cmd/internal/ld: cache file name construction in linkgetline

This avoids repeated allocation and map lookups
when constructing the pcln tables.

For 6g compiling cmd/internal/gc/*.go this saves about 8% wall time.

Change-Id: I6a1a80e278ae2c2a44bd1537015ea7b4e7a4d6ca
Reviewed-on: https://go-review.googlesource.com/6793Reviewed-by: Rob Pike <r@golang.org>

src/cmd/internal/obj/link.go

@@ -154,14 +154,6 @@ type Auto struct {
 	Gotype  *LSym
 }
 
-type Hist struct {
-	Link    *Hist
-	Name    string
-	Line    int32
-	Offset  int32
-	Printed uint8
-}
-
 type Link struct {
 	Thechar            int32
 	Thestring          string

src/cmd/internal/obj/obj.go

@@ -15,6 +15,15 @@ const (
 	NSYM   = 50
 )
 
+type Hist struct {
+	Link    *Hist
+	Name    string
+	Sym     *LSym
+	Line    int32
+	Offset  int32
+	Printed uint8
+}
+
 func Linklinefmt(ctxt *Link, lno0 int, showAll, showFullPath bool) string {
 	var a [HISTSZ]struct {
 		incl *Hist
@@ -174,13 +183,17 @@ func linkgetline(ctxt *Link, line int32, f **LSym, l *int32) {
 	n--
 	var dlno int32
 	var file string
+	var sym *LSym
 	if a[n].line != nil {
 		file = a[n].line.Name
+		sym = a[n].line.Sym
 		dlno = a[n].ldel - 1
 	} else {
 		file = a[n].incl.Name
+		sym = a[n].incl.Sym
 		dlno = a[n].idel - 1
 	}
+	if sym == nil {
 		var buf string
 		if filepath.IsAbs(file) || strings.HasPrefix(file, "<") {
 			buf = file
@@ -202,8 +215,15 @@ func linkgetline(ctxt *Link, line int32, f **LSym, l *int32) {
 			buf1 := fmt.Sprintf("%s%s", ctxt.Goroot_final, buf[len(ctxt.Goroot):])
 			buf = buf1
 		}
+		sym = Linklookup(ctxt, buf, HistVersion)
+		if a[n].line != nil {
+			a[n].line.Sym = sym
+		} else {
+			a[n].incl.Sym = sym
+		}
+	}
 	lno -= dlno
-	*f = Linklookup(ctxt, buf, HistVersion)
+	*f = sym
 	*l = lno
 }