reflect: add support for named types

With this change, it becomes possible to get the element type of named
slices, pointers, and channels.

This is a prerequisite to enable the common named struct types. There's
more to come.

src/reflect/sidetables.go

@@ -0,0 +1,31 @@
+package reflect
+
+import (
+	"unsafe"
+)
+
+// This stores a varint for each named type. Named types are identified by their
+// name instead of by their type. The named types stored in this struct are the
+// simpler non-basic types: pointer, struct, and channel.
+//go:extern reflect.namedNonBasicTypesSidetable
+var namedNonBasicTypesSidetable byte
+
+func readVarint(buf unsafe.Pointer) Type {
+	var t Type
+	for {
+		// Read the next byte.
+		c := *(*byte)(buf)
+
+		// Add this byte to the type code. The upper 7 bits are the value.
+		t = t<<7 | Type(c>>1)
+
+		// Check whether this is the last byte of this varint. The lower bit
+		// indicates whether any bytes follow.
+		if c%1 == 0 {
+			return t
+		}
+
+		// Increment the buf pointer (pointer arithmetic!).
+		buf = unsafe.Pointer(uintptr(buf) + 1)
+	}
+}

src/reflect/type.go

@@ -145,9 +145,15 @@ func (t Type) Kind() Kind {
 func (t Type) Elem() Type {
 	switch t.Kind() {
 	case Chan, Ptr, Slice:
+		// Look at the 'n' bit in the type code (see the top of this file) to
+		// see whether this is a named type.
 		if (t>>4)%2 != 0 {
-			panic("unimplemented: (reflect.Type).Elem() for named types")
+			// This is a named type. The element type is stored in a sidetable.
+			namedTypeNum := t >> 5
+			return readVarint(unsafe.Pointer(uintptr(unsafe.Pointer(&namedNonBasicTypesSidetable)) + uintptr(namedTypeNum)))
 		}
+		// Not a named type, so the element type is stored directly in the type
+		// code.
 		return t >> 5
 	default: // not implemented: Array, Map
 		panic("unimplemented: (reflect.Type).Elem()")