arena_malloc
void __arena *arena_malloc(size_t size);
nodec = arena_malloc(sizeof(*nodec));
node_ctx nodec = arena_malloc(sizeof(*nodec));
rbnode = (struct rbnode __arena *)arena_malloc(sizeof(*rbnode));
rbtree = arena_malloc(sizeof(*rbtree));
spmc = arena_malloc(sizeof(*spmc));
arr->data = arena_malloc((SPMC_ARR_BASESZ << order) * sizeof(*arr->data));