cmd/internal/obj: add a flag to not write referenced symbol names in object file

The Go object file references (some of) symbols from other
packages by indices, not by names. The linker doesn't need the
symbol names to do the linking. The names are included in the
object file so it is self-contained and tools (objdump, nm) can
read the referenced symbol names. Including the names increases
object file size. Add a flag to disable it on demand (off by
default).

Change-Id: I143a0eb656997497c750b8eb1541341b2aee8f30
Reviewed-on: https://go-review.googlesource.com/c/go/+/404297
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Cherry Mui <cherryyz@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/base/debug.go

@@ -31,6 +31,7 @@ type DebugFlags struct {
 	LocationLists        int    `help:"print information about DWARF location list creation"`
 	Nil                  int    `help:"print information about nil checks"`
 	NoOpenDefer          int    `help:"disable open-coded defers"`
+	NoRefName            int    `help:"do not include referenced symbol names in object file"`
 	PCTab                string `help:"print named pc-value table\nOne of: pctospadj, pctofile, pctoline, pctoinline, pctopcdata"`
 	Panic                int    `help:"show all compiler panics"`
 	Slice                int    `help:"print information about slice compilation"`

src/cmd/compile/internal/base/flag.go

@@ -192,6 +192,7 @@ func ParseFlags() {
 	Ctxt.Flag_optimize = Flag.N == 0
 	Ctxt.Debugasm = int(Flag.S)
 	Ctxt.Flag_maymorestack = Debug.MayMoreStack
+	Ctxt.Flag_noRefName = Debug.NoRefName != 0
 
 	if flag.NArg() < 1 {
 		usage()

src/cmd/internal/goobj/objfile.go

@@ -177,6 +177,7 @@ const (
 	PkgIdxHashed                        // Hashed (content-addressable) symbols
 	PkgIdxBuiltin                       // Predefined runtime symbols (ex: runtime.newobject)
 	PkgIdxSelf                          // Symbols defined in the current package
+	PkgIdxSpecial  = PkgIdxSelf         // Indices above it has special meanings
 	PkgIdxInvalid  = 0
 	// The index of other referenced packages starts from 1.
 )

src/cmd/internal/obj/link.go

@@ -899,6 +899,7 @@ type Link struct {
 	Flag_linkshared    bool
 	Flag_optimize      bool
 	Flag_locationlists bool
+	Flag_noRefName     bool   // do not include referenced symbol names in object file
 	Retpoline          bool   // emit use of retpoline stubs for indirect jmp/call
 	Flag_maymorestack  string // If not "", call this function before stack checks
 	Bso                *bufio.Writer

src/cmd/internal/obj/objfile.go

@@ -269,17 +269,21 @@ func (w *writer) StringTable() {
 		w.AddString(pkg)
 	}
 	w.ctxt.traverseSyms(traverseAll, func(s *LSym) {
-		// TODO: this includes references of indexed symbols from other packages,
-		// for which the linker doesn't need the name. Consider moving them to
-		// a separate block (for tools only).
-		if w.pkgpath != "" {
-			s.Name = strings.Replace(s.Name, "\"\".", w.pkgpath+".", -1)
-		}
 		// Don't put names of builtins into the string table (to save
 		// space).
 		if s.PkgIdx == goobj.PkgIdxBuiltin {
 			return
 		}
+		// TODO: this includes references of indexed symbols from other packages,
+		// for which the linker doesn't need the name. Consider moving them to
+		// a separate block (for tools only).
+		if w.ctxt.Flag_noRefName && s.PkgIdx < goobj.PkgIdxSpecial {
+			// Don't include them if Flag_noRefName
+			return
+		}
+		if w.pkgpath != "" {
+			s.Name = strings.Replace(s.Name, "\"\".", w.pkgpath+".", -1)
+		}
 		w.AddString(s.Name)
 	})
 
@@ -625,6 +629,9 @@ func (w *writer) refFlags() {
 // Emits names of referenced indexed symbols, used by tools (objdump, nm)
 // only.
 func (w *writer) refNames() {
+	if w.ctxt.Flag_noRefName {
+		return
+	}
 	seen := make(map[*LSym]bool)
 	w.ctxt.traverseSyms(traverseRefs, func(rs *LSym) { // only traverse refs, not auxs, as tools don't need auxs
 		switch rs.PkgIdx {

src/cmd/internal/obj/objfile_test.go

@@ -121,3 +121,25 @@ func TestSymbolTooLarge(t *testing.T) { // Issue 42054
 		t.Errorf("unexpected error message: want: %q, got: %s", want, out)
 	}
 }
+
+func TestNoRefName(t *testing.T) {
+	// Test that the norefname flag works.
+	testenv.MustHaveGoBuild(t)
+
+	tmpdir := t.TempDir()
+
+	src := filepath.Join(tmpdir, "x.go")
+	err := ioutil.WriteFile(src, []byte("package main; import \"fmt\"; func main() { fmt.Println(123) }\n"), 0666)
+	if err != nil {
+		t.Fatalf("failed to write source file: %v\n", err)
+	}
+	exe := filepath.Join(tmpdir, "x.exe")
+
+	// Build the fmt package with norefname. Not rebuilding all packages to save time.
+	// Also testing that norefname and non-norefname packages can link together.
+	cmd := exec.Command(testenv.GoToolPath(t), "build", "-gcflags=fmt=-d=norefname", "-o", exe, src)
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Fatalf("build failed: %v, output:\n%s", err, out)
+	}
+}