diff --git a/cmake/google/protobuf/descriptor.upb.c b/cmake/google/protobuf/descriptor.upb.c
index 98d5444bd2..932fbb3559 100644
--- a/cmake/google/protobuf/descriptor.upb.c
+++ b/cmake/google/protobuf/descriptor.upb.c
@@ -23,7 +23,7 @@ static const upb_msglayout_field google_protobuf_FileDescriptorSet__fields[1] =
 const upb_msglayout google_protobuf_FileDescriptorSet_msginit = {
   &google_protobuf_FileDescriptorSet_submsgs[0],
   &google_protobuf_FileDescriptorSet__fields[0],
-  UPB_SIZE(8, 8), 1, false, 255,
+  UPB_SIZE(8, 8), 1, false, 1, 255,
 };
 
 static const upb_msglayout *const google_protobuf_FileDescriptorProto_submsgs[6] = {
@@ -53,7 +53,7 @@ static const upb_msglayout_field google_protobuf_FileDescriptorProto__fields[12]
 const upb_msglayout google_protobuf_FileDescriptorProto_msginit = {
   &google_protobuf_FileDescriptorProto_submsgs[0],
   &google_protobuf_FileDescriptorProto__fields[0],
-  UPB_SIZE(64, 128), 12, false, 255,
+  UPB_SIZE(64, 128), 12, false, 12, 255,
 };
 
 static const upb_msglayout *const google_protobuf_DescriptorProto_submsgs[7] = {
@@ -82,7 +82,7 @@ static const upb_msglayout_field google_protobuf_DescriptorProto__fields[10] = {
 const upb_msglayout google_protobuf_DescriptorProto_msginit = {
   &google_protobuf_DescriptorProto_submsgs[0],
   &google_protobuf_DescriptorProto__fields[0],
-  UPB_SIZE(48, 96), 10, false, 255,
+  UPB_SIZE(48, 96), 10, false, 10, 255,
 };
 
 static const upb_msglayout *const google_protobuf_DescriptorProto_ExtensionRange_submsgs[1] = {
@@ -98,7 +98,7 @@ static const upb_msglayout_field google_protobuf_DescriptorProto_ExtensionRange_
 const upb_msglayout google_protobuf_DescriptorProto_ExtensionRange_msginit = {
   &google_protobuf_DescriptorProto_ExtensionRange_submsgs[0],
   &google_protobuf_DescriptorProto_ExtensionRange__fields[0],
-  UPB_SIZE(16, 24), 3, false, 255,
+  UPB_SIZE(16, 24), 3, false, 3, 255,
 };
 
 static const upb_msglayout_field google_protobuf_DescriptorProto_ReservedRange__fields[2] = {
@@ -109,7 +109,7 @@ static const upb_msglayout_field google_protobuf_DescriptorProto_ReservedRange__
 const upb_msglayout google_protobuf_DescriptorProto_ReservedRange_msginit = {
   NULL,
   &google_protobuf_DescriptorProto_ReservedRange__fields[0],
-  UPB_SIZE(16, 16), 2, false, 255,
+  UPB_SIZE(16, 16), 2, false, 2, 255,
 };
 
 static const upb_msglayout *const google_protobuf_ExtensionRangeOptions_submsgs[1] = {
@@ -123,7 +123,7 @@ static const upb_msglayout_field google_protobuf_ExtensionRangeOptions__fields[1
 const upb_msglayout google_protobuf_ExtensionRangeOptions_msginit = {
   &google_protobuf_ExtensionRangeOptions_submsgs[0],
   &google_protobuf_ExtensionRangeOptions__fields[0],
-  UPB_SIZE(8, 8), 1, false, 255,
+  UPB_SIZE(8, 8), 1, false, 0, 255,
 };
 
 static const upb_msglayout *const google_protobuf_FieldDescriptorProto_submsgs[1] = {
@@ -147,7 +147,7 @@ static const upb_msglayout_field google_protobuf_FieldDescriptorProto__fields[11
 const upb_msglayout google_protobuf_FieldDescriptorProto_msginit = {
   &google_protobuf_FieldDescriptorProto_submsgs[0],
   &google_protobuf_FieldDescriptorProto__fields[0],
-  UPB_SIZE(72, 112), 11, false, 255,
+  UPB_SIZE(72, 112), 11, false, 10, 255,
 };
 
 static const upb_msglayout *const google_protobuf_OneofDescriptorProto_submsgs[1] = {
@@ -162,7 +162,7 @@ static const upb_msglayout_field google_protobuf_OneofDescriptorProto__fields[2]
 const upb_msglayout google_protobuf_OneofDescriptorProto_msginit = {
   &google_protobuf_OneofDescriptorProto_submsgs[0],
   &google_protobuf_OneofDescriptorProto__fields[0],
-  UPB_SIZE(16, 32), 2, false, 255,
+  UPB_SIZE(16, 32), 2, false, 2, 255,
 };
 
 static const upb_msglayout *const google_protobuf_EnumDescriptorProto_submsgs[3] = {
@@ -182,7 +182,7 @@ static const upb_msglayout_field google_protobuf_EnumDescriptorProto__fields[5]
 const upb_msglayout google_protobuf_EnumDescriptorProto_msginit = {
   &google_protobuf_EnumDescriptorProto_submsgs[0],
   &google_protobuf_EnumDescriptorProto__fields[0],
-  UPB_SIZE(32, 64), 5, false, 255,
+  UPB_SIZE(32, 64), 5, false, 5, 255,
 };
 
 static const upb_msglayout_field google_protobuf_EnumDescriptorProto_EnumReservedRange__fields[2] = {
@@ -193,7 +193,7 @@ static const upb_msglayout_field google_protobuf_EnumDescriptorProto_EnumReserve
 const upb_msglayout google_protobuf_EnumDescriptorProto_EnumReservedRange_msginit = {
   NULL,
   &google_protobuf_EnumDescriptorProto_EnumReservedRange__fields[0],
-  UPB_SIZE(16, 16), 2, false, 255,
+  UPB_SIZE(16, 16), 2, false, 2, 255,
 };
 
 static const upb_msglayout *const google_protobuf_EnumValueDescriptorProto_submsgs[1] = {
@@ -209,7 +209,7 @@ static const upb_msglayout_field google_protobuf_EnumValueDescriptorProto__field
 const upb_msglayout google_protobuf_EnumValueDescriptorProto_msginit = {
   &google_protobuf_EnumValueDescriptorProto_submsgs[0],
   &google_protobuf_EnumValueDescriptorProto__fields[0],
-  UPB_SIZE(24, 32), 3, false, 255,
+  UPB_SIZE(24, 32), 3, false, 3, 255,
 };
 
 static const upb_msglayout *const google_protobuf_ServiceDescriptorProto_submsgs[2] = {
@@ -226,7 +226,7 @@ static const upb_msglayout_field google_protobuf_ServiceDescriptorProto__fields[
 const upb_msglayout google_protobuf_ServiceDescriptorProto_msginit = {
   &google_protobuf_ServiceDescriptorProto_submsgs[0],
   &google_protobuf_ServiceDescriptorProto__fields[0],
-  UPB_SIZE(24, 48), 3, false, 255,
+  UPB_SIZE(24, 48), 3, false, 3, 255,
 };
 
 static const upb_msglayout *const google_protobuf_MethodDescriptorProto_submsgs[1] = {
@@ -245,7 +245,7 @@ static const upb_msglayout_field google_protobuf_MethodDescriptorProto__fields[6
 const upb_msglayout google_protobuf_MethodDescriptorProto_msginit = {
   &google_protobuf_MethodDescriptorProto_submsgs[0],
   &google_protobuf_MethodDescriptorProto__fields[0],
-  UPB_SIZE(32, 64), 6, false, 255,
+  UPB_SIZE(32, 64), 6, false, 6, 255,
 };
 
 static const upb_msglayout *const google_protobuf_FileOptions_submsgs[1] = {
@@ -279,7 +279,7 @@ static const upb_msglayout_field google_protobuf_FileOptions__fields[21] = {
 const upb_msglayout google_protobuf_FileOptions_msginit = {
   &google_protobuf_FileOptions_submsgs[0],
   &google_protobuf_FileOptions__fields[0],
-  UPB_SIZE(104, 192), 21, false, 255,
+  UPB_SIZE(104, 192), 21, false, 1, 255,
 };
 
 static const upb_msglayout *const google_protobuf_MessageOptions_submsgs[1] = {
@@ -297,7 +297,7 @@ static const upb_msglayout_field google_protobuf_MessageOptions__fields[5] = {
 const upb_msglayout google_protobuf_MessageOptions_msginit = {
   &google_protobuf_MessageOptions_submsgs[0],
   &google_protobuf_MessageOptions__fields[0],
-  UPB_SIZE(16, 16), 5, false, 255,
+  UPB_SIZE(16, 16), 5, false, 3, 255,
 };
 
 static const upb_msglayout *const google_protobuf_FieldOptions_submsgs[1] = {
@@ -317,7 +317,7 @@ static const upb_msglayout_field google_protobuf_FieldOptions__fields[7] = {
 const upb_msglayout google_protobuf_FieldOptions_msginit = {
   &google_protobuf_FieldOptions_submsgs[0],
   &google_protobuf_FieldOptions__fields[0],
-  UPB_SIZE(24, 24), 7, false, 255,
+  UPB_SIZE(24, 24), 7, false, 3, 255,
 };
 
 static const upb_msglayout *const google_protobuf_OneofOptions_submsgs[1] = {
@@ -331,7 +331,7 @@ static const upb_msglayout_field google_protobuf_OneofOptions__fields[1] = {
 const upb_msglayout google_protobuf_OneofOptions_msginit = {
   &google_protobuf_OneofOptions_submsgs[0],
   &google_protobuf_OneofOptions__fields[0],
-  UPB_SIZE(8, 8), 1, false, 255,
+  UPB_SIZE(8, 8), 1, false, 0, 255,
 };
 
 static const upb_msglayout *const google_protobuf_EnumOptions_submsgs[1] = {
@@ -347,7 +347,7 @@ static const upb_msglayout_field google_protobuf_EnumOptions__fields[3] = {
 const upb_msglayout google_protobuf_EnumOptions_msginit = {
   &google_protobuf_EnumOptions_submsgs[0],
   &google_protobuf_EnumOptions__fields[0],
-  UPB_SIZE(8, 16), 3, false, 255,
+  UPB_SIZE(8, 16), 3, false, 0, 255,
 };
 
 static const upb_msglayout *const google_protobuf_EnumValueOptions_submsgs[1] = {
@@ -362,7 +362,7 @@ static const upb_msglayout_field google_protobuf_EnumValueOptions__fields[2] = {
 const upb_msglayout google_protobuf_EnumValueOptions_msginit = {
   &google_protobuf_EnumValueOptions_submsgs[0],
   &google_protobuf_EnumValueOptions__fields[0],
-  UPB_SIZE(8, 16), 2, false, 255,
+  UPB_SIZE(8, 16), 2, false, 1, 255,
 };
 
 static const upb_msglayout *const google_protobuf_ServiceOptions_submsgs[1] = {
@@ -377,7 +377,7 @@ static const upb_msglayout_field google_protobuf_ServiceOptions__fields[2] = {
 const upb_msglayout google_protobuf_ServiceOptions_msginit = {
   &google_protobuf_ServiceOptions_submsgs[0],
   &google_protobuf_ServiceOptions__fields[0],
-  UPB_SIZE(8, 16), 2, false, 255,
+  UPB_SIZE(8, 16), 2, false, 0, 255,
 };
 
 static const upb_msglayout *const google_protobuf_MethodOptions_submsgs[1] = {
@@ -393,7 +393,7 @@ static const upb_msglayout_field google_protobuf_MethodOptions__fields[3] = {
 const upb_msglayout google_protobuf_MethodOptions_msginit = {
   &google_protobuf_MethodOptions_submsgs[0],
   &google_protobuf_MethodOptions__fields[0],
-  UPB_SIZE(16, 24), 3, false, 255,
+  UPB_SIZE(16, 24), 3, false, 0, 255,
 };
 
 static const upb_msglayout *const google_protobuf_UninterpretedOption_submsgs[1] = {
@@ -413,7 +413,7 @@ static const upb_msglayout_field google_protobuf_UninterpretedOption__fields[7]
 const upb_msglayout google_protobuf_UninterpretedOption_msginit = {
   &google_protobuf_UninterpretedOption_submsgs[0],
   &google_protobuf_UninterpretedOption__fields[0],
-  UPB_SIZE(64, 96), 7, false, 255,
+  UPB_SIZE(64, 96), 7, false, 0, 255,
 };
 
 static const upb_msglayout_field google_protobuf_UninterpretedOption_NamePart__fields[2] = {
@@ -424,7 +424,7 @@ static const upb_msglayout_field google_protobuf_UninterpretedOption_NamePart__f
 const upb_msglayout google_protobuf_UninterpretedOption_NamePart_msginit = {
   NULL,
   &google_protobuf_UninterpretedOption_NamePart__fields[0],
-  UPB_SIZE(16, 32), 2, false, 255,
+  UPB_SIZE(16, 32), 2, false, 2, 255,
 };
 
 static const upb_msglayout *const google_protobuf_SourceCodeInfo_submsgs[1] = {
@@ -438,7 +438,7 @@ static const upb_msglayout_field google_protobuf_SourceCodeInfo__fields[1] = {
 const upb_msglayout google_protobuf_SourceCodeInfo_msginit = {
   &google_protobuf_SourceCodeInfo_submsgs[0],
   &google_protobuf_SourceCodeInfo__fields[0],
-  UPB_SIZE(8, 8), 1, false, 255,
+  UPB_SIZE(8, 8), 1, false, 1, 255,
 };
 
 static const upb_msglayout_field google_protobuf_SourceCodeInfo_Location__fields[5] = {
@@ -452,7 +452,7 @@ static const upb_msglayout_field google_protobuf_SourceCodeInfo_Location__fields
 const upb_msglayout google_protobuf_SourceCodeInfo_Location_msginit = {
   NULL,
   &google_protobuf_SourceCodeInfo_Location__fields[0],
-  UPB_SIZE(32, 64), 5, false, 255,
+  UPB_SIZE(32, 64), 5, false, 4, 255,
 };
 
 static const upb_msglayout *const google_protobuf_GeneratedCodeInfo_submsgs[1] = {
@@ -466,7 +466,7 @@ static const upb_msglayout_field google_protobuf_GeneratedCodeInfo__fields[1] =
 const upb_msglayout google_protobuf_GeneratedCodeInfo_msginit = {
   &google_protobuf_GeneratedCodeInfo_submsgs[0],
   &google_protobuf_GeneratedCodeInfo__fields[0],
-  UPB_SIZE(8, 8), 1, false, 255,
+  UPB_SIZE(8, 8), 1, false, 1, 255,
 };
 
 static const upb_msglayout_field google_protobuf_GeneratedCodeInfo_Annotation__fields[4] = {
@@ -479,7 +479,7 @@ static const upb_msglayout_field google_protobuf_GeneratedCodeInfo_Annotation__f
 const upb_msglayout google_protobuf_GeneratedCodeInfo_Annotation_msginit = {
   NULL,
   &google_protobuf_GeneratedCodeInfo_Annotation__fields[0],
-  UPB_SIZE(24, 48), 4, false, 255,
+  UPB_SIZE(24, 48), 4, false, 4, 255,
 };
 
 #include "upb/port_undef.inc"
diff --git a/upb/decode.c b/upb/decode.c
index ecbfc9b1c2..2eeeda7379 100644
--- a/upb/decode.c
+++ b/upb/decode.c
@@ -299,24 +299,42 @@ static void decode_munge(int type, wireval *val) {
 }
 
 static const upb_msglayout_field *upb_find_field(const upb_msglayout *l,
-                                                 uint32_t field_number) {
+                                                 uint32_t field_number,
+                                                 int *last_field_index) {
   static upb_msglayout_field none = {0, 0, 0, 0, 0, 0};
 
-  /* Lots of optimization opportunities here. */
-  int i;
   if (l == NULL) return &none;
-  for (i = 0; i < l->field_count; i++) {
-    if (l->fields[i].number == field_number) {
-      return &l->fields[i];
+
+  size_t idx = (size_t)field_number - 1;  // 0 wraps to SIZE_MAX
+  if (idx < l->dense_below) {
+    goto found;
+  }
+
+  int last = *last_field_index;
+  for (idx = last; idx < l->field_count; idx++) {
+    if (l->fields[idx].number == field_number) {
+      goto found;
+    }
+  }
+
+  for (idx = 0; idx < last; idx++) {
+    if (l->fields[idx].number == field_number) {
+      goto found;
     }
   }
 
   return &none; /* Unknown field. */
+
+ found:
+  UPB_ASSERT(l->fields[idx].number == field_number);
+  *last_field_index = idx;
+  return &l->fields[idx];
 }
 
-static upb_msg *decode_newsubmsg(upb_decstate *d, const upb_msglayout *layout,
+static upb_msg *decode_newsubmsg(upb_decstate *d,
+                                 const upb_msglayout *const *submsgs,
                                  const upb_msglayout_field *field) {
-  const upb_msglayout *subl = layout->submsgs[field->submsg_index];
+  const upb_msglayout *subl = submsgs[field->submsg_index];
   return _upb_msg_new_inl(subl, &d->arena);
 }
 
@@ -346,9 +364,10 @@ static const char *decode_readstr(upb_decstate *d, const char *ptr, int size,
 
 UPB_FORCEINLINE
 static const char *decode_tosubmsg(upb_decstate *d, const char *ptr,
-                                   upb_msg *submsg, const upb_msglayout *layout,
+                                   upb_msg *submsg, 
+                                   const upb_msglayout *const *submsgs,
                                    const upb_msglayout_field *field, int size) {
-  const upb_msglayout *subl = layout->submsgs[field->submsg_index];
+  const upb_msglayout *subl = submsgs[field->submsg_index];
   int saved_delta = decode_pushlimit(d, ptr, size);
   if (--d->depth < 0) decode_err(d);
   if (!decode_isdone(d, &ptr)) {
@@ -377,15 +396,17 @@ static const char *decode_group(upb_decstate *d, const char *ptr,
 
 UPB_FORCEINLINE
 static const char *decode_togroup(upb_decstate *d, const char *ptr,
-                                  upb_msg *submsg, const upb_msglayout *layout,
+                                  upb_msg *submsg,
+                                  const upb_msglayout *const *submsgs,
                                   const upb_msglayout_field *field) {
-  const upb_msglayout *subl = layout->submsgs[field->submsg_index];
+  const upb_msglayout *subl = submsgs[field->submsg_index];
   return decode_group(d, ptr, submsg, subl, field->number);
 }
 
 static const char *decode_toarray(upb_decstate *d, const char *ptr,
-                                  upb_msg *msg, const upb_msglayout *layout,
-                                  const upb_msglayout_field *field, wireval val,
+                                  upb_msg *msg,
+                                  const upb_msglayout *const *submsgs,
+                                  const upb_msglayout_field *field, wireval *val,
                                   int op) {
   upb_array **arrp = UPB_PTR_AT(msg, field->offset, void);
   upb_array *arr = *arrp;
@@ -407,27 +428,27 @@ static const char *decode_toarray(upb_decstate *d, const char *ptr,
       /* Append scalar value. */
       mem = UPB_PTR_AT(_upb_array_ptr(arr), arr->len << op, void);
       arr->len++;
-      memcpy(mem, &val, 1 << op);
+      memcpy(mem, val, 1 << op);
       return ptr;
     case OP_STRING:
-      decode_verifyutf8(d, ptr, val.size);
+      decode_verifyutf8(d, ptr, val->size);
       /* Fallthrough. */
     case OP_BYTES: {
       /* Append bytes. */
       upb_strview *str = (upb_strview*)_upb_array_ptr(arr) + arr->len;
       arr->len++;
-      return decode_readstr(d, ptr, val.size, str);
+      return decode_readstr(d, ptr, val->size, str);
     }
     case OP_SUBMSG: {
       /* Append submessage / group. */
-      upb_msg *submsg = decode_newsubmsg(d, layout, field);
+      upb_msg *submsg = decode_newsubmsg(d, submsgs, field);
       *UPB_PTR_AT(_upb_array_ptr(arr), arr->len * sizeof(void *), upb_msg *) =
           submsg;
       arr->len++;
       if (UPB_UNLIKELY(field->descriptortype == UPB_DTYPE_GROUP)) {
-        return decode_togroup(d, ptr, submsg, layout, field);
+        return decode_togroup(d, ptr, submsg, submsgs, field);
       } else {
-        return decode_tosubmsg(d, ptr, submsg, layout, field, val.size);
+        return decode_tosubmsg(d, ptr, submsg, submsgs, field, val->size);
       }
     }
     case OP_FIXPCK_LG2(2):
@@ -435,15 +456,15 @@ static const char *decode_toarray(upb_decstate *d, const char *ptr,
       /* Fixed packed. */
       int lg2 = op - OP_FIXPCK_LG2(0);
       int mask = (1 << lg2) - 1;
-      size_t count = val.size >> lg2;
-      if ((val.size & mask) != 0) {
+      size_t count = val->size >> lg2;
+      if ((val->size & mask) != 0) {
         decode_err(d); /* Length isn't a round multiple of elem size. */
       }
       decode_reserve(d, arr, count);
       mem = UPB_PTR_AT(_upb_array_ptr(arr), arr->len << lg2, void);
       arr->len += count;
-      memcpy(mem, ptr, val.size);  /* XXX: ptr boundary. */
-      return ptr + val.size;
+      memcpy(mem, ptr, val->size);  /* XXX: ptr boundary. */
+      return ptr + val->size;
     }
     case OP_VARPCK_LG2(0):
     case OP_VARPCK_LG2(2):
@@ -451,7 +472,7 @@ static const char *decode_toarray(upb_decstate *d, const char *ptr,
       /* Varint packed. */
       int lg2 = op - OP_VARPCK_LG2(0);
       int scale = 1 << lg2;
-      int saved_limit = decode_pushlimit(d, ptr, val.size);
+      int saved_limit = decode_pushlimit(d, ptr, val->size);
       char *out = UPB_PTR_AT(_upb_array_ptr(arr), arr->len << lg2, void);
       while (!decode_isdone(d, &ptr)) {
         wireval elem;
@@ -473,16 +494,15 @@ static const char *decode_toarray(upb_decstate *d, const char *ptr,
 }
 
 static const char *decode_tomap(upb_decstate *d, const char *ptr, upb_msg *msg,
-                                const upb_msglayout *layout,
-                                const upb_msglayout_field *field, wireval val) {
+                                const upb_msglayout *const *submsgs,
+                                const upb_msglayout_field *field, wireval *val) {
   upb_map **map_p = UPB_PTR_AT(msg, field->offset, upb_map *);
   upb_map *map = *map_p;
   upb_map_entry ent;
-  const upb_msglayout *entry = layout->submsgs[field->submsg_index];
+  const upb_msglayout *entry = submsgs[field->submsg_index];
 
   if (!map) {
     /* Lazily create map. */
-    const upb_msglayout *entry = layout->submsgs[field->submsg_index];
     const upb_msglayout_field *key_field = &entry->fields[0];
     const upb_msglayout_field *val_field = &entry->fields[1];
     char key_size = desctype_to_mapsize[key_field->descriptortype];
@@ -502,28 +522,28 @@ static const char *decode_tomap(upb_decstate *d, const char *ptr, upb_msg *msg,
     ent.v.val = upb_value_ptr(_upb_msg_new(entry->submsgs[0], &d->arena));
   }
 
-  ptr = decode_tosubmsg(d, ptr, &ent.k, layout, field, val.size);
+  ptr = decode_tosubmsg(d, ptr, &ent.k, submsgs, field, val->size);
   _upb_map_set(map, &ent.k, map->key_size, &ent.v, map->val_size, &d->arena);
   return ptr;
 }
 
 static const char *decode_tomsg(upb_decstate *d, const char *ptr, upb_msg *msg,
-                                const upb_msglayout *layout,
-                                const upb_msglayout_field *field, wireval val,
+                                const upb_msglayout *const *submsgs,
+                                const upb_msglayout_field *field, wireval *val,
                                 int op) {
   void *mem = UPB_PTR_AT(msg, field->offset, void);
   int type = field->descriptortype;
 
   /* Set presence if necessary. */
-  if (field->presence < 0) {
+  if (field->presence > 0) {
+    _upb_sethas_field(msg, field);
+  } else if (field->presence < 0) {
     /* Oneof case */
     uint32_t *oneof_case = _upb_oneofcase_field(msg, field);
     if (op == OP_SUBMSG && *oneof_case != field->number) {
       memset(mem, 0, sizeof(void*));
     }
     *oneof_case = field->number;
-  } else if (field->presence > 0) {
-    _upb_sethas_field(msg, field);
   }
 
   /* Store into message. */
@@ -532,29 +552,29 @@ static const char *decode_tomsg(upb_decstate *d, const char *ptr, upb_msg *msg,
       upb_msg **submsgp = mem;
       upb_msg *submsg = *submsgp;
       if (!submsg) {
-        submsg = decode_newsubmsg(d, layout, field);
+        submsg = decode_newsubmsg(d, submsgs, field);
         *submsgp = submsg;
       }
       if (UPB_UNLIKELY(type == UPB_DTYPE_GROUP)) {
-        ptr = decode_togroup(d, ptr, submsg, layout, field);
+        ptr = decode_togroup(d, ptr, submsg, submsgs, field);
       } else {
-        ptr = decode_tosubmsg(d, ptr, submsg, layout, field, val.size);
+        ptr = decode_tosubmsg(d, ptr, submsg, submsgs, field, val->size);
       }
       break;
     }
     case OP_STRING:
-      decode_verifyutf8(d, ptr, val.size);
+      decode_verifyutf8(d, ptr, val->size);
       /* Fallthrough. */
     case OP_BYTES:
-      return decode_readstr(d, ptr, val.size, mem);
+      return decode_readstr(d, ptr, val->size, mem);
     case OP_SCALAR_LG2(3):
-      memcpy(mem, &val, 8);
+      memcpy(mem, val, 8);
       break;
     case OP_SCALAR_LG2(2):
-      memcpy(mem, &val, 4);
+      memcpy(mem, val, 4);
       break;
     case OP_SCALAR_LG2(0):
-      memcpy(mem, &val, 1);
+      memcpy(mem, val, 1);
       break;
     default:
       UPB_UNREACHABLE();
@@ -580,6 +600,7 @@ static bool decode_tryfastdispatch(upb_decstate *d, const char **ptr,
 UPB_NOINLINE
 static const char *decode_msg(upb_decstate *d, const char *ptr, upb_msg *msg,
                               const upb_msglayout *layout) {
+  int last_field_index = 0;
   while (true) {
     uint32_t tag;
     const upb_msglayout_field *field;
@@ -594,7 +615,7 @@ static const char *decode_msg(upb_decstate *d, const char *ptr, upb_msg *msg,
     field_number = tag >> 3;
     wire_type = tag & 7;
 
-    field = upb_find_field(layout, field_number);
+    field = upb_find_field(layout, field_number, &last_field_index);
 
     switch (wire_type) {
       case UPB_WIRE_TYPE_VARINT:
@@ -646,13 +667,13 @@ static const char *decode_msg(upb_decstate *d, const char *ptr, upb_msg *msg,
       switch (field->label) {
         case UPB_LABEL_REPEATED:
         case _UPB_LABEL_PACKED:
-          ptr = decode_toarray(d, ptr, msg, layout, field, val, op);
+          ptr = decode_toarray(d, ptr, msg, layout->submsgs, field, &val, op);
           break;
         case _UPB_LABEL_MAP:
-          ptr = decode_tomap(d, ptr, msg, layout, field, val);
+          ptr = decode_tomap(d, ptr, msg, layout->submsgs, field, &val);
           break;
         default:
-          ptr = decode_tomsg(d, ptr, msg, layout, field, val, op);
+          ptr = decode_tomsg(d, ptr, msg, layout->submsgs, field, &val, op);
           break;
       }
     } else {
diff --git a/upb/def.c b/upb/def.c
index d803559440..5973e7a078 100644
--- a/upb/def.c
+++ b/upb/def.c
@@ -1018,14 +1018,20 @@ static int field_number_cmp(const void *p1, const void *p2) {
   return f1->number - f2->number;
 }
 
-static void assign_layout_indices(const upb_msgdef *m, upb_msglayout_field *fields) {
+static void assign_layout_indices(const upb_msgdef *m, upb_msglayout *l,
+                                  upb_msglayout_field *fields) {
   int i;
   int n = upb_msgdef_numfields(m);
+  int dense_below = 0;
   for (i = 0; i < n; i++) {
     upb_fielddef *f = (upb_fielddef*)upb_msgdef_itof(m, fields[i].number);
     UPB_ASSERT(f);
     f->layout_index = i;
+    if (upb_fielddef_number(f) == i + 1) {
+      dense_below = upb_fielddef_number(f);
+    }
   }
+  l->dense_below = dense_below;
 }
 
 /* This function is the dynamic equivalent of message_layout.{cc,h} in upbc.
@@ -1197,7 +1203,7 @@ static void make_layout(symtab_addctx *ctx, const upb_msgdef *m) {
 
   /* Sort fields by number. */
   qsort(fields, upb_msgdef_numfields(m), sizeof(*fields), field_number_cmp);
-  assign_layout_indices(m, fields);
+  assign_layout_indices(m, l, fields);
 }
 
 static char *strviewdup(symtab_addctx *ctx, upb_strview view) {
diff --git a/upb/msg_internal.h b/upb/msg_internal.h
index a279f50b9a..97d711980b 100644
--- a/upb/msg_internal.h
+++ b/upb/msg_internal.h
@@ -65,6 +65,7 @@ struct upb_msglayout {
   uint16_t size;
   uint16_t field_count;
   bool extendable;
+  uint8_t dense_below;
   uint8_t table_mask;
   /* To constant-initialize the tables of variable length, we need a flexible
    * array member, and we need to compile in C99 mode. */
@@ -131,7 +132,11 @@ UPB_INLINE bool _upb_hasbit(const upb_msg *msg, size_t idx) {
 }
 
 UPB_INLINE void _upb_sethas(const upb_msg *msg, size_t idx) {
-  (*UPB_PTR_AT(msg, idx / 8, char)) |= (char)(1 << (idx % 8));
+  if (UPB_LIKELY(idx < 32)) {
+    (*UPB_PTR_AT(msg, 0, uint32_t)) |= (1UL << idx);
+  } else {
+    (*UPB_PTR_AT(msg, idx / 8, char)) |= (char)(1 << (idx % 8));
+  }
 }
 
 UPB_INLINE void _upb_clearhas(const upb_msg *msg, size_t idx) {
diff --git a/upbc/protoc-gen-upb.cc b/upbc/protoc-gen-upb.cc
index 0d4fede363..863bd2eb5a 100644
--- a/upbc/protoc-gen-upb.cc
+++ b/upbc/protoc-gen-upb.cc
@@ -867,6 +867,7 @@ void WriteSource(const protobuf::FileDescriptor* file, Output& output,
     std::string msgname = ToCIdent(message->full_name());
     std::string fields_array_ref = "NULL";
     std::string submsgs_array_ref = "NULL";
+    uint8_t dense_below = 0;
     MessageLayout layout(message);
     SubmsgArray submsg_array(message);
 
@@ -896,6 +897,12 @@ void WriteSource(const protobuf::FileDescriptor* file, Output& output,
         int submsg_index = 0;
         std::string presence = "0";
 
+        if (field->number() <=
+                std::numeric_limits<decltype(dense_below)>::max() &&
+            dense_below + 1 == field->number()) {
+          dense_below = field->number();
+        }
+
         if (field->cpp_type() == protobuf::FieldDescriptor::CPPTYPE_MESSAGE) {
           submsg_index = submsg_array.GetIndex(field);
         }
@@ -951,9 +958,10 @@ void WriteSource(const protobuf::FileDescriptor* file, Output& output,
     output("const upb_msglayout $0 = {\n", MessageInit(message));
     output("  $0,\n", submsgs_array_ref);
     output("  $0,\n", fields_array_ref);
-    output("  $0, $1, $2, $3,\n", GetSizeInit(layout.message_size()),
+    output("  $0, $1, $2, $3, $4,\n", GetSizeInit(layout.message_size()),
            field_number_order.size(),
            "false",  // TODO: extendable
+           dense_below,
            table_mask
     );
     if (!table.empty()) {