diff --git a/generated_for_cmake/google/protobuf/descriptor.upb.c b/generated_for_cmake/google/protobuf/descriptor.upb.c
index 58137f58e4..ac95b2ec9e 100644
--- a/generated_for_cmake/google/protobuf/descriptor.upb.c
+++ b/generated_for_cmake/google/protobuf/descriptor.upb.c
@@ -91,7 +91,7 @@ const upb_msglayout google_protobuf_FileDescriptorSet_msginit = {
   },
   &google_protobuf_FileDescriptorSet_submsgs[0],
   &google_protobuf_FileDescriptorSet__fields[0],
-  UPB_SIZE(4, 8), 1, false,
+  UPB_SIZE(8, 8), 1, false,
 };
 
 static const upb_msglayout *const google_protobuf_FileDescriptorProto_submsgs[6] = {
@@ -449,7 +449,7 @@ const upb_msglayout google_protobuf_DescriptorProto_ReservedRange_msginit = {
   },
   NULL,
   &google_protobuf_DescriptorProto_ReservedRange__fields[0],
-  UPB_SIZE(12, 12), 2, false,
+  UPB_SIZE(16, 16), 2, false,
 };
 
 static const upb_msglayout *const google_protobuf_ExtensionRangeOptions_submsgs[1] = {
@@ -531,7 +531,7 @@ const upb_msglayout google_protobuf_ExtensionRangeOptions_msginit = {
   },
   &google_protobuf_ExtensionRangeOptions_submsgs[0],
   &google_protobuf_ExtensionRangeOptions__fields[0],
-  UPB_SIZE(4, 8), 1, false,
+  UPB_SIZE(8, 8), 1, false,
 };
 
 static const upb_msglayout *const google_protobuf_FieldDescriptorProto_submsgs[1] = {
@@ -873,7 +873,7 @@ const upb_msglayout google_protobuf_EnumDescriptorProto_EnumReservedRange_msgini
   },
   NULL,
   &google_protobuf_EnumDescriptorProto_EnumReservedRange__fields[0],
-  UPB_SIZE(12, 12), 2, false,
+  UPB_SIZE(16, 16), 2, false,
 };
 
 static const upb_msglayout *const google_protobuf_EnumValueDescriptorProto_submsgs[1] = {
@@ -1317,7 +1317,7 @@ const upb_msglayout google_protobuf_MessageOptions_msginit = {
   },
   &google_protobuf_MessageOptions_submsgs[0],
   &google_protobuf_MessageOptions__fields[0],
-  UPB_SIZE(12, 16), 5, false,
+  UPB_SIZE(16, 16), 5, false,
 };
 
 static const upb_msglayout *const google_protobuf_FieldOptions_submsgs[1] = {
@@ -1487,7 +1487,7 @@ const upb_msglayout google_protobuf_OneofOptions_msginit = {
   },
   &google_protobuf_OneofOptions_submsgs[0],
   &google_protobuf_OneofOptions__fields[0],
-  UPB_SIZE(4, 8), 1, false,
+  UPB_SIZE(8, 8), 1, false,
 };
 
 static const upb_msglayout *const google_protobuf_EnumOptions_submsgs[1] = {
@@ -2070,7 +2070,7 @@ const upb_msglayout google_protobuf_SourceCodeInfo_msginit = {
   },
   &google_protobuf_SourceCodeInfo_submsgs[0],
   &google_protobuf_SourceCodeInfo__fields[0],
-  UPB_SIZE(4, 8), 1, false,
+  UPB_SIZE(8, 8), 1, false,
 };
 
 static const upb_msglayout_field google_protobuf_SourceCodeInfo_Location__fields[5] = {
@@ -2234,7 +2234,7 @@ const upb_msglayout google_protobuf_GeneratedCodeInfo_msginit = {
   },
   &google_protobuf_GeneratedCodeInfo_submsgs[0],
   &google_protobuf_GeneratedCodeInfo__fields[0],
-  UPB_SIZE(4, 8), 1, false,
+  UPB_SIZE(8, 8), 1, false,
 };
 
 static const upb_msglayout_field google_protobuf_GeneratedCodeInfo_Annotation__fields[4] = {
diff --git a/upb/decode.c b/upb/decode.c
index 8b87ed2a01..25b07e0a1a 100644
--- a/upb/decode.c
+++ b/upb/decode.c
@@ -185,12 +185,93 @@ void decode_verifyutf8(upb_decstate *d, const char *buf, int len) {
   if (i != len) decode_err(d);
 }
 
-static bool decode_reserve(upb_decstate *d, upb_array *arr, size_t elem) {
-  bool need_realloc = arr->size - arr->len < elem;
-  if (need_realloc && !_upb_array_realloc(arr, arr->len + elem, d->arena)) {
+static void decode_stealmem(upb_decstate *d) {
+  _upb_arena_head *a = (_upb_arena_head*)d->arena;
+  d->arena_ptr = a->ptr;
+  d->arena_end = a->end;
+  a->ptr = a->end;
+}
+
+static void decode_donatemem(upb_decstate *d) {
+  _upb_arena_head *a = (_upb_arena_head*)d->arena;
+  UPB_ASSERT(a->end == d->arena_end);
+  a->ptr = d->arena_ptr;
+}
+
+UPB_NOINLINE
+static void *decode_mallocfallback(upb_decstate *d, size_t size) {
+  char *ptr = _upb_arena_slowmalloc(d->arena, size);
+  if (!ptr) decode_err(d);
+  decode_stealmem(d);
+  return ptr;
+}
+
+UPB_FORCEINLINE
+static void *decode_malloc(upb_decstate *d, size_t size) {
+  UPB_ASSERT((size & 7) == 0);
+  char *ptr = d->arena_ptr;
+  if (UPB_UNLIKELY((size_t)(d->arena_end - d->arena_ptr) < size)) {
+    return decode_mallocfallback(d, size);
+  }
+  d->arena_ptr += size;
+  return ptr;
+}
+
+static upb_msg *decode_newmsg(upb_decstate *d, const upb_msglayout *l) {
+  size_t size = l->size + sizeof(upb_msg_internal);
+  char *msg_data = decode_malloc(d, size);
+  memset(msg_data, 0, size);
+  return msg_data + sizeof(upb_msg_internal);
+}
+
+UPB_NOINLINE
+static void decode_realloc(upb_decstate *d, upb_array *arr, size_t need_elem) {
+  decode_donatemem(d);
+  bool ok = _upb_array_realloc(arr, arr->len + need_elem, d->arena);
+  decode_stealmem(d);
+  if (!ok) decode_err(d);
+}
+
+UPB_FORCEINLINE
+static bool decode_reserve(upb_decstate *d, upb_array *arr, size_t need_elem) {
+  if (arr->size - arr->len < need_elem) {
+    decode_realloc(d, arr, need_elem);
+    return true;
+  }
+  return false;
+}
+
+static upb_array *decode_newarr(upb_decstate *d, upb_fieldtype_t type) {
+  size_t elem_size_lg2 = _upb_fieldtype_to_sizelg2[type];
+  size_t count = type == UPB_TYPE_BOOL ? 8 : 4;
+  size_t size = sizeof(upb_array) + (count * (1 << elem_size_lg2));
+  upb_array *arr = decode_malloc(d, size);
+
+  if (!arr) {
     decode_err(d);
   }
-  return need_realloc;
+
+  arr->data = _upb_array_tagptr(arr + 1, elem_size_lg2);
+  arr->len = 0;
+  arr->size = count;
+
+  return arr;
+}
+
+static void decode_addunknown(upb_decstate *d, upb_msg *msg, const char *ptr,
+                              size_t len) {
+  upb_msg_internal *in = upb_msg_getinternal(msg);
+  if (!in->unknown || in->unknown->size - in->unknown->len < len) {
+    bool ok;
+    decode_donatemem(d);
+    ok = _upb_msg_addunknown(msg, ptr, len, d->arena);
+    decode_stealmem(d);
+    if (!ok) decode_err(d);
+  } else {
+    char *dst = UPB_PTR_AT(in->unknown + 1, in->unknown->len, char);
+    memcpy(dst, ptr, len);
+    in->unknown->len += len;
+  }
 }
 
 UPB_NOINLINE
@@ -276,8 +357,7 @@ static const upb_msglayout_field *upb_find_field(const upb_msglayout *l,
 
 static upb_msg *decode_newsubmsg(upb_decstate *d, const upb_msglayout *layout,
                                  const upb_msglayout_field *field) {
-  const upb_msglayout *subl = layout->submsgs[field->submsg_index];
-  return _upb_msg_new(subl, d->arena);
+  return decode_newmsg(d, layout->submsgs[field->submsg_index]);
 }
 
 static void decode_tosubmsg(upb_decstate *d, upb_msg *submsg,
@@ -324,8 +404,7 @@ static const char *decode_toarray(upb_decstate *d, const char *ptr,
 
   if (!arr) {
     upb_fieldtype_t type = desctype_to_fieldtype[field->descriptortype];
-    arr = _upb_array_new(d->arena, type);
-    if (!arr) decode_err(d);
+    arr = decode_newarr(d, type);
     *arrp = arr;
   }
 
@@ -423,7 +502,9 @@ static void decode_tomap(upb_decstate *d, upb_msg *msg,
     char val_size = desctype_to_mapsize[val_field->descriptortype];
     UPB_ASSERT(key_field->offset == 0);
     UPB_ASSERT(val_field->offset == sizeof(upb_strview));
+    decode_donatemem(d);  /* We'll let map use the actual arena. */
     map = _upb_map_new(d->arena, key_size, val_size);
+    decode_stealmem(d);
     *map_p = map;
   }
 
@@ -433,13 +514,15 @@ static void decode_tomap(upb_decstate *d, upb_msg *msg,
   if (entry->fields[1].descriptortype == UPB_DESCRIPTOR_TYPE_MESSAGE ||
       entry->fields[1].descriptortype == UPB_DESCRIPTOR_TYPE_GROUP) {
     /* Create proactively to handle the case where it doesn't appear. */
-    ent.v.val = upb_value_ptr(_upb_msg_new(entry->submsgs[0], d->arena));
+    ent.v.val = upb_value_ptr(decode_newmsg(d, entry->submsgs[0]));
   }
 
   decode_tosubmsg(d, &ent.k, layout, field, val.str_val);
 
   /* Insert into map. */
+  decode_donatemem(d);  /* We'll let map use the actual arena. */
   _upb_map_set(map, &ent.k, map->key_size, &ent.v, map->val_size, d->arena);
+  decode_stealmem(d);
 }
 
 UPB_FORCEINLINE
@@ -587,10 +670,7 @@ static const char *decode_field(upb_decstate *d, const char *ptr, upb_msg *msg,
       ptr = decode_group(d, ptr, NULL, NULL, field_number);
     }
     if (msg) {
-      if (!_upb_msg_addunknown(msg, field_start, ptr - field_start,
-                               d->arena)) {
-        decode_err(d);
-      }
+      decode_addunknown(d, msg, field_start, ptr - field_start);
     }
   }
 
@@ -618,7 +698,9 @@ static const char *decode_msg(upb_decstate *d, const char *ptr, upb_msg *msg,
 
 bool upb_decode(const char *buf, size_t size, void *msg, const upb_msglayout *l,
                 upb_arena *arena) {
+  bool ret;
   upb_decstate state;
+
   state.limit = buf + size;
   state.fastend = buf + size - 16;
   state.fastlimit = state.fastend;
@@ -626,12 +708,19 @@ bool upb_decode(const char *buf, size_t size, void *msg, const upb_msglayout *l,
   state.depth = 64;
   state.end_group = 0;
 
-  if (setjmp(state.err)) return false;
-
   if (size == 0) return true;
-  decode_msg(&state, buf, msg, l);
 
-  return state.end_group == 0;
+  decode_stealmem(&state);
+
+  if (setjmp(state.err)) {
+    ret = false;
+  } else {
+    decode_msg(&state, buf, msg, l);
+    ret = state.end_group == 0;
+  }
+
+  decode_donatemem(&state);
+  return ret;
 }
 
 #undef OP_SCALAR_LG2
diff --git a/upb/decode.h b/upb/decode.h
index 9a8473e026..432604c71c 100644
--- a/upb/decode.h
+++ b/upb/decode.h
@@ -21,6 +21,8 @@ typedef struct upb_decstate {
   const char *limit;       /* End of delimited region or end of buffer. */
   const char *fastlimit;   /* End of delimited region or end of buffer. */
   const char *fastend;
+  char *arena_ptr;
+  char *arena_end;
   upb_array *arr;
   upb_arena *arena;
   int depth;
diff --git a/upb/msg.c b/upb/msg.c
index 25747c8481..b9310c9f7b 100644
--- a/upb/msg.c
+++ b/upb/msg.c
@@ -7,7 +7,7 @@
 
 /** upb_msg *******************************************************************/
 
-static const char _upb_fieldtype_to_sizelg2[12] = {
+const char _upb_fieldtype_to_sizelg2[12] = {
   0,
   0,  /* UPB_TYPE_BOOL */
   2,  /* UPB_TYPE_FLOAT */
@@ -22,17 +22,10 @@ static const char _upb_fieldtype_to_sizelg2[12] = {
   UPB_SIZE(3, 4),  /* UPB_TYPE_BYTES */
 };
 
-static uintptr_t tag_arrptr(void* ptr, int elem_size_lg2) {
-  UPB_ASSERT(elem_size_lg2 <= 4);
-  return (uintptr_t)ptr | elem_size_lg2;
-}
-
-static int upb_msg_internalsize(const upb_msglayout *l) {
-  return sizeof(upb_msg_internal) - l->extendable * sizeof(void *);
-}
+static const size_t overhead = sizeof(upb_msg_internal);
 
 static size_t upb_msg_sizeof(const upb_msglayout *l) {
-  return l->size + upb_msg_internalsize(l);
+  return l->size + overhead;
 }
 
 static const upb_msg_internal *upb_msg_getinternal_const(const upb_msg *msg) {
@@ -40,14 +33,9 @@ static const upb_msg_internal *upb_msg_getinternal_const(const upb_msg *msg) {
   return UPB_PTR_AT(msg, -size, upb_msg_internal);
 }
 
-static upb_msg_internal *upb_msg_getinternal(upb_msg *msg) {
-  return (upb_msg_internal*)upb_msg_getinternal_const(msg);
-}
-
 void _upb_msg_clear(upb_msg *msg, const upb_msglayout *l) {
-  ptrdiff_t internal = upb_msg_internalsize(l);
-  void *mem = UPB_PTR_AT(msg, -internal, char);
-  memset(mem, 0, l->size + internal);
+  void *mem = UPB_PTR_AT(msg, -overhead, char);
+  memset(mem, 0, l->size + overhead);
 }
 
 upb_msg *_upb_msg_new(const upb_msglayout *l, upb_arena *a) {
@@ -58,37 +46,51 @@ upb_msg *_upb_msg_new(const upb_msglayout *l, upb_arena *a) {
     return NULL;
   }
 
-  msg = UPB_PTR_AT(mem, upb_msg_internalsize(l), upb_msg);
+  msg = UPB_PTR_AT(mem, overhead, upb_msg);
   _upb_msg_clear(msg, l);
   return msg;
 }
 
 bool _upb_msg_addunknown(upb_msg *msg, const char *data, size_t len,
                          upb_arena *arena) {
+
   upb_msg_internal *in = upb_msg_getinternal(msg);
-  if (len > in->unknown_size - in->unknown_len) {
-    upb_alloc *alloc = upb_arena_alloc(arena);
-    size_t need = in->unknown_size + len;
-    size_t newsize = UPB_MAX(in->unknown_size * 2, need);
-    void *mem = upb_realloc(alloc, in->unknown, in->unknown_size, newsize);
-    if (!mem) return false;
-    in->unknown = mem;
-    in->unknown_size = newsize;
+  if (!in->unknown) {
+    size_t size = 128;
+    while (size < len) size *= 2;
+    in->unknown = upb_arena_malloc(arena, size + overhead);
+    if (!in->unknown) return false;
+    in->unknown->size = size;
+    in->unknown->len = 0;
+  } else if (in->unknown->size - in->unknown->len < len) {
+    size_t need = in->unknown->len + len;
+    size_t size = in->unknown->size;;
+    while (size < need)  size *= 2;
+    in->unknown = upb_arena_realloc(
+        arena, in->unknown, in->unknown->size + overhead, size + overhead);
+    if (!in->unknown) return false;
   }
-  memcpy(in->unknown + in->unknown_len, data, len);
-  in->unknown_len += len;
+  memcpy(UPB_PTR_AT(in->unknown + 1, in->unknown->len, char), data, len);
+  in->unknown->len += len;
   return true;
 }
 
 void _upb_msg_discardunknown_shallow(upb_msg *msg) {
   upb_msg_internal *in = upb_msg_getinternal(msg);
-  in->unknown_len = 0;
+  if (in->unknown) {
+    in->unknown->len = 0;
+  }
 }
 
 const char *upb_msg_getunknown(const upb_msg *msg, size_t *len) {
   const upb_msg_internal *in = upb_msg_getinternal_const(msg);
-  *len = in->unknown_len;
-  return in->unknown;
+  if (in->unknown) {
+    *len = in->unknown->len;
+    return (char*)(in->unknown + 1);
+  } else {
+    *len = 0;
+    return NULL;
+  }
 }
 
 /** upb_array *****************************************************************/
@@ -100,7 +102,7 @@ upb_array *_upb_array_new(upb_arena *a, upb_fieldtype_t type) {
     return NULL;
   }
 
-  arr->data = tag_arrptr(NULL, _upb_fieldtype_to_sizelg2[type]);
+  arr->data = _upb_array_tagptr(NULL, _upb_fieldtype_to_sizelg2[type]);
   arr->len = 0;
   arr->size = 0;
 
@@ -124,7 +126,7 @@ bool _upb_array_realloc(upb_array *arr, size_t min_size, upb_arena *arena) {
     return false;
   }
 
-  arr->data = tag_arrptr(ptr, elem_size_lg2);
+  arr->data = _upb_array_tagptr(ptr, elem_size_lg2);
   arr->size = new_size;
   return true;
 }
diff --git a/upb/msg.h b/upb/msg.h
index 7a1ec760ed..b132afe604 100644
--- a/upb/msg.h
+++ b/upb/msg.h
@@ -72,25 +72,28 @@ typedef struct upb_msglayout {
  * 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 {
-  char *unknown;
-  size_t unknown_len;
-  size_t unknown_size;
-} upb_msg_internal;
+  uint32_t len;
+  uint32_t size;
+  /* Data follows. */
+} upb_msg_unknowndata;
 
-/* Used when a message is extendable. */
+/* Used when a message is not extendable. */
 typedef struct {
-  upb_inttable *extdict;
-  upb_msg_internal base;
-} upb_msg_internal_withext;
+  upb_msg_unknowndata *unknown;
+} upb_msg_internal;
 
 /* Maps upb_fieldtype_t -> memory size. */
 extern char _upb_fieldtype_to_size[12];
+extern const char _upb_fieldtype_to_sizelg2[12];
 
 /* Creates a new messages with the given layout on the given arena. */
 upb_msg *_upb_msg_new(const upb_msglayout *l, upb_arena *a);
 
+UPB_INLINE upb_msg_internal *upb_msg_getinternal(upb_msg *msg) {
+  return UPB_PTR_AT(msg, -sizeof(upb_msg_internal), upb_msg_internal);
+}
+
 /* Clears the given message. */
 void _upb_msg_clear(upb_msg *msg, const upb_msglayout *l);
 
@@ -189,6 +192,11 @@ UPB_INLINE const void *_upb_array_constptr(const upb_array *arr) {
   return (void*)(arr->data & ~(uintptr_t)7);
 }
 
+UPB_INLINE uintptr_t _upb_array_tagptr(void* ptr, int elem_size_lg2) {
+  UPB_ASSERT(elem_size_lg2 <= 4);
+  return (uintptr_t)ptr | elem_size_lg2;
+}
+
 UPB_INLINE void *_upb_array_ptr(upb_array *arr) {
   return (void*)_upb_array_constptr(arr);
 }
diff --git a/upbc/message_layout.cc b/upbc/message_layout.cc
index cb7f7f9c43..14aa3e1a3e 100644
--- a/upbc/message_layout.cc
+++ b/upbc/message_layout.cc
@@ -105,7 +105,7 @@ int64_t MessageLayout::FieldLayoutRank(const protobuf::FieldDescriptor* field) {
 
 void MessageLayout::ComputeLayout(const protobuf::Descriptor* descriptor) {
   size_ = Size{0, 0};
-  maxalign_ = Size{0, 0};
+  maxalign_ = Size{8, 8};
 
   if (descriptor->options().map_entry()) {
     // Map entries aren't actually stored, they are only used during parsing.