cmd: emit dwarf for string constants

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

@@ -176,22 +176,34 @@ func dumpGlobalConst(n *ir.Name) {
 		return
 	}
 	// only export integer constants for now
-	if !t.IsInteger() {
+	if !t.IsInteger() && !t.IsString() {
 		return
 	}
 	v := n.Val()
 	if t.IsUntyped() {
-		// Export untyped integers as int (if they fit).
-		t = types.Types[types.TINT]
-		if ir.ConstOverflow(v, t) {
-			return
+		if t.IsInteger() {
+			// Export untyped integers as int (if they fit).
+			t = types.Types[types.TINT]
+			if ir.ConstOverflow(v, t) {
+				return
+			}
+		} else {
+			t = types.Types[types.TSTRING]
 		}
 	} else {
-		// If the type of the constant is an instantiated generic, we need to emit
-		// that type so the linker knows about it. See issue 51245.
-		_ = reflectdata.TypeLinksym(t)
+		if t.IsInteger() {
+			// If the type of the constant is an instantiated generic, we need to emit
+			// that type so the linker knows about it. See issue 51245.
+			_ = reflectdata.TypeLinksym(t)
+		}
+		// For const string, type:string isn't the real type.
+	}
+
+	if t.IsInteger() {
+		base.Ctxt.DwarfIntConst(n.Sym().Name, types.TypeSymName(t), ir.IntVal(t, v))
+	} else if t.IsString() {
+		base.Ctxt.DwarfStringConst(n.Sym().Name, types.TypeSymName(t), ir.StringVal(n))
 	}
-	base.Ctxt.DwarfIntConst(n.Sym().Name, types.TypeSymName(t), ir.IntVal(t, v))
 }
 
 // addGCLocals adds gcargs, gclocals, gcregs, and stack object symbols to Ctxt.Data.

src/cmd/internal/dwarf/dwarf.go

@@ -27,6 +27,10 @@ const InfoPrefix = "go:info."
 // entries that contain constants.
 const ConstInfoPrefix = "go:constinfo."
 
+// ConstStringInfoPrefix is the prefix for all symbols containing
+// DWARF info entries that are referred by string constants.
+const ConstStringInfoPrefix = "$const."
+
 // CUInfoPrefix is the prefix for symbols containing information to
 // populate the DWARF compilation unit info entries.
 const CUInfoPrefix = "go:cuinfo."
@@ -336,6 +340,7 @@ const (
 	DW_ABRV_INLINED_SUBROUTINE_RANGES
 	DW_ABRV_VARIABLE
 	DW_ABRV_INT_CONSTANT
+	DW_ABRV_STRING_CONSTANT
 	DW_ABRV_LEXICAL_BLOCK_RANGES
 	DW_ABRV_LEXICAL_BLOCK_SIMPLE
 	DW_ABRV_STRUCTFIELD
@@ -354,6 +359,7 @@ const (
 	DW_ABRV_BARE_PTRTYPE // only for void*, no DW_AT_type attr to please gdb 6.
 	DW_ABRV_SLICETYPE
 	DW_ABRV_STRINGTYPE
+	DW_ABRV_CONSTANT_STRINGTYPE
 	DW_ABRV_STRUCTTYPE
 	DW_ABRV_TYPEDECL
 	DW_ABRV_DICT_INDEX
@@ -575,7 +581,7 @@ var abbrevs = []dwAbbrev{
 		},
 	},
 
-	/* INT CONSTANT */
+	/* INT_CONSTANT */
 	{
 		DW_TAG_constant,
 		DW_CHILDREN_no,
@@ -586,6 +592,17 @@ var abbrevs = []dwAbbrev{
 		},
 	},
 
+	/* STRING_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_block1},
+		},
+	},
+
 	/* LEXICAL_BLOCK_RANGES */
 	{
 		DW_TAG_lexical_block,
@@ -807,6 +824,16 @@ var abbrevs = []dwAbbrev{
 		},
 	},
 
+	/* CONSTANT_STRINGTYPE */
+	{
+		DW_TAG_string_type,
+		DW_CHILDREN_no,
+		[]dwAttrForm{
+			{DW_AT_name, DW_FORM_string},
+			{DW_AT_byte_size, DW_FORM_udata},
+		},
+	},
+
 	/* STRUCTTYPE */
 	{
 		DW_TAG_structure_type,
@@ -1031,6 +1058,14 @@ func PutIntConst(ctxt Context, info, typ Sym, name string, val int64) {
 	putattr(ctxt, info, DW_ABRV_INT_CONSTANT, DW_FORM_sdata, DW_CLS_CONSTANT, val, nil)
 }
 
+// PutStringConst writes a DIE for a string constant
+func PutStringConst(ctxt Context, info, typ Sym, name string, val string) {
+	Uleb128put(ctxt, info, DW_ABRV_STRING_CONSTANT)
+	putattr(ctxt, info, DW_ABRV_STRING_CONSTANT, DW_FORM_string, DW_CLS_STRING, int64(len(name)), name)
+	putattr(ctxt, info, DW_ABRV_STRING_CONSTANT, DW_FORM_ref_addr, DW_CLS_REFERENCE, 0, typ)
+	putattr(ctxt, info, DW_ABRV_STRING_CONSTANT, DW_FORM_block1, DW_CLS_BLOCK, int64(len(val)), []byte(val))
+}
+
 // PutGlobal writes a DIE for a global variable.
 func PutGlobal(ctxt Context, info, typ, gvar Sym, name string) {
 	Uleb128put(ctxt, info, DW_ABRV_VARIABLE)

src/cmd/internal/obj/dwarf.go

@@ -12,6 +12,7 @@ import (
 	"cmd/internal/src"
 	"fmt"
 	"slices"
+	"strconv"
 	"strings"
 	"sync"
 )
@@ -393,18 +394,38 @@ func (ctxt *Link) populateDWARF(curfn Func, s *LSym) {
 	ctxt.generateDebugLinesSymbol(s, lines)
 }
 
-// DwarfIntConst creates a link symbol for an integer constant with the
-// given name, type and value.
-func (ctxt *Link) DwarfIntConst(name, typename string, val int64) {
+// ensureConstInfoSym ensures that the DWARF constant info symbol exists
+func (ctxt *Link) ensureConstInfoSym() *LSym {
 	myimportpath := ctxt.Pkgpath
 	if myimportpath == "" {
-		return
+		return nil
 	}
 	s := ctxt.LookupInit(dwarf.ConstInfoPrefix+myimportpath, func(s *LSym) {
 		s.Type = objabi.SDWARFCONST
 		ctxt.Data = append(ctxt.Data, s)
 	})
-	dwarf.PutIntConst(dwCtxt{ctxt}, s, ctxt.Lookup(dwarf.InfoPrefix+typename), myimportpath+"."+name, val)
+	return s
+}
+
+// DwarfIntConst creates a link symbol for an integer constant with the
+// given name, type and value.
+func (ctxt *Link) DwarfIntConst(name, typename string, val int64) {
+	s := ctxt.ensureConstInfoSym()
+	if s == nil {
+		return
+	}
+	dwarf.PutIntConst(dwCtxt{ctxt}, s, ctxt.Lookup(dwarf.InfoPrefix+typename), ctxt.Pkgpath+"."+name, val)
+}
+
+// DwarfStringConst creates a link symbol for a string constant with the
+// given name and value.
+func (ctxt *Link) DwarfStringConst(name, typename, value string) {
+	s := ctxt.ensureConstInfoSym()
+	if s == nil {
+		return
+	}
+	typSym := ctxt.Lookup(dwarf.InfoPrefix + dwarf.ConstStringInfoPrefix + typename + "." + strconv.Itoa(len(value)))
+	dwarf.PutStringConst(dwCtxt{ctxt}, s, typSym, ctxt.Pkgpath+"."+name, value)
 }
 
 // DwarfGlobal creates a link symbol containing a DWARF entry for

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

@@ -29,6 +29,7 @@ import (
 	"path"
 	"runtime"
 	"slices"
+	"strconv"
 	"strings"
 	"sync"
 )
@@ -285,13 +286,16 @@ func (d *dwctxt) newdie(parent *dwarf.DWDie, abbrev int, name string) *dwarf.DWD
 	die.Link = parent.Child
 	parent.Child = die
 
-	newattr(die, dwarf.DW_AT_name, dwarf.DW_CLS_STRING, int64(len(name)), name)
-
 	// Sanity check: all DIEs created in the linker should be named.
 	if name == "" {
 		panic("nameless DWARF DIE")
 	}
 
+	// for constant string types, we emit the nams later since it didn't use symbol name as DW_AT_name
+	if abbrev != dwarf.DW_ABRV_CONSTANT_STRINGTYPE {
+		newattr(die, dwarf.DW_AT_name, dwarf.DW_CLS_STRING, int64(len(name)), name)
+	}
+
 	var st sym.SymKind
 	switch abbrev {
 	case dwarf.DW_ABRV_FUNCTYPEPARAM, dwarf.DW_ABRV_FUNCTYPEOUTPARAM, dwarf.DW_ABRV_DOTDOTDOT, dwarf.DW_ABRV_STRUCTFIELD, dwarf.DW_ABRV_ARRAYRANGE:
@@ -1183,6 +1187,24 @@ func getCompilationDir() string {
 	return "."
 }
 
+func (d *dwctxt) genConstStringType(name string) {
+	if d.find(name) != 0 {
+		return
+	}
+	i := strings.LastIndex(name, ".")
+	if i < 0 {
+		log.Fatalf("error: invalid constant string type name %q", name)
+	}
+	size, err := strconv.ParseInt(name[i+1:], 10, 64)
+	if err != nil {
+		log.Fatalf("error: invalid constant string type name %q: %v", name, err)
+	}
+	atname := name[len(dwarf.ConstStringInfoPrefix):i]
+	die := d.newdie(&dwtypes, dwarf.DW_ABRV_CONSTANT_STRINGTYPE, name)
+	newattr(die, dwarf.DW_AT_name, dwarf.DW_CLS_STRING, int64(len(atname)), atname)
+	newattr(die, dwarf.DW_AT_byte_size, dwarf.DW_CLS_CONSTANT, size, 0)
+}
+
 func (d *dwctxt) importInfoSymbol(dsym loader.Sym) {
 	d.ldr.SetAttrReachable(dsym, true)
 	d.ldr.SetAttrNotInSymbolTable(dsym, true)
@@ -1207,6 +1229,15 @@ func (d *dwctxt) importInfoSymbol(dsym loader.Sym) {
 		// symbol name here?
 		sn := d.ldr.SymName(rsym)
 		tn := sn[len(dwarf.InfoPrefix):]
+
+		// If the symbol is a constant string type, we generate it
+		// These types do not exist in go,
+		// but can tell gdb how to interpret the corresponding values
+		if strings.HasPrefix(tn, dwarf.ConstStringInfoPrefix) {
+			d.genConstStringType(tn)
+			continue
+		}
+
 		ts := d.ldr.Lookup("type:"+tn, 0)
 		d.defgotype(ts)
 	}

src/runtime/runtime-gdb_test.go

@@ -662,6 +662,9 @@ package main
 const aConstant int = 42
 const largeConstant uint64 = ^uint64(0)
 const minusOne int64 = -1
+const typedS string = "typed string"
+const untypedS = "untyped string"
+const nulS string = "\x00str"
 
 func main() {
 	println("hello world")
@@ -700,6 +703,9 @@ func TestGdbConst(t *testing.T) {
 		"-ex", "print main.minusOne",
 		"-ex", "print 'runtime.mSpanInUse'",
 		"-ex", "print 'runtime._PageSize'",
+		"-ex", "print main.typedS",
+		"-ex", "print main.untypedS",
+		"-ex", "print main.nulS",
 		filepath.Join(dir, "a.exe"),
 	}
 	gdbArgsFixup(args)
@@ -711,7 +717,14 @@ func TestGdbConst(t *testing.T) {
 
 	sgot := strings.ReplaceAll(string(got), "\r\n", "\n")
 
-	if !strings.Contains(sgot, "\n$1 = 42\n$2 = 18446744073709551615\n$3 = -1\n$4 = 1 '\\001'\n$5 = 8192") {
+	if !strings.Contains(sgot, `$1 = 42
+$2 = 18446744073709551615
+$3 = -1
+$4 = 1 '\001'
+$5 = 8192
+$6 = "typed string"
+$7 = "untyped string"
+$8 = "\000str"`) {
 		t.Fatalf("output mismatch")
 	}
 }