cmd/compile: don't emit autom's into object file

Don't write Autom records when writing a function to the object file;
we no longer need them in the linker for DWARF processing. So as to
keep the object file format unchanged, write out a zero-length list of
automs to the object, as opposed to removing all references.

Updates #34554.

Change-Id: I42a1d67207ea7114ae4f3a315cf37effba57f190
Reviewed-on: https://go-review.googlesource.com/c/go/+/197499Reviewed-by: Jeremy Faller <jeremy@golang.org>

src/cmd/compile/internal/gc/pgen.go

@@ -384,13 +384,12 @@ func debuginfo(fnsym *obj.LSym, infosym *obj.LSym, curfn interface{}) ([]dwarf.S
 		}
 	}
 
-	var automDecls []*Node
-	// Populate Automs for fn.
+	var apdecls []*Node
+	// Populate decls for fn.
 	for _, n := range fn.Func.Dcl {
 		if n.Op != ONAME { // might be OTYPE or OLITERAL
 			continue
 		}
-		var name obj.AddrName
 		switch n.Class() {
 		case PAUTO:
 			if !n.Name.Used() {
@@ -400,24 +399,15 @@ func debuginfo(fnsym *obj.LSym, infosym *obj.LSym, curfn interface{}) ([]dwarf.S
 				}
 				continue
 			}
-			name = obj.NAME_AUTO
 		case PPARAM, PPARAMOUT:
-			name = obj.NAME_PARAM
 		default:
 			continue
 		}
-		automDecls = append(automDecls, n)
-		gotype := ngotype(n).Linksym()
-		fnsym.Func.Autom = append(fnsym.Func.Autom, &obj.Auto{
-			Asym:    Ctxt.Lookup(n.Sym.Name),
-			Aoffset: int32(n.Xoffset),
-			Name:    name,
-			Gotype:  gotype,
-		})
-		fnsym.Func.RecordAutoType(gotype)
+		apdecls = append(apdecls, n)
+		fnsym.Func.RecordAutoType(ngotype(n).Linksym())
 	}
 
-	decls, dwarfVars := createDwarfVars(fnsym, fn.Func, automDecls)
+	decls, dwarfVars := createDwarfVars(fnsym, fn.Func, apdecls)
 
 	// For each type referenced by the functions auto vars, attach a
 	// dummy relocation to the function symbol to insure that the type
@@ -467,11 +457,11 @@ func debuginfo(fnsym *obj.LSym, infosym *obj.LSym, curfn interface{}) ([]dwarf.S
 
 // createSimpleVars creates a DWARF entry for every variable declared in the
 // function, claiming that they are permanently on the stack.
-func createSimpleVars(automDecls []*Node) ([]*Node, []*dwarf.Var, map[*Node]bool) {
+func createSimpleVars(apDecls []*Node) ([]*Node, []*dwarf.Var, map[*Node]bool) {
 	var vars []*dwarf.Var
 	var decls []*Node
 	selected := make(map[*Node]bool)
-	for _, n := range automDecls {
+	for _, n := range apDecls {
 		if n.IsAutoTmp() {
 			continue
 		}
@@ -559,7 +549,7 @@ func createComplexVars(fn *Func) ([]*Node, []*dwarf.Var, map[*Node]bool) {
 
 // createDwarfVars process fn, returning a list of DWARF variables and the
 // Nodes they represent.
-func createDwarfVars(fnsym *obj.LSym, fn *Func, automDecls []*Node) ([]*Node, []*dwarf.Var) {
+func createDwarfVars(fnsym *obj.LSym, fn *Func, apDecls []*Node) ([]*Node, []*dwarf.Var) {
 	// Collect a raw list of DWARF vars.
 	var vars []*dwarf.Var
 	var decls []*Node
@@ -567,10 +557,10 @@ func createDwarfVars(fnsym *obj.LSym, fn *Func, automDecls []*Node) ([]*Node, []
 	if Ctxt.Flag_locationlists && Ctxt.Flag_optimize && fn.DebugInfo != nil {
 		decls, vars, selected = createComplexVars(fn)
 	} else {
-		decls, vars, selected = createSimpleVars(automDecls)
+		decls, vars, selected = createSimpleVars(apDecls)
 	}
 
-	dcl := automDecls
+	dcl := apDecls
 	if fnsym.WasInlined() {
 		dcl = preInliningDcls(fnsym)
 	}
@@ -649,17 +639,8 @@ func createDwarfVars(fnsym *obj.LSym, fn *Func, automDecls []*Node) ([]*Node, []
 			InlIndex:      int32(inlIndex),
 			ChildIndex:    -1,
 		})
-		// Append a "deleted auto" entry to the autom list so as to
-		// insure that the type in question is picked up by the linker.
-		// See issue 22941.
-		gotype := ngotype(n).Linksym()
-		fnsym.Func.Autom = append(fnsym.Func.Autom, &obj.Auto{
-			Asym:    Ctxt.Lookup(n.Sym.Name),
-			Aoffset: int32(-1),
-			Name:    obj.NAME_DELETED_AUTO,
-			Gotype:  gotype,
-		})
-		fnsym.Func.RecordAutoType(gotype)
+		// Record go type of to insure that it gets emitted by the linker.
+		fnsym.Func.RecordAutoType(ngotype(n).Linksym())
 	}
 
 	return decls, vars

src/cmd/internal/obj/link.go

@@ -211,9 +211,6 @@ const (
 	// A reference to name@GOT(SB) is a reference to the entry in the global offset
 	// table for 'name'.
 	NAME_GOTREF
-	// Indicates auto that was optimized away, but whose type
-	// we want to preserve in the DWARF debug info.
-	NAME_DELETED_AUTO
 	// Indicates that this is a reference to a TOC anchor.
 	NAME_TOCREF
 )
@@ -398,7 +395,6 @@ type FuncInfo struct {
 	Args     int32
 	Locals   int32
 	Text     *Prog
-	Autom    []*Auto
 	Autot    map[*LSym]struct{}
 	Pcln     Pcln
 	InlMarks []InlMark

src/cmd/internal/obj/objfile.go

@@ -31,7 +31,6 @@ type objWriter struct {
 	nData     int
 	nReloc    int
 	nPcdata   int
-	nAutom    int
 	nFuncdata int
 	nFile     int
 
@@ -60,7 +59,6 @@ func (w *objWriter) addLengths(s *LSym) {
 	w.nData += data
 	w.nPcdata += len(pc.Pcdata)
 
-	w.nAutom += len(s.Func.Autom)
 	w.nFuncdata += len(pc.Funcdataoff)
 	w.nFile += len(pc.File)
 }
@@ -69,7 +67,7 @@ func (w *objWriter) writeLengths() {
 	w.writeInt(int64(w.nData))
 	w.writeInt(int64(w.nReloc))
 	w.writeInt(int64(w.nPcdata))
-	w.writeInt(int64(w.nAutom))
+	w.writeInt(int64(0)) // TODO: remove at next object file rev
 	w.writeInt(int64(w.nFuncdata))
 	w.writeInt(int64(w.nFile))
 }
@@ -206,10 +204,6 @@ func (w *objWriter) writeRefs(s *LSym) {
 	}
 
 	if s.Type == objabi.STEXT {
-		for _, a := range s.Func.Autom {
-			w.writeRef(a.Asym, false)
-			w.writeRef(a.Gotype, false)
-		}
 		pc := &s.Func.Pcln
 		for _, d := range pc.Funcdata {
 			w.writeRef(d, false)
@@ -364,21 +358,7 @@ func (w *objWriter) writeSym(s *LSym) {
 		flags |= 1 << 4
 	}
 	w.writeInt(flags)
-	w.writeInt(int64(len(s.Func.Autom)))
-	for _, a := range s.Func.Autom {
-		w.writeRefIndex(a.Asym)
-		w.writeInt(int64(a.Aoffset))
-		if a.Name == NAME_AUTO {
-			w.writeInt(objabi.A_AUTO)
-		} else if a.Name == NAME_PARAM {
-			w.writeInt(objabi.A_PARAM)
-		} else if a.Name == NAME_DELETED_AUTO {
-			w.writeInt(objabi.A_DELETED_AUTO)
-		} else {
-			log.Fatalf("%s: invalid local variable type %d", s.Name, a.Name)
-		}
-		w.writeRefIndex(a.Gotype)
-	}
+	w.writeInt(int64(0)) // TODO: remove at next object file rev
 
 	pc := &s.Func.Pcln
 	w.writeInt(int64(len(pc.Pcsp.P)))