[dev.link] cmd/internal/obj, cmd/link: use aux symbol for DWARF symbols

Use the auxiliary symbol mechanism to connect the text symbol and
its associated DWARF symbols. This way, the linker can track the
DWARF symbols from the text symbol, without looking up special
names.

Currently, in the linker this is only used in the deadcode pass
to track which DWARF symbols are used and need to load. Later
passes still use name lookup for now.

Change-Id: I2fe49f3b1f0ecc1472ae8aa93907cff740022d8d
Reviewed-on: https://go-review.googlesource.com/c/go/+/199801
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>

src/cmd/internal/goobj/readnew.go

@@ -125,6 +125,8 @@ func (r *objReader) readNew() {
 				isym = int(a.Sym.SymIdx)
 			case goobj2.AuxFuncdata:
 				funcdata = append(funcdata, a.Sym)
+			case goobj2.AuxDwarfInfo, goobj2.AuxDwarfLoc, goobj2.AuxDwarfRanges, goobj2.AuxDwarfLines:
+				// nothing to do
 			default:
 				panic("unknown aux type")
 			}

src/cmd/internal/goobj2/objfile.go

@@ -280,8 +280,12 @@ const (
 	AuxGotype = iota
 	AuxFuncInfo
 	AuxFuncdata
+	AuxDwarfInfo
+	AuxDwarfLoc
+	AuxDwarfRanges
+	AuxDwarfLines
 
-	// TODO: more. DWARF? Pcdata?
+	// TODO: more. Pcdata?
 )
 
 func (a *Aux) Write(w *Writer) {

src/cmd/internal/obj/objfile2.go

@@ -95,13 +95,7 @@ func WriteObjFile2(ctxt *Link, b *bio.Writer, pkgpath string) {
 	for _, list := range lists {
 		for _, s := range list {
 			w.Uint32(naux)
-			if s.Gotype != nil {
-				naux++
-			}
-			if s.Func != nil {
-				// FuncInfo is an aux symbol, each Funcdata is an aux symbol
-				naux += 1 + uint32(len(s.Func.Pcln.Funcdata))
-			}
+			naux += uint32(nAuxSym(s))
 		}
 	}
 	w.Uint32(naux)
@@ -301,7 +295,61 @@ func (w *writer) Aux(s *LSym) {
 			}
 			o.Write(w.Writer)
 		}
+
+		if s.Func.dwarfInfoSym != nil {
+			o := goobj2.Aux{
+				Type: goobj2.AuxDwarfInfo,
+				Sym:  makeSymRef(s.Func.dwarfInfoSym),
+			}
+			o.Write(w.Writer)
+		}
+		if s.Func.dwarfLocSym != nil {
+			o := goobj2.Aux{
+				Type: goobj2.AuxDwarfLoc,
+				Sym:  makeSymRef(s.Func.dwarfLocSym),
+			}
+			o.Write(w.Writer)
+		}
+		if s.Func.dwarfRangesSym != nil {
+			o := goobj2.Aux{
+				Type: goobj2.AuxDwarfRanges,
+				Sym:  makeSymRef(s.Func.dwarfRangesSym),
+			}
+			o.Write(w.Writer)
+		}
+		if s.Func.dwarfDebugLinesSym != nil {
+			o := goobj2.Aux{
+				Type: goobj2.AuxDwarfLines,
+				Sym:  makeSymRef(s.Func.dwarfDebugLinesSym),
+			}
+			o.Write(w.Writer)
+		}
+	}
+}
+
+// return the number of aux symbols s have.
+func nAuxSym(s *LSym) int {
+	n := 0
+	if s.Gotype != nil {
+		n++
+	}
+	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 {
+			n++
+		}
+		if s.Func.dwarfLocSym != nil {
+			n++
+		}
+		if s.Func.dwarfRangesSym != nil {
+			n++
+		}
+		if s.Func.dwarfDebugLinesSym != nil {
+			n++
+		}
 	}
+	return n
 }
 
 // generate symbols for FuncInfo.

src/cmd/link/internal/ld/deadcode2.go

@@ -126,6 +126,12 @@ func (d *deadcodePass2) flood() {
 				i += 2
 				continue
 			}
+			if r.Type == objabi.R_USETYPE {
+				// type symbol used for DWARF. we need to load the symbol but it may not
+				// be otherwise reachable in the program.
+				// do nothing for now as we still load all type symbols.
+				continue
+			}
 			d.mark(r.Sym)
 		}
 		naux := d.loader.NAux(symIdx)

src/cmd/link/internal/objfile/objfile2.go

@@ -462,10 +462,10 @@ func LoadFull(l *Loader, arch *sys.Arch, syms *sym.Symbols) {
 	// external symbols
 	for i := l.extStart; i <= l.max; i++ {
 		nv := l.extSyms[i-l.extStart]
-		if l.Reachable.Has(i) || strings.HasPrefix(nv.name, "go.info.") || strings.HasPrefix(nv.name, "gofile..") { // XXX some go.info and file symbols are used but not marked
+		if l.Reachable.Has(i) || strings.HasPrefix(nv.name, "gofile..") { // XXX file symbols are used but not marked
 			s := syms.Newsym(nv.name, nv.v)
 			preprocess(arch, s)
-			s.Attr.Set(sym.AttrReachable, true)
+			s.Attr.Set(sym.AttrReachable, l.Reachable.Has(i))
 			l.Syms[i] = s
 		}
 	}
@@ -499,10 +499,9 @@ func loadObjSyms(l *Loader, syms *sym.Symbols, r *oReader) {
 		if t == 0 {
 			log.Fatalf("missing type for %s in %s", name, lib)
 		}
-		if !l.Reachable.Has(istart+Sym(i)) && (t < sym.SDWARFSECT || t > sym.SDWARFLINES) && !(t == sym.SRODATA && strings.HasPrefix(name, "type.")) && name != "runtime.addmoduledata" && name != "runtime.lastmoduledatap" {
+		if !l.Reachable.Has(istart+Sym(i)) && !(t == sym.SRODATA && strings.HasPrefix(name, "type.")) && name != "runtime.addmoduledata" && name != "runtime.lastmoduledatap" {
 			// No need to load unreachable symbols.
-			// XXX DWARF symbols may be used but are not marked reachable.
-			// XXX type symbol's content may be needed in DWARF code, but they are not marked.
+			// XXX some type symbol's content may be needed in DWARF code, but they are not marked.
 			// XXX reference to runtime.addmoduledata may be generated later by the linker in plugin mode.
 			continue
 		}
@@ -612,6 +611,8 @@ func loadObjFull(l *Loader, r *oReader) {
 					panic("funcinfo symbol not defined in current package")
 				}
 				isym = int(a.Sym.SymIdx)
+			case goobj2.AuxDwarfInfo, goobj2.AuxDwarfLoc, goobj2.AuxDwarfRanges, goobj2.AuxDwarfLines:
+				// ignored for now
 			default:
 				panic("unknown aux type")
 			}