cmd/compile: distinguish args and return values in DWARF

Set DW_AT_variable_parameter on DW_TAG_formal_parameters that are
actually return values. variable_parameter is supposed to indicate inout
parameters, but Go doesn't really have those, and DWARF doesn't have
explicit support for multiple return values. This seems to be the best
compromise, especially since the implementation of the two is very
similar -- both are stack slots.

Fixes #21100

Change-Id: Icebabc92b7b397e0aa00a7237478cce84ad1a670
Reviewed-on: https://go-review.googlesource.com/71670
Run-TryBot: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

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

@@ -400,11 +400,12 @@ func createSimpleVars(automDecls []*Node) ([]*Node, []*dwarf.Var) {
 		typename := dwarf.InfoPrefix + typesymname(n.Type)
 		decls = append(decls, n)
 		vars = append(vars, &dwarf.Var{
-			Name:        n.Sym.Name,
-			Abbrev:      abbrev,
-			StackOffset: int32(offs),
-			Type:        Ctxt.Lookup(typename),
-			DeclLine:    n.Pos.Line(),
+			Name:          n.Sym.Name,
+			IsReturnValue: n.Class() == PPARAMOUT,
+			Abbrev:        abbrev,
+			StackOffset:   int32(offs),
+			Type:          Ctxt.Lookup(typename),
+			DeclLine:      n.Pos.Line(),
 		})
 	}
 	return decls, vars

src/cmd/internal/dwarf/dwarf.go

@@ -49,13 +49,14 @@ type Piece struct {
 
 // A Var represents a local variable or a function parameter.
 type Var struct {
-	Name         string
-	Abbrev       int // Either DW_ABRV_AUTO or DW_ABRV_PARAM
-	StackOffset  int32
-	LocationList []Location
-	Scope        int32
-	Type         Sym
-	DeclLine     uint
+	Name          string
+	Abbrev        int // Either DW_ABRV_AUTO[_LOCLIST] or DW_ABRV_PARAM[_LOCLIST]
+	IsReturnValue bool
+	StackOffset   int32
+	LocationList  []Location
+	Scope         int32
+	Type          Sym
+	DeclLine      uint
 }
 
 // A Scope represents a lexical scope. All variables declared within a
@@ -355,6 +356,7 @@ var abbrevs = [DW_NABRV]dwAbbrev{
 		DW_CHILDREN_no,
 		[]dwAttrForm{
 			{DW_AT_name, DW_FORM_string},
+			{DW_AT_variable_parameter, DW_FORM_flag},
 			{DW_AT_decl_line, DW_FORM_udata},
 			{DW_AT_location, DW_FORM_block1},
 			{DW_AT_type, DW_FORM_ref_addr},
@@ -367,6 +369,7 @@ var abbrevs = [DW_NABRV]dwAbbrev{
 		DW_CHILDREN_no,
 		[]dwAttrForm{
 			{DW_AT_name, DW_FORM_string},
+			{DW_AT_variable_parameter, DW_FORM_flag},
 			{DW_AT_decl_line, DW_FORM_udata},
 			{DW_AT_location, DW_FORM_sec_offset},
 			{DW_AT_type, DW_FORM_ref_addr},
@@ -833,8 +836,6 @@ func putscope(ctxt Context, info, loc, ranges, startPC Sym, curscope int32, scop
 }
 
 func putvar(ctxt Context, info, loc Sym, v *Var, startPC Sym, encbuf []byte) {
-	n := v.Name
-
 	// If the variable was entirely optimized out, don't emit a location list;
 	// convert to an inline abbreviation and emit an empty location.
 	missing := false
@@ -848,7 +849,14 @@ func putvar(ctxt Context, info, loc Sym, v *Var, startPC Sym, encbuf []byte) {
 	}
 
 	Uleb128put(ctxt, info, int64(v.Abbrev))
-	putattr(ctxt, info, v.Abbrev, DW_FORM_string, DW_CLS_STRING, int64(len(n)), n)
+	putattr(ctxt, info, v.Abbrev, DW_FORM_string, DW_CLS_STRING, int64(len(v.Name)), v.Name)
+	if v.Abbrev == DW_ABRV_PARAM || v.Abbrev == DW_ABRV_PARAM_LOCLIST {
+		var isReturn int64
+		if v.IsReturnValue {
+			isReturn = 1
+		}
+		putattr(ctxt, info, v.Abbrev, DW_FORM_flag, DW_CLS_FLAG, isReturn, nil)
+	}
 	putattr(ctxt, info, v.Abbrev, DW_FORM_udata, DW_CLS_CONSTANT, int64(v.DeclLine), nil)
 	if v.Abbrev == DW_ABRV_AUTO_LOCLIST || v.Abbrev == DW_ABRV_PARAM_LOCLIST {
 		putattr(ctxt, info, v.Abbrev, DW_FORM_sec_offset, DW_CLS_PTR, int64(loc.Len()), loc)