Properly align the arena.

upb/msg.c

@@ -17,8 +17,6 @@ bool upb_fieldtype_mapkeyok(upb_fieldtype_t type) {
 
 /** upb_msgval ****************************************************************/
 
-#define upb_alignof(t) offsetof(struct { char c; t x; }, x)
-
 /* These functions will generate real memcpy() calls on ARM sadly, because
  * the compiler assumes they might not be aligned. */
 

upb/upb.c

@@ -177,11 +177,17 @@ static void *upb_arena_doalloc(upb_alloc *alloc, void *ptr, size_t oldsize,
 
 /* Public Arena API ***********************************************************/
 
+#define upb_alignof(type) offsetof (struct { char c; type member; }, member)
+
 upb_arena *upb_arena_init(void *mem, size_t n, upb_alloc *alloc) {
   const size_t first_block_overhead = sizeof(upb_arena) + sizeof(mem_block);
   upb_arena *a;
   bool owned = false;
 
+  /* Round block size down to alignof(*a) since we will allocate the arena
+   * itself at the end. */
+  n &= ~(upb_alignof(upb_arena) - 1);
+
   if (n < first_block_overhead) {
     /* We need to malloc the initial block. */
     n = first_block_overhead + 256;
@@ -208,6 +214,8 @@ upb_arena *upb_arena_init(void *mem, size_t n, upb_alloc *alloc) {
   return a;
 }
 
+#undef upb_alignof
+
 void upb_arena_free(upb_arena *a) {
   cleanup_ent *ent = a->cleanup_head;
   mem_block *block = a->block_head;