Shrunk the arena state that needs to be synced.

upb/decode.c

@@ -141,7 +141,6 @@ static const int8_t delim_ops[37] = {
 typedef struct {
   const char *limit;       /* End of delimited region or end of buffer. */
   upb_arena arena;
-  upb_arena *orig_parent;
   int depth;
   uint32_t end_group; /* Set to field number of END_GROUP tag, if any. */
   jmp_buf err;
@@ -613,11 +612,9 @@ bool upb_decode(const char *buf, size_t size, void *msg, const upb_msglayout *l,
   state.limit = buf + size;
   state.depth = 64;
   state.end_group = 0;
-  state.orig_parent = arena->parent;
-  state.arena = *arena;
-  if (arena->parent == arena) {
-    state.arena.parent = &state.arena;
-  }
+  state.arena.head = arena->head;
+  state.arena.last_size = arena->last_size;
+  state.arena.parent = arena;
 
   if (setjmp(state.err)) {
     ok = false;
@@ -626,8 +623,7 @@ bool upb_decode(const char *buf, size_t size, void *msg, const upb_msglayout *l,
     ok = state.end_group == 0;
   }
 
-  *arena = state.arena;
-  arena->parent = state.orig_parent;
+  arena->head = state.arena.head;
   return ok;
 }
 

upb/upb.c

@@ -98,9 +98,9 @@ static upb_arena *arena_findroot(upb_arena *a) {
   return a;
 }
 
-static void upb_arena_addblock(upb_arena *a, void *ptr, size_t size) {
+static void upb_arena_addblock(upb_arena *a, upb_arena *root, void *ptr,
+                               size_t size) {
   mem_block *block = ptr;
-  upb_arena *root = arena_findroot(a);
 
   /* The block is for arena |a|, but should appear in the freelist of |root|. */
   block->next = root->freelist;
@@ -118,11 +118,12 @@ static void upb_arena_addblock(upb_arena *a, void *ptr, size_t size) {
 }
 
 static bool upb_arena_allocblock(upb_arena *a, size_t size) {
+  upb_arena *root = arena_findroot(a);
   size_t block_size = UPB_MAX(size, a->last_size * 2) + memblock_reserve;
-  mem_block *block = upb_malloc(a->block_alloc, block_size);
+  mem_block *block = upb_malloc(root->block_alloc, block_size);
 
   if (!block) return false;
-  upb_arena_addblock(a, block, block_size);
+  upb_arena_addblock(a, root, block, block_size);
   return true;
 }
 
@@ -165,7 +166,7 @@ upb_arena *arena_initslow(void *mem, size_t n, upb_alloc *alloc) {
   a->freelist = NULL;
   a->freelist_tail = NULL;
 
-  upb_arena_addblock(a, mem, n);
+  upb_arena_addblock(a, a, mem, n);
 
   return a;
 }