diff --git a/Makefile b/Makefile
index 09b71c4545..581993bffe 100644
--- a/Makefile
+++ b/Makefile
@@ -463,7 +463,7 @@ testlua: lua
 	  echo LUA $$test; \
 	  LUA_PATH="third_party/lunit/?.lua;upb/bindings/lua/?.lua" \
 	    LUA_CPATH=upb/bindings/lua/?.so \
-	    lua $$test; \
+	    $(RUN_UNDER) lua $$test; \
 	done
 
 clean: clean_lua
diff --git a/tests/bindings/lua/test_upb.lua b/tests/bindings/lua/test_upb.lua
index 1dc07178f4..07d794ce91 100644
--- a/tests/bindings/lua/test_upb.lua
+++ b/tests/bindings/lua/test_upb.lua
@@ -565,13 +565,20 @@ function test_msg_primitives()
 
   local symtab = upb.SymbolTable{
     upb.MessageDef{full_name = "TestMessage", fields = {
-      upb.FieldDef{name = "i32", number = 1, type = upb.TYPE_INT32},
-      upb.FieldDef{name = "u32", number = 2, type = upb.TYPE_UINT32},
-      upb.FieldDef{name = "i64", number = 3, type = upb.TYPE_INT64},
-      upb.FieldDef{name = "u64", number = 4, type = upb.TYPE_UINT64},
-      upb.FieldDef{name = "dbl", number = 5, type = upb.TYPE_DOUBLE},
-      upb.FieldDef{name = "flt", number = 6, type = upb.TYPE_FLOAT},
-      upb.FieldDef{name = "bool", number = 7, type = upb.TYPE_BOOL},
+      upb.FieldDef{
+          name = "i32", number = 1, type = upb.TYPE_INT32, default = 1},
+      upb.FieldDef{
+          name = "u32", number = 2, type = upb.TYPE_UINT32, default = 2},
+      upb.FieldDef{
+          name = "i64", number = 3, type = upb.TYPE_INT64, default = 3},
+      upb.FieldDef{
+          name = "u64", number = 4, type = upb.TYPE_UINT64, default = 4},
+      upb.FieldDef{
+          name = "dbl", number = 5, type = upb.TYPE_DOUBLE, default = 5},
+      upb.FieldDef{
+          name = "flt", number = 6, type = upb.TYPE_FLOAT, default = 6},
+      upb.FieldDef{
+          name = "bool", number = 7, type = upb.TYPE_BOOL, default = true},
       }
     }
   }
@@ -581,13 +588,13 @@ function test_msg_primitives()
   msg = TestMessage()
 
   -- Unset member returns default value.
-  assert_equal(0, msg.i32)
-  assert_equal(0, msg.u32)
-  assert_equal(0, msg.i64)
-  assert_equal(0, msg.u64)
-  assert_equal(0, msg.dbl)
-  assert_equal(0, msg.flt)
-  assert_equal(false, msg.bool)
+  assert_equal(1, msg.i32)
+  assert_equal(2, msg.u32)
+  assert_equal(3, msg.i64)
+  assert_equal(4, msg.u64)
+  assert_equal(5, msg.dbl)
+  assert_equal(6, msg.flt)
+  assert_equal(true, msg.bool)
 
   -- Attempts to access non-existent fields fail.
   assert_error_match("no such field", function() msg.no_such = 1 end)
diff --git a/upb/bindings/lua/msg.c b/upb/bindings/lua/msg.c
index 64c8e7c285..c222ca5444 100644
--- a/upb/bindings/lua/msg.c
+++ b/upb/bindings/lua/msg.c
@@ -189,7 +189,8 @@ typedef struct lupb_msgfactory {
   upb_msgfactory *factory;
 } lupb_msgfactory;
 
-static int lupb_msgclass_pushnew(lua_State *L, int factory, const upb_msglayout *l);
+static int lupb_msgclass_pushnew(lua_State *L, int factory,
+                                 const upb_msgdef *md);
 
 /* lupb_msgfactory helpers. */
 
@@ -199,8 +200,6 @@ static lupb_msgfactory *lupb_msgfactory_check(lua_State *L, int narg) {
 
 static void lupb_msgfactory_pushmsgclass(lua_State *L, int narg,
                                          const upb_msgdef *md) {
-  const lupb_msgfactory *lfactory = lupb_msgfactory_check(L, narg);
-
   lupb_getuservalue(L, narg);
   lua_pushlightuserdata(L, (void*)md);
   lua_rawget(L, -2);
@@ -208,8 +207,7 @@ static void lupb_msgfactory_pushmsgclass(lua_State *L, int narg,
   if (lua_isnil(L, -1)) {
     lua_pop(L, 1);
     /* TODO: verify md is in symtab? */
-    lupb_msgclass_pushnew(L, narg,
-                          upb_msgfactory_getlayout(lfactory->factory, md));
+    lupb_msgclass_pushnew(L, narg, md);
 
     /* Set in userval. */
     lua_pushlightuserdata(L, (void*)md);
@@ -317,6 +315,10 @@ const upb_handlers *lupb_msgclass_getmergehandlers(lua_State *L, int narg) {
       lmsgclass->lfactory->factory, upb_msglayout_msgdef(lmsgclass->layout));
 }
 
+upb_msgfactory *lupb_msgclass_getfactory(const lupb_msgclass *lmsgclass) {
+  return lmsgclass->lfactory->factory;
+}
+
 /**
  * lupb_msgclass_typecheck()
  *
@@ -360,13 +362,13 @@ static const lupb_msgclass *lupb_msgclass_getsubmsgclass(lua_State *L, int narg,
   return lupb_msgclass_msgclassfor(L, narg, upb_fielddef_msgsubdef(f));
 }
 
-static int lupb_msgclass_pushnew(lua_State *L, int factory, const upb_msglayout *l) {
+static int lupb_msgclass_pushnew(lua_State *L, int factory,
+                                 const upb_msgdef *md) {
   const lupb_msgfactory *lfactory = lupb_msgfactory_check(L, factory);
   lupb_msgclass *lmc = lupb_newuserdata(L, sizeof(*lmc), LUPB_MSGCLASS);
 
-  UPB_ASSERT(l);
   lupb_uservalseti(L, -1, LUPB_MSGCLASS_FACTORY, factory);
-  lmc->layout = l;
+  lmc->layout = upb_msgfactory_getlayout(lfactory->factory, md);
   lmc->lfactory = lfactory;
 
   return 1;
@@ -933,7 +935,7 @@ static const lupb_msgclass *lupb_msg_getsubmsgclass(lua_State *L, int narg,
   return lupb_msgclass_getsubmsgclass(L, -1, f);
 }
 
-int lupb_msg_pushref(lua_State *L, int msgclass, void *msg) {
+int lupb_msg_pushref(lua_State *L, int msgclass, upb_msg *msg) {
   const lupb_msgclass *lmsgclass = lupb_msgclass_check(L, msgclass);
   lupb_msg *lmsg = lupb_newuserdata(L, sizeof(lupb_msg), LUPB_MSG);
 
@@ -966,8 +968,8 @@ static int lupb_msg_pushnew(lua_State *L, int narg) {
   lupb_msg *lmsg = lupb_newuserdata(L, size, LUPB_MSG);
 
   lmsg->lmsgclass = lmsgclass;
-  lmsg->msg = ADD_BYTES(lmsg, sizeof(*lmsg));
-  upb_msg_init(lmsg->msg, lmsgclass->layout, lupb_alloc_get(L));
+  lmsg->msg = upb_msg_init(
+      ADD_BYTES(lmsg, sizeof(*lmsg)), lmsgclass->layout, lupb_alloc_get(L));
 
   lupb_uservalseti(L, -1, LUPB_MSG_MSGCLASSINDEX, narg);
 
diff --git a/upb/bindings/lua/upb.h b/upb/bindings/lua/upb.h
index 88a201cee8..982ae57b5b 100644
--- a/upb/bindings/lua/upb.h
+++ b/upb/bindings/lua/upb.h
@@ -135,6 +135,7 @@ const upb_msg *lupb_msg_checkmsg(lua_State *L, int narg,
 const lupb_msgclass *lupb_msgclass_check(lua_State *L, int narg);
 const upb_msglayout *lupb_msgclass_getlayout(lua_State *L, int narg);
 const upb_handlers *lupb_msgclass_getmergehandlers(lua_State *L, int narg);
+upb_msgfactory *lupb_msgclass_getfactory(const lupb_msgclass *lmsgclass);
 void lupb_msg_registertypes(lua_State *L);
 
 #endif  /* UPB_LUA_UPB_H_ */
diff --git a/upb/bindings/lua/upb/pb.c b/upb/bindings/lua/upb/pb.c
index a25d1ac8a6..731430d691 100644
--- a/upb/bindings/lua/upb/pb.c
+++ b/upb/bindings/lua/upb/pb.c
@@ -106,7 +106,7 @@ static int lupb_pb_makemsgtostrencoder(lua_State *L) {
   const upb_msglayout *layout = lupb_msgclass_getlayout(L, 1);
   const lupb_msgclass *lmsgclass = lupb_msgclass_check(L, 1);
   const upb_msgdef *md = upb_msglayout_msgdef(layout);
-  upb_msgfactory *factory = upb_msglayout_factory(layout);
+  upb_msgfactory *factory = lupb_msgclass_getfactory(lmsgclass);
   const upb_handlers *encode_handlers;
   const upb_visitorplan *vp;
 
diff --git a/upb/msg.c b/upb/msg.c
index 39e3035779..26d0e9847f 100644
--- a/upb/msg.c
+++ b/upb/msg.c
@@ -24,7 +24,7 @@ bool upb_fieldtype_mapkeyok(upb_fieldtype_t type) {
 void *upb_array_pack(const upb_array *arr, void *p, size_t *ofs, size_t size);
 void *upb_map_pack(const upb_map *map, void *p, size_t *ofs, size_t size);
 
-#define CHARPTR_AT(msg, ofs) ((char*)msg + ofs)
+#define VOIDPTR_AT(msg, ofs) (void*)((char*)msg + ofs)
 #define ENCODE_MAX_NESTING 64
 #define CHECK_TRUE(x) if (!(x)) { return false; }
 
@@ -150,16 +150,8 @@ static upb_msgval upb_msgval_fromdefault(const upb_fielddef *f) {
 /** upb_msglayout *************************************************************/
 
 struct upb_msglayout {
-  upb_msgfactory *factory;
-  const upb_msgdef *msgdef;
-  size_t size;
-  size_t extdict_offset;
-  void *default_msg;
-  uint32_t *field_offsets;
-  uint32_t *case_offsets;
-  uint32_t *hasbits;
-  bool has_extdict;
-  uint8_t align;
+  const upb_msgdef *msgdef;  /* TODO(haberman): remove. */
+  struct upb_msglayout_msginit_v1 data;
 };
 
 static void upb_msg_checkfield(const upb_msglayout *l, const upb_fielddef *f) {
@@ -167,7 +159,7 @@ static void upb_msg_checkfield(const upb_msglayout *l, const upb_fielddef *f) {
 }
 
 static void upb_msglayout_free(upb_msglayout *l) {
-  upb_gfree(l->default_msg);
+  upb_gfree(l->data.default_msg);
   upb_gfree(l);
 }
 
@@ -178,35 +170,35 @@ const upb_msgdef *upb_msglayout_msgdef(const upb_msglayout *l) {
 static size_t upb_msglayout_place(upb_msglayout *l, size_t size) {
   size_t ret;
 
-  l->size = align_up(l->size, size);
-  l->align = align_up(l->align, size);
-  ret = l->size;
-  l->size += size;
+  l->data.size = align_up(l->data.size, size);
+  l->data.align = align_up(l->data.align, size);
+  ret = l->data.size;
+  l->data.size += size;
   return ret;
 }
 
 static uint32_t upb_msglayout_offset(const upb_msglayout *l,
                                      const upb_fielddef *f) {
-  return l->field_offsets[upb_fielddef_index(f)];
+  return l->data.fields[upb_fielddef_index(f)].offset;
 }
 
 static uint32_t upb_msglayout_hasbit(const upb_msglayout *l,
                                      const upb_fielddef *f) {
-  return l->hasbits[upb_fielddef_index(f)];
+  return l->data.fields[upb_fielddef_index(f)].hasbit;
 }
 
 static bool upb_msglayout_initdefault(upb_msglayout *l) {
   const upb_msgdef *m = l->msgdef;
   upb_msg_field_iter it;
 
-  if (upb_msgdef_syntax(m) == UPB_SYNTAX_PROTO2 && l->size) {
+  if (upb_msgdef_syntax(m) == UPB_SYNTAX_PROTO2 && l->data.size) {
     /* Allocate default message and set default values in it. */
-    l->default_msg = upb_gmalloc(l->size);
-    if (!l->default_msg) {
+    l->data.default_msg = upb_gmalloc(l->data.size);
+    if (!l->data.default_msg) {
       return false;
     }
 
-    memset(l->default_msg, 0, l->size);
+    memset(l->data.default_msg, 0, l->data.size);
 
     for (upb_msg_field_begin(&it, m); !upb_msg_field_done(&it);
          upb_msg_field_next(&it)) {
@@ -219,7 +211,7 @@ static bool upb_msglayout_initdefault(upb_msglayout *l) {
       if (!upb_fielddef_isstring(f) &&
           !upb_fielddef_issubmsg(f) &&
           !upb_fielddef_isseq(f)) {
-        upb_msg_set(l->default_msg, f, upb_msgval_fromdefault(f), l);
+        upb_msg_set(l->data.default_msg, f, upb_msgval_fromdefault(f), l);
       }
     }
   }
@@ -232,22 +224,29 @@ static upb_msglayout *upb_msglayout_new(const upb_msgdef *m) {
   upb_msg_oneof_iter oit;
   upb_msglayout *l;
   size_t hasbit;
-  size_t array_size = upb_msgdef_numfields(m) + upb_msgdef_numoneofs(m);
+  size_t submsg_count = 0;
 
-  if (upb_msgdef_syntax(m) == UPB_SYNTAX_PROTO2) {
-    array_size += upb_msgdef_numfields(m);  /* hasbits. */
+  for (upb_msg_field_begin(&it, m), hasbit = sizeof(void*) * 8;
+       !upb_msg_field_done(&it);
+       upb_msg_field_next(&it)) {
+    const upb_fielddef* f = upb_msg_iter_field(&it);
+    if (upb_fielddef_issubmsg(f)) {
+      submsg_count++;
+    }
   }
 
-  l = upb_gmalloc(sizeof(*l) + (sizeof(uint32_t) * array_size));
+  l = upb_gmalloc(sizeof(*l));
   if (!l) return NULL;
 
   memset(l, 0, sizeof(*l));
-
   l->msgdef = m;
-  l->align = 1;
-  l->field_offsets = (uint32_t*)CHARPTR_AT(l, sizeof(*l));
-  l->case_offsets = l->field_offsets + upb_msgdef_numfields(m);
-  l->hasbits = l->case_offsets + upb_msgdef_numoneofs(m);
+
+  /* TODO(haberman): check OOM. */
+  l->data.fields = upb_gmalloc(upb_msgdef_numfields(m) *
+                               sizeof(struct upb_msglayout_fieldinit_v1));
+  l->data.submsgs = upb_gmalloc(submsg_count * sizeof(void*));
+  l->data.case_offsets = upb_gmalloc(upb_msgdef_numoneofs(m) *
+                                     sizeof(*l->data.case_offsets));
 
   /* Allocate data offsets in three stages:
    *
@@ -265,12 +264,13 @@ static upb_msglayout *upb_msglayout_new(const upb_msgdef *m) {
     const upb_fielddef* f = upb_msg_iter_field(&it);
 
     if (upb_fielddef_haspresence(f) && !upb_fielddef_containingoneof(f)) {
-      l->hasbits[upb_fielddef_index(f)] = hasbit++;
+      l->data.fields[upb_fielddef_index(f)].hasbit = hasbit++;
     }
   }
 
   /* Account for space used by hasbits. */
-  l->size = div_round_up(hasbit, 8);
+  l->data.size = div_round_up(hasbit, 8);
+  l->data.align = 1;
 
   /* Allocate non-oneof fields. */
   for (upb_msg_field_begin(&it, m); !upb_msg_field_done(&it);
@@ -279,13 +279,12 @@ static upb_msglayout *upb_msglayout_new(const upb_msgdef *m) {
     size_t field_size = upb_msg_fieldsize(f);
     size_t index = upb_fielddef_index(f);
 
-
     if (upb_fielddef_containingoneof(f)) {
       /* Oneofs are handled separately below. */
       continue;
     }
 
-    l->field_offsets[index] = upb_msglayout_place(l, field_size);
+    l->data.fields[index].offset = upb_msglayout_place(l, field_size);
   }
 
   /* Allocate oneof fields.  Each oneof field consists of a uint32 for the case
@@ -311,19 +310,19 @@ static upb_msglayout *upb_msglayout_new(const upb_msgdef *m) {
     case_offset = upb_msglayout_place(l, case_size);
     val_offset = upb_msglayout_place(l, field_size);
 
-    l->case_offsets[upb_oneofdef_index(oneof)] = case_offset;
+    l->data.case_offsets[upb_oneofdef_index(oneof)] = case_offset;
 
     /* Assign all fields in the oneof this same offset. */
     for (upb_oneof_begin(&fit, oneof); !upb_oneof_done(&fit);
          upb_oneof_next(&fit)) {
       const upb_fielddef* f = upb_oneof_iter_field(&fit);
-      l->field_offsets[upb_fielddef_index(f)] = val_offset;
+      l->data.fields[upb_fielddef_index(f)].offset = val_offset;
     }
   }
 
   /* Size of the entire structure should be a multiple of its greatest
    * alignment. */
-  l->size = align_up(l->size, l->align);
+  l->data.size = align_up(l->data.size, l->data.align);
 
   if (upb_msglayout_initdefault(l)) {
     return l;
@@ -333,8 +332,17 @@ static upb_msglayout *upb_msglayout_new(const upb_msgdef *m) {
   }
 }
 
-upb_msgfactory *upb_msglayout_factory(const upb_msglayout *layout) {
-  return layout->factory;
+upb_msglayout *upb_msglayout_frominit_v1(
+    const struct upb_msglayout_msginit_v1 *init, upb_alloc *a) {
+  UPB_UNUSED(a);
+  /* If upb upgrades to a v2, this would create a heap-allocated v2. */
+  return (upb_msglayout*)init;
+}
+
+void upb_msglayout_uninit_v1(upb_msglayout *layout, upb_alloc *a) {
+  UPB_UNUSED(layout);
+  UPB_UNUSED(a);
+  /* If upb upgrades to a v2, this would free the heap-allocated v2. */
 }
 
 
@@ -393,7 +401,6 @@ const upb_msglayout *upb_msgfactory_getlayout(upb_msgfactory *f,
     upb_msglayout *l = upb_msglayout_new(m);
     upb_inttable_insertptr(&mutable_f->layouts, m, upb_value_ptr(l));
     UPB_ASSERT(l);
-    l->factory = f;
     return l;
   }
 }
@@ -402,8 +409,7 @@ const upb_msglayout *upb_msgfactory_getlayout(upb_msgfactory *f,
 
 void *upb_msg_startstr(void *msg, const void *hd, size_t size_hint) {
   uint32_t ofs = (uintptr_t)hd;
-  /* We pass NULL here because we know we can get away with it. */
-  upb_alloc *alloc = upb_msg_alloc(msg, NULL);
+  upb_alloc *alloc = upb_msg_alloc(msg);
   upb_msgval val;
   UPB_UNUSED(size_hint);
 
@@ -420,8 +426,7 @@ void *upb_msg_startstr(void *msg, const void *hd, size_t size_hint) {
 size_t upb_msg_str(void *msg, const void *hd, const char *ptr, size_t size,
                    const upb_bufhandle *handle) {
   uint32_t ofs = (uintptr_t)hd;
-  /* We pass NULL here because we know we can get away with it. */
-  upb_alloc *alloc = upb_msg_alloc(msg, NULL);
+  upb_alloc *alloc = upb_msg_alloc(msg);
   upb_msgval val;
   size_t newsize;
   UPB_UNUSED(handle);
@@ -628,20 +633,58 @@ bool upb_visitor_visitmsg(upb_visitor *visitor, const upb_msg *msg) {
 /* If we always read/write as a consistent type to each address, this shouldn't
  * violate aliasing.
  */
-#define DEREF(msg, ofs, type) *(type*)CHARPTR_AT(msg, ofs)
+#define DEREF(msg, ofs, type) *(type*)VOIDPTR_AT(msg, ofs)
+
+/* Internal members of a upb_msg.  We can change this without breaking binary
+ * compatibility.  We put these before the user's data.  The user's upb_msg*
+ * points after the upb_msg_internal. */
+
+/* Used when a message is not extendable. */
+typedef struct {
+  /* TODO(haberman): add unknown fields. */
+  upb_alloc *alloc;
+} upb_msg_internal;
+
+/* Used when a message is extendable. */
+typedef struct {
+  upb_inttable *extdict;
+  upb_msg_internal base;
+} upb_msg_internal_withext;
+
+#define INTERNAL_MEMBERS_SIZE(l) \
+    sizeof(upb_msg_internal) - (l->data.extendable * sizeof(void*))
+
+static upb_msg_internal *upb_msg_getinternal(upb_msg *msg) {
+  return VOIDPTR_AT(msg, -sizeof(upb_msg_internal));
+}
+
+static const upb_msg_internal *upb_msg_getinternal_const(const upb_msg *msg) {
+  return VOIDPTR_AT(msg, -sizeof(upb_msg_internal));
+}
+
+static upb_msg_internal_withext *upb_msg_getinternalwithext(
+    upb_msg *msg, const upb_msglayout *l) {
+  UPB_ASSERT(l->data.extendable);
+  return VOIDPTR_AT(msg, -sizeof(upb_msg_internal_withext));
+}
+
+static const upb_msg_internal_withext *upb_msg_getinternalwithext_const(
+    const upb_msg *msg, const upb_msglayout *l) {
+  UPB_ASSERT(l->data.extendable);
+  return VOIDPTR_AT(msg, -sizeof(upb_msg_internal_withext));
+}
 
 static upb_inttable *upb_msg_trygetextdict(const upb_msg *msg,
                                            const upb_msglayout *l) {
-  return l->has_extdict ? DEREF(msg, l->extdict_offset, upb_inttable*) : NULL;
+  return upb_msg_getinternalwithext_const(msg, l)->extdict;
 }
 
 static upb_inttable *upb_msg_getextdict(upb_msg *msg,
                                         const upb_msglayout *l,
                                         upb_alloc *a) {
   upb_inttable *ext_dict;
-  UPB_ASSERT(l->has_extdict);
 
-  ext_dict = upb_msg_trygetextdict(msg, l);
+  ext_dict = upb_msg_getinternalwithext(msg, l)->extdict;
 
   if (!ext_dict) {
     ext_dict = upb_malloc(a, sizeof(upb_inttable));
@@ -656,7 +699,7 @@ static upb_inttable *upb_msg_getextdict(upb_msg *msg,
       return NULL;
     }
 
-    DEREF(msg, l->extdict_offset, upb_inttable*) = ext_dict;
+    upb_msg_getinternalwithext(msg, l)->extdict = ext_dict;
   }
 
   return ext_dict;
@@ -665,7 +708,7 @@ static upb_inttable *upb_msg_getextdict(upb_msg *msg,
 static uint32_t upb_msg_getoneofint(const upb_msg *msg,
                                     const upb_oneofdef *o,
                                     const upb_msglayout *l) {
-  size_t oneof_ofs = l->case_offsets[upb_oneofdef_index(o)];
+  size_t oneof_ofs = l->data.case_offsets[upb_oneofdef_index(o)];
   return DEREF(msg, oneof_ofs, uint8_t);
 }
 
@@ -673,7 +716,7 @@ static void upb_msg_setoneofcase(const upb_msg *msg,
                                  const upb_oneofdef *o,
                                  const upb_msglayout *l,
                                  uint32_t val) {
-  size_t oneof_ofs = l->case_offsets[upb_oneofdef_index(o)];
+  size_t oneof_ofs = l->data.case_offsets[upb_oneofdef_index(o)];
   DEREF(msg, oneof_ofs, uint8_t) = val;
 }
 
@@ -683,46 +726,49 @@ static bool upb_msg_oneofis(const upb_msg *msg, const upb_msglayout *l,
   return upb_msg_getoneofint(msg, o, l) == upb_fielddef_number(f);
 }
 
-size_t upb_msg_sizeof(const upb_msglayout *l) { return l->size; }
+size_t upb_msg_sizeof(const upb_msglayout *l) {
+  return l->data.size + INTERNAL_MEMBERS_SIZE(l);
+}
 
-void upb_msg_init(upb_msg *msg, const upb_msglayout *l, upb_alloc *a) {
-  if (l->default_msg) {
-    memcpy(msg, l->default_msg, l->size);
+upb_msg *upb_msg_init(void *mem, const upb_msglayout *l, upb_alloc *a) {
+  upb_msg *msg = VOIDPTR_AT(mem, INTERNAL_MEMBERS_SIZE(l));
+  if (l->data.default_msg) {
+    memcpy(msg, l->data.default_msg, l->data.size);
   } else {
-    memset(msg, 0, l->size);
+    memset(msg, 0, l->data.size);
+  }
+
+  upb_msg_getinternal(msg)->alloc = a;
+  if (l->data.extendable) {
+    upb_msg_getinternalwithext(msg, l)->extdict = NULL;
   }
 
-  /* Set arena pointer. */
-  memcpy(msg, &a, sizeof(a));
+  return msg;
 }
 
-void upb_msg_uninit(upb_msg *msg, const upb_msglayout *l) {
-  upb_inttable *ext_dict = upb_msg_trygetextdict(msg, l);
-  if (ext_dict) {
-    upb_inttable_uninit2(ext_dict, upb_msg_alloc(msg, l));
+void *upb_msg_uninit(upb_msg *msg, const upb_msglayout *l) {
+  if (l->data.extendable) {
+    upb_inttable *ext_dict = upb_msg_getinternalwithext(msg, l)->extdict;
+    if (ext_dict) {
+      upb_inttable_uninit2(ext_dict, upb_msg_alloc(msg));
+      upb_free(upb_msg_alloc(msg), ext_dict);
+    }
   }
+
+  return VOIDPTR_AT(msg, -INTERNAL_MEMBERS_SIZE(l));
 }
 
 upb_msg *upb_msg_new(const upb_msglayout *l, upb_alloc *a) {
-  upb_msg *msg = upb_malloc(a, upb_msg_sizeof(l));
-
-  if (msg) {
-    upb_msg_init(msg, l, a);
-  }
-
-  return msg;
+  void *mem = upb_malloc(a, upb_msg_sizeof(l));
+  return mem ? upb_msg_init(mem, l, a) : NULL;
 }
 
 void upb_msg_free(upb_msg *msg, const upb_msglayout *l) {
-  upb_msg_uninit(msg, l);
-  upb_free(upb_msg_alloc(msg, l), msg);
+  upb_free(upb_msg_alloc(msg), upb_msg_uninit(msg, l));
 }
 
-upb_alloc *upb_msg_alloc(const upb_msg *msg, const upb_msglayout *l) {
-  upb_alloc *alloc;
-  UPB_UNUSED(l);
-  memcpy(&alloc, msg, sizeof(alloc));
-  return alloc;
+upb_alloc *upb_msg_alloc(const upb_msg *msg) {
+  return upb_msg_getinternal_const(msg)->alloc;
 }
 
 bool upb_msg_has(const upb_msg *msg,
@@ -743,7 +789,7 @@ bool upb_msg_has(const upb_msg *msg,
     return upb_msg_getoneofint(msg, o, l) == upb_fielddef_number(f);
   } else {
     /* Other fields are set when their hasbit is set. */
-    uint32_t hasbit = l->hasbits[upb_fielddef_index(f)];
+    uint32_t hasbit = l->data.fields[upb_fielddef_index(f)].hasbit;
     return DEREF(msg, hasbit / 8, char) | (1 << (hasbit % 8));
   }
 }
@@ -761,7 +807,7 @@ upb_msgval upb_msg_get(const upb_msg *msg, const upb_fielddef *f,
       return upb_msgval_fromdefault(f);
     }
   } else {
-    size_t ofs = l->field_offsets[upb_fielddef_index(f)];
+    size_t ofs = l->data.fields[upb_fielddef_index(f)].offset;
     const upb_oneofdef *o = upb_fielddef_containingoneof(f);
     upb_msgval ret;
 
@@ -780,10 +826,10 @@ bool upb_msg_set(upb_msg *msg,
                  const upb_fielddef *f,
                  upb_msgval val,
                  const upb_msglayout *l) {
-  upb_alloc *a = upb_msg_alloc(msg, l);
   upb_msg_checkfield(l, f);
 
   if (upb_fielddef_isextension(f)) {
+    upb_alloc *a = upb_msg_alloc(msg);
     /* TODO(haberman): introduce table API that can do this in one call. */
     upb_inttable *ext = upb_msg_getextdict(msg, l, a);
     upb_value val2 = upb_toval(val);
@@ -792,7 +838,7 @@ bool upb_msg_set(upb_msg *msg,
       return false;
     }
   } else {
-    size_t ofs = l->field_offsets[upb_fielddef_index(f)];
+    size_t ofs = l->data.fields[upb_fielddef_index(f)].offset;
     const upb_oneofdef *o = upb_fielddef_containingoneof(f);
 
     if (o) {
diff --git a/upb/msg.h b/upb/msg.h
index 36470ae58a..c2b5cf0063 100644
--- a/upb/msg.h
+++ b/upb/msg.h
@@ -65,18 +65,6 @@ typedef void upb_msg;
  * instances of this from a upb_msgfactory, and the factory always owns the
  * msglayout. */
 
-/* Gets the factory for this layout */
-upb_msgfactory *upb_msglayout_factory(const upb_msglayout *l);
-
-/* Get the msglayout for a submessage.  This requires that this field is a
- * submessage, ie. upb_fielddef_issubmsg(upb_msglayout_msgdef(l)) == true.
- *
- * Since map entry messages don't have layouts, if upb_fielddef_ismap(f) == true
- * then this function will return the layout for the map's value.  It requires
- * that the value type of the map field is a submessage. */
-const upb_msglayout *upb_msglayout_sublayout(const upb_msglayout *l,
-                                             const upb_fielddef *f);
-
 /* Returns the msgdef for this msglayout. */
 const upb_msgdef *upb_msglayout_msgdef(const upb_msglayout *l);
 
@@ -212,19 +200,29 @@ size_t upb_msg_sizeof(const upb_msglayout *l);
  * upb_msg_uninit() must be called to release internally-allocated memory
  * unless the allocator is an arena that does not require freeing.
  *
+ * Please note that upb_msg_init() may return a value that is different than
+ * |msg|, so you must assign the return value and not cast your memory block
+ * to upb_msg* directly!
+ *
  * Please note that upb_msg_uninit() does *not* free any submessages, maps,
  * or arrays referred to by this message's fields.  You must free them manually
- * yourself. */
-void upb_msg_init(upb_msg *msg, const upb_msglayout *l, upb_alloc *a);
-void upb_msg_uninit(upb_msg *msg, const upb_msglayout *l);
+ * yourself.
+ *
+ * upb_msg_uninit returns the original memory block, which may be useful if
+ * you dynamically allocated it (though upb_msg_new() would normally be more
+ * appropriate in this case). */
+upb_msg *upb_msg_init(void *msg, const upb_msglayout *l, upb_alloc *a);
+void *upb_msg_uninit(upb_msg *msg, const upb_msglayout *l);
 
 /* Like upb_msg_init() / upb_msg_uninit(), except the message's memory is
  * allocated / freed from the given upb_alloc. */
 upb_msg *upb_msg_new(const upb_msglayout *l, upb_alloc *a);
 void upb_msg_free(upb_msg *msg, const upb_msglayout *l);
 
-/* Returns the upb_alloc for the given message. */
-upb_alloc *upb_msg_alloc(const upb_msg *msg, const upb_msglayout *l);
+/* Returns the upb_alloc for the given message.
+ * TODO(haberman): get rid of this?  Not sure we want to be storing this
+ * for every message. */
+upb_alloc *upb_msg_alloc(const upb_msg *msg);
 
 /* Packs the tree of messages rooted at "msg" into a single hunk of memory,
  * allocated from the given allocator. */
@@ -400,6 +398,43 @@ bool upb_msg_getscalarhandlerdata(const upb_handlers *h,
                                   size_t *offset,
                                   int32_t *hasbit);
 
+
+/** Interfaces for generated code *********************************************/
+
+struct upb_msglayout_strinit_v1 {
+  const char *ptr;
+  uint32_t length;
+};
+
+struct upb_msglayout_fieldinit_v1 {
+  uint32_t number;
+  uint32_t offset;
+  uint16_t hasbit;
+  uint16_t oneof_index;
+  uint16_t submsg_index;
+  uint8_t type;
+  uint8_t label;
+};
+
+struct upb_msglayout_msginit_v1 {
+  struct upb_msglayout_fieldinit_v1 *fields;
+  struct upb_msglayout_msginit_v1 **submsgs;
+  uint32_t *case_offsets;
+  void *default_msg;
+  /* Must be aligned to 8.  Doesn't include internal members like unknown
+   * fields, extension dict, pointer to msglayout, etc. */
+  uint32_t size;
+  bool extendable;
+  char align;
+};
+
+/* Initialize/uninitialize a msglayout from a msginit.  If upb uses v1
+ * internally, this will not allocate any memory.  Should only be used by
+ * generated code. */
+upb_msglayout *upb_msglayout_frominit_v1(
+    const struct upb_msglayout_msginit_v1 *init, upb_alloc *a);
+void upb_msglayout_uninit_v1(upb_msglayout *layout, upb_alloc *a);
+
 UPB_END_EXTERN_C
 
 #endif /* UPB_MSG_H_ */