We have a properly structured algorithm, but perf regresses by 20%.

upb/decode.c

@@ -275,7 +275,6 @@ static upb_msg *decode_newsubmsg(upb_decstate *d, const upb_msglayout *layout,
   return _upb_msg_new(subl, d->arena);
 }
 
-UPB_FORCEINLINE
 static void decode_tosubmsg(upb_decstate *d, upb_msg *submsg,
                             const upb_msglayout *layout,
                             const upb_msglayout_field *field, upb_strview val) {
@@ -283,13 +282,14 @@ static void decode_tosubmsg(upb_decstate *d, upb_msg *submsg,
   const char *saved_limit = d->limit;
   if (--d->depth < 0) decode_err(d);
   d->limit = val.data + val.size;
+  d->fastlimit = UPB_MIN(d->limit, d->fastend);
   decode_msg(d, val.data, submsg, subl);
   d->limit = saved_limit;
+  d->fastlimit = UPB_MIN(d->limit, d->fastend);
   if (d->end_group != 0) decode_err(d);
   d->depth++;
 }
 
-UPB_FORCEINLINE
 static const char *decode_group(upb_decstate *d, const char *ptr,
                                 upb_msg *submsg, const upb_msglayout *subl,
                                 uint32_t number) {
@@ -301,7 +301,6 @@ static const char *decode_group(upb_decstate *d, const char *ptr,
   return ptr;
 }
 
-UPB_FORCEINLINE
 static const char *decode_togroup(upb_decstate *d, const char *ptr,
                                   upb_msg *submsg, const upb_msglayout *layout,
                                   const upb_msglayout_field *field) {
@@ -402,7 +401,6 @@ static const char *decode_toarray(upb_decstate *d, const char *ptr,
   }
 }
 
-UPB_FORCEINLINE
 static void decode_tomap(upb_decstate *d, upb_msg *msg,
                          const upb_msglayout *layout,
                          const upb_msglayout_field *field, wireval val) {
@@ -594,19 +592,23 @@ static const char *decode_field(upb_decstate *d, const char *ptr, upb_msg *msg,
   return ptr;
 }
 
+UPB_NOINLINE
 const char *fastdecode_generic(upb_decstate *d, const char *ptr, upb_msg *msg,
                                const upb_msglayout *table, uint64_t hasbits,
                                uint64_t data) {
-  *(uint32_t*)msg |= hasbits;  /* Sync hasbits. */
+  if (hasbits) *(uint32_t*)msg |= hasbits;  /* Sync hasbits. */
   (void)data;
-  return decode_field(d, ptr, msg, table);
+  ptr = decode_field(d, ptr, msg, table);
+  return fastdecode_dispatch(d, ptr, msg, table, hasbits);
 }
 
+UPB_NOINLINE
 static const char *decode_msg(upb_decstate *d, const char *ptr, upb_msg *msg,
                               const upb_msglayout *layout) {
-  while (ptr < d->limit) {
+  while (1) {
     ptr = fastdecode_dispatch(d, ptr, msg, layout, 0);
-    if (ptr < d->limit) ptr = decode_field(d, ptr, msg, layout);
+    if (ptr == d->limit) break;
+    ptr = decode_field(d, ptr, msg, layout);
   }
 
   if (ptr != d->limit) decode_err(d);
@@ -617,7 +619,8 @@ bool upb_decode(const char *buf, size_t size, void *msg, const upb_msglayout *l,
                 upb_arena *arena) {
   upb_decstate state;
   state.limit = buf + size;
-  state.fastlimit = buf + size - 16;
+  state.fastend = buf + size - 16;
+  state.fastlimit = state.fastend;
   state.arena = arena;
   state.depth = 64;
   state.end_group = 0;

upb/decode.h

@@ -22,6 +22,7 @@ typedef struct upb_decstate {
   const void *rep_end;
   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;
   upb_array *arr;
   _upb_field_parser *resume;
   upb_arena *arena;