cmd/internal/obj: simplify filename handling

The old Go object file format used linker symbols like "gofile..foo"
to record references to the filename "foo". But the current object
file format has a dedicated section for file names, so we don't need
these useless prefixes anymore.

Also, change DWARF generation to pass around the src.Pos directly,
rather than the old file symbols, which it just turned back into a
file index before writing out anyway.

Finally, directly record the FileIndex into src.PosBase, so that we
can skip the map lookups.

Change-Id: Ia4a5ebfa95da271f2522e45befdb9f137c16d373
Reviewed-on: https://go-review.googlesource.com/c/go/+/523378
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>

1 files changed, 18 insertions, 31 deletions

diff --git a/src/cmd/internal/obj/dwarf.go b/src/cmd/internal/obj/dwarf.go
index 825f0133f1..f5caa08f0a 100644
--- a/src/cmd/internal/obj/dwarf.go
+++ b/src/cmd/internal/obj/dwarf.go
@@ -48,7 +48,7 @@ func (ctxt *Link) generateDebugLinesSymbol(s, lines *LSym) {
 	line := int64(1)
 	pc := s.Func().Text.Pc
 	var lastpc int64 // last PC written to line table, not last PC in func
-	name := ""
+	fileIndex := 1
 	prologue, wrotePrologue := false, false
 	// Walk the progs, generating the DWARF table.
 	for p := s.Func().Text; p != nil; p = p.Link {
@@ -58,15 +58,15 @@ func (ctxt *Link) generateDebugLinesSymbol(s, lines *LSym) {
 			continue
 		}
 		newStmt := p.Pos.IsStmt() != src.PosNotStmt
-		newName, newLine := ctxt.getFileSymbolAndLine(p.Pos)
+		newFileIndex, newLine := ctxt.getFileIndexAndLine(p.Pos)
+		newFileIndex++ // 1 indexing for the table
 
 		// Output debug info.
 		wrote := false
-		if name != newName {
-			newFile := ctxt.PosTable.FileIndex(newName) + 1 // 1 indexing for the table.
+		if newFileIndex != fileIndex {
 			dctxt.AddUint8(lines, dwarf.DW_LNS_set_file)
-			dwarf.Uleb128put(dctxt, lines, int64(newFile))
-			name = newName
+			dwarf.Uleb128put(dctxt, lines, int64(newFileIndex))
+			fileIndex = newFileIndex
 			wrote = true
 		}
 		if prologue && !wrotePrologue {
@@ -258,16 +258,6 @@ func (c dwCtxt) AddDWARFAddrSectionOffset(s dwarf.Sym, t interface{}, ofs int64)
 	r.Type = objabi.R_DWARFSECREF
 }
 
-func (c dwCtxt) AddFileRef(s dwarf.Sym, f interface{}) {
-	ls := s.(*LSym)
-	rsym := f.(*LSym)
-	fidx := c.Link.PosTable.FileIndex(rsym.Name)
-	// Note the +1 here -- the value we're writing is going to be an
-	// index into the DWARF line table file section, whose entries
-	// are numbered starting at 1, not 0.
-	ls.WriteInt(c.Link, ls.Size, 4, int64(fidx+1))
-}
-
 func (c dwCtxt) CurrentOffset(s dwarf.Sym) int64 {
 	ls := s.(*LSym)
 	return ls.Size
@@ -329,17 +319,13 @@ func (s *LSym) Length(dwarfContext interface{}) int64 {
 	return s.Size
 }
 
-// fileSymbol returns a symbol corresponding to the source file of the
-// first instruction (prog) of the specified function. This will
-// presumably be the file in which the function is defined.
-func (ctxt *Link) fileSymbol(fn *LSym) *LSym {
-	p := fn.Func().Text
-	if p != nil {
-		f, _ := ctxt.getFileSymbolAndLine(p.Pos)
-		fsym := ctxt.Lookup(f)
-		return fsym
+// textPos returns the source position of the first instruction (prog)
+// of the specified function.
+func textPos(fn *LSym) src.XPos {
+	if p := fn.Func().Text; p != nil {
+		return p.Pos
 	}
-	return nil
+	return src.NoXPos
 }
 
 // populateDWARF fills in the DWARF Debugging Information Entries for
@@ -362,17 +348,19 @@ func (ctxt *Link) populateDWARF(curfn Func, s *LSym) {
 	}
 	var err error
 	dwctxt := dwCtxt{ctxt}
-	filesym := ctxt.fileSymbol(s)
+	startPos := ctxt.InnermostPos(textPos(s))
+	if !startPos.IsKnown() || startPos.RelLine() != uint(s.Func().StartLine) {
+		panic("bad startPos")
+	}
 	fnstate := &dwarf.FnState{
 		Name:          s.Name,
 		Info:          info,
-		Filesym:       filesym,
 		Loc:           loc,
 		Ranges:        ranges,
 		Absfn:         absfunc,
 		StartPC:       s,
 		Size:          s.Size,
-		StartLine:     s.Func().StartLine,
+		StartPos:      startPos,
 		External:      !s.Static(),
 		Scopes:        scopes,
 		InlCalls:      inlcalls,
@@ -434,13 +422,12 @@ func (ctxt *Link) DwarfAbstractFunc(curfn Func, s *LSym) {
 		s.NewFuncInfo()
 	}
 	scopes, _ := ctxt.DebugInfo(s, absfn, curfn)
-	_, startLine := ctxt.getFileSymbolAndLine(curfn.Pos())
 	dwctxt := dwCtxt{ctxt}
 	fnstate := dwarf.FnState{
 		Name:          s.Name,
 		Info:          absfn,
 		Absfn:         absfn,
-		StartLine:     startLine,
+		StartPos:      ctxt.InnermostPos(curfn.Pos()),
 		External:      !s.Static(),
 		Scopes:        scopes,
 		UseBASEntries: ctxt.UseBASEntries,