cmd/compile,cmd/link: export int global consts to DWARF

Updates #14517

Change-Id: I23ef88e71c89da12dffcadf5562ea2d7557b62cf
Reviewed-on: https://go-review.googlesource.com/61019Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -203,24 +203,57 @@ func addptabs() {
 	}
 }
 
-func dumpglobls() {
-	// add globals
-	for _, n := range externdcl {
-		if n.Op != ONAME {
-			continue
-		}
-
+func dumpGlobal(n *Node) {
 	if n.Type == nil {
 		Fatalf("external %v nil type\n", n)
 	}
 	if n.Class() == PFUNC {
-			continue
+		return
 	}
 	if n.Sym.Pkg != localpkg {
-			continue
+		return
 	}
 	dowidth(n.Type)
 	ggloblnod(n)
+}
+
+func dumpGlobalConst(n *Node) {
+	// only export typed constants
+	if n.Type == nil {
+		return
+	}
+	if n.Sym.Pkg != localpkg {
+		return
+	}
+	// only export integer constants for now
+	switch n.Type.Etype {
+	case TINT8:
+	case TINT16:
+	case TINT32:
+	case TINT64:
+	case TINT:
+	case TUINT8:
+	case TUINT16:
+	case TUINT32:
+	case TUINT64:
+	case TUINT:
+	case TUINTPTR:
+		// ok
+	default:
+		return
+	}
+	Ctxt.DwarfIntConst(myimportpath, n.Sym.Name, typesymname(n.Type), n.Int64())
+}
+
+func dumpglobls() {
+	// add globals
+	for _, n := range externdcl {
+		switch n.Op {
+		case ONAME:
+			dumpGlobal(n)
+		case OLITERAL:
+			dumpGlobalConst(n)
+		}
 	}
 
 	obj.SortSlice(funcsyms, func(i, j int) bool {

src/cmd/dist/build.go

@@ -685,7 +685,7 @@ func install(dir string) {
 	// For package runtime, this writes go_asm.h, which
 	// the assembly files will need.
 	pkg := dir
-	if strings.HasPrefix(dir, "cmd/") {
+	if strings.HasPrefix(dir, "cmd/") && strings.Count(dir, "/") == 1 {
 		pkg = "main"
 	}
 	b := pathf("%s/_go_.a", workdir)

src/cmd/internal/dwarf/dwarf.go

@@ -21,6 +21,10 @@ const LocPrefix = "go.loc."
 // RangePrefix is the prefix for all the symbols containing DWARF range lists.
 const RangePrefix = "go.range."
 
+// InfoConstPrefix is the prefix for all symbols containing DWARF info
+// entries that contain constants.
+const ConstInfoPrefix = "go.constinfo."
+
 // Sym represents a symbol.
 type Sym interface {
 	Len() int64
@@ -234,6 +238,7 @@ const (
 	DW_ABRV_COMPUNIT
 	DW_ABRV_FUNCTION
 	DW_ABRV_VARIABLE
+	DW_ABRV_INT_CONSTANT
 	DW_ABRV_AUTO
 	DW_ABRV_AUTO_LOCLIST
 	DW_ABRV_PARAM
@@ -310,6 +315,17 @@ var abbrevs = [DW_NABRV]dwAbbrev{
 		},
 	},
 
+	/* INT CONSTANT */
+	{
+		DW_TAG_constant,
+		DW_CHILDREN_no,
+		[]dwAttrForm{
+			{DW_AT_name, DW_FORM_string},
+			{DW_AT_type, DW_FORM_ref_addr},
+			{DW_AT_const_value, DW_FORM_sdata},
+		},
+	},
+
 	/* AUTO */
 	{
 		DW_TAG_variable,
@@ -734,6 +750,14 @@ func HasChildren(die *DWDie) bool {
 	return abbrevs[die.Abbrev].children != 0
 }
 
+// PutIntConst writes a DIE for an integer constant
+func PutIntConst(ctxt Context, info, typ Sym, name string, val int64) {
+	Uleb128put(ctxt, info, DW_ABRV_INT_CONSTANT)
+	putattr(ctxt, info, DW_ABRV_INT_CONSTANT, DW_FORM_string, DW_CLS_STRING, int64(len(name)), name)
+	putattr(ctxt, info, DW_ABRV_INT_CONSTANT, DW_FORM_ref_addr, DW_CLS_REFERENCE, 0, typ)
+	putattr(ctxt, info, DW_ABRV_INT_CONSTANT, DW_FORM_sdata, DW_CLS_CONSTANT, val, nil)
+}
+
 // PutFunc writes a DIE for a function to s.
 // It also writes child DIEs for each variable in vars.
 func PutFunc(ctxt Context, info, loc, ranges Sym, name string, external bool, startPC Sym, size int64, scopes []Scope) error {

src/cmd/internal/obj/objfile.go

@@ -499,3 +499,16 @@ func (ctxt *Link) populateDWARF(curfn interface{}, s *LSym) {
 		ctxt.Diag("emitting DWARF for %s failed: %v", s.Name, err)
 	}
 }
+
+// DwarfIntConst creates a link symbol for an integer constant with the
+// given name, type and value.
+func (ctxt *Link) DwarfIntConst(myimportpath, name, typename string, val int64) {
+	if myimportpath == "" {
+		return
+	}
+	s := ctxt.LookupInit(dwarf.ConstInfoPrefix+myimportpath, func(s *LSym) {
+		s.Type = objabi.SDWARFINFO
+		ctxt.Data = append(ctxt.Data, s)
+	})
+	dwarf.PutIntConst(dwCtxt{ctxt}, s, ctxt.Lookup(dwarf.InfoPrefix+typename), myimportpath+"."+name, val)
+}

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

@@ -972,6 +972,21 @@ func getCompilationDir() string {
 	return "/"
 }
 
+func importInfoSymbol(ctxt *Link, dsym *Symbol) {
+	dsym.Attr |= AttrNotInSymbolTable | AttrReachable
+	dsym.Type = SDWARFINFO
+	for _, r := range dsym.R {
+		if r.Type == objabi.R_DWARFREF && r.Sym.Size == 0 {
+			if Buildmode == BuildmodeShared {
+				// These type symbols may not be present in BuildmodeShared. Skip.
+				continue
+			}
+			n := nameFromDIESym(r.Sym)
+			defgotype(ctxt, ctxt.Syms.Lookup("type."+n, 0))
+		}
+	}
+}
+
 func writelines(ctxt *Link, syms []*Symbol) ([]*Symbol, []*Symbol) {
 	var dwarfctxt dwarf.Context = dwctxt{ctxt}
 	ls := ctxt.Syms.Lookup(".debug_line", 0)
@@ -1064,18 +1079,7 @@ func writelines(ctxt *Link, syms []*Symbol) ([]*Symbol, []*Symbol) {
 		epcs = s
 
 		dsym := ctxt.Syms.Lookup(dwarf.InfoPrefix+s.Name, int(s.Version))
-		dsym.Attr |= AttrNotInSymbolTable | AttrReachable
-		dsym.Type = SDWARFINFO
-		for _, r := range dsym.R {
-			if r.Type == objabi.R_DWARFREF && r.Sym.Size == 0 {
-				if Buildmode == BuildmodeShared {
-					// These type symbols may not be present in BuildmodeShared. Skip.
-					continue
-				}
-				n := nameFromDIESym(r.Sym)
-				defgotype(ctxt, ctxt.Syms.Lookup("type."+n, 0))
-			}
-		}
+		importInfoSymbol(ctxt, dsym)
 		funcs = append(funcs, dsym)
 
 		finddebugruntimepath(s)
@@ -1296,7 +1300,7 @@ const (
 	COMPUNITHEADERSIZE = 4 + 2 + 4 + 1
 )
 
-func writeinfo(ctxt *Link, syms []*Symbol, funcs []*Symbol, abbrevsym *Symbol) []*Symbol {
+func writeinfo(ctxt *Link, syms []*Symbol, funcs, consts []*Symbol, abbrevsym *Symbol) []*Symbol {
 	infosec := ctxt.Syms.Lookup(".debug_info", 0)
 	infosec.R = infosec.R[:0]
 	infosec.Type = SDWARFINFO
@@ -1330,6 +1334,10 @@ func writeinfo(ctxt *Link, syms []*Symbol, funcs []*Symbol, abbrevsym *Symbol) [
 			cu = append(cu, funcs...)
 			funcs = nil
 		}
+		if consts != nil {
+			cu = append(cu, consts...)
+			consts = nil
+		}
 		cu = putdies(ctxt, dwarfctxt, cu, compunit.Child)
 		var cusize int64
 		for _, child := range cu {
@@ -1544,6 +1552,14 @@ func dwarfgeneratedebugsyms(ctxt *Link) {
 
 	genasmsym(ctxt, defdwsymb)
 
+	var consts []*Symbol
+	for _, lib := range ctxt.Library {
+		if s := ctxt.Syms.Lookup(dwarf.ConstInfoPrefix+lib.Pkg, 0); s != nil {
+			importInfoSymbol(ctxt, s)
+			consts = append(consts, s)
+		}
+	}
+
 	abbrev := writeabbrev(ctxt)
 	syms := []*Symbol{abbrev}
 	syms, funcs := writelines(ctxt, syms)
@@ -1563,7 +1579,7 @@ func dwarfgeneratedebugsyms(ctxt *Link) {
 
 	// Need to reorder symbols so SDWARFINFO is after all SDWARFSECT
 	// (but we need to generate dies before writepub)
-	infosyms := writeinfo(ctxt, nil, funcs, abbrev)
+	infosyms := writeinfo(ctxt, nil, funcs, consts, abbrev)
 
 	syms = writepub(ctxt, ".debug_pubnames", ispubname, syms)
 	syms = writepub(ctxt, ".debug_pubtypes", ispubtype, syms)

src/runtime/runtime-gdb_test.go

@@ -381,3 +381,61 @@ func TestGdbAutotmpTypes(t *testing.T) {
 		}
 	}
 }
+
+const constsSource = `
+package main
+
+const aConstant int = 42
+const largeConstant uint64 = ^uint64(0)
+const minusOne int64 = -1
+
+func main() {
+	println("hello world")
+}
+`
+
+func TestGdbConst(t *testing.T) {
+	t.Parallel()
+	checkGdbEnvironment(t)
+	checkGdbVersion(t)
+
+	dir, err := ioutil.TempDir("", "go-build")
+	if err != nil {
+		t.Fatalf("failed to create temp directory: %v", err)
+	}
+	defer os.RemoveAll(dir)
+
+	// Build the source code.
+	src := filepath.Join(dir, "main.go")
+	err = ioutil.WriteFile(src, []byte(constsSource), 0644)
+	if err != nil {
+		t.Fatalf("failed to create file: %v", err)
+	}
+	cmd := exec.Command(testenv.GoToolPath(t), "build", "-gcflags=-N -l", "-o", "a.exe")
+	cmd.Dir = dir
+	out, err := testenv.CleanCmdEnv(cmd).CombinedOutput()
+	if err != nil {
+		t.Fatalf("building source %v\n%s", err, out)
+	}
+
+	// Execute gdb commands.
+	args := []string{"-nx", "-batch",
+		"-ex", "set startup-with-shell off",
+		"-ex", "break main.main",
+		"-ex", "run",
+		"-ex", "print main.aConstant",
+		"-ex", "print main.largeConstant",
+		"-ex", "print main.minusOne",
+		"-ex", "print 'runtime._MSpanInUse'",
+		filepath.Join(dir, "a.exe"),
+	}
+	got, _ := exec.Command("gdb", args...).CombinedOutput()
+
+	sgot := string(got)
+
+	t.Logf("output %q", sgot)
+
+	if strings.Index(sgot, "\n$1 = 42\n$2 = 18446744073709551615\n$3 = -1\n$4 = 1 '\\001'") < 0 {
+		t.Fatalf("output mismatch")
+	}
+}