runtime: introduce unsafe.New and unsafe.NewArray

to provide functionality previously hacked in to
    reflect and gob.

R=r
https://golang.org/cl/165076

src/cmd/gc/builtin.c.boot

@@ -86,5 +86,7 @@ char *unsafeimport =
 	"func unsafe.Typeof (i interface { }) (typ interface { })\n"
 	"func unsafe.Reflect (i interface { }) (typ interface { }, addr unsafe.Pointer)\n"
 	"func unsafe.Unreflect (typ interface { }, addr unsafe.Pointer) (ret interface { })\n"
+	"func unsafe.New (typ interface { }) (? unsafe.Pointer)\n"
+	"func unsafe.NewArray (typ interface { }, n int) (? unsafe.Pointer)\n"
 	"\n"
 	"$$\n";

src/cmd/gc/unsafe.go

@@ -12,3 +12,5 @@ func Alignof(any) int
 func Typeof(i interface{}) (typ interface{})
 func Reflect(i interface{}) (typ interface{}, addr Pointer)
 func Unreflect(typ interface{}, addr Pointer) (ret interface{})
+func New(typ interface{}) Pointer
+func NewArray(typ interface{}, n int) Pointer

src/pkg/gob/decode.go

@@ -13,6 +13,7 @@ import (
 	"math";
 	"os";
 	"reflect";
+	"runtime";
 	"unsafe";
 )
 
@@ -361,10 +362,8 @@ func decodeStruct(engine *decEngine, rtyp *reflect.StructType, b *bytes.Buffer,
 			up = decIndirect(up, indir)
 		}
 		if *(*unsafe.Pointer)(up) == nil {
-			// Allocate object by making a slice of bytes and recording the
-			// address of the beginning of the array. TODO(rsc).
-			b := make([]byte, rtyp.Size());
-			*(*unsafe.Pointer)(up) = unsafe.Pointer(&b[0]);
+			// Allocate object.
+			*(*unsafe.Pointer)(up) = unsafe.New((*runtime.StructType)(unsafe.Pointer(rtyp)))
 		}
 		p = *(*uintptr)(up);
 	}
@@ -437,10 +436,8 @@ func decodeArray(atyp *reflect.ArrayType, state *decodeState, p uintptr, elemOp
 	if indir > 0 {
 		up := unsafe.Pointer(p);
 		if *(*unsafe.Pointer)(up) == nil {
-			// Allocate the array by making a slice of bytes of the correct size
-			// and taking the address of the beginning of the array. TODO(rsc).
-			b := make([]byte, atyp.Size());
-			*(**byte)(up) = &b[0];
+			// Allocate object.
+			*(*unsafe.Pointer)(up) = unsafe.New(atyp)
 		}
 		p = *(*uintptr)(up);
 	}
@@ -466,23 +463,22 @@ func ignoreArray(state *decodeState, elemOp decOp, length int) os.Error {
 }
 
 func decodeSlice(atyp *reflect.SliceType, state *decodeState, p uintptr, elemOp decOp, elemWid uintptr, indir, elemIndir int, ovfl os.ErrorString) os.Error {
-	length := uintptr(decodeUint(state));
+	n := int(uintptr(decodeUint(state)));
 	if indir > 0 {
 		up := unsafe.Pointer(p);
 		if *(*unsafe.Pointer)(up) == nil {
 			// Allocate the slice header.
-			*(*unsafe.Pointer)(up) = unsafe.Pointer(new(reflect.SliceHeader))
+			*(*unsafe.Pointer)(up) = unsafe.Pointer(new([]unsafe.Pointer))
 		}
 		p = *(*uintptr)(up);
 	}
 	// Allocate storage for the slice elements, that is, the underlying array.
-	data := make([]byte, length*atyp.Elem().Size());
 	// Always write a header at p.
 	hdrp := (*reflect.SliceHeader)(unsafe.Pointer(p));
-	hdrp.Data = uintptr(unsafe.Pointer(&data[0]));
-	hdrp.Len = int(length);
-	hdrp.Cap = int(length);
-	return decodeArrayHelper(state, hdrp.Data, elemOp, elemWid, int(length), elemIndir, ovfl);
+	hdrp.Data = uintptr(unsafe.NewArray(atyp.Elem(), n));
+	hdrp.Len = n;
+	hdrp.Cap = n;
+	return decodeArrayHelper(state, hdrp.Data, elemOp, elemWid, n, elemIndir, ovfl);
 }
 
 func ignoreSlice(state *decodeState, elemOp decOp) os.Error {

src/pkg/reflect/value.go

@@ -595,15 +595,11 @@ func (v *SliceValue) Elem(i int) Value {
 // MakeSlice creates a new zero-initialized slice value
 // for the specified slice type, length, and capacity.
 func MakeSlice(typ *SliceType, len, cap int) *SliceValue {
-	s := new(SliceHeader);
-	size := typ.Elem().Size() * uintptr(cap);
-	if size == 0 {
-		size = 1
-	}
-	data := make([]uint8, size);
-	s.Data = uintptr(addr(&data[0]));
-	s.Len = len;
-	s.Cap = cap;
+	s := &SliceHeader{
+		Data: uintptr(unsafe.NewArray(typ.Elem(), cap)),
+		Len: len,
+		Cap: cap,
+	};
 	return newValue(typ, addr(s), true).(*SliceValue);
 }
 
@@ -1270,13 +1266,8 @@ func newValue(typ Type, addr addr, canSet bool) Value {
 
 // MakeZero returns a zero Value for the specified Type.
 func MakeZero(typ Type) Value {
-	// TODO: this will have to move into
-	// the runtime proper in order to play nicely
-	// with the garbage collector.
-	size := typ.Size();
-	if size == 0 {
-		size = 1
+	if typ == nil {
+		return nil
 	}
-	data := make([]uint8, size);
-	return newValue(typ, addr(&data[0]), true);
+	return newValue(typ, addr(unsafe.New(typ)), true);
 }

src/pkg/runtime/iface.c

@@ -4,6 +4,7 @@
 
 #include "runtime.h"
 #include "type.h"
+#include "malloc.h"
 
 static void
 printiface(Iface i)
@@ -605,7 +606,7 @@ unsafe·Reflect(Eface e, Eface rettype, void *retaddr)
 }
 
 void
-unsafe·Unreflect(Iface typ, void *addr, Eface e)
+unsafe·Unreflect(Eface typ, void *addr, Eface e)
 {
 	// Reflect library has reinterpreted typ
 	// as its own kind of type structure.
@@ -625,3 +626,41 @@ unsafe·Unreflect(Iface typ, void *addr, Eface e)
 
 	FLUSH(&e);
 }
+
+void
+unsafe·New(Eface typ, void *ret)
+{
+	Type *t;
+
+	// Reflect library has reinterpreted typ
+	// as its own kind of type structure.
+	// We know that the pointer to the original
+	// type structure sits before the data pointer.
+	t = (Type*)((Eface*)typ.data-1);
+
+	if(t->kind&KindNoPointers)
+		ret = mallocgc(t->size, RefNoPointers, 1);
+	else
+		ret = mal(t->size);
+	FLUSH(&ret);
+}
+
+void
+unsafe·NewArray(Eface typ, uint32 n, void *ret)
+{
+	uint64 size;
+	Type *t;
+
+	// Reflect library has reinterpreted typ
+	// as its own kind of type structure.
+	// We know that the pointer to the original
+	// type structure sits before the data pointer.
+	t = (Type*)((Eface*)typ.data-1);
+	
+	size = n*t->size;
+	if(t->kind&KindNoPointers)
+		ret = mallocgc(size, RefNoPointers, 1);
+	else
+		ret = mal(size);
+	FLUSH(&ret);
+}

src/pkg/runtime/slice.c

@@ -8,6 +8,7 @@
 
 static	int32	debug	= 0;
 
+// see also unsafe·NewArray
 // makeslice(typ *Type, nel int, cap int) (ary []any);
 void
 runtime·makeslice(SliceType *t, uint32 nel, uint32 cap, Slice ret)
@@ -21,9 +22,7 @@ runtime·makeslice(SliceType *t, uint32 nel, uint32 cap, Slice ret)
 	ret.len = nel;
 	ret.cap = cap;
 
-	// TODO(rsc): Disabled because reflect and gob cast []byte
-	// to data structures with pointers.
-	if(0 && (t->elem->kind&KindNoPointers))
+	if((t->elem->kind&KindNoPointers))
 		ret.array = mallocgc(size, RefNoPointers, 1);
 	else
 		ret.array = mal(size);

src/pkg/unsafe/unsafe.go

@@ -43,5 +43,17 @@ func Typeof(i interface{}) (typ interface{})
 func Reflect(i interface{}) (typ interface{}, addr uintptr)
 
 // Unreflect inverts Reflect: Given a type and a pointer, it returns an empty interface value
-// with those contents.
+// with those contents.  The typ is assumed to contain a pointer to a runtime type;
+// the type information in the interface{} is ignored, so that, for example, both
+// *reflect.StructType and *runtime.StructType can be passed for typ.
 func Unreflect(typ interface{}, addr uintptr) (ret interface{})
+
+// New allocates and returns a pointer to memory for a new value of the given type.
+// The typ is assumed to hold a pointer to a runtime type.
+// Callers should use reflect.MakeZero instead of invoking unsafe.New directly.
+func New(typ interface{}) Pointer
+
+// NewArray allocates and returns a pointer to an array of n elements of the given type.
+// The typ is assumed to hold a pointer to a runtime type.
+// Callers should use reflect.MakeSlice instead of invoking unsafe.NewArray directly.
+func NewArray(typ interface{}, n int) Pointer