diff --git a/generated_for_cmake/upb/json/parser.c b/generated_for_cmake/upb/json/parser.c
index 9955a024aa..0efc7301d1 100644
--- a/generated_for_cmake/upb/json/parser.c
+++ b/generated_for_cmake/upb/json/parser.c
@@ -391,7 +391,7 @@ static upb_selector_t getsel_for_handlertype(upb_json_parser *p,
                                              upb_handlertype_t type) {
   upb_selector_t sel;
   bool ok = upb_handlers_getselector(p->top->f, type, &sel);
-  UPB_ASSERT(ok);
+  UPB_ASSUME(ok);
   return sel;
 }
 
@@ -416,7 +416,7 @@ static void set_name_table(upb_json_parser *p, upb_jsonparser_frame *frame) {
   const upb_json_parsermethod *method;
 
   ok = upb_inttable_lookupptr(&cache->methods, frame->m, &v);
-  UPB_ASSERT(ok);
+  UPB_ASSUME(ok);
   method = upb_value_getconstptr(v);
 
   frame->name_table = &method->name_table;
@@ -2021,7 +2021,7 @@ static void end_member(upb_json_parser *p) {
     /* send ENDSUBMSG in repeated-field-of-mapentries frame. */
     p->top--;
     ok = upb_handlers_getselector(mapfield, UPB_HANDLER_ENDSUBMSG, &sel);
-    UPB_ASSERT(ok);
+    UPB_ASSUME(ok);
     upb_sink_endsubmsg(p->top->sink, (p->top + 1)->sink, sel);
   }
 
diff --git a/upb/decode.c b/upb/decode.c
index f1be7502c3..9f1e986e73 100644
--- a/upb/decode.c
+++ b/upb/decode.c
@@ -53,7 +53,6 @@ static const uint8_t desctype_to_mapsize[] = {
 
 /* Data pertaining to the parse. */
 typedef struct {
-  const char *ptr;           /* Current parsing position. */
   const char *field_start;   /* Start of this field. */
   const char *limit;         /* End of delimited region or end of buffer. */
   upb_arena *arena;
@@ -61,60 +60,52 @@ typedef struct {
   uint32_t end_group;  /* Set to field number of END_GROUP tag, if any. */
 } upb_decstate;
 
-/* Data passed by value to each parsing function. */
-typedef struct {
-  char *msg;
-  const upb_msglayout *layout;
-  upb_decstate *state;
-} upb_decframe;
-
 #define CHK(x) if (!(x)) { return 0; }
+#define PTR_AT(msg, ofs, type) (type*)((const char*)msg + ofs)
 
-static bool upb_skip_unknowngroup(upb_decstate *d, int field_number);
-static bool upb_decode_message(upb_decstate *d, char *msg,
-                               const upb_msglayout *l);
+static const char *upb_skip_unknowngroup(const char *ptr, upb_decstate *d,
+                                         int field_number);
+static const char *upb_decode_message(const char *ptr, const upb_msglayout *l,
+                                      upb_msg *msg, upb_decstate *d);
 
-static bool upb_decode_varint(const char **ptr, const char *limit,
-                              uint64_t *val) {
+static const char *upb_decode_varint(const char *ptr, const char *limit,
+                                     uint64_t *val) {
   uint8_t byte;
   int bitpos = 0;
-  const char *p = *ptr;
   *val = 0;
 
   do {
-    CHK(bitpos < 70 && p < limit);
-    byte = *p;
+    CHK(bitpos < 70 && ptr < limit);
+    byte = *ptr;
     *val |= (uint64_t)(byte & 0x7F) << bitpos;
-    p++;
+    ptr++;
     bitpos += 7;
   } while (byte & 0x80);
 
-  *ptr = p;
-  return true;
+  return ptr;
 }
 
-static bool upb_decode_varint32(const char **ptr, const char *limit,
-                                uint32_t *val) {
+static const char *upb_decode_varint32(const char *ptr, const char *limit,
+                                       uint32_t *val) {
   uint64_t u64;
-  CHK(upb_decode_varint(ptr, limit, &u64) && u64 <= UINT32_MAX);
+  CHK(ptr = upb_decode_varint(ptr, limit, &u64))
+  CHK(u64 <= UINT32_MAX);
   *val = (uint32_t)u64;
-  return true;
+  return ptr;
 }
 
-static bool upb_decode_64bit(const char **ptr, const char *limit,
-                             uint64_t *val) {
-  CHK(limit - *ptr >= 8);
-  memcpy(val, *ptr, 8);
-  *ptr += 8;
-  return true;
+static const char *upb_decode_64bit(const char *ptr, const char *limit,
+                                    uint64_t *val) {
+  CHK(limit - ptr >= 8);
+  memcpy(val, ptr, 8);
+  return ptr + 8;
 }
 
-static bool upb_decode_32bit(const char **ptr, const char *limit,
-                             uint32_t *val) {
-  CHK(limit - *ptr >= 4);
-  memcpy(val, *ptr, 4);
-  *ptr += 4;
-  return true;
+static const char *upb_decode_32bit(const char *ptr, const char *limit,
+                                    uint32_t *val) {
+  CHK(limit - ptr >= 4);
+  memcpy(val, ptr, 4);
+  return ptr + 4;
 }
 
 static int32_t upb_zzdecode_32(uint32_t n) {
@@ -125,68 +116,68 @@ static int64_t upb_zzdecode_64(uint64_t n) {
   return (n >> 1) ^ -(int64_t)(n & 1);
 }
 
-static bool upb_decode_string(const char **ptr, const char *limit,
-                              int *outlen) {
+static const char *upb_decode_string(const char *ptr, const char *limit,
+                                     int *outlen) {
   uint32_t len;
 
-  CHK(upb_decode_varint32(ptr, limit, &len) &&
-      len < INT32_MAX &&
-      limit - *ptr >= (int32_t)len);
+  CHK(ptr = upb_decode_varint32(ptr, limit, &len));
+  CHK(len < INT32_MAX);
+  CHK(limit - ptr >= (int32_t)len);
 
   *outlen = len;
-  return true;
+  return ptr;
 }
 
 static void upb_set32(void *msg, size_t ofs, uint32_t val) {
   memcpy((char*)msg + ofs, &val, sizeof(val));
 }
 
-static bool upb_append_unknown(upb_decstate *d, upb_decframe *frame) {
-  upb_msg_addunknown(frame->msg, d->field_start, d->ptr - d->field_start,
-                     d->arena);
-  return true;
+static const char *upb_append_unknown(const char *ptr, upb_msg *msg, upb_decstate *d) {
+  upb_msg_addunknown(msg, d->field_start, ptr - d->field_start, d->arena);
+  return ptr;
 }
 
-
-static bool upb_skip_unknownfielddata(upb_decstate *d, uint32_t tag,
-                                      uint32_t group_fieldnum) {
+static const char *upb_skip_unknownfielddata(const char *ptr, upb_decstate *d,
+                                             uint32_t tag,
+                                             uint32_t group_fieldnum) {
   switch (tag & 7) {
     case UPB_WIRE_TYPE_VARINT: {
       uint64_t val;
-      return upb_decode_varint(&d->ptr, d->limit, &val);
+      return upb_decode_varint(ptr, d->limit, &val);
     }
     case UPB_WIRE_TYPE_32BIT: {
       uint32_t val;
-      return upb_decode_32bit(&d->ptr, d->limit, &val);
+      return upb_decode_32bit(ptr, d->limit, &val);
     }
     case UPB_WIRE_TYPE_64BIT: {
       uint64_t val;
-      return upb_decode_64bit(&d->ptr, d->limit, &val);
+      return upb_decode_64bit(ptr, d->limit, &val);
     }
     case UPB_WIRE_TYPE_DELIMITED: {
       int len;
-      CHK(upb_decode_string(&d->ptr, d->limit, &len));
-      d->ptr += len;
-      return true;
+      CHK(ptr = upb_decode_string(ptr, d->limit, &len));
+      return ptr + len;
     }
     case UPB_WIRE_TYPE_START_GROUP:
-      return upb_skip_unknowngroup(d, tag >> 3);
+      return upb_skip_unknowngroup(ptr, d, tag >> 3);
     case UPB_WIRE_TYPE_END_GROUP:
-      return (tag >> 3) == group_fieldnum;
+      CHK((tag >> 3) == group_fieldnum);
+      return ptr;
   }
   return false;
 }
 
-static bool upb_skip_unknowngroup(upb_decstate *d, int field_number) {
-  while (d->ptr < d->limit && d->end_group == 0) {
+static const char *upb_skip_unknowngroup(const char *ptr, upb_decstate *d,
+                                         int field_number) {
+  while (ptr < d->limit && d->end_group == 0) {
     uint32_t tag = 0;
-    CHK(upb_decode_varint32(&d->ptr, d->limit, &tag));
-    CHK(upb_skip_unknownfielddata(d, tag, field_number));
+    CHK(ptr = upb_decode_varint32(ptr, d->limit, &tag));
+    CHK(ptr = upb_skip_unknownfielddata(ptr, d, tag, field_number));
   }
 
   CHK(d->end_group == field_number);
   d->end_group = 0;
-  return true;
+  return ptr;
 }
 
 static void *upb_array_reserve(upb_array *arr, size_t elements,
@@ -208,83 +199,80 @@ bool upb_array_add(upb_array *arr, size_t elements, size_t elem_size,
   return true;
 }
 
-static upb_array *upb_getarr(upb_decframe *frame,
-                             const upb_msglayout_field *field) {
+static upb_array *upb_getarr(upb_msg *msg, const upb_msglayout_field *field) {
   UPB_ASSERT(field->label == UPB_LABEL_REPEATED);
-  return *(upb_array**)&frame->msg[field->offset];
+  return *PTR_AT(msg, field->offset, upb_array*);
 }
 
-static upb_array *upb_getorcreatearr(upb_decframe *frame,
-                                     const upb_msglayout_field *field) {
-  upb_array *arr = upb_getarr(frame, field);
+static upb_array *upb_getorcreatearr(upb_msg *msg,
+                                     const upb_msglayout_field *field,
+                                     upb_decstate *d) {
+  upb_array *arr = upb_getarr(msg, field);
 
   if (!arr) {
     upb_fieldtype_t type = desctype_to_fieldtype[field->descriptortype];
-    arr = _upb_array_new(frame->state->arena, type);
+    arr = _upb_array_new(d->arena, type);
     CHK(arr);
-    *(upb_array**)&frame->msg[field->offset] = arr;
+    *PTR_AT(msg, field->offset, upb_array*) = arr;
   }
 
   return arr;
 }
 
-static upb_msg *upb_getorcreatemsg(upb_decframe *frame,
+static upb_msg *upb_getorcreatemsg(upb_msg *msg,
                                    const upb_msglayout_field *field,
-                                   const upb_msglayout **subm) {
-  upb_msg **submsg = (void*)(frame->msg + field->offset);
-  *subm = frame->layout->submsgs[field->submsg_index];
+                                   const upb_msglayout *layout,
+                                   upb_decstate *d) {
+  upb_msg **submsg = PTR_AT(msg, field->offset, upb_msg*);
 
   UPB_ASSERT(field->label != UPB_LABEL_REPEATED);
 
   if (!*submsg) {
-    *submsg = _upb_msg_new(*subm, frame->state->arena);
+    *submsg = _upb_msg_new(layout, d->arena);
     CHK(*submsg);
   }
 
   return *submsg;
 }
 
-static upb_msg *upb_addmsg(upb_decframe *frame,
+static upb_msg *upb_addmsg(upb_msg *msg,
                            const upb_msglayout_field *field,
-                           const upb_msglayout **subm) {
+                           const upb_msglayout *layout,
+                           upb_decstate *d) {
   upb_msg *submsg;
-  upb_array *arr = upb_getorcreatearr(frame, field);
+  upb_array *arr = upb_getorcreatearr(msg, field, d);
 
   UPB_ASSERT(field->label == UPB_LABEL_REPEATED);
   UPB_ASSERT(field->descriptortype == UPB_DESCRIPTOR_TYPE_MESSAGE ||
              field->descriptortype == UPB_DESCRIPTOR_TYPE_GROUP);
 
-  *subm = frame->layout->submsgs[field->submsg_index];
-  submsg = _upb_msg_new(*subm, frame->state->arena);
+  submsg = _upb_msg_new(layout, d->arena);
   CHK(submsg);
-  upb_array_add(arr, 1, sizeof(submsg), &submsg, frame->state->arena);
+  upb_array_add(arr, 1, sizeof(submsg), &submsg, d->arena);
 
   return submsg;
 }
 
-static void upb_sethasbit(upb_decframe *frame,
-                          const upb_msglayout_field *field) {
+static void upb_sethasbit(upb_msg *msg, const upb_msglayout_field *field) {
   int32_t hasbit = field->presence;
   UPB_ASSERT(field->presence > 0);
-  frame->msg[hasbit / 8] |= (1 << (hasbit % 8));
+  *PTR_AT(msg, hasbit / 8, char) |= (1 << (hasbit % 8));
 }
 
-static void upb_setoneofcase(upb_decframe *frame,
-                             const upb_msglayout_field *field) {
+static void upb_setoneofcase(upb_msg *msg, const upb_msglayout_field *field) {
   UPB_ASSERT(field->presence < 0);
-  upb_set32(frame->msg, ~field->presence, field->number);
+  upb_set32(msg, ~field->presence, field->number);
 }
 
-static bool upb_decode_addval(upb_decframe *frame,
-                               const upb_msglayout_field *field, void *val,
-                               size_t size) {
-  char *field_mem = frame->msg + field->offset;
+static bool upb_decode_addval(upb_msg *msg, const upb_msglayout_field *field,
+                              void *val, size_t size, upb_decstate *d) {
+  char *field_mem = PTR_AT(msg, field->offset, char);
   upb_array *arr;
 
   if (field->label == UPB_LABEL_REPEATED) {
-    arr = upb_getorcreatearr(frame, field);
+    arr = upb_getorcreatearr(msg, field, d);
     CHK(arr);
-    field_mem = upb_array_reserve(arr, 1, size, frame->state->arena);
+    field_mem = upb_array_reserve(arr, 1, size, d->arena);
     CHK(field_mem);
   }
 
@@ -292,142 +280,147 @@ static bool upb_decode_addval(upb_decframe *frame,
   return true;
 }
 
-static void upb_decode_setpresent(upb_decframe *frame,
+static void upb_decode_setpresent(upb_msg *msg,
                                   const upb_msglayout_field *field) {
   if (field->label == UPB_LABEL_REPEATED) {
-   upb_array *arr = upb_getarr(frame, field);
+   upb_array *arr = upb_getarr(msg, field);
    UPB_ASSERT(arr->len < arr->size);
    arr->len++;
   } else if (field->presence < 0) {
-    upb_setoneofcase(frame, field);
+    upb_setoneofcase(msg, field);
   } else if (field->presence > 0) {
-    upb_sethasbit(frame, field);
+    upb_sethasbit(msg, field);
   }
 }
 
-static bool upb_decode_msgfield(upb_decstate *d, upb_msg *msg,
-                                const upb_msglayout *layout, int limit) {
+static const char *upb_decode_msgfield(const char *ptr,
+                                       const upb_msglayout *layout, int limit,
+                                       upb_msg *msg, upb_decstate *d) {
   const char* saved_limit = d->limit;
-  d->limit = d->ptr + limit;
+  d->limit = ptr + limit;
   CHK(--d->depth >= 0);
-  upb_decode_message(d, msg, layout);
+  ptr = upb_decode_message(ptr, layout, msg, d);
   d->depth++;
   d->limit = saved_limit;
   CHK(d->end_group == 0);
-  return true;
+  return ptr;
 }
 
-static bool upb_decode_groupfield(upb_decstate *d, upb_msg *msg,
-                                  const upb_msglayout *layout,
-                                  int field_number) {
+static const char *upb_decode_groupfield(const char *ptr,
+                                         const upb_msglayout *layout,
+                                         int field_number, upb_msg *msg,
+                                         upb_decstate *d) {
   CHK(--d->depth >= 0);
-  upb_decode_message(d, msg, layout);
+  ptr = upb_decode_message(ptr, layout, msg, d);
   d->depth++;
   CHK(d->end_group == field_number);
   d->end_group = 0;
-  return true;
+  return ptr;
 }
 
-static bool upb_decode_varintfield(upb_decstate *d, upb_decframe *frame,
-                                   const upb_msglayout_field *field) {
+static const char *upb_decode_varintfield(const char *ptr, upb_msg *msg,
+                                          const upb_msglayout_field *field,
+                                          upb_decstate *d) {
   uint64_t val;
-  CHK(upb_decode_varint(&d->ptr, d->limit, &val));
+  CHK(ptr = upb_decode_varint(ptr, d->limit, &val));
 
   switch (field->descriptortype) {
     case UPB_DESCRIPTOR_TYPE_INT64:
     case UPB_DESCRIPTOR_TYPE_UINT64:
-      CHK(upb_decode_addval(frame, field, &val, sizeof(val)));
+      CHK(upb_decode_addval(msg, field, &val, sizeof(val), d));
       break;
     case UPB_DESCRIPTOR_TYPE_INT32:
     case UPB_DESCRIPTOR_TYPE_UINT32:
     case UPB_DESCRIPTOR_TYPE_ENUM: {
       uint32_t val32 = (uint32_t)val;
-      CHK(upb_decode_addval(frame, field, &val32, sizeof(val32)));
+      CHK(upb_decode_addval(msg, field, &val32, sizeof(val32), d));
       break;
     }
     case UPB_DESCRIPTOR_TYPE_BOOL: {
       bool valbool = val != 0;
-      CHK(upb_decode_addval(frame, field, &valbool, sizeof(valbool)));
+      CHK(upb_decode_addval(msg, field, &valbool, sizeof(valbool), d));
       break;
     }
     case UPB_DESCRIPTOR_TYPE_SINT32: {
       int32_t decoded = upb_zzdecode_32((uint32_t)val);
-      CHK(upb_decode_addval(frame, field, &decoded, sizeof(decoded)));
+      CHK(upb_decode_addval(msg, field, &decoded, sizeof(decoded), d));
       break;
     }
     case UPB_DESCRIPTOR_TYPE_SINT64: {
       int64_t decoded = upb_zzdecode_64(val);
-      CHK(upb_decode_addval(frame, field, &decoded, sizeof(decoded)));
+      CHK(upb_decode_addval(msg, field, &decoded, sizeof(decoded), d));
       break;
     }
     default:
-      return upb_append_unknown(d, frame);
+      return upb_append_unknown(ptr, msg, d);
   }
 
-  upb_decode_setpresent(frame, field);
-  return true;
+  upb_decode_setpresent(msg, field);
+  return ptr;
 }
 
-static bool upb_decode_64bitfield(upb_decstate *d, upb_decframe *frame,
-                                  const upb_msglayout_field *field) {
+static const char *upb_decode_64bitfield(const char *ptr,
+                                         const upb_msglayout_field *field,
+                                         upb_msg *msg, upb_decstate *d) {
   uint64_t val;
-  CHK(upb_decode_64bit(&d->ptr, d->limit, &val));
+  CHK(ptr = upb_decode_64bit(ptr, d->limit, &val));
 
   switch (field->descriptortype) {
     case UPB_DESCRIPTOR_TYPE_DOUBLE:
     case UPB_DESCRIPTOR_TYPE_FIXED64:
     case UPB_DESCRIPTOR_TYPE_SFIXED64:
-      CHK(upb_decode_addval(frame, field, &val, sizeof(val)));
+      CHK(upb_decode_addval(msg, field, &val, sizeof(val), d));
       break;
     default:
-      return upb_append_unknown(d, frame);
+      return upb_append_unknown(ptr, msg, d);
   }
 
-  upb_decode_setpresent(frame, field);
-  return true;
+  upb_decode_setpresent(msg, field);
+  return ptr;
 }
 
-static bool upb_decode_32bitfield(upb_decstate *d, upb_decframe *frame,
-                                  const upb_msglayout_field *field) {
+static const char *upb_decode_32bitfield(const char *ptr,
+                                         const upb_msglayout_field *field,
+                                         upb_msg *msg, upb_decstate *d) {
   uint32_t val;
-  CHK(upb_decode_32bit(&d->ptr, d->limit, &val));
+  CHK(ptr = upb_decode_32bit(ptr, d->limit, &val));
 
   switch (field->descriptortype) {
     case UPB_DESCRIPTOR_TYPE_FLOAT:
     case UPB_DESCRIPTOR_TYPE_FIXED32:
     case UPB_DESCRIPTOR_TYPE_SFIXED32:
-      CHK(upb_decode_addval(frame, field, &val, sizeof(val)));
+      CHK(upb_decode_addval(msg, field, &val, sizeof(val), d));
       break;
     default:
-      return upb_append_unknown(d, frame);
+      return upb_append_unknown(ptr, msg, d);
   }
 
-  upb_decode_setpresent(frame, field);
-  return true;
+  upb_decode_setpresent(msg, field);
+  return ptr;
 }
 
-static bool upb_decode_fixedpacked(upb_decstate *d, upb_array *arr,
-                                   uint32_t len, int elem_size) {
+static const char *upb_decode_fixedpacked(const char *ptr, upb_decstate *d,
+                                          upb_array *arr, uint32_t len,
+                                          int elem_size) {
   size_t elements = len / elem_size;
 
   CHK((size_t)(elements * elem_size) == len);
-  CHK(upb_array_add(arr, elements, elem_size, d->ptr, d->arena));
-  d->ptr += len;
-
-  return true;
+  CHK(upb_array_add(arr, elements, elem_size, ptr, d->arena));
+  return ptr + len;
 }
 
-static upb_strview upb_decode_strfield(upb_decstate *d, uint32_t len) {
-  upb_strview ret;
-  ret.data = d->ptr;
-  ret.size = len;
-  d->ptr += len;
-  return ret;
+static const char *upb_decode_strfield(const char *ptr, upb_decstate *d,
+                                       uint32_t len, upb_strview *str) {
+  str->data = ptr;
+  str->size = len;
+  return ptr + len;
 }
 
-static bool upb_decode_toarray(upb_decstate *d, upb_decframe *frame,
-                               const upb_msglayout_field *field, int len) {
-  upb_array *arr = upb_getorcreatearr(frame, field);
+static const char *upb_decode_toarray(const char *ptr,
+                                      const upb_msglayout *layout,
+                                      const upb_msglayout_field *field, int len,
+                                      upb_msg *msg, upb_decstate *d) {
+  upb_array *arr = upb_getorcreatearr(msg, field, d);
   CHK(arr);
 
 #define VARINT_CASE(ctype, decode) \
@@ -435,33 +428,33 @@ static bool upb_decode_toarray(upb_decstate *d, upb_decframe *frame,
 
 #define VARINT_CASE_EX(ctype, decode, dtype)                           \
   {                                                                    \
-    const char *ptr = d->ptr;                                          \
     const char *limit = ptr + len;                                     \
     while (ptr < limit) {                                              \
       uint64_t val;                                                    \
       ctype decoded;                                                   \
-      CHK(upb_decode_varint(&ptr, limit, &val));                       \
+      CHK(ptr = upb_decode_varint(ptr, limit, &val));                  \
       decoded = (decode)((dtype)val);                                  \
       CHK(upb_array_add(arr, 1, sizeof(decoded), &decoded, d->arena)); \
     }                                                                  \
-    d->ptr = ptr;                                                      \
-    return true;                                                       \
+    return ptr;                                                        \
   }
 
   switch (field->descriptortype) {
     case UPB_DESCRIPTOR_TYPE_STRING:
     case UPB_DESCRIPTOR_TYPE_BYTES: {
-      upb_strview str = upb_decode_strfield(d, len);
-      return upb_array_add(arr, 1, sizeof(str), &str, d->arena);
+      upb_strview str;
+      ptr = upb_decode_strfield(ptr, d, len, &str);
+      CHK(upb_array_add(arr, 1, sizeof(str), &str, d->arena));
+      return ptr;
     }
     case UPB_DESCRIPTOR_TYPE_FLOAT:
     case UPB_DESCRIPTOR_TYPE_FIXED32:
     case UPB_DESCRIPTOR_TYPE_SFIXED32:
-      return upb_decode_fixedpacked(d, arr, len, sizeof(int32_t));
+      return upb_decode_fixedpacked(ptr, d, arr, len, sizeof(int32_t));
     case UPB_DESCRIPTOR_TYPE_DOUBLE:
     case UPB_DESCRIPTOR_TYPE_FIXED64:
     case UPB_DESCRIPTOR_TYPE_SFIXED64:
-      return upb_decode_fixedpacked(d, arr, len, sizeof(int64_t));
+      return upb_decode_fixedpacked(ptr, d, arr, len, sizeof(int64_t));
     case UPB_DESCRIPTOR_TYPE_INT32:
     case UPB_DESCRIPTOR_TYPE_UINT32:
     case UPB_DESCRIPTOR_TYPE_ENUM:
@@ -476,22 +469,24 @@ static bool upb_decode_toarray(upb_decstate *d, upb_decframe *frame,
     case UPB_DESCRIPTOR_TYPE_SINT64:
       VARINT_CASE_EX(int64_t, upb_zzdecode_64, uint64_t);
     case UPB_DESCRIPTOR_TYPE_MESSAGE: {
-      const upb_msglayout *subm;
-      upb_msg *submsg = upb_addmsg(frame, field, &subm);
+      const upb_msglayout *subl = layout->submsgs[field->submsg_index];
+      upb_msg *submsg = upb_addmsg(msg, field, subl, d);
       CHK(submsg);
-      return upb_decode_msgfield(d, submsg, subm, len);
+      return upb_decode_msgfield(ptr, subl, len, submsg, d);
     }
     case UPB_DESCRIPTOR_TYPE_GROUP:
-      return upb_append_unknown(d, frame);
+      return upb_append_unknown(ptr, msg, d);
   }
 #undef VARINT_CASE
   UPB_UNREACHABLE();
 }
 
-static bool upb_decode_mapfield(upb_decstate *d, upb_decframe *frame,
-                                const upb_msglayout_field *field, int len) {
-  upb_map *map = *(upb_map**)&frame->msg[field->offset];
-  const upb_msglayout *entry = frame->layout->submsgs[field->submsg_index];
+static const char *upb_decode_mapfield(const char *ptr,
+                                       const upb_msglayout *layout,
+                                       const upb_msglayout_field *field,
+                                       int len, upb_msg *msg, upb_decstate *d) {
+  upb_map *map = *PTR_AT(msg, field->offset, upb_map*);
+  const upb_msglayout *entry = layout->submsgs[field->submsg_index];
   upb_map_entry ent;
 
   if (!map) {
@@ -504,51 +499,53 @@ static bool upb_decode_mapfield(upb_decstate *d, upb_decframe *frame,
     UPB_ASSERT(val_field->number == 2);
     UPB_ASSERT(key_field->offset == 0);
     UPB_ASSERT(val_field->offset == sizeof(upb_strview));
-    map = _upb_map_new(frame->state->arena, key_size, val_size);
-    *(upb_map**)&frame->msg[field->offset] = map;
+    map = _upb_map_new(d->arena, key_size, val_size);
+    *PTR_AT(msg, field->offset, upb_map*) = map;
   }
 
   /* Parse map entry. */
   memset(&ent, 0, sizeof(ent));
-  CHK(upb_decode_msgfield(d, &ent.k, entry, len));
+  CHK(ptr = upb_decode_msgfield(ptr, entry, len, &ent.k, d));
 
   /* Insert into map. */
   _upb_map_set(map, &ent.k, map->key_size, &ent.v, map->val_size, d->arena);
-  return true;
+  return ptr;
 }
 
-static bool upb_decode_delimitedfield(upb_decstate *d, upb_decframe *frame,
-                                      const upb_msglayout_field *field) {
+static const char *upb_decode_delimitedfield(const char *ptr,
+                                             const upb_msglayout *layout,
+                                             const upb_msglayout_field *field,
+                                             upb_msg *msg, upb_decstate *d) {
   int len;
 
-  CHK(upb_decode_string(&d->ptr, d->limit, &len));
+  CHK(ptr = upb_decode_string(ptr, d->limit, &len));
 
   if (field->label == UPB_LABEL_REPEATED) {
-    return upb_decode_toarray(d, frame, field, len);
+    return upb_decode_toarray(ptr, layout, field, len, msg, d);
   } else if (field->label == UPB_LABEL_MAP) {
-    return upb_decode_mapfield(d, frame, field, len);
+    return upb_decode_mapfield(ptr, layout, field, len, msg, d);
   } else {
     switch (field->descriptortype) {
       case UPB_DESCRIPTOR_TYPE_STRING:
       case UPB_DESCRIPTOR_TYPE_BYTES: {
-        upb_strview str = upb_decode_strfield(d, len);
-        CHK(upb_decode_addval(frame, field, &str, sizeof(str)));
+        upb_strview str;
+        ptr = upb_decode_strfield(ptr, d, len, &str);
+        CHK(upb_decode_addval(msg, field, &str, sizeof(str), d));
         break;
       }
       case UPB_DESCRIPTOR_TYPE_MESSAGE: {
-        const upb_msglayout *subm;
-        upb_msg *submsg = upb_getorcreatemsg(frame, field, &subm);
+        const upb_msglayout *subl = layout->submsgs[field->submsg_index];
+        upb_msg *submsg = upb_getorcreatemsg(msg, field, subl, d);
         CHK(submsg);
-        CHK(upb_decode_msgfield(d, submsg, subm, len));
+        CHK(ptr = upb_decode_msgfield(ptr, subl, len, submsg, d));
         break;
       }
       default:
         /* TODO(haberman): should we accept the last element of a packed? */
-        d->ptr += len;
-        return upb_append_unknown(d, frame);
+        return upb_append_unknown(ptr + len, msg, d);
     }
-    upb_decode_setpresent(frame, field);
-    return true;
+    upb_decode_setpresent(msg, field);
+    return ptr;
   }
 }
 
@@ -565,77 +562,75 @@ static const upb_msglayout_field *upb_find_field(const upb_msglayout *l,
   return NULL;  /* Unknown field. */
 }
 
-static bool upb_decode_field(upb_decstate *d, upb_decframe *frame) {
+static const char *upb_decode_field(const char *ptr,
+                                    const upb_msglayout *layout, upb_msg *msg,
+                                    upb_decstate *d) {
   uint32_t tag;
   const upb_msglayout_field *field;
   int field_number;
 
-  d->field_start = d->ptr;
-  CHK(upb_decode_varint32(&d->ptr, d->limit, &tag));
+  d->field_start = ptr;
+  CHK(ptr = upb_decode_varint32(ptr, d->limit, &tag));
   field_number = tag >> 3;
-  field = upb_find_field(frame->layout, field_number);
+  field = upb_find_field(layout, field_number);
 
   if (field) {
     switch (tag & 7) {
       case UPB_WIRE_TYPE_VARINT:
-        return upb_decode_varintfield(d, frame, field);
+        return upb_decode_varintfield(ptr, msg, field, d);
       case UPB_WIRE_TYPE_32BIT:
-        return upb_decode_32bitfield(d, frame, field);
+        return upb_decode_32bitfield(ptr, field, msg, d);
       case UPB_WIRE_TYPE_64BIT:
-        return upb_decode_64bitfield(d, frame, field);
+        return upb_decode_64bitfield(ptr, field, msg, d);
       case UPB_WIRE_TYPE_DELIMITED:
-        return upb_decode_delimitedfield(d, frame, field);
+        return upb_decode_delimitedfield(ptr, layout, field, msg, d);
       case UPB_WIRE_TYPE_START_GROUP: {
-        const upb_msglayout *layout;
+        const upb_msglayout *subl = layout->submsgs[field->submsg_index];
         upb_msg *group;
 
         if (field->label == UPB_LABEL_REPEATED) {
-          group = upb_addmsg(frame, field, &layout);
+          group = upb_addmsg(msg, field, subl, d);
         } else {
-          group = upb_getorcreatemsg(frame, field, &layout);
+          group = upb_getorcreatemsg(msg, field, subl, d);
         }
 
-        return upb_decode_groupfield(d, group, layout, field_number);
+        return upb_decode_groupfield(ptr, subl, field_number, group, d);
       }
       case UPB_WIRE_TYPE_END_GROUP:
         d->end_group = field_number;
-        return true;
+        return ptr;
       default:
         CHK(false);
     }
   } else {
     CHK(field_number != 0);
-    CHK(upb_skip_unknownfielddata(d, tag, -1));
-    CHK(upb_append_unknown(d, frame));
-    return true;
+    CHK(ptr = upb_skip_unknownfielddata(ptr, d, tag, -1));
+    CHK(ptr = upb_append_unknown(ptr, msg, d));
+    return ptr;
   }
   UPB_UNREACHABLE();
 }
 
-static bool upb_decode_message(upb_decstate *d, char *msg, const upb_msglayout *l) {
-  upb_decframe frame;
-  frame.msg = msg;
-  frame.layout = l;
-  frame.state = d;
-
-  while (d->ptr < d->limit) {
-    CHK(upb_decode_field(d, &frame));
+static const char *upb_decode_message(const char *ptr, const upb_msglayout *l,
+                                      upb_msg *msg, upb_decstate *d) {
+  while (ptr < d->limit) {
+    CHK(ptr = upb_decode_field(ptr, l, msg, d));
   }
 
-  return true;
+  return ptr;
 }
 
 bool upb_decode(const char *buf, size_t size, void *msg, const upb_msglayout *l,
                 upb_arena *arena) {
   upb_decstate state;
-  state.ptr = buf;
   state.limit = buf + size;
   state.arena = arena;
   state.depth = 64;
   state.end_group = 0;
 
-  CHK(upb_decode_message(&state, msg, l));
+  CHK(upb_decode_message(buf, l, msg, &state));
   return state.end_group == 0;
 }
 
 #undef CHK
+#undef PTR_AT
diff --git a/upb/def.c b/upb/def.c
index 1a83687788..c3c9d104fc 100644
--- a/upb/def.c
+++ b/upb/def.c
@@ -1331,7 +1331,7 @@ static bool create_fielddef(
           break;
         }
       }
-      assert(found);
+      UPB_ASSERT(found);
     }
   } else {
     /* extension field. */
diff --git a/upb/json/parser.rl b/upb/json/parser.rl
index 1180ce137f..eefdadf577 100644
--- a/upb/json/parser.rl
+++ b/upb/json/parser.rl
@@ -389,7 +389,7 @@ static upb_selector_t getsel_for_handlertype(upb_json_parser *p,
                                              upb_handlertype_t type) {
   upb_selector_t sel;
   bool ok = upb_handlers_getselector(p->top->f, type, &sel);
-  UPB_ASSERT(ok);
+  UPB_ASSUME(ok);
   return sel;
 }
 
@@ -414,7 +414,7 @@ static void set_name_table(upb_json_parser *p, upb_jsonparser_frame *frame) {
   const upb_json_parsermethod *method;
 
   ok = upb_inttable_lookupptr(&cache->methods, frame->m, &v);
-  UPB_ASSERT(ok);
+  UPB_ASSUME(ok);
   method = upb_value_getconstptr(v);
 
   frame->name_table = &method->name_table;
@@ -2019,7 +2019,7 @@ static void end_member(upb_json_parser *p) {
     /* send ENDSUBMSG in repeated-field-of-mapentries frame. */
     p->top--;
     ok = upb_handlers_getselector(mapfield, UPB_HANDLER_ENDSUBMSG, &sel);
-    UPB_ASSERT(ok);
+    UPB_ASSUME(ok);
     upb_sink_endsubmsg(p->top->sink, (p->top + 1)->sink, sel);
   }
 
diff --git a/upb/pb/compile_decoder.c b/upb/pb/compile_decoder.c
index f5c7d65646..44b310fe3f 100644
--- a/upb/pb/compile_decoder.c
+++ b/upb/pb/compile_decoder.c
@@ -910,10 +910,10 @@ const upb_pbdecodermethod *upb_pbcodecache_get(upb_pbcodecache *c,
   } else {
     g = mgroup_new(h, c->lazy);
     ok = upb_inttable_insertptr(&c->groups, md, upb_value_constptr(g));
-    UPB_ASSERT(ok);
+    UPB_ASSUME(ok);
   }
 
   ok = upb_inttable_lookupptr(&g->methods, h, &v);
-  UPB_ASSERT(ok);
+  UPB_ASSUME(ok);
   return upb_value_getptr(v);
 }
diff --git a/upb/port_def.inc b/upb/port_def.inc
index 992148e736..51f8eac96f 100644
--- a/upb/port_def.inc
+++ b/upb/port_def.inc
@@ -128,6 +128,18 @@ int msvc_vsnprintf(char* s, size_t n, const char* format, va_list arg);
 
 #define UPB_UNUSED(var) (void)var
 
+/* UPB_ASSUME(): in release mode, we tell the compiler to assume this is true.
+ */
+#ifdef NDEBUG
+#ifdef __GNUC__
+#define UPB_ASSUME(expr) if (!(expr)) __builtin_unreachable()
+#else
+#define UPB_ASSUME(expr) do {} if (false && (expr))
+#endif
+#else
+#define UPB_ASSUME(expr) assert(expr)
+#endif
+
 /* UPB_ASSERT(): in release mode, we use the expression without letting it be
  * evaluated.  This prevents "unused variable" warnings. */
 #ifdef NDEBUG
diff --git a/upb/port_undef.inc b/upb/port_undef.inc
index 6a4daa5076..e91e9644bb 100644
--- a/upb/port_undef.inc
+++ b/upb/port_undef.inc
@@ -12,6 +12,7 @@
 #undef UPB_MAX
 #undef UPB_MIN
 #undef UPB_UNUSED
+#undef UPB_ASSUME
 #undef UPB_ASSERT
 #undef UPB_ASSERT_DEBUGVAR
 #undef UPB_UNREACHABLE