[dev.link] cmd/compile, cmd/link: reference type symbol of defined type by index

The type descriptor symbol of a defined (named) type (and pointer
to it) is defined only in the package that defines the type. It
is not dupOK, unlike other type descriptors. So it can be
referenced by index. Currently it is referenced by name for
cross-package references, because the index is not exported and
so not known to the referencing package.

This CL passes the index through the export data, so the symbol
can be referenced by index, and does not need to be looked up by
name. This also makes such symbol references consistent: it is
referenced by index within the defining package and also cross-
package, which makes it easier for content hashing (in later CLs).

One complication is that we need to set flags on referenced
symbols (specifically, the UsedInIface flag). Before, they are
non-package refs, which naturally carry flags in the object file.
For indexed refs, we currently don't put their flags in the
object file. Introduce a new block for this.

Change-Id: I8126f8e318ac4e6609eb2ac136201fd6c264c256
Reviewed-on: https://go-review.googlesource.com/c/go/+/245718
Reviewed-by: Jeremy Faller <jeremy@golang.org>

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

@@ -484,6 +484,7 @@ func (p *iexporter) doDecl(n *Node) {
 
 		t := n.Type
 		if t.IsInterface() {
+			w.typeExt(t)
 			break
 		}
 
@@ -496,6 +497,7 @@ func (p *iexporter) doDecl(n *Node) {
 			w.signature(m.Type)
 		}
 
+		w.typeExt(t)
 		for _, m := range ms.Slice() {
 			w.methExt(m)
 		}
@@ -1014,6 +1016,17 @@ func (w *exportWriter) symIdx(s *types.Sym) {
 	}
 }
 
+func (w *exportWriter) typeExt(t *types.Type) {
+	// For type T, export the index of type descriptor symbols of T and *T.
+	if i, ok := typeSymIdx[t]; ok {
+		w.int64(i[0])
+		w.int64(i[1])
+		return
+	}
+	w.symIdx(typesym(t))
+	w.symIdx(typesym(t.PtrTo()))
+}
+
 // Inline bodies.
 
 func (w *exportWriter) stmtList(list Nodes) {

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

@@ -316,6 +316,7 @@ func (r *importReader) doDecl(n *Node) {
 		resumecheckwidth()
 
 		if underlying.IsInterface() {
+			r.typeExt(t)
 			break
 		}
 
@@ -346,6 +347,7 @@ func (r *importReader) doDecl(n *Node) {
 		}
 		t.Methods().Set(ms)
 
+		r.typeExt(t)
 		for _, m := range ms {
 			r.methExt(m)
 		}
@@ -708,6 +710,17 @@ func (r *importReader) symIdx(s *types.Sym) {
 	}
 }
 
+func (r *importReader) typeExt(t *types.Type) {
+	i, pi := r.int64(), r.int64()
+	if i != -1 && pi != -1 {
+		typeSymIdx[t] = [2]int64{i, pi}
+	}
+}
+
+// Map imported type T to the index of type descriptor symbols of T and *T,
+// so we can use index to reference the symbol.
+var typeSymIdx = make(map[*types.Type][2]int64)
+
 func (r *importReader) doInline(n *Node) {
 	if len(n.Func.Inl.Body) != 0 {
 		Fatalf("%v already has inline body", n)

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

@@ -1168,6 +1168,15 @@ func dtypesym(t *types.Type) *obj.LSym {
 	if myimportpath != "runtime" || (tbase != types.Types[tbase.Etype] && tbase != types.Bytetype && tbase != types.Runetype && tbase != types.Errortype) { // int, float, etc
 		// named types from other files are defined only by those files
 		if tbase.Sym != nil && tbase.Sym.Pkg != localpkg {
+			if i, ok := typeSymIdx[tbase]; ok {
+				lsym.Pkg = tbase.Sym.Pkg.Prefix
+				if t != tbase {
+					lsym.SymIdx = int32(i[1])
+				} else {
+					lsym.SymIdx = int32(i[0])
+				}
+				lsym.Set(obj.AttrIndexed, true)
+			}
 			return lsym
 		}
 		// TODO(mdempsky): Investigate whether this can happen.

src/cmd/internal/goobj2/objfile.go

@@ -41,11 +41,12 @@ import (
 //    DwarfFiles [...]string
 //
 //    SymbolDefs [...]struct {
-//       Name string
-//       ABI  uint16
-//       Type uint8
-//       Flag uint8
-//       Size uint32
+//       Name  string
+//       ABI   uint16
+//       Type  uint8
+//       Flag  uint8
+//       Flag2 uint8
+//       Size  uint32
 //    }
 //    Hashed64Defs [...]struct { // short hashed (content-addressable) symbol definitions
 //       ... // same as SymbolDefs
@@ -60,6 +61,12 @@ import (
 //       ... // same as SymbolDefs
 //    }
 //
+//    RefFlags [...]struct { // referenced symbol flags
+//       Sym   symRef
+//       Flag  uint8
+//       Flag2 uint8
+//    }
+//
 //    Hash64 [...][8]byte
 //    Hash   [...][N]byte
 //
@@ -176,6 +183,7 @@ const (
 	BlkHasheddef
 	BlkNonpkgdef
 	BlkNonpkgref
+	BlkRefFlags
 	BlkHash64
 	BlkHash
 	BlkRelocIdx
@@ -431,6 +439,33 @@ func (a *Aux) Write(w *Writer) { w.Bytes(a[:]) }
 // for testing
 func (a *Aux) fromBytes(b []byte) { copy(a[:], b) }
 
+// Referenced symbol flags.
+//
+// Serialized format:
+// RefFlags struct {
+//    Sym   symRef
+//    Flag  uint8
+//    Flag2 uint8
+// }
+type RefFlags [RefFlagsSize]byte
+
+const RefFlagsSize = 8 + 1 + 1
+
+func (r *RefFlags) Sym() SymRef {
+	return SymRef{binary.LittleEndian.Uint32(r[:]), binary.LittleEndian.Uint32(r[4:])}
+}
+func (r *RefFlags) Flag() uint8  { return r[8] }
+func (r *RefFlags) Flag2() uint8 { return r[9] }
+
+func (r *RefFlags) SetSym(x SymRef) {
+	binary.LittleEndian.PutUint32(r[:], x.PkgIdx)
+	binary.LittleEndian.PutUint32(r[4:], x.SymIdx)
+}
+func (r *RefFlags) SetFlag(x uint8)  { r[8] = x }
+func (r *RefFlags) SetFlag2(x uint8) { r[9] = x }
+
+func (r *RefFlags) Write(w *Writer) { w.Bytes(r[:]) }
+
 // Referenced symbol name.
 //
 // Serialized format:
@@ -689,6 +724,18 @@ func (r *Reader) Sym(i uint32) *Sym {
 	return (*Sym)(unsafe.Pointer(&r.b[off]))
 }
 
+// NRefFlags returns the number of referenced symbol flags.
+func (r *Reader) NRefFlags() int {
+	return int(r.h.Offsets[BlkRefFlags+1]-r.h.Offsets[BlkRefFlags]) / RefFlagsSize
+}
+
+// RefFlags returns a pointer to the i-th referenced symbol flags.
+// Note: here i is not a local symbol index, just a counter.
+func (r *Reader) RefFlags(i int) *RefFlags {
+	off := r.h.Offsets[BlkRefFlags] + uint32(i*RefFlagsSize)
+	return (*RefFlags)(unsafe.Pointer(&r.b[off]))
+}
+
 // Hash64 returns the i-th short hashed symbol's hash.
 // Note: here i is the index of short hashed symbols, not all symbols
 // (unlike other accessors).

src/cmd/internal/obj/objfile2.go

@@ -105,6 +105,10 @@ func WriteObjFile(ctxt *Link, b *bio.Writer) {
 		w.Sym(s)
 	}
 
+	// Referenced package symbol flags
+	h.Offsets[goobj2.BlkRefFlags] = w.Offset()
+	w.refFlags()
+
 	// Hashes
 	h.Offsets[goobj2.BlkHash64] = w.Offset()
 	for _, s := range ctxt.hashed64defs {
@@ -468,10 +472,9 @@ func (w *writer) Aux(s *LSym) {
 	}
 }
 
-// Emits names of referenced indexed symbols, used by tools (objdump, nm)
-// only.
-func (w *writer) refNames() {
-	seen := make(map[goobj2.SymRef]bool)
+// Emits flags of referenced indexed symbols.
+func (w *writer) refFlags() {
+	seen := make(map[*LSym]bool)
 	w.ctxt.traverseSyms(traverseRefs, func(rs *LSym) { // only traverse refs, not auxs, as tools don't need auxs
 		switch rs.PkgIdx {
 		case goobj2.PkgIdxNone, goobj2.PkgIdxHashed64, goobj2.PkgIdxHashed, goobj2.PkgIdxBuiltin, goobj2.PkgIdxSelf: // not an external indexed reference
@@ -479,11 +482,41 @@ func (w *writer) refNames() {
 		case goobj2.PkgIdxInvalid:
 			panic("unindexed symbol reference")
 		}
-		symref := makeSymRef(rs)
-		if seen[symref] {
+		if seen[rs] {
 			return
 		}
-		seen[symref] = true
+		seen[rs] = true
+		symref := makeSymRef(rs)
+		flag2 := uint8(0)
+		if rs.UsedInIface() {
+			flag2 |= goobj2.SymFlagUsedInIface
+		}
+		if flag2 == 0 {
+			return // no need to write zero flags
+		}
+		var o goobj2.RefFlags
+		o.SetSym(symref)
+		o.SetFlag2(flag2)
+		o.Write(w.Writer)
+	})
+}
+
+// Emits names of referenced indexed symbols, used by tools (objdump, nm)
+// only.
+func (w *writer) refNames() {
+	seen := make(map[*LSym]bool)
+	w.ctxt.traverseSyms(traverseRefs, func(rs *LSym) { // only traverse refs, not auxs, as tools don't need auxs
+		switch rs.PkgIdx {
+		case goobj2.PkgIdxNone, goobj2.PkgIdxHashed64, goobj2.PkgIdxHashed, goobj2.PkgIdxBuiltin, goobj2.PkgIdxSelf: // not an external indexed reference
+			return
+		case goobj2.PkgIdxInvalid:
+			panic("unindexed symbol reference")
+		}
+		if seen[rs] {
+			return
+		}
+		seen[rs] = true
+		symref := makeSymRef(rs)
 		var o goobj2.RefName
 		o.SetSym(symref)
 		o.SetName(rs.Name, w.Writer)

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

@@ -2149,6 +2149,7 @@ func (l *Loader) LoadNonpkgSyms(arch *sys.Arch) {
 }
 
 func loadObjRefs(l *Loader, r *oReader, arch *sys.Arch) {
+	// load non-package refs
 	ndef := uint32(r.NAlldef())
 	needNameExpansion := r.NeedNameExpansion()
 	for i, n := uint32(0), uint32(r.NNonpkgref()); i < n; i++ {
@@ -2167,6 +2168,15 @@ func loadObjRefs(l *Loader, r *oReader, arch *sys.Arch) {
 			l.SetAttrUsedInIface(gi, true)
 		}
 	}
+
+	// load flags of package refs
+	for i, n := 0, r.NRefFlags(); i < n; i++ {
+		rf := r.RefFlags(i)
+		gi := l.resolve(r, rf.Sym())
+		if rf.Flag2()&goobj2.SymFlagUsedInIface != 0 {
+			l.SetAttrUsedInIface(gi, true)
+		}
+	}
 }
 
 func abiToVer(abi uint16, localSymVersion int) int {