[dev.link] cmd/compile, cmd/link: move DWARF info sym to anonymous aux data

Switch the primary subprogram die DWARF symbol emitted by the compiler
from named+dupOK to anonymous aux. This should help performance wise
by not having to add these symbols to the linker's symbol name lookup
tables.

Change-Id: Idf66662b8bf60b3dee9a55e6cd5137b24a9f5ab6
Reviewed-on: https://go-review.googlesource.com/c/go/+/223669
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/internal/obj/objfile.go

@@ -207,7 +207,9 @@ func (ctxt *Link) dwarfSym(s *LSym) (dwarfInfoSym, dwarfLocSym, dwarfRangesSym,
 		ctxt.Diag("dwarfSym of non-TEXT %v", s)
 	}
 	if s.Func.dwarfInfoSym == nil {
-		s.Func.dwarfInfoSym = ctxt.LookupDerived(s, dwarf.InfoPrefix+s.Name)
+		s.Func.dwarfInfoSym = &LSym{
+			Type: objabi.SDWARFINFO,
+		}
 		if ctxt.Flag_locationlists {
 			s.Func.dwarfLocSym = &LSym{
 				Type: objabi.SDWARFLOC,

src/cmd/internal/obj/objfile2.go

@@ -299,7 +299,7 @@ func (w *writer) Aux(s *LSym) {
 			o.Write(w.Writer)
 		}
 
-		if s.Func.dwarfInfoSym != nil {
+		if s.Func.dwarfInfoSym != nil && s.Func.dwarfInfoSym.Size != 0 {
 			o := goobj2.Aux{
 				Type: goobj2.AuxDwarfInfo,
 				Sym:  makeSymRef(s.Func.dwarfInfoSym),
@@ -339,7 +339,7 @@ func nAuxSym(s *LSym) int {
 	if s.Func != nil {
 		// FuncInfo is an aux symbol, each Funcdata is an aux symbol
 		n += 1 + len(s.Func.Pcln.Funcdata)
-		if s.Func.dwarfInfoSym != nil {
+		if s.Func.dwarfInfoSym != nil && s.Func.dwarfInfoSym.Size != 0 {
 			n++
 		}
 		if s.Func.dwarfLocSym != nil && s.Func.dwarfLocSym.Size != 0 {
@@ -419,7 +419,7 @@ func genFuncInfoSyms(ctxt *Link) {
 		s.Func.FuncInfoSym = isym
 		b.Reset()
 
-		dwsyms := []*LSym{s.Func.dwarfRangesSym, s.Func.dwarfLocSym, s.Func.dwarfDebugLinesSym}
+		dwsyms := []*LSym{s.Func.dwarfRangesSym, s.Func.dwarfLocSym, s.Func.dwarfDebugLinesSym, s.Func.dwarfInfoSym}
 		for _, s := range dwsyms {
 			if s == nil || s.Size == 0 {
 				continue

src/cmd/internal/obj/plist.go

@@ -139,10 +139,7 @@ func (ctxt *Link) InitTextSym(s *LSym, flag int) {
 	ctxt.Text = append(ctxt.Text, s)
 
 	// Set up DWARF entries for s.
-	info, _, _, _, _ := ctxt.dwarfSym(s)
+	ctxt.dwarfSym(s)
-	info.Type = objabi.SDWARFINFO
-	info.Set(AttrDuplicateOK, s.DuplicateOK())
-	ctxt.Data = append(ctxt.Data, info)
 }
 
 func (ctxt *Link) Globl(s *LSym, size int64, flag int) {

src/cmd/internal/obj/sym.go

@@ -330,7 +330,7 @@ func (ctxt *Link) traverseFuncAux(flag traverseFlag, fsym *LSym, fn func(parent
 			fn(fsym, filesym)
 		}
 	}
-	dwsyms := []*LSym{fsym.Func.dwarfRangesSym, fsym.Func.dwarfLocSym, fsym.Func.dwarfDebugLinesSym}
+	dwsyms := []*LSym{fsym.Func.dwarfRangesSym, fsym.Func.dwarfLocSym, fsym.Func.dwarfDebugLinesSym, fsym.Func.dwarfInfoSym}
 	for _, dws := range dwsyms {
 		if dws == nil || dws.Size == 0 {
 			continue

src/cmd/link/internal/ld/dwarf.go

@@ -1912,6 +1912,9 @@ func dwarfGenerateDebugInfo(ctxt *Link) {
 			for _, s := range unit.Textp2 { // textp2 has been dead-code-eliminated already.
 				fnSym := loader.Sym(s)
 				infosym, _, rangesym, _ := d.ldr.GetFuncDwarfAuxSyms(fnSym)
+				if infosym == 0 {
+					continue
+				}
 				d.ldr.SetAttrNotInSymbolTable(infosym, true)
 				d.ldr.SetAttrReachable(infosym, true)
 

src/cmd/link/internal/loader/loader.go

@@ -2140,11 +2140,17 @@ func loadObjSyms(l *Loader, syms *sym.Symbols, r *oReader) int {
 		osym := goobj2.Sym{}
 		osym.Read(r.Reader, r.SymOff(i))
 		name := strings.Replace(osym.Name, "\"\".", r.pkgprefix, -1)
-		if name == "" {
+		t := sym.AbiSymKindToSymKind[objabi.SymKind(osym.Type)]
+		// NB: for the test below, we can skip most anonymous symbols
+		// since they will never be turned into sym.Symbols (ex:
+		// funcdata), however DWARF subprogram DIE symbols (which are
+		// nameless) will eventually need to be turned into
+		// sym.Symbols (with relocations), so the simplest thing to do
+		// is include them as part of this loop.
+		if name == "" && t != sym.SDWARFINFO {
 			continue
 		}
 		ver := abiToVer(osym.ABI, r.version)
-		t := sym.AbiSymKindToSymKind[objabi.SymKind(osym.Type)]
 		if t == sym.SXREF {
 			log.Fatalf("bad sxref")
 		}