reflect: generated unrolled GC bitmask directly

The code for a generated type is already generating an
unrolled GC bitmask.  Rather than unrolling the the source
type bitmasks and copying them, just generate the required
bitmask directly.  Don't mark it as an unrolled GC program,
since there is no need to do so.

Fixes #8917.

LGTM=rsc
R=dvyukov, rsc
CC=golang-codereviews
https://golang.org/cl/156930044

src/reflect/all_test.go

@@ -4018,3 +4018,9 @@ func TestInvalid(t *testing.T) {
 		t.Errorf("field elem: IsValid=%v, Kind=%v, want false, Invalid", v.IsValid(), v.Kind())
 	}
 }
+
+// Issue 8917.
+func TestLargeGCProg(t *testing.T) {
+	fv := ValueOf(func([256]*byte) {})
+	fv.Call([]Value{ValueOf([256]*byte{})})
+}

src/reflect/type.go

@@ -1514,20 +1514,36 @@ func (gc *gcProg) appendProg(t *rtype) {
 		gc.size += t.size
 		return
 	}
-	nptr := t.size / unsafe.Sizeof(uintptr(0))
-	var prog []byte
-	if t.kind&kindGCProg != 0 {
-		// Ensure that the runtime has unrolled GC program.
-		// TODO(rsc): Do not allocate.
-		unsafe_New(t)
-		// The program is stored in t.gc[0], skip unroll flag.
-		prog = (*[1 << 30]byte)(unsafe.Pointer(t.gc[0]))[1:]
-	} else {
-		// The mask is linked directly in t.gc.
-		prog = (*[2 * ptrSize]byte)(unsafe.Pointer(t.gc[0]))[:]
-	}
-	for i := uintptr(0); i < nptr; i++ {
-		gc.appendWord(extractGCWord(prog, i))
+	switch t.Kind() {
+	default:
+		panic("reflect: non-pointer type marked as having pointers")
+	case Ptr, UnsafePointer, Chan, Func, Map:
+		gc.appendWord(bitsPointer)
+	case Slice:
+		gc.appendWord(bitsPointer)
+		gc.appendWord(bitsScalar)
+		gc.appendWord(bitsScalar)
+	case String:
+		gc.appendWord(bitsPointer)
+		gc.appendWord(bitsScalar)
+	case Array:
+		c := t.Len()
+		e := t.Elem().common()
+		for i := 0; i < c; i++ {
+			gc.appendProg(e)
+		}
+	case Interface:
+		gc.appendWord(bitsMultiWord)
+		if t.NumMethod() == 0 {
+			gc.appendWord(bitsEface)
+		} else {
+			gc.appendWord(bitsIface)
+		}
+	case Struct:
+		c := t.NumField()
+		for i := 0; i < c; i++ {
+			gc.appendProg(t.Field(i).Type.common())
+		}
 	}
 }
 
@@ -1562,7 +1578,6 @@ func (gc *gcProg) finalize() unsafe.Pointer {
 			gc.appendWord(extractGCWord(gc.gc, i))
 		}
 	}
-	gc.gc = append([]byte{1}, gc.gc...) // prepend unroll flag
 	return unsafe.Pointer(&gc.gc[0])
 }
 
@@ -1574,9 +1589,14 @@ func (gc *gcProg) align(a uintptr) {
 	gc.size = align(gc.size, a)
 }
 
+// These constants must stay in sync with ../runtime/mgc0.h.
 const (
-	bitsScalar  = 1
-	bitsPointer = 2
+	bitsScalar    = 1
+	bitsPointer   = 2
+	bitsMultiWord = 3
+
+	bitsIface = 2
+	bitsEface = 3
 )
 
 // Make sure these routines stay in sync with ../../runtime/hashmap.go!
@@ -1619,7 +1639,6 @@ func bucketOf(ktyp, etyp *rtype) *rtype {
 	b := new(rtype)
 	b.size = gc.size
 	b.gc[0] = gc.finalize()
-	b.kind |= kindGCProg
 	s := "bucket(" + *ktyp.string + "," + *etyp.string + ")"
 	b.string = &s
 	return b
@@ -1821,7 +1840,6 @@ func funcLayout(t *rtype, rcvr *rtype) (frametype *rtype, argSize, retOffset uin
 	x := new(rtype)
 	x.size = gc.size
 	x.gc[0] = gc.finalize()
-	x.kind |= kindGCProg
 	var s string
 	if rcvr != nil {
 		s = "methodargs(" + *rcvr.string + ")(" + *t.string + ")"