cmd/compile: emit DW_AT_decl_line

Some debuggers use the declaration line to avoid showing variables
before they're declared. Emit them for local variables and function
parameters.

DW_AT_decl_file would be nice too, but since its value is an index
into a table built by the linker, that's dramatically harder. In
practice, with inlining disabled it's safe to assume that all a
function's variables are declared in the same file, so this should still
be pretty useful.

Change-Id: I8105818c8940cd71bc5473ec98797cce2f3f9872
Reviewed-on: https://go-review.googlesource.com/44350Reviewed-by: David Chase <drchase@google.com>

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

@@ -397,6 +397,7 @@ func createSimpleVars(automDecls []*Node) ([]*Node, []*dwarf.Var) {
 			Abbrev:      abbrev,
 			StackOffset: int32(offs),
 			Type:        Ctxt.Lookup(typename),
+			DeclLine:    n.Pos.Line(),
 		})
 	}
 	return decls, vars
@@ -513,6 +514,7 @@ func createComplexVar(debugInfo *ssa.FuncDebug, n *Node, parts []varPart) *dwarf
 		Abbrev:      abbrev,
 		Type:        Ctxt.Lookup(typename),
 		StackOffset: int32(stackOffset),
+		DeclLine:    n.Pos.Line(),
 	}
 
 	if Debug_locationlist != 0 {

src/cmd/internal/dwarf/dwarf.go

@@ -51,6 +51,7 @@ type Var struct {
 	LocationList []Location
 	Scope        int32
 	Type         Sym
+	DeclLine     uint
 }
 
 // A Scope represents a lexical scope. All variables declared within a
@@ -315,6 +316,7 @@ var abbrevs = [DW_NABRV]dwAbbrev{
 		DW_CHILDREN_no,
 		[]dwAttrForm{
 			{DW_AT_name, DW_FORM_string},
+			{DW_AT_decl_line, DW_FORM_udata},
 			{DW_AT_location, DW_FORM_block1},
 			{DW_AT_type, DW_FORM_ref_addr},
 		},
@@ -337,6 +339,7 @@ var abbrevs = [DW_NABRV]dwAbbrev{
 		DW_CHILDREN_no,
 		[]dwAttrForm{
 			{DW_AT_name, DW_FORM_string},
+			{DW_AT_decl_line, DW_FORM_udata},
 			{DW_AT_location, DW_FORM_block1},
 			{DW_AT_type, DW_FORM_ref_addr},
 		},
@@ -794,6 +797,7 @@ 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_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)
 		addLocList(ctxt, loc, startPC, v, encbuf)

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

@@ -83,7 +83,7 @@ func gobuild(t *testing.T, dir string, testfile string) *objfilepkg.File {
 		t.Fatal(err)
 	}
 
-	cmd := exec.Command(testenv.GoToolPath(t), "build", "-o", dst, src)
+	cmd := exec.Command(testenv.GoToolPath(t), "build", "-gcflags", "-N -l", "-o", dst, src)
 	if b, err := cmd.CombinedOutput(); err != nil {
 		t.Logf("build: %s\n", b)
 		t.Fatalf("build error: %v", err)
@@ -298,3 +298,60 @@ func main() {
 		}
 	}
 }
+
+func TestVarDeclCoords(t *testing.T) {
+	testenv.MustHaveGoBuild(t)
+
+	if runtime.GOOS == "plan9" {
+		t.Skip("skipping on plan9; no DWARF symbol table in executables")
+	}
+
+	const prog = `
+package main
+
+func main() {
+	var i int
+	i = i
+}
+`
+	dir, err := ioutil.TempDir("", "TestVarDeclCoords")
+	if err != nil {
+		t.Fatalf("could not create directory: %v", err)
+	}
+	defer os.RemoveAll(dir)
+
+	f := gobuild(t, dir, prog)
+
+	d, err := f.DWARF()
+	if err != nil {
+		t.Fatalf("error reading DWARF: %v", err)
+	}
+
+	rdr := d.Reader()
+	var iEntry *dwarf.Entry
+	foundMain := false
+	for entry, err := rdr.Next(); entry != nil; entry, err = rdr.Next() {
+		if err != nil {
+			t.Fatalf("error reading DWARF: %v", err)
+		}
+		if entry.Tag == dwarf.TagSubprogram && entry.Val(dwarf.AttrName).(string) == "main.main" {
+			foundMain = true
+			continue
+		}
+		if !foundMain {
+			continue
+		}
+		if entry.Tag == dwarf.TagSubprogram {
+			t.Fatalf("didn't find DW_TAG_variable for i in main.main")
+		}
+		if foundMain && entry.Tag == dwarf.TagVariable && entry.Val(dwarf.AttrName).(string) == "i" {
+			iEntry = entry
+			break
+		}
+	}
+
+	line := iEntry.Val(dwarf.AttrDeclLine)
+	if line == nil || line.(int64) != 5 {
+		t.Errorf("DW_AT_decl_line for i is %v, want 5", line)
+	}
+}