cmd/link, cmd/internal/obj: use aux symbol for global variable DWARF info

Currently, for a global variable, its debug info symbol is a named
symbol with the variable's name with a special prefix. And the
linker looks it up by name. This CL makes the debug info symbol an
aux symbol of the variable symbol.

Change-Id: I55614d0ef2af9c53eb40144ad80e09339bf3cbee
Reviewed-on: https://go-review.googlesource.com/c/go/+/490816
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>

src/cmd/internal/obj/dwarf.go

@@ -412,12 +412,11 @@ func (ctxt *Link) DwarfGlobal(myimportpath, typename string, varSym *LSym) {
 		return
 	}
 	varname := varSym.Name
-	dieSymName := dwarf.InfoPrefix + varname
-	dieSym := ctxt.LookupInit(dieSymName, func(s *LSym) {
-		s.Type = objabi.SDWARFVAR
-		s.Set(AttrDuplicateOK, true) // needed for shared linkage
-		ctxt.Data = append(ctxt.Data, s)
-	})
+	dieSym := &LSym{
+		Type: objabi.SDWARFVAR,
+	}
+	varSym.NewVarInfo().dwarfInfoSym = dieSym
+	ctxt.Data = append(ctxt.Data, dieSym)
 	typeSym := ctxt.Lookup(dwarf.InfoPrefix + typename)
 	dwarf.PutGlobal(dwCtxt{ctxt}, dieSym, typeSym, varSym, varname)
 }

src/cmd/internal/obj/link.go

@@ -465,7 +465,7 @@ type LSym struct {
 	P      []byte
 	R      []Reloc
 
-	Extra *interface{} // *FuncInfo or *FileInfo, if present
+	Extra *interface{} // *FuncInfo, *VarInfo, or *FileInfo, if present
 
 	Pkg    string
 	PkgIdx int32
@@ -537,6 +537,30 @@ func (s *LSym) Func() *FuncInfo {
 	return f
 }
 
+type VarInfo struct {
+	dwarfInfoSym *LSym
+}
+
+// NewVarInfo allocates and returns a VarInfo for LSym.
+func (s *LSym) NewVarInfo() *VarInfo {
+	if s.Extra != nil {
+		panic(fmt.Sprintf("invalid use of LSym - NewVarInfo with Extra of type %T", *s.Extra))
+	}
+	f := new(VarInfo)
+	s.Extra = new(interface{})
+	*s.Extra = f
+	return f
+}
+
+// VarInfo returns the *VarInfo associated with s, or else nil.
+func (s *LSym) VarInfo() *VarInfo {
+	if s.Extra == nil {
+		return nil
+	}
+	f, _ := (*s.Extra).(*VarInfo)
+	return f
+}
+
 // A FileInfo contains extra fields for SDATA symbols backed by files.
 // (If LSym.Extra is a *FileInfo, LSym.P == nil.)
 type FileInfo struct {

src/cmd/internal/obj/objfile.go

@@ -615,6 +615,10 @@ func (w *writer) Aux(s *LSym) {
 			}
 			w.aux1(goobj.AuxWasmImport, fn.WasmImportSym)
 		}
+	} else if v := s.VarInfo(); v != nil {
+		if v.dwarfInfoSym != nil && v.dwarfInfoSym.Size != 0 {
+			w.aux1(goobj.AuxDwarfInfo, v.dwarfInfoSym)
+		}
 	}
 }
 
@@ -721,6 +725,10 @@ func nAuxSym(s *LSym) int {
 			}
 			n++
 		}
+	} else if v := s.VarInfo(); v != nil {
+		if v.dwarfInfoSym != nil && v.dwarfInfoSym.Size != 0 {
+			n++
+		}
 	}
 	return n
 }
@@ -795,11 +803,14 @@ func genFuncInfoSyms(ctxt *Link) {
 func writeAuxSymDebug(ctxt *Link, par *LSym, aux *LSym) {
 	// Most aux symbols (ex: funcdata) are not interesting--
 	// pick out just the DWARF ones for now.
-	if aux.Type != objabi.SDWARFLOC &&
-		aux.Type != objabi.SDWARFFCN &&
-		aux.Type != objabi.SDWARFABSFCN &&
-		aux.Type != objabi.SDWARFLINES &&
-		aux.Type != objabi.SDWARFRANGE {
+	switch aux.Type {
+	case objabi.SDWARFLOC,
+		objabi.SDWARFFCN,
+		objabi.SDWARFABSFCN,
+		objabi.SDWARFLINES,
+		objabi.SDWARFRANGE,
+		objabi.SDWARFVAR:
+	default:
 		return
 	}
 	ctxt.writeSymDebugNamed(aux, "aux for "+par.Name)

src/cmd/internal/obj/sym.go

@@ -367,6 +367,8 @@ func (ctxt *Link) traverseSyms(flag traverseFlag, fn func(*LSym)) {
 						fn(aux)
 					}
 					ctxt.traverseFuncAux(flag, s, f, files)
+				} else if v := s.VarInfo(); v != nil {
+					fnNoNil(v.dwarfInfoSym)
 				}
 			}
 			if flag&traversePcdata != 0 && s.Type == objabi.STEXT {
@@ -443,10 +445,11 @@ func (ctxt *Link) traverseAuxSyms(flag traverseFlag, fn func(parent *LSym, aux *
 					fn(s, s.Gotype)
 				}
 			}
-			if s.Type != objabi.STEXT {
-				continue
+			if s.Type == objabi.STEXT {
+				ctxt.traverseFuncAux(flag, s, fn, files)
+			} else if v := s.VarInfo(); v != nil && v.dwarfInfoSym != nil {
+				fn(s, v.dwarfInfoSym)
 			}
-			ctxt.traverseFuncAux(flag, s, fn, files)
 		}
 	}
 }

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

@@ -1936,21 +1936,13 @@ func dwarfGenerateDebugInfo(ctxt *Link) {
 		if d.ldr.IsFileLocal(idx) {
 			continue
 		}
-		sn := d.ldr.SymName(idx)
-		if sn == "" {
-			// skip aux symbols
-			continue
-		}
 
 		// Find compiler-generated DWARF info sym for global in question,
 		// and tack it onto the appropriate unit.  Note that there are
 		// circumstances under which we can't find the compiler-generated
 		// symbol-- this typically happens as a result of compiler options
 		// (e.g. compile package X with "-dwarf=0").
-
-		// FIXME: use an aux sym or a relocation here instead of a
-		// name lookup.
-		varDIE := d.ldr.Lookup(dwarf.InfoPrefix+sn, 0)
+		varDIE := d.ldr.GetVarDwarfAuxSym(idx)
 		if varDIE != 0 {
 			unit := d.ldr.SymUnit(idx)
 			d.defgotype(gt)

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

@@ -1685,6 +1685,15 @@ func (l *Loader) GetFuncDwarfAuxSyms(fnSymIdx Sym) (auxDwarfInfo, auxDwarfLoc, a
 	return
 }
 
+func (l *Loader) GetVarDwarfAuxSym(i Sym) Sym {
+	aux := l.aux1(i, goobj.AuxDwarfInfo)
+	if aux != 0 && l.SymType(aux) != sym.SDWARFVAR {
+		fmt.Println(l.SymName(i), l.SymType(i), l.SymType(aux), sym.SDWARFVAR)
+		panic("aux dwarf info sym with wrong type")
+	}
+	return aux
+}
+
 // AddInteriorSym sets up 'interior' as an interior symbol of
 // container/payload symbol 'container'. An interior symbol does not
 // itself have data, but gives a name to a subrange of the data in its