Allow zero hash table size

This avoids any extra allocation for such vectors, including empty
tables read by the Lisp reader, and provides extra safety essentially
for free.

* src/fns.c (make_hash_table): Allow tables to be 0-sized.  The index
will always have at least one entry, to avoid extra look-up costs.
* src/alloc.c (process_mark_stack): Don't mark pure objects,
because empty vectors are pure.

1 files changed, 2 insertions, 1 deletions

diff --git a/src/alloc.c b/src/alloc.c
index 17ed711a318..636b4972c84 100644
--- a/src/alloc.c
+++ b/src/alloc.c
@@ -7242,7 +7242,8 @@ process_mark_stack (ptrdiff_t base_sp)
                       eassert (h->next_weak == NULL);
                       h->next_weak = weak_hash_tables;
                       weak_hash_tables = h;
-                      set_vector_marked (XVECTOR (h->key_and_value));
+                      if (!PURE_P (h->key_and_value))
+                        set_vector_marked (XVECTOR (h->key_and_value));
                     }
                   break;
                 }