cmd/link: generate DIE for types referenced only through dictionaries

Generate debug_info entries for types that are only referenced through
dictionaries.

Change-Id: Ic36c2e6d9588ec6746793bb213c2dc0e17a8a850
Reviewed-on: https://go-review.googlesource.com/c/go/+/350532
Run-TryBot: Alessandro Arzilli <alessandro.arzilli@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Trust: Dan Scales <danscales@google.com>
Trust: David Chase <drchase@google.com>

src/cmd/compile/internal/noder/stencil.go

@@ -1479,6 +1479,7 @@ func markTypeUsed(t *types.Type, lsym *obj.LSym) {
 	} else {
 		// TODO: This is somewhat overkill, we really only need it
 		// for types that are put into interfaces.
+		// Note: this relocation is also used in cmd/link/internal/ld/dwarf.go
 		reflectdata.MarkTypeUsedInInterface(t, lsym)
 	}
 }

src/cmd/internal/goobj/objfile.go

@@ -304,6 +304,7 @@ const (
 const (
 	SymFlagUsedInIface = 1 << iota
 	SymFlagItab
+	SymFlagDict
 )
 
 // Returns the length of the name of the symbol.
@@ -333,6 +334,7 @@ func (s *Sym) ReflectMethod() bool { return s.Flag()&SymFlagReflectMethod != 0 }
 func (s *Sym) IsGoType() bool      { return s.Flag()&SymFlagGoType != 0 }
 func (s *Sym) UsedInIface() bool   { return s.Flag2()&SymFlagUsedInIface != 0 }
 func (s *Sym) IsItab() bool        { return s.Flag2()&SymFlagItab != 0 }
+func (s *Sym) IsDict() bool        { return s.Flag2()&SymFlagDict != 0 }
 
 func (s *Sym) SetName(x string, w *Writer) {
 	binary.LittleEndian.PutUint32(s[:], uint32(len(x)))

src/cmd/internal/obj/objfile.go

@@ -340,6 +340,9 @@ func (w *writer) Sym(s *LSym) {
 	if strings.HasPrefix(s.Name, "go.itab.") && s.Type == objabi.SRODATA {
 		flag2 |= goobj.SymFlagItab
 	}
+	if strings.HasPrefix(s.Name, w.ctxt.Pkgpath) && strings.HasPrefix(s.Name[len(w.ctxt.Pkgpath):], "..dict") {
+		flag2 |= goobj.SymFlagDict
+	}
 	name := s.Name
 	if strings.HasPrefix(name, "gofile..") {
 		name = filepath.ToSlash(name)

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

@@ -1890,6 +1890,8 @@ func dwarfGenerateDebugInfo(ctxt *Link) {
 	// global variables. For each global of this sort, locate
 	// the corresponding compiler-generated DIE symbol and tack
 	// it onto the list associated with the unit.
+	// Also looks for dictionary symbols and generates DIE symbols for each
+	// type they reference.
 	for idx := loader.Sym(1); idx < loader.Sym(d.ldr.NDef()); idx++ {
 		if !d.ldr.AttrReachable(idx) ||
 			d.ldr.AttrNotInSymbolTable(idx) ||
@@ -1903,9 +1905,21 @@ func dwarfGenerateDebugInfo(ctxt *Link) {
 		default:
 			continue
 		}
-		// Skip things with no type
+		// Skip things with no type, unless it's a dictionary
 		gt := d.ldr.SymGoType(idx)
 		if gt == 0 {
+			if t == sym.SRODATA {
+				if d.ldr.IsDict(idx) {
+					// This is a dictionary, make sure that all types referenced by this dictionary are reachable
+					relocs := d.ldr.Relocs(idx)
+					for i := 0; i < relocs.Count(); i++ {
+						reloc := relocs.At(i)
+						if reloc.Type() == objabi.R_USEIFACE {
+							d.defgotype(reloc.Sym())
+						}
+					}
+				}
+			}
 			continue
 		}
 		// Skip file local symbols (this includes static tmps, stack

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

@@ -1749,7 +1749,9 @@ func main() {
 }
 
 func TestDictIndex(t *testing.T) {
-	// Check that variables with a parametric type have a dictionary index attribute
+	// Check that variables with a parametric type have a dictionary index
+	// attribute and that types that are only referenced through dictionaries
+	// have DIEs.
 	testenv.MustHaveGoBuild(t)
 
 	if runtime.GOOS == "plan9" {
@@ -1765,6 +1767,8 @@ package main
 
 import "fmt"
 
+type CustomInt int
+
 func testfn[T any](arg T) {
 	var mapvar = make(map[int]T)
 	mapvar[0] = arg
@@ -1772,7 +1776,7 @@ func testfn[T any](arg T) {
 }
 
 func main() {
-	testfn("test")
+	testfn(CustomInt(3))
 }
 `
 
@@ -1829,4 +1833,19 @@ func main() {
 			t.Errorf("could not find DW_AT_go_dict_index attribute offset %#x (%T)", off, entry.Val(intdwarf.DW_AT_go_dict_index))
 		}
 	}
+
+	rdr.Seek(0)
+	ex := examiner{}
+	if err := ex.populate(rdr); err != nil {
+		t.Fatalf("error reading DWARF: %v", err)
+	}
+	for _, typeName := range []string{"main.CustomInt", "map[int]main.CustomInt"} {
+		dies := ex.Named(typeName)
+		if len(dies) != 1 {
+			t.Errorf("wanted 1 DIE named %s, found %v", typeName, len(dies))
+		}
+		if dies[0].Val(intdwarf.DW_AT_go_runtime_type).(uint64) == 0 {
+			t.Errorf("type %s does not have DW_AT_go_runtime_type", typeName)
+		}
+	}
 }

src/cmd/link/internal/loader/loader.go

@@ -1209,6 +1209,15 @@ func (l *Loader) IsItab(i Sym) bool {
 	return r.Sym(li).IsItab()
 }
 
+// Returns whether this symbol is a dictionary symbol.
+func (l *Loader) IsDict(i Sym) bool {
+	if l.IsExternal(i) {
+		return false
+	}
+	r, li := l.toLocal(i)
+	return r.Sym(li).IsDict()
+}
+
 // Return whether this is a trampoline of a deferreturn call.
 func (l *Loader) IsDeferReturnTramp(i Sym) bool {
 	return l.deferReturnTramp[i]