diff --git a/docs/migration.rst b/docs/migration.rst
index 2b00fba..29fb5d3 100644
--- a/docs/migration.rst
+++ b/docs/migration.rst
@@ -11,7 +11,22 @@
 
 .. contents ::
 
-Nanopb-0.4.2 (2020-xx-xx)
+Nanopb-0.4.3 (2020-xx-xx)
+=========================
+
+pb_msgdesc_t struct has new fields
+----------------------------------
+
+**Changes:** New fields `required_field_count` and `largest_tag` were added
+to `pb_msgdesc_t` and existing fields were reordered.
+
+**Required actions:** All `.pb.c` files must be recompiled. Regeneration is not
+needed.
+
+**Error indications:** Messages may fail to encode or decode, or the code can
+crash inside `load_descriptor_values()` in `pb_common.c`.
+
+Nanopb-0.4.2 (2020-06-23)
 =========================
 
 Generator now uses Python 3 by default
diff --git a/pb.h b/pb.h
index cc78303..3cc130f 100644
--- a/pb.h
+++ b/pb.h
@@ -276,17 +276,18 @@
 /* This structure is used in auto-generated constants
  * to specify struct fields.
  */
-PB_PACKED_STRUCT_START
 typedef struct pb_msgdesc_s pb_msgdesc_t;
 struct pb_msgdesc_s {
-    pb_size_t field_count;
     const uint32_t *field_info;
     const pb_msgdesc_t * const * submsg_info;
     const pb_byte_t *default_value;
 
     bool (*field_callback)(pb_istream_t *istream, pb_ostream_t *ostream, const pb_field_iter_t *field);
-} pb_packed;
-PB_PACKED_STRUCT_END
+
+    pb_size_t field_count;
+    pb_size_t required_field_count;
+    pb_size_t largest_tag;
+};
 
 /* Iterator for message descriptor */
 struct pb_field_iter_s {
@@ -469,15 +470,21 @@
     }; \
     const pb_msgdesc_t structname ## _msg = \
     { \
-       0 msgname ## _FIELDLIST(PB_GEN_FIELD_COUNT, structname), \
        structname ## _field_info, \
        structname ## _submsg_info, \
        msgname ## _DEFAULT, \
        msgname ## _CALLBACK, \
+       0 msgname ## _FIELDLIST(PB_GEN_FIELD_COUNT, structname), \
+       0 msgname ## _FIELDLIST(PB_GEN_REQ_FIELD_COUNT, structname), \
+       0 msgname ## _FIELDLIST(PB_GEN_LARGEST_TAG, structname), \
     }; \
     msgname ## _FIELDLIST(PB_GEN_FIELD_INFO_ASSERT_ ## width, structname)
 
 #define PB_GEN_FIELD_COUNT(structname, atype, htype, ltype, fieldname, tag) +1
+#define PB_GEN_REQ_FIELD_COUNT(structname, atype, htype, ltype, fieldname, tag) \
+    + (PB_HTYPE_ ## htype == PB_HTYPE_REQUIRED)
+#define PB_GEN_LARGEST_TAG(structname, atype, htype, ltype, fieldname, tag) \
+    * 0 + tag
 
 /* X-macro for generating the entries in struct_field_info[] array. */
 #define PB_GEN_FIELD_INFO_1(structname, atype, htype, ltype, fieldname, tag) \
diff --git a/pb_common.c b/pb_common.c
index c718eb1..6aee76b 100644
--- a/pb_common.c
+++ b/pb_common.c
@@ -198,6 +198,10 @@
     {
         return true; /* Nothing to do, correct field already. */
     }
+    else if (tag > iter->descriptor->largest_tag)
+    {
+        return false;
+    }
     else
     {
         pb_size_t start = iter->index;
diff --git a/pb_decode.c b/pb_decode.c
index 8055295..2dab0e3 100644
--- a/pb_decode.c
+++ b/pb_decode.c
@@ -1091,27 +1091,15 @@
 
     /* Check that all required fields were present. */
     {
-        /* First figure out the number of required fields by
-         * seeking to the end of the field array. Usually we
-         * are already close to end after decoding.
-         */
-        pb_size_t req_field_count;
-        pb_type_t last_type;
-        pb_size_t i;
-        do {
-            req_field_count = iter.required_field_index;
-            last_type = iter.type;
-        } while (pb_field_iter_next(&iter));
-
-        /* Fixup if last field was also required. */
-        if (PB_HTYPE(last_type) == PB_HTYPE_REQUIRED && iter.tag != 0)
-            req_field_count++;
-
-        if (req_field_count > PB_MAX_REQUIRED_FIELDS)
-            req_field_count = PB_MAX_REQUIRED_FIELDS;
+        pb_size_t req_field_count = iter.descriptor->required_field_count;
 
         if (req_field_count > 0)
         {
+            pb_size_t i;
+
+            if (req_field_count > PB_MAX_REQUIRED_FIELDS)
+                req_field_count = PB_MAX_REQUIRED_FIELDS;
+
             /* Check the whole words */
             for (i = 0; i < (req_field_count >> 5); i++)
             {