runtime: remove bad field from itab

Just use fun[0]==0 to indicate a bad itab.

Change-Id: I28ecb2d2d857090c1ecc40b1d1866ac24a844848
Reviewed-on: https://go-review.googlesource.com/44473
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

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

@@ -1459,16 +1459,14 @@ func dumptabs() {
 		//   _type  *_type
 		//   _      uintptr TODO: remove
 		//   hash   uint32
-		//   bad    bool
-		//   _      [3]byte
+		//   _      [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 += 1                                 // bad is false
-		o += 3                                 // skip unused fields
+		o += 4                                 // skip unused field
 		o += len(imethods(i.itype)) * Widthptr // skip fun method pointers
 		// at runtime the itab will contain pointers to types, other itabs and
 		// method functions. None are allocated on heap, so we can use obj.NOPTR.

src/reflect/value.go

@@ -184,8 +184,7 @@ type nonEmptyInterface struct {
 		typ  *rtype // dynamic concrete type
 		_    uintptr
 		hash uint32 // copy of typ.hash
-		bad  bool
-		_    [3]byte
+		_    [4]byte
 		fun  [100000]unsafe.Pointer // method table
 	}
 	word unsafe.Pointer

src/runtime/iface.go

@@ -70,7 +70,7 @@ func getitab(inter *interfacetype, typ *_type, canfail bool) *itab {
 	itabAdd(m)
 	unlock(&itabLock)
 finish:
-	if !m.bad {
+	if m.fun[0] != 0 {
 		return m
 	}
 	if canfail {
@@ -219,7 +219,7 @@ imethods:
 			}
 		}
 		// didn't find method
-		m.bad = true
+		m.fun[0] = 0
 		return iname
 	}
 	return ""

src/runtime/runtime2.go

@@ -628,9 +628,8 @@ type itab struct {
 	_type *_type
 	_     uintptr
 	hash  uint32 // copy of _type.hash. Used for type switches.
-	bad   bool   // type does not implement interface
-	_     [3]byte
-	fun   [1]uintptr // variable sized
+	_     [4]byte
+	fun   [1]uintptr // variable sized. fun[0]==0 means _type does not implement inter.
 }
 
 // Lock-free stack node.