cmd/internal/obj: use string instead of LSym in Pcln

In a concurrent backend, Ctxt.Lookup will need some
form of concurrency protection, which will make it
more expensive.

This CL changes the pcln table builder to track
filenames as strings rather than LSyms.
Those strings are then converted into LSyms
at the last moment, for writing the object file.

This CL removes over 85% of the calls to Ctxt.Lookup
in a run of make.bash.

Passes toolstash-check.

Updates #15756

Change-Id: I3c53deff6f16f2643169f3bdfcc7aca2ca58b0a4
Reviewed-on: https://go-review.googlesource.com/39291
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/internal/obj/line.go

@@ -74,11 +74,11 @@ func (ctxt *Link) AddImport(pkg string) {
 	ctxt.Imports = append(ctxt.Imports, pkg)
 }
 
-func linkgetlineFromPos(ctxt *Link, xpos src.XPos) (f *LSym, l int32) {
+func linkgetlineFromPos(ctxt *Link, xpos src.XPos) (f string, l int32) {
 	pos := ctxt.PosTable.Pos(xpos)
 	if !pos.IsKnown() {
 		pos = src.Pos{}
 	}
 	// TODO(gri) Should this use relative or absolute line number?
-	return Linklookup(ctxt, pos.SymFilename(), 0), int32(pos.RelLine())
+	return pos.SymFilename(), int32(pos.RelLine())
 }

src/cmd/internal/obj/line_test.go

@@ -31,7 +31,7 @@ func TestLinkgetlineFromPos(t *testing.T) {
 
 	for _, test := range tests {
 		f, l := linkgetlineFromPos(ctxt, ctxt.PosTable.XPos(test.pos))
-		got := fmt.Sprintf("%s:%d", f.Name, l)
+		got := fmt.Sprintf("%s:%d", f, l)
 		if got != src.FileSymPrefix+test.want {
 			t.Errorf("linkgetline(%v) = %q, want %q", test.pos, got, test.want)
 		}

src/cmd/internal/obj/link.go

@@ -401,8 +401,8 @@ type Pcln struct {
 	Pcdata      []Pcdata
 	Funcdata    []*LSym
 	Funcdataoff []int64
-	File        []*LSym
-	Lastfile    *LSym
+	File        []string
+	Lastfile    string
 	Lastindex   int
 	InlTree     InlTree // per-function inlining tree extracted from the global tree
 }

src/cmd/internal/obj/objfile.go

@@ -299,12 +299,14 @@ func (w *objWriter) writeRefs(s *LSym) {
 			w.writeRef(d, false)
 		}
 		for _, f := range pc.File {
-			w.writeRef(f, true)
+			fsym := Linklookup(w.ctxt, f, 0)
+			w.writeRef(fsym, true)
 		}
 		for _, call := range pc.InlTree.nodes {
 			w.writeRef(call.Func, false)
 			f, _ := linkgetlineFromPos(w.ctxt, call.Pos)
-			w.writeRef(f, true)
+			fsym := Linklookup(w.ctxt, f, 0)
+			w.writeRef(fsym, true)
 		}
 	}
 }
@@ -467,13 +469,15 @@ func (w *objWriter) writeSym(s *LSym) {
 	}
 	w.writeInt(int64(len(pc.File)))
 	for _, f := range pc.File {
-		w.writeRefIndex(f)
+		fsym := Linklookup(ctxt, f, 0)
+		w.writeRefIndex(fsym)
 	}
 	w.writeInt(int64(len(pc.InlTree.nodes)))
 	for _, call := range pc.InlTree.nodes {
 		w.writeInt(int64(call.Parent))
 		f, l := linkgetlineFromPos(w.ctxt, call.Pos)
-		w.writeRefIndex(f)
+		fsym := Linklookup(ctxt, f, 0)
+		w.writeRefIndex(fsym)
 		w.writeInt(int64(l))
 		w.writeRefIndex(call.Func)
 	}

src/cmd/internal/obj/pcln.go

@@ -131,11 +131,6 @@ func pctofileline(ctxt *Link, sym *LSym, oldval int32, p *Prog, phase int32, arg
 		return oldval
 	}
 	f, l := linkgetlineFromPos(ctxt, p.Pos)
-	if f == nil {
-		//	print("getline failed for %s %v\n", ctxt->cursym->name, p);
-		return oldval
-	}
-
 	if arg == nil {
 		return l
 	}