runtime: remove link field from itab

We don't use it any more, remove it.

Change-Id: I76ce1a4c2e7048fdd13a37d3718b5abf39ed9d26
Reviewed-on: https://go-review.googlesource.com/44474
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

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

@@ -1408,7 +1408,7 @@ func itabsym(it *obj.LSym, offset int64) *obj.LSym {
 	}
 
 	// keep this arithmetic in sync with *itab layout
-	methodnum := int((offset - 3*int64(Widthptr) - 8) / int64(Widthptr))
+	methodnum := int((offset - 2*int64(Widthptr) - 8) / int64(Widthptr))
 	if methodnum >= len(syms) {
 		return nil
 	}
@@ -1457,14 +1457,12 @@ func dumptabs() {
 		// type itab struct {
 		//   inter  *interfacetype
 		//   _type  *_type
-		//   _      uintptr TODO: remove
 		//   hash   uint32
 		//   _      [4]byte
 		//   fun    [1]uintptr // variable sized
 		// }
 		o := dsymptr(i.lsym, 0, dtypesym(i.itype).Linksym(), 0)
 		o = dsymptr(i.lsym, o, dtypesym(i.t).Linksym(), 0)
-		o = duintptr(i.lsym, o, 0)             // unused
 		o = duint32(i.lsym, o, typehash(i.t))  // copy of type hash
 		o += 4                                 // skip unused field
 		o += len(imethods(i.itype)) * Widthptr // skip fun method pointers

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

@@ -3122,7 +3122,7 @@ func (s *state) call(n *Node, k callKind) *ssa.Value {
 		if k != callNormal {
 			s.nilCheck(itab)
 		}
-		itabidx := fn.Xoffset + 3*int64(Widthptr) + 8 // offset of fun field in runtime.itab
+		itabidx := fn.Xoffset + 2*int64(Widthptr) + 8 // offset of fun field in runtime.itab
 		itab = s.newValue1I(ssa.OpOffPtr, s.f.Config.Types.UintptrPtr, itabidx, itab)
 		if k == callNormal {
 			codeptr = s.newValue2(ssa.OpLoad, types.Types[TUINTPTR], itab, s.mem())

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

@@ -757,7 +757,7 @@ func (s *typeSwitch) walk(sw *Node) {
 	if cond.Right.Type.IsEmptyInterface() {
 		h.Xoffset = int64(2 * Widthptr) // offset of hash in runtime._type
 	} else {
-		h.Xoffset = int64(3 * Widthptr) // offset of hash in runtime.itab
+		h.Xoffset = int64(2 * Widthptr) // offset of hash in runtime.itab
 	}
 	h.SetBounded(true) // guaranteed not to fault
 	a = nod(OAS, s.hashname, h)

src/reflect/value.go

@@ -182,7 +182,6 @@ type nonEmptyInterface struct {
 	itab *struct {
 		ityp *rtype // static interface type
 		typ  *rtype // dynamic concrete type
-		_    uintptr
 		hash uint32 // copy of typ.hash
 		_    [4]byte
 		fun  [100000]unsafe.Pointer // method table

src/runtime/runtime2.go

@@ -626,7 +626,6 @@ type _func struct {
 type itab struct {
 	inter *interfacetype
 	_type *_type
-	_     uintptr
 	hash  uint32 // copy of _type.hash. Used for type switches.
 	_     [4]byte
 	fun   [1]uintptr // variable sized. fun[0]==0 means _type does not implement inter.