[dev.link] cmd: convert symbol "shared" flag to object file flag

For the new object file format, don't tag individual symbols with a
"shared" flag, since that characteristic is better off as an attribute
of the containing object file as opposed to the individual symbol. Add
a new flags field in the object file header and put a bit in the flags
if the shared flags is in effect during compilation.

Change-Id: I2cf6d33bf7bf2fd8a7614ae0cd6ef03914777498
Reviewed-on: https://go-review.googlesource.com/c/go/+/201398
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

@@ -20,6 +20,7 @@ import (
 //
 //    Header struct {
 //       Magic   [...]byte   // "\x00go114LD"
+//       Flags   uint32
 //       // TODO: Fingerprint
 //       Offsets [...]uint32 // byte offset of each block below
 //    }
@@ -148,6 +149,7 @@ const (
 // TODO: probably no need to export this.
 type Header struct {
 	Magic   string
+	Flags   uint32
 	Offsets [NBlk]uint32
 }
 
@@ -155,6 +157,7 @@ const Magic = "\x00go114LD"
 
 func (h *Header) Write(w *Writer) {
 	w.RawString(h.Magic)
+	w.Uint32(h.Flags)
 	for _, x := range h.Offsets {
 		w.Uint32(x)
 	}
@@ -167,6 +170,8 @@ func (h *Header) Read(r *Reader) error {
 		return errors.New("wrong magic, not a Go object file")
 	}
 	off := uint32(len(h.Magic))
+	h.Flags = r.uint32At(off)
+	off += 4
 	for i := range h.Offsets {
 		h.Offsets[i] = r.uint32At(off)
 		off += 4
@@ -175,7 +180,7 @@ func (h *Header) Read(r *Reader) error {
 }
 
 func (h *Header) Size() int {
-	return len(h.Magic) + 4*len(h.Offsets)
+	return len(h.Magic) + 4 + 4*len(h.Offsets)
 }
 
 // Symbol definition.
@@ -189,6 +194,10 @@ type Sym struct {
 
 const SymABIstatic = ^uint16(0)
 
+const (
+	ObjFlagShared = 1 << iota
+)
+
 const (
 	SymFlagDupok = 1 << iota
 	SymFlagLocal
@@ -196,7 +205,6 @@ const (
 	SymFlagLeaf
 	SymFlagCFunc
 	SymFlagReflectMethod
-	SymFlagShared // This is really silly
 	SymFlagTopFrame
 )
 
@@ -226,7 +234,6 @@ 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) Shared() bool        { return s.Flag&SymFlagShared != 0 }
 func (s *Sym) TopFrame() bool      { return s.Flag&SymFlagTopFrame != 0 }
 
 // Symbol reference.
@@ -596,3 +603,8 @@ func (r *Reader) PcdataBase() uint32 {
 func (r *Reader) ReadOnly() bool {
 	return r.readonly
 }
+
+// Flags returns the flag bits read from the object file header.
+func (r *Reader) Flags() uint32 {
+	return r.h.Flags
+}

src/cmd/internal/obj/objfile2.go

@@ -35,7 +35,11 @@ func WriteObjFile2(ctxt *Link, b *bio.Writer, pkgpath string) {
 
 	// Header
 	// We just reserve the space. We'll fill in the offsets later.
-	h := goobj2.Header{Magic: goobj2.Magic}
+	flags := uint32(0)
+	if ctxt.Flag_shared {
+		flags |= goobj2.ObjFlagShared
+	}
+	h := goobj2.Header{Magic: goobj2.Magic, Flags: flags}
 	h.Write(w.Writer)
 
 	// String table
@@ -231,9 +235,6 @@ func (w *writer) Sym(s *LSym) {
 	if s.ReflectMethod() {
 		flag |= goobj2.SymFlagReflectMethod
 	}
-	if w.ctxt.Flag_shared { // This is really silly
-		flag |= goobj2.SymFlagShared
-	}
 	if s.TopFrame() {
 		flag |= goobj2.SymFlagTopFrame
 	}

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

@@ -57,6 +57,7 @@ type oReader struct {
 	*goobj2.Reader
 	unit      *sym.CompilationUnit
 	version   int    // version of static symbol
+	flags     uint32 // read from object file
 	pkgprefix string
 }
 
@@ -460,7 +461,7 @@ func (l *Loader) Preload(arch *sys.Arch, syms *sym.Symbols, f *bio.Reader, lib *
 	}
 	localSymVersion := syms.IncVersion()
 	pkgprefix := objabi.PathToPrefix(lib.Pkg) + "."
-	or := &oReader{r, unit, localSymVersion, pkgprefix}
+	or := &oReader{r, unit, localSymVersion, r.Flags(), pkgprefix}
 
 	// Autolib
 	lib.ImportStrings = append(lib.ImportStrings, r.Autolib()...)
@@ -770,7 +771,7 @@ func loadObjFull(l *Loader, r *oReader) {
 		if osym.ReflectMethod() {
 			s.Attr |= sym.AttrReflectMethod
 		}
-		if osym.Shared() {
+		if r.Flags()&goobj2.ObjFlagShared != 0 {
 			s.Attr |= sym.AttrShared
 		}
 		if osym.TopFrame() {