cmd/compile: treat empty and absent struct field tags as identical

Fixes #15439.

Change-Id: I5a32384c46e20f8db6968e5a9e854c45ab262fe4
Reviewed-on: https://go-review.googlesource.com/22429
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -720,18 +720,7 @@ func (p *exporter) field(f *Field) {
 	p.pos(f.Sym.Def)
 	p.fieldName(f.Sym, f)
 	p.typ(f.Type)
-	// TODO(gri) Do we care that a non-present tag cannot be distinguished
-	// from a present but empty ta string? (reflect doesn't seem to make
-	// a difference). Investigate.
-	p.note(f.Note)
-}
-
-func (p *exporter) note(n *string) {
-	var s string
-	if n != nil {
-		s = *n
-	}
-	p.string(s)
+	p.string(f.Note)
 }
 
 func (p *exporter) methodList(t *Type) {
@@ -847,7 +836,7 @@ func (p *exporter) param(q *Field, n int, numbered bool) {
 	// TODO(gri) This is compiler-specific (escape info).
 	// Move into compiler-specific section eventually?
 	// (Not having escape info causes tests to fail, e.g. runtime GCInfoTest)
-	p.note(q.Note)
+	p.string(q.Note)
 }
 
 func parName(f *Field, numbered bool) string {

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

@@ -457,7 +457,7 @@ func (p *importer) field() *Node {
 	p.pos()
 	sym := p.fieldName()
 	typ := p.typ()
-	note := p.note()
+	note := p.string()
 
 	var n *Node
 	if sym.Name != "" {
@@ -475,18 +475,11 @@ func (p *importer) field() *Node {
 		n = embedded(s, pkg)
 		n.Right = typenod(typ)
 	}
-	n.SetVal(note)
+	n.SetVal(Val{U: note})
 
 	return n
 }
 
-func (p *importer) note() (v Val) {
-	if s := p.string(); s != "" {
-		v.U = s
-	}
-	return
-}
-
 // parser.go:hidden_interfacedcl_list
 func (p *importer) methodList() (methods []*Node) {
 	if n := p.int(); n > 0 {
@@ -572,7 +565,7 @@ func (p *importer) param(named bool) *Node {
 
 	// TODO(gri) This is compiler-specific (escape info).
 	// Move into compiler-specific section eventually?
-	n.SetVal(p.note())
+	n.SetVal(Val{U: p.string()})
 
 	return n
 }

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

@@ -757,7 +757,7 @@ func structfield(n *Node) *Field {
 
 	switch u := n.Val().U.(type) {
 	case string:
-		f.Note = &u
+		f.Note = u
 	default:
 		Yyerror("field annotation must be string")
 	case nil:

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

@@ -1181,7 +1181,7 @@ func escassign(e *EscState, dst, src *Node, step *EscStep) {
 var tags [1 << (bitsPerOutputInTag + EscReturnBits)]string
 
 // mktag returns the string representation for an escape analysis tag.
-func mktag(mask int) *string {
+func mktag(mask int) string {
 	switch mask & EscMask {
 	case EscNone, EscReturn:
 		break
@@ -1191,22 +1191,22 @@ func mktag(mask int) *string {
 	}
 
 	if mask < len(tags) && tags[mask] != "" {
-		return &tags[mask]
+		return tags[mask]
 	}
 
 	s := fmt.Sprintf("esc:0x%x", mask)
 	if mask < len(tags) {
 		tags[mask] = s
 	}
-	return &s
+	return s
 }
 
 // parsetag decodes an escape analysis tag and returns the esc value.
-func parsetag(note *string) uint16 {
-	if note == nil || !strings.HasPrefix(*note, "esc:") {
+func parsetag(note string) uint16 {
+	if !strings.HasPrefix(note, "esc:") {
 		return EscUnknown
 	}
-	n, _ := strconv.ParseInt((*note)[4:], 0, 0)
+	n, _ := strconv.ParseInt(note[4:], 0, 0)
 	em := uint16(n)
 	if em == 0 {
 		return EscNone
@@ -1268,7 +1268,7 @@ func describeEscape(em uint16) string {
 
 // escassignfromtag models the input-to-output assignment flow of one of a function
 // calls arguments, where the flow is encoded in "note".
-func escassignfromtag(e *EscState, note *string, dsts Nodes, src *Node) uint16 {
+func escassignfromtag(e *EscState, note string, dsts Nodes, src *Node) uint16 {
 	em := parsetag(note)
 	if src.Op == OLITERAL {
 		return em
@@ -1997,7 +1997,7 @@ func esctag(e *EscState, func_ *Node) {
 					}
 					Warnl(func_.Lineno, "%v assuming %v is unsafe uintptr", funcSym(func_), name)
 				}
-				t.Note = &unsafeUintptrTag
+				t.Note = unsafeUintptrTag
 			}
 		}
 

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

@@ -1700,8 +1700,8 @@ func Fldconv(f *Field, flag FmtFlag) string {
 	// (The escape analysis tags do not apply to func vars.)
 	// But it must not suppress struct field tags.
 	// See golang.org/issue/13777 and golang.org/issue/14331.
-	if flag&FmtShort == 0 && (!fmtbody || !f.Funarg) && f.Note != nil {
-		str += " " + strconv.Quote(*f.Note)
+	if flag&FmtShort == 0 && (!fmtbody || !f.Funarg) && f.Note != "" {
+		str += " " + strconv.Quote(f.Note)
 	}
 
 	if fmtmode == FTypeId && (sf&FmtUnsigned != 0) {

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

@@ -373,7 +373,7 @@ func ordercall(n *Node, order *Order) {
 			if t == nil {
 				break
 			}
-			if t.Note != nil && *t.Note == unsafeUintptrTag {
+			if t.Note == unsafeUintptrTag {
 				xp := n.List.Addr(i)
 				for (*xp).Op == OCONVNOP && !(*xp).Type.IsPtr() {
 					xp = &(*xp).Left

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

@@ -501,14 +501,11 @@ func isExportedField(ft *Field) bool {
 
 // dnameField dumps a reflect.name for a struct field.
 func dnameField(s *Sym, ot int, ft *Field) int {
-	var name, tag string
+	var name string
 	if ft.Sym != nil && ft.Embedded == 0 {
 		name = ft.Sym.Name
 	}
-	if ft.Note != nil {
-		tag = *ft.Note
-	}
-	nsym := dname(name, tag, nil, isExportedField(ft))
+	nsym := dname(name, ft.Note, nil, isExportedField(ft))
 	return dsymptrLSym(Linksym(s), ot, nsym, 0)
 }
 

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

@@ -619,10 +619,6 @@ func cplxsubtype(et EType) EType {
 	return 0
 }
 
-func eqnote(a, b *string) bool {
-	return a == b || a != nil && b != nil && *a == *b
-}
-
 // Eqtype reports whether t1 and t2 are identical, following the spec rules.
 //
 // Any cyclic type must go through a named type, and if one is
@@ -670,7 +666,7 @@ func eqtype1(t1, t2 *Type, assumedEqual map[typePair]struct{}) bool {
 		t1, i1 := IterFields(t1)
 		t2, i2 := IterFields(t2)
 		for ; t1 != nil && t2 != nil; t1, t2 = i1.Next(), i2.Next() {
-			if t1.Sym != t2.Sym || t1.Embedded != t2.Embedded || !eqtype1(t1.Type, t2.Type, assumedEqual) || !eqnote(t1.Note, t2.Note) {
+			if t1.Sym != t2.Sym || t1.Embedded != t2.Embedded || !eqtype1(t1.Type, t2.Type, assumedEqual) || t1.Note != t2.Note {
 				return false
 			}
 		}

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

@@ -300,7 +300,7 @@ type Field struct {
 	// or interface Type.
 	Offset int64
 
-	Note *string // literal string annotation
+	Note string // literal string annotation
 }
 
 // End returns the offset of the first byte immediately after this field.
@@ -1003,15 +1003,7 @@ func (t *Type) cmp(x *Type) ssa.Cmp {
 				return cmpForNe(t1.Embedded < x1.Embedded)
 			}
 			if t1.Note != x1.Note {
-				if t1.Note == nil {
-					return ssa.CMPlt
-				}
-				if x1.Note == nil {
-					return ssa.CMPgt
-				}
-				if *t1.Note != *x1.Note {
-					return cmpForNe(*t1.Note < *x1.Note)
-				}
+				return cmpForNe(t1.Note < x1.Note)
 			}
 			if c := t1.Sym.cmpsym(x1.Sym); c != ssa.CMPeq {
 				return c

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

@@ -3807,7 +3807,7 @@ func usefield(n *Node) {
 	if field == nil {
 		Fatalf("usefield %v %v without paramfld", n.Left.Type, n.Sym)
 	}
-	if field.Note == nil || !strings.Contains(*field.Note, "go:\"track\"") {
+	if !strings.Contains(field.Note, "go:\"track\"") {
 		return
 	}
 

test/fixedbugs/issue15439.go

@@ -0,0 +1,25 @@
+// run
+
+// Copyright 2016 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 main
+
+import "reflect"
+
+func main() {
+	a := &struct{ x int }{}
+	b := &struct{ x int "" }{}
+
+	ta := reflect.TypeOf(a)
+	tb := reflect.TypeOf(b)
+
+	// Ensure cmd/compile treats absent and empty tags as equivalent.
+	a = b
+
+	// Ensure package reflect treats absent and empty tags as equivalent.
+	if !tb.AssignableTo(ta) {
+		panic("fail")
+	}
+}