cmd/compile: deduplicate symbol references

Reduces size of archives in pkg/linux_amd64 by 1.4MB (3.2%),
slightly improving link time.

name       old s/op   new s/op   delta
LinkCmdGo  0.52 ± 3%  0.51 ± 2%  -0.65%  (p=0.000 n=98+99)

Change-Id: I7e265f4d4dd08967c5c5d55c1045e533466bbbec
Reviewed-on: https://go-review.googlesource.com/20802
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/internal/obj/objfile.go

@@ -345,10 +345,31 @@ func Writeobjfile(ctxt *Link, b *Biobuf) {
 	fmt.Fprintf(b, "go13ld")
 }
 
+// Provide the the index of a symbol reference by symbol name.
+// One map for versioned symbols and one for unversioned symbols.
+// Used for deduplicating the symbol reference list.
+var refIdx = make(map[string]int)
+var vrefIdx = make(map[string]int)
+
 func wrref(ctxt *Link, b *Biobuf, s *LSym, isPath bool) {
 	if s == nil || s.RefIdx != 0 {
 		return
 	}
+	var m map[string]int
+	switch s.Version {
+	case 0:
+		m = refIdx
+	case 1:
+		m = vrefIdx
+	default:
+		log.Fatalf("%s: invalid version number %d", s.Name, s.Version)
+	}
+
+	idx := m[s.Name]
+	if idx != 0 {
+		s.RefIdx = idx
+		return
+	}
 	Bputc(b, 0xfe)
 	if isPath {
 		wrstring(b, filepath.ToSlash(s.Name))
@@ -358,6 +379,7 @@ func wrref(ctxt *Link, b *Biobuf, s *LSym, isPath bool) {
 	wrint(b, int64(s.Version))
 	ctxt.RefsWritten++
 	s.RefIdx = ctxt.RefsWritten
+	m[s.Name] = ctxt.RefsWritten
 }
 
 func writerefs(ctxt *Link, b *Biobuf, s *LSym) {