reflect: allow Slice of arrays

R=r
CC=golang-dev
https://golang.org/cl/4444049

src/cmd/gc/reflect.c

@@ -690,7 +690,7 @@ dtypesym(Type *t)
 	int ot, xt, n, isddd, dupok;
 	Sym *s, *s1, *s2;
 	Sig *a, *m;
-	Type *t1, *tbase;
+	Type *t1, *tbase, *t2;
 
 	if(isideal(t))
 		fatal("dtypesym %T", t);
@@ -727,15 +727,25 @@ ok:
 		break;
 
 	case TARRAY:
+		if(t->bound >= 0) {
 			// ../../pkg/runtime/type.go:/ArrayType
 			s1 = dtypesym(t->type);
+			t2 = typ(TARRAY);
+			t2->type = t->type;
+			t2->bound = -1;  // slice
+			s2 = dtypesym(t2);
+			ot = dcommontype(s, ot, t);
+			xt = ot - 2*widthptr;
+			ot = dsymptr(s, ot, s1, 0);
+			ot = dsymptr(s, ot, s2, 0);
+			ot = duintptr(s, ot, t->bound);
+		} else {
+			// ../../pkg/runtime/type.go:/SliceType
+			s1 = dtypesym(t->type);
 			ot = dcommontype(s, ot, t);
 			xt = ot - 2*widthptr;
 			ot = dsymptr(s, ot, s1, 0);
-		if(t->bound < 0)
-			ot = duintptr(s, ot, -1);
-		else
-			ot = duintptr(s, ot, t->bound);
+		}
 		break;
 
 	case TCHAN:

src/pkg/reflect/all_test.go

@@ -1435,3 +1435,17 @@ func TestSmallNegativeInt(t *testing.T) {
 		t.Errorf("int16(-1).Int() returned %v", v.Int())
 	}
 }
+
+func TestSlice(t *testing.T) {
+	xs := []int{1, 2, 3, 4, 5, 6, 7, 8}
+	v := NewValue(xs).Slice(3, 5).Interface().([]int)
+	if len(v) != 2 || v[0] != 4 || v[1] != 5 {
+		t.Errorf("xs.Slice(3, 5) = %v", v)
+	}
+
+	xa := [7]int{10, 20, 30, 40, 50, 60, 70}
+	v = NewValue(&xa).Elem().Slice(2, 5).Interface().([]int)
+	if len(v) != 3 || v[0] != 30 || v[1] != 40 || v[2] != 50 {
+		t.Errorf("xa.Slice(2, 5) = %v", v)
+	}
+}

src/pkg/reflect/type.go

@@ -260,6 +260,7 @@ const (
 type arrayType struct {
 	commonType "array"
 	elem       *runtime.Type
+	slice      *runtime.Type
 	len        uintptr
 }
 

src/pkg/reflect/value.go

@@ -1244,20 +1244,32 @@ func (v Value) SetString(x string) {
 	*(*string)(iv.addr) = x
 }
 
-// BUG(rsc): Value.Slice should allow slicing arrays.
-
 // Slice returns a slice of v.
-// It panics if v's Kind is not Slice.
+// It panics if v's Kind is not Array or Slice.
 func (v Value) Slice(beg, end int) Value {
 	iv := v.internal()
-	iv.mustBe(Slice)
+	if iv.kind != Array && iv.kind != Slice {
+		panic(&ValueError{"reflect.Value.Slice", iv.kind})
+	}
 	cap := v.Cap()
 	if beg < 0 || end < beg || end > cap {
 		panic("reflect.Value.Slice: slice index out of bounds")
 	}
-	typ := iv.typ.toType()
+	var typ Type
+	var base uintptr
+	switch iv.kind {
+	case Array:
+		if iv.flag&flagAddr == 0 {
+			panic("reflect.Value.Slice: slice of unaddressable array")
+		}
+		typ = toType((*arrayType)(unsafe.Pointer(iv.typ)).slice)
+		base = uintptr(iv.addr)
+	case Slice:
+		typ = iv.typ.toType()
+		base = (*SliceHeader)(iv.addr).Data
+	}
 	s := new(SliceHeader)
-	s.Data = uintptr((*SliceHeader)(iv.addr).Data) + uintptr(beg)*typ.Elem().Size()
+	s.Data = base + uintptr(beg)*typ.Elem().Size()
 	s.Len = end - beg
 	s.Cap = cap - beg
 	return valueFromAddr(iv.flag&flagRO, typ, unsafe.Pointer(s))

src/pkg/runtime/type.go

@@ -118,6 +118,7 @@ type UnsafePointerType commonType
 type ArrayType struct {
 	commonType
 	elem  *Type // array element type
+	slice *Type // slice type
 	len   uintptr
 }