cmd/compile: rework how fieldtrack is implemented

Shrinks gc.Type and gc.Func slightly.

Passes "GOEXPERIMENT=fieldtrack ./all.bash" and "go test -a
-toolexec='toolstash -cmp' -ldflags=-k=rsc.io/tmp/fieldtrack.tracked
rsc.io/tmp/fieldtrack".

Change-Id: I785fe8a18eb830d9867d34247e4cd41a6a7921d4
Reviewed-on: https://go-review.googlesource.com/20557
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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

@@ -459,8 +459,15 @@ func compile(fn *Node) {
 	gcargs := makefuncdatasym("gcargs·%d", obj.FUNCDATA_ArgsPointerMaps)
 	gclocals := makefuncdatasym("gclocals·%d", obj.FUNCDATA_LocalsPointerMaps)
 
-	for _, t := range Curfn.Func.Fieldtrack {
-		gtrack(tracksym(t))
+	if obj.Fieldtrack_enabled != 0 && len(Curfn.Func.FieldTrack) > 0 {
+		trackSyms := make([]*Sym, 0, len(Curfn.Func.FieldTrack))
+		for sym := range Curfn.Func.FieldTrack {
+			trackSyms = append(trackSyms, sym)
+		}
+		sort.Sort(symByName(trackSyms))
+		for _, sym := range trackSyms {
+			gtrack(sym)
+		}
 	}
 
 	for _, n := range fn.Func.Dcl {

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

@@ -798,8 +798,10 @@ func typesym(t *Type) *Sym {
 	return Pkglookup(Tconv(t, obj.FmtLeft), typepkg)
 }
 
-func tracksym(t *Type) *Sym {
-	return Pkglookup(Tconv(t.Outer, obj.FmtLeft)+"."+t.Sym.Name, trackpkg)
+// tracksym returns the symbol for tracking use of field/method f, assumed
+// to be a member of struct/interface type t.
+func tracksym(t, f *Type) *Sym {
+	return Pkglookup(Tconv(t, obj.FmtLeft)+"."+f.Sym.Name, trackpkg)
 }
 
 func typelinksym(t *Type) *Sym {

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

@@ -23,11 +23,11 @@ func TestSizeof(t *testing.T) {
 		_64bit uintptr     // size on 64bit platforms
 	}{
 		{Flow{}, 52, 88},
-		{Func{}, 104, 184},
+		{Func{}, 96, 168},
 		{Name{}, 52, 80},
 		{Node{}, 92, 144},
 		{Sym{}, 60, 112},
-		{Type{}, 144, 240},
+		{Type{}, 136, 224},
 	}
 
 	for _, tt := range tests {

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

@@ -156,7 +156,7 @@ type Func struct {
 	Inldcl     Nodes   // copy of dcl for use in inlining
 	Closgen    int
 	Outerfunc  *Node
-	Fieldtrack []*Type
+	FieldTrack map[*Sym]struct{}
 	Outer      *Node // outer func for closure
 	Ntype      *Node // signature
 	Top        int   // top context (Ecall, Eproc, etc)

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

@@ -141,9 +141,8 @@ type Type struct {
 	Width int64 // offset in TFIELD, width in all others
 
 	// TFIELD
-	Down  *Type   // next struct field, also key type in TMAP
-	Outer *Type   // outer struct
-	Note  *string // literal string annotation
+	Down *Type   // next struct field, also key type in TMAP
+	Note *string // literal string annotation
 
 	// TARRAY
 	Bound int64 // negative is slice
@@ -159,8 +158,6 @@ type Type struct {
 
 	// for TFORW, where to copy the eventual value to
 	Copyto []*Node
-
-	Lastfn *Node // for usefield
 }
 
 // typ returns a new Type of the specified kind.

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

@@ -3838,23 +3838,22 @@ func usefield(n *Node) {
 		return
 	}
 
-	// dedup on list
-	if field.Lastfn == Curfn {
-		return
-	}
-	field.Lastfn = Curfn
-	field.Outer = n.Left.Type
-	if Isptr[field.Outer.Etype] {
-		field.Outer = field.Outer.Type
+	outer := n.Left.Type
+	if Isptr[outer.Etype] {
+		outer = outer.Type
 	}
-	if field.Outer.Sym == nil {
+	if outer.Sym == nil {
 		Yyerror("tracked field must be in named struct type")
 	}
 	if !exportname(field.Sym.Name) {
 		Yyerror("tracked field must be exported (upper case)")
 	}
 
-	Curfn.Func.Fieldtrack = append(Curfn.Func.Fieldtrack, field)
+	sym := tracksym(outer, field)
+	if Curfn.Func.FieldTrack == nil {
+		Curfn.Func.FieldTrack = make(map[*Sym]struct{})
+	}
+	Curfn.Func.FieldTrack[sym] = struct{}{}
 }
 
 func candiscardlist(l Nodes) bool {