dextratype(s *Sym, ot int, t *Type, dataAdd int) int {
 	m := methods(t)
 	if t.Sym == nil && len(m) == 0 {
-		return off
+		return ot
 	}
-	noff := int(Rnd(int64(off), int64(Widthptr)))
-	if noff != off {
-		panic("dextratype rounding does something. :-(")
+	noff := int(Rnd(int64(ot), int64(Widthptr)))
+	if noff != ot {
+		Fatalf("unexpected alignment in dextratype for %s", t)
 	}
-	off = noff
 
 	for _, a := range m {
 		dtypesym(a.type_)
 	}
 
-	ot := off
-	s := sym
-	if t.Sym != nil && t != Types[t.Etype] && t != errortype {
-		ot = dgopkgpath(s, ot, t.Sym.Pkg)
-	} else {
-		ot = dgostringptr(s, ot, "")
-	}
+	ot = dgopkgpath(s, ot, typePkg(t))
 
 	// slice header
 	ot = dsymptr(s, ot, s, ot+Widthptr+2*Widthint+dataAdd)
@@ -495,15 +577,28 @@ func dextratype(sym *Sym, off int, t *Type, dataAdd int) int {
 	return ot
 }
 
+func typePkg(t *Type) *Pkg {
+	tsym := t.Sym
+	if tsym == nil && t.Type != nil {
+		tsym = t.Type.Sym
+	}
+	if tsym != nil && t != Types[t.Etype] && t != errortype {
+		return tsym.Pkg
+	}
+	return nil
+}
+
 // dextratypeData dumps the backing array for the []method field of
 // runtime.uncommontype.
 func dextratypeData(s *Sym, ot int, t *Type) int {
 	for _, a := range methods(t) {
-		// method
 		// ../../../../runtime/type.go:/method
-		ot = dgostringptr(s, ot, a.name)
-
-		ot = dgopkgpath(s, ot, a.pkg)
+		exported := exportname(a.name)
+		var pkg *Pkg
+		if !exported && a.pkg != typePkg(t) {
+			pkg = a.pkg
+		}
+		ot = dname(s, ot, a.name, "", pkg, exported)
 		ot = dmethodptr(s, ot, dtypesym(a.mtype))
 		ot = dmethodptr(s, ot, a.isym)
 		ot = dmethodptr(s, ot, a.tsym)
@@ -1076,6 +1171,12 @@ ok:
 		// ../../../../runtime/type.go:/interfaceType
 		ot = dcommontype(s, ot, t)
 
+		var tpkg *Pkg
+		if t.Sym != nil && t != Types[t.Etype] && t != errortype {
+			tpkg = t.Sym.Pkg
+		}
+		ot = dgopkgpath(s, ot, tpkg)
+
 		ot = dsymptr(s, ot, s, ot+Widthptr+2*Widthint+uncommonSize(t))
 		ot = duintxx(s, ot, uint64(n), Widthint)
 		ot = duintxx(s, ot, uint64(n), Widthint)
@@ -1084,8 +1185,12 @@ ok:
 
 		for _, a := range m {
 			// ../../../../runtime/type.go:/imethod
-			ot = dgostringptr(s, ot, a.name)
-			ot = dgopkgpath(s, ot, a.pkg)
+			exported := exportname(a.name)
+			var pkg *Pkg
+			if !exported && a.pkg != tpkg {
+				pkg = a.pkg
+			}
+			ot = dname(s, ot, a.name, "", pkg, exported)
 			ot = dsymptr(s, ot, dtypesym(a.type_), 0)
 		}
 
@@ -1148,6 +1253,11 @@ ok:
 		}
 
 		ot = dcommontype(s, ot, t)
+		var pkg *Pkg
+		if t.Sym != nil {
+			pkg = t.Sym.Pkg
+		}
+		ot = dgopkgpath(s, ot, pkg)
 		ot = dsymptr(s, ot, s, ot+Widthptr+2*Widthint+uncommonSize(t))
 		ot = duintxx(s, ot, uint64(n), Widthint)
 		ot = duintxx(s, ot, uint64(n), Widthint)
@@ -1155,28 +1265,11 @@ ok:
 		dataAdd := n * structfieldSize()
 		ot = dextratype(s, ot, t, dataAdd)
 
-		for _, t1 := range t.Fields().Slice() {
+		for _, f := range t.Fields().Slice() {
 			// ../../../../runtime/type.go:/structField
-			if t1.Sym != nil && t1.Embedded == 0 {
-				ot = dgostringptr(s, ot, t1.Sym.Name)
-				if exportname(t1.Sym.Name) {
-					ot = dgostringptr(s, ot, "")
-				} else {
-					ot = dgopkgpath(s, ot, t1.Sym.Pkg)
-				}
-			} else {
-				ot = dgostringptr(s, ot, "")
-				if t1.Type.Sym != nil &&
-					(t1.Type.Sym.Pkg == builtinpkg || !exportname(t1.Type.Sym.Name)) {
-					ot = dgopkgpath(s, ot, localpkg)
-				} else {
-					ot = dgostringptr(s, ot, "")
-				}
-			}
-
-			ot = dsymptr(s, ot, dtypesym(t1.Type), 0)
-			ot = dgostrlitptr(s, ot, t1.Note)
-			ot = duintptr(s, ot, uint64(t1.Width)) // field offset
+			ot = dnameField(s, ot, f)
+			ot = dsymptr(s, ot, dtypesym(f.Type), 0)
+			ot = duintptr(s, ot, uint64(f.Width)) // field offset
 		}
 	}
 

src/cmd/link/internal/ld/decodesym.go

@@ -47,7 +47,7 @@ func decode_inuxi(p []byte, sz int) uint64 {
 }
 
 func commonsize() int      { return 6*Thearch.Ptrsize + 8 }                 // runtime._type
-func structfieldSize() int { return 5 * Thearch.Ptrsize }                   // runtime.structfield
+func structfieldSize() int { return 3 * Thearch.Ptrsize }                   // runtime.structfield
 func uncommonSize() int    { return 2*Thearch.Ptrsize + 2*Thearch.Intsize } // runtime.uncommontype
 
 // Type.commonType.kind
@@ -203,11 +203,11 @@ func decodetype_funcouttype(s *LSym, i int) *LSym {
 
 // Type.StructType.fields.Slice::length
 func decodetype_structfieldcount(s *LSym) int {
-	return int(decode_inuxi(s.P[commonsize()+Thearch.Ptrsize:], Thearch.Intsize))
+	return int(decode_inuxi(s.P[commonsize()+2*Thearch.Ptrsize:], Thearch.Intsize))
 }
 
 func decodetype_structfieldarrayoff(s *LSym, i int) int {
-	off := commonsize() + Thearch.Ptrsize + 2*Thearch.Intsize
+	off := commonsize() + 2*Thearch.Ptrsize + 2*Thearch.Intsize
 	if decodetype_hasUncommon(s) {
 		off += uncommonSize()
 	}
@@ -228,24 +228,37 @@ func decodetype_stringptr(s *LSym, off int) string {
 	return string(r.Sym.P[r.Add : r.Add+strlen])
 }
 
+// decodetype_name decodes the name from a reflect.name.
+func decodetype_name(s *LSym, off int) string {
+	r := decode_reloc(s, int32(off))
+	if r == nil {
+		return ""
+	}
+
+	data := r.Sym.P
+	namelen := int(uint16(data[1]<<8) | uint16(data[2]))
+	return string(data[3 : 3+namelen])
+
+}
+
 func decodetype_structfieldname(s *LSym, i int) string {
 	off := decodetype_structfieldarrayoff(s, i)
-	return decodetype_stringptr(s, off)
+	return decodetype_name(s, off)
 }
 
 func decodetype_structfieldtype(s *LSym, i int) *LSym {
 	off := decodetype_structfieldarrayoff(s, i)
-	return decode_reloc_sym(s, int32(off+2*Thearch.Ptrsize))
+	return decode_reloc_sym(s, int32(off+Thearch.Ptrsize))
 }
 
 func decodetype_structfieldoffs(s *LSym, i int) int64 {
 	off := decodetype_structfieldarrayoff(s, i)
-	return int64(decode_inuxi(s.P[off+4*Thearch.Ptrsize:], Thearch.Intsize))
+	return int64(decode_inuxi(s.P[off+2*Thearch.Ptrsize:], Thearch.Intsize))
 }
 
 // InterfaceType.methods.length
 func decodetype_ifacemethodcount(s *LSym) int64 {
-	return int64(decode_inuxi(s.P[commonsize()+Thearch.Ptrsize:], Thearch.Intsize))
+	return int64(decode_inuxi(s.P[commonsize()+2*Thearch.Ptrsize:], Thearch.Intsize))
 }
 
 // methodsig is a fully qualified typed method signature, like
@@ -266,16 +279,16 @@ const (
 )
 
 // decode_methodsig decodes an array of method signature information.
-// Each element of the array is size bytes. The first word is a *string
-// for the name, the third word is a *rtype for the funcType.
+// Each element of the array is size bytes. The first word is a
+// reflect.name for the name, the second word is a *rtype for the funcType.
 //
 // Conveniently this is the layout of both runtime.method and runtime.imethod.
 func decode_methodsig(s *LSym, off, size, count int) []methodsig {
 	var buf bytes.Buffer
 	var methods []methodsig
 	for i := 0; i < count; i++ {
-		buf.WriteString(decodetype_stringptr(s, off))
-		mtypSym := decode_reloc_sym(s, int32(off+2*Thearch.Ptrsize))
+		buf.WriteString(decodetype_name(s, off))
+		mtypSym := decode_reloc_sym(s, int32(off+Thearch.Ptrsize))
 
 		buf.WriteRune('(')
 		inCount := decodetype_funcincount(mtypSym)
@@ -306,7 +319,7 @@ func decodetype_ifacemethods(s *LSym) []methodsig {
 	if decodetype_kind(s)&kindMask != kindInterface {
 		panic(fmt.Sprintf("symbol %q is not an interface", s.Name))
 	}
-	r := decode_reloc(s, int32(commonsize()))
+	r := decode_reloc(s, int32(commonsize()+Thearch.Ptrsize))
 	if r == nil {
 		return nil
 	}
@@ -315,7 +328,7 @@ func decodetype_ifacemethods(s *LSym) []methodsig {
 	}
 	off := int(r.Add) // array of reflect.imethod values
 	numMethods := int(decodetype_ifacemethodcount(s))
-	sizeofIMethod := 3 * Thearch.Ptrsize
+	sizeofIMethod := 2 * Thearch.Ptrsize
 	return decode_methodsig(s, off, sizeofIMethod, numMethods)
 }
 
@@ -326,7 +339,7 @@ func decodetype_methods(s *LSym) []methodsig {
 	off := commonsize() // reflect.rtype
 	switch decodetype_kind(s) & kindMask {
 	case kindStruct: // reflect.structType
-		off += Thearch.Ptrsize + 2*Thearch.Intsize
+		off += 2*Thearch.Ptrsize + 2*Thearch.Intsize
 	case kindPtr: // reflect.ptrType
 		off += Thearch.Ptrsize
 	case kindFunc: // reflect.funcType
@@ -351,6 +364,6 @@ func decodetype_methods(s *LSym) []methodsig {
 		panic(fmt.Sprintf("method slice pointer in %s leads to a different symbol %s", s, r.Sym))
 	}
 	off = int(r.Add)                    // array of reflect.method values
-	sizeofMethod := 5 * Thearch.Ptrsize // sizeof reflect.method in program
+	sizeofMethod := 4 * Thearch.Ptrsize // sizeof reflect.method in program
 	return decode_methodsig(s, off, sizeofMethod, numMethods)
 }

src/reflect/type.go

@@ -288,11 +288,10 @@ type typeAlg struct {
 
 // Method on non-interface type
 type method struct {
-	name    *string        // name of method
-	pkgPath *string        // nil for exported Names; otherwise import path
-	mtyp    *rtype         // method type (without receiver)
-	ifn     unsafe.Pointer // fn used in interface call (one-word receiver)
-	tfn     unsafe.Pointer // fn used for normal method call
+	name name           // name of method
+	mtyp *rtype         // method type (without receiver)
+	ifn  unsafe.Pointer // fn used in interface call (one-word receiver)
+	tfn  unsafe.Pointer // fn used for normal method call
 }
 
 // uncommonType is present only for types with names or methods
@@ -347,14 +346,14 @@ type funcType struct {
 
 // imethod represents a method on an interface type
 type imethod struct {
-	name    *string // name of method
-	pkgPath *string // nil for exported Names; otherwise import path
-	typ     *rtype  // .(*FuncType) underneath
+	name name   // name of method
+	typ  *rtype // .(*FuncType) underneath
 }
 
 // interfaceType represents an interface type.
 type interfaceType struct {
 	rtype   `reflect:"interface"`
+	pkgPath *string   // import path
 	methods []imethod // sorted by hash
 }
 
@@ -388,17 +387,101 @@ type sliceType struct {
 
 // Struct field
 type structField struct {
-	name    *string // nil for embedded fields
-	pkgPath *string // nil for exported Names; otherwise import path
-	typ     *rtype  // type of field
-	tag     *string // nil if no tag
-	offset  uintptr // byte offset of field within struct
+	name   name    // name is empty for embedded fields
+	typ    *rtype  // type of field
+	offset uintptr // byte offset of field within struct
 }
 
 // structType represents a struct type.
 type structType struct {
-	rtype  `reflect:"struct"`
-	fields []structField // sorted by offset
+	rtype   `reflect:"struct"`
+	pkgPath *string
+	fields  []structField // sorted by offset
+}
+
+// name is an encoded type name with optional extra data.
+//
+// The first byte is a bit field containing:
+//
+//	1<<0 the name is exported
+//	1<<1 tag data follows the name
+//	1<<2 pkgPath *string follow the name and tag
+//
+// The next two bytes are the data length:
+//
+//	 l := uint16(data[1])<<8 | uint16(data[2])
+//
+// Bytes [3:3+l] are the string data.
+//
+// If tag data follows then bytes 3+l and 3+l+1 are the tag length,
+// with the data following.
+//
+// If the import path follows, then ptrSize bytes at the end of
+// the data form a *string. The pointer is aligned to its width.
+// The import path is only set for concrete methods that are defined
+// in a different package than their type.
+type name struct {
+	bytes *byte
+}
+
+func (n *name) data(off int) *byte {
+	return (*byte)(add(unsafe.Pointer(n.bytes), uintptr(off)))
+}
+
+func (n *name) isExported() bool {
+	return (*n.bytes)&(1<<0) != 0
+}
+
+func (n *name) nameLen() int {
+	return int(uint16(*n.data(1))<<8 | uint16(*n.data(2)))
+}
+
+func (n *name) tagLen() int {
+	if *n.data(0)&(1<<1) == 0 {
+		return 0
+	}
+	off := 3 + n.nameLen()
+	return int(uint16(*n.data(off))<<8 | uint16(*n.data(off + 1)))
+}
+
+func (n *name) name() (s string) {
+	nl := n.nameLen()
+	if nl == 0 {
+		return ""
+	}
+	hdr := (*stringHeader)(unsafe.Pointer(&s))
+	hdr.Data = unsafe.Pointer(n.data(3))
+	hdr.Len = nl
+	return s
+}
+
+func (n *name) tag() (s string) {
+	tl := n.tagLen()
+	if tl == 0 {
+		return ""
+	}
+	nl := n.nameLen()
+	hdr := (*stringHeader)(unsafe.Pointer(&s))
+	hdr.Data = unsafe.Pointer(n.data(3 + nl + 2))
+	hdr.Len = tl
+	return s
+}
+
+func (n *name) pkgPath() *string {
+	if *n.data(0)&(1<<2) == 0 {
+		return nil
+	}
+	off := 3 + n.nameLen()
+	if tl := n.tagLen(); tl > 0 {
+		off += 2 + tl
+	}
+	off = int(round(uintptr(off), ptrSize))
+	return *(**string)(unsafe.Pointer(n.data(off)))
+}
+
+// round n up to a multiple of a.  a must be a power of 2.
+func round(n, a uintptr) uintptr {
+	return (n + a - 1) &^ (a - 1)
 }
 
 /*
@@ -583,12 +666,14 @@ func (t *rtype) Method(i int) (m Method) {
 		panic("reflect: Method index out of range")
 	}
 	p := &ut.methods[i]
-	if p.name != nil {
-		m.Name = *p.name
-	}
+	m.Name = p.name.name()
 	fl := flag(Func)
-	if p.pkgPath != nil {
-		m.PkgPath = *p.pkgPath
+	if !p.name.isExported() {
+		pkgPath := p.name.pkgPath()
+		if pkgPath == nil {
+			pkgPath = ut.pkgPath
+		}
+		m.PkgPath = *pkgPath
 		fl |= flagStickyRO
 	}
 	if p.mtyp != nil {
@@ -620,10 +705,9 @@ func (t *rtype) MethodByName(name string) (m Method, ok bool) {
 	if ut == nil {
 		return Method{}, false
 	}
-	var p *method
 	for i := range ut.methods {
-		p = &ut.methods[i]
-		if p.name != nil && *p.name == name {
+		p := &ut.methods[i]
+		if p.name.name() == name {
 			return t.Method(i), true
 		}
 	}
@@ -832,9 +916,13 @@ func (t *interfaceType) Method(i int) (m Method) {
 		return
 	}
 	p := &t.methods[i]
-	m.Name = *p.name
-	if p.pkgPath != nil {
-		m.PkgPath = *p.pkgPath
+	m.Name = p.name.name()
+	if !p.name.isExported() {
+		pkgPath := p.name.pkgPath()
+		if pkgPath == nil {
+			pkgPath = t.pkgPath
+		}
+		m.PkgPath = *pkgPath
 	}
 	m.Type = toType(p.typ)
 	m.Index = i
@@ -852,7 +940,7 @@ func (t *interfaceType) MethodByName(name string) (m Method, ok bool) {
 	var p *imethod
 	for i := range t.methods {
 		p = &t.methods[i]
-		if *p.name == name {
+		if p.name.name() == name {
 			return t.Method(i), true
 		}
 	}
@@ -950,8 +1038,8 @@ func (t *structType) Field(i int) (f StructField) {
 	}
 	p := &t.fields[i]
 	f.Type = toType(p.typ)
-	if p.name != nil {
-		f.Name = *p.name
+	if name := p.name.name(); name != "" {
+		f.Name = name
 	} else {
 		t := f.Type
 		if t.Kind() == Ptr {
@@ -960,11 +1048,12 @@ func (t *structType) Field(i int) (f StructField) {
 		f.Name = t.Name()
 		f.Anonymous = true
 	}
-	if p.pkgPath != nil {
-		f.PkgPath = *p.pkgPath
+	if t.pkgPath != nil && !p.name.isExported() {
+		// Fields never have an import path in their name.
+		f.PkgPath = *t.pkgPath
 	}
-	if p.tag != nil {
-		f.Tag = StructTag(*p.tag)
+	if tag := p.name.tag(); tag != "" {
+		f.Tag = StructTag(tag)
 	}
 	f.Offset = p.offset
 
@@ -1056,8 +1145,8 @@ func (t *structType) FieldByNameFunc(match func(string) bool) (result StructFiel
 				// Find name and type for field f.
 				var fname string
 				var ntyp *rtype
-				if f.name != nil {
-					fname = *f.name
+				if name := f.name.name(); name != "" {
+					fname = name
 				} else {
 					// Anonymous field of type T or *T.
 					// Name taken from type.
@@ -1122,11 +1211,12 @@ func (t *structType) FieldByName(name string) (f StructField, present bool) {
 	if name != "" {
 		for i := range t.fields {
 			tf := &t.fields[i]
-			if tf.name == nil {
+			tfname := tf.name.name()
+			if tfname == "" {
 				hasAnon = true
 				continue
 			}
-			if *tf.name == name {
+			if tfname == name {
 				return t.Field(i), true
 			}
 		}
@@ -1278,7 +1368,7 @@ func implements(T, V *rtype) bool {
 		for j := 0; j < len(v.methods); j++ {
 			tm := &t.methods[i]
 			vm := &v.methods[j]
-			if *vm.name == *tm.name && vm.pkgPath == tm.pkgPath && vm.typ == tm.typ {
+			if vm.name.name() == tm.name.name() && vm.typ == tm.typ {
 				if i++; i >= len(t.methods) {
 					return true
 				}
@@ -1295,7 +1385,7 @@ func implements(T, V *rtype) bool {
 	for j := 0; j < len(v.methods); j++ {
 		tm := &t.methods[i]
 		vm := &v.methods[j]
-		if *vm.name == *tm.name && vm.pkgPath == tm.pkgPath && vm.mtyp == tm.typ {
+		if vm.name.name() == tm.name.name() && vm.mtyp == tm.typ {
 			if i++; i >= len(t.methods) {
 				return true
 			}
@@ -1400,16 +1490,13 @@ func haveIdenticalUnderlyingType(T, V *rtype) bool {
 		for i := range t.fields {
 			tf := &t.fields[i]
 			vf := &v.fields[i]
-			if tf.name != vf.name && (tf.name == nil || vf.name == nil || *tf.name != *vf.name) {
-				return false
-			}
-			if tf.pkgPath != vf.pkgPath && (tf.pkgPath == nil || vf.pkgPath == nil || *tf.pkgPath != *vf.pkgPath) {
+			if tf.name.name() != vf.name.name() {
 				return false
 			}
 			if tf.typ != vf.typ {
 				return false
 			}
-			if tf.tag != vf.tag && (tf.tag == nil || vf.tag == nil || *tf.tag != *vf.tag) {
+			if tf.name.tag() != vf.name.tag() {
 				return false
 			}
 			if tf.offset != vf.offset {

src/reflect/value.go

@@ -554,7 +554,7 @@ func methodReceiver(op string, v Value, methodIndex int) (rcvrtype, t *rtype, fn
 			panic("reflect: internal error: invalid method index")
 		}
 		m := &tt.methods[i]
-		if m.pkgPath != nil {
+		if !m.name.isExported() {
 			panic("reflect: " + op + " of unexported method")
 		}
 		iface := (*nonEmptyInterface)(v.ptr)
@@ -571,7 +571,7 @@ func methodReceiver(op string, v Value, methodIndex int) (rcvrtype, t *rtype, fn
 			panic("reflect: internal error: invalid method index")
 		}
 		m := &ut.methods[i]
-		if m.pkgPath != nil {
+		if !m.name.isExported() {
 			panic("reflect: " + op + " of unexported method")
 		}
 		fn = unsafe.Pointer(&m.ifn)
@@ -750,8 +750,8 @@ func (v Value) Field(i int) Value {
 	// Inherit permission bits from v, but clear flagEmbedRO.
 	fl := v.flag&(flagStickyRO|flagIndir|flagAddr) | flag(typ.Kind())
 	// Using an unexported field forces flagRO.
-	if field.pkgPath != nil {
-		if field.name == nil {
+	if !field.name.isExported() {
+		if field.name.name() == "" {
 			fl |= flagEmbedRO
 		} else {
 			fl |= flagStickyRO

src/runtime/iface.go

@@ -30,7 +30,7 @@ func getitab(inter *interfacetype, typ *_type, canfail bool) *itab {
 		if canfail {
 			return nil
 		}
-		panic(&TypeAssertionError{"", typ._string, inter.typ._string, *inter.mhdr[0].name})
+		panic(&TypeAssertionError{"", typ._string, inter.typ._string, inter.mhdr[0].name.name()})
 	}
 
 	// compiler has provided some good hash codes for us.
@@ -84,19 +84,25 @@ search:
 	j := 0
 	for k := 0; k < ni; k++ {
 		i := &inter.mhdr[k]
-		iname := i.name
-		ipkgpath := i.pkgpath
+		iname := i.name.name()
 		itype := i._type
+		ipkg := i.name.pkgPath()
+		if ipkg == nil {
+			ipkg = inter.pkgpath
+		}
 		for ; j < nt; j++ {
 			t := &x.mhdr[j]
-			if t.name == nil {
-				throw("itab t.name is nil")
-			}
-			if t.mtyp == itype && (t.name == iname || *t.name == *iname) && t.pkgpath == ipkgpath {
-				if m != nil {
-					*(*unsafe.Pointer)(add(unsafe.Pointer(&m.fun[0]), uintptr(k)*sys.PtrSize)) = t.ifn
+			if t.mtyp == itype && t.name.name() == iname {
+				pkgPath := t.name.pkgPath()
+				if pkgPath == nil {
+					pkgPath = x.pkgpath
+				}
+				if t.name.isExported() || pkgPath == ipkg {
+					if m != nil {
+						*(*unsafe.Pointer)(add(unsafe.Pointer(&m.fun[0]), uintptr(k)*sys.PtrSize)) = t.ifn
+					}
+					goto nextimethod
 				}
-				goto nextimethod
 			}
 		}
 		// didn't find method
@@ -104,7 +110,7 @@ search:
 			if locked != 0 {
 				unlock(&ifaceLock)
 			}
-			panic(&TypeAssertionError{"", typ._string, inter.typ._string, *iname})
+			panic(&TypeAssertionError{"", typ._string, inter.typ._string, iname})
 		}
 		m.bad = 1
 		break

src/runtime/type.go

@@ -6,7 +6,10 @@
 
 package runtime
 
-import "unsafe"
+import (
+	"runtime/internal/sys"
+	"unsafe"
+)
 
 // tflag is documented in ../reflect/type.go.
 type tflag uint8
@@ -152,11 +155,10 @@ func (t *functype) dotdotdot() bool {
 }
 
 type method struct {
-	name    *string
-	pkgpath *string
-	mtyp    *_type
-	ifn     unsafe.Pointer
-	tfn     unsafe.Pointer
+	name name
+	mtyp *_type
+	ifn  unsafe.Pointer
+	tfn  unsafe.Pointer
 }
 
 type uncommontype struct {
@@ -165,14 +167,14 @@ type uncommontype struct {
 }
 
 type imethod struct {
-	name    *string
-	pkgpath *string
-	_type   *_type
+	name  name
+	_type *_type
 }
 
 type interfacetype struct {
-	typ  _type
-	mhdr []imethod
+	typ     _type
+	pkgpath *string
+	mhdr    []imethod
 }
 
 type maptype struct {
@@ -220,14 +222,62 @@ type ptrtype struct {
 }
 
 type structfield struct {
-	name    *string
-	pkgpath *string
-	typ     *_type
-	tag     *string
-	offset  uintptr
+	name   name
+	typ    *_type
+	offset uintptr
 }
 
 type structtype struct {
-	typ    _type
-	fields []structfield
+	typ     _type
+	pkgPath *string
+	fields  []structfield
+}
+
+// name is an encoded type name with optional extra data.
+// See reflect/type.go for details.
+type name struct {
+	bytes *byte
+}
+
+func (n *name) data(off int) *byte {
+	return (*byte)(add(unsafe.Pointer(n.bytes), uintptr(off)))
+}
+
+func (n *name) isExported() bool {
+	return (*n.bytes)&(1<<0) != 0
+}
+
+func (n *name) nameLen() int {
+	return int(uint16(*n.data(1))<<8 | uint16(*n.data(2)))
+}
+
+func (n *name) tagLen() int {
+	if *n.data(0)&(1<<1) == 0 {
+		return 0
+	}
+	off := 3 + n.nameLen()
+	return int(uint16(*n.data(off))<<8 | uint16(*n.data(off + 1)))
+}
+
+func (n *name) name() (s string) {
+	nl := n.nameLen()
+	if nl == 0 {
+		return ""
+	}
+	hdr := (*stringStruct)(unsafe.Pointer(&s))
+	hdr.str = unsafe.Pointer(n.data(3))
+	hdr.len = nl
+	return s
+}
+
+func (n *name) pkgPath() *string {
+	if *n.data(0)&(1<<2) == 0 {
+		return nil
+	}
+	off := 3 + n.nameLen()
+	if tl := n.tagLen(); tl > 0 {
+		off += 2 + tl
+	}
+	off = int(round(uintptr(off), sys.PtrSize))
+	return *(**string)(unsafe.Pointer(n.data(off)))
 }