Fix clrhash bug when hash table needs rehashing

Problem reported by Pip Cet in:
https://lists.gnu.org/r/emacs-devel/2019-08/msg00316.html
* src/fns.c (maybe_resize_hash_table): Prefer ASET to gc_aset
where either will do.  Simplify appending of Qunbound values.
Put index_size calculation closer to where it’s needed.
(hash_clear): If hash_rehash_needed_p (h), don’t clear the
nonexistent hash vector.  Use memclear to speed up clearing.
* src/lisp.h (HASH_TABLE_SIZE): Check that the size is positive,
and tell that to the compiler.

1 files changed, 5 insertions, 4 deletions

diff --git a/src/lisp.h b/src/lisp.h
index 56ad99b8e39..ae5a81e7b5e 100644
--- a/src/lisp.h
+++ b/src/lisp.h
@@ -2307,7 +2307,7 @@ struct Lisp_Hash_Table
      weakness of the table.  */
   Lisp_Object weak;
 
-  /* Vector of hash codes.
+  /* Vector of hash codes, or nil if the table needs rehashing.
      If the I-th entry is unused, then hash[I] should be nil.  */
   Lisp_Object hash;
 
@@ -2327,8 +2327,7 @@ struct Lisp_Hash_Table
      'index' are special and are either ignored by the GC or traced in
      a special way (e.g. because of weakness).  */
 
-  /* Number of key/value entries in the table.  This number is
-     negated if the table needs rehashing.  */
+  /* Number of key/value entries in the table.  */
   ptrdiff_t count;
 
   /* Index of first free entry in free list, or -1 if none.  */
@@ -2413,7 +2412,9 @@ HASH_HASH (const struct Lisp_Hash_Table *h, ptrdiff_t idx)
 INLINE ptrdiff_t
 HASH_TABLE_SIZE (const struct Lisp_Hash_Table *h)
 {
-  return ASIZE (h->next);
+  ptrdiff_t size = ASIZE (h->next);
+  eassume (0 < size);
+  return size;
 }
 
 void hash_table_rehash (struct Lisp_Hash_Table *h);