[dev.link] cmd: add flag to mark gotype symbols

Add a flag bit to mark symbols in the new object file as containing Go
type information. The use of a flag eliminates the need to do symbol
name matching as part of the new dead code elimination pass, which
should produce a minor speedup.

Change-Id: Iec8700e1139e2c4e310644c0766379865d2d6f82
Reviewed-on: https://go-review.googlesource.com/c/go/+/201399
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jeremy Faller <jeremy@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/internal/goobj2/objfile.go

@@ -205,6 +205,7 @@ const (
 	SymFlagLeaf
 	SymFlagCFunc
 	SymFlagReflectMethod
+	SymFlagGoType
 	SymFlagTopFrame
 )
 
@@ -234,6 +235,7 @@ func (s *Sym) Typelink() bool      { return s.Flag&SymFlagTypelink != 0 }
 func (s *Sym) Leaf() bool          { return s.Flag&SymFlagLeaf != 0 }
 func (s *Sym) CFunc() bool         { return s.Flag&SymFlagCFunc != 0 }
 func (s *Sym) ReflectMethod() bool { return s.Flag&SymFlagReflectMethod != 0 }
+func (s *Sym) IsGoType() bool      { return s.Flag&SymFlagGoType != 0 }
 func (s *Sym) TopFrame() bool      { return s.Flag&SymFlagTopFrame != 0 }
 
 // Symbol reference.

src/cmd/internal/obj/objfile2.go

@@ -238,6 +238,9 @@ func (w *writer) Sym(s *LSym) {
 	if s.TopFrame() {
 		flag |= goobj2.SymFlagTopFrame
 	}
+	if strings.HasPrefix(s.Name, "type.") && s.Name[5] != '.' && s.Type == objabi.SRODATA {
+		flag |= goobj2.SymFlagGoType
+	}
 	name := s.Name
 	if strings.HasPrefix(name, "gofile..") {
 		name = filepath.ToSlash(name)

src/cmd/link/internal/ld/deadcode2.go

@@ -108,9 +108,7 @@ func (d *deadcodePass2) flood() {
 		symIdx := d.wq.pop()
 
 		d.reflectSeen = d.reflectSeen || d.ldr.IsReflectMethod(symIdx)
-
-		name := d.ldr.RawSymName(symIdx)
-		if strings.HasPrefix(name, "type.") && name[5] != '.' { // TODO: use an attribute instead of checking name
+		if d.ldr.IsGoType(symIdx) {
 			p := d.ldr.Data(symIdx)
 			if len(p) != 0 && decodetypeKind(d.ctxt.Arch, p)&kindMask == kindInterface {
 				for _, sig := range decodeIfaceMethods2(d.ldr, d.ctxt.Arch, symIdx) {

src/cmd/link/internal/loader/loader.go

@@ -364,6 +364,11 @@ func (l *Loader) IsReflectMethod(i Sym) bool {
 	return l.SymAttr(i)&goobj2.SymFlagReflectMethod != 0
 }
 
+// Returns whether this is a Go type symbol.
+func (l *Loader) IsGoType(i Sym) bool {
+	return l.SymAttr(i)&goobj2.SymFlagGoType != 0
+}
+
 // Returns the symbol content of the i-th symbol. i is global index.
 func (l *Loader) Data(i Sym) []byte {
 	if l.isExternal(i) {