Subject: bug#25743: rehash-size ignored

From: Stefan Monnier <[email-masked]>
Date: Wed, 15 Feb 2017 14:18:54 -0500

Package: Emacs
Version: 26.0.50

Looking accidentally at maybe_resize_hash_table, I noticed that we don't
obey resize_hash, although we do all the work needed for that.
Worse, we make dangerous assumptions about the behavior of
larger_vector:

maybe_resize_hash_table takes `old_size' from `ASIZE (h->next)' and then
uses rehash_size to compute the desired new_size.  The problem comes
here:

      set_hash_key_and_value (h, larger_vector (h->key_and_value,
						2 * (new_size - old_size), -1));
      set_hash_next (h, larger_vector (h->next, new_size - old_size, -1));

This says, that h->next and h->key_and_value are replaced by new vectors
that are larger than the previous one so that they are large enough to
accomodate new_size.

The problem is that larger_vector's promise is to return a vector at
least as large as requested, but sometimes larger.  In practice the new
vector is twice the size of the old vector.  This means that rehash_size
is ignored when it means to something smaller than twice the size.
It *also* means that h->next may end up larger than planned.  But `ASIZE
(h->next)' is the definition of the hash-table's size, so if it's larger
than planned, then it's indispensible to make sure that all other
related tables are equally larger: h->key_and_value must always be at
least twice the size of h->next.  But the call above only makes sure the
new h->key_and_value is at least twice the desired new_size but not
twice the size of the new h->next.

I think the right fix is to change the calls to larger_vector so that
instead of requesting vectors "at least as large as new_size", we should
request vectors "of size exactly new_size".  That will fix both problems
at the same time.  The patch below implements it.

Any objection?

        Stefan

diff --git a/src/fns.c b/src/fns.c
index 760a017dedf..62c834fac60 100644
--- a/src/fns.c
+++ b/src/fns.c
@@ -3536,19 +3535,19 @@ get_key_arg (Lisp_Object key, ptrdiff_t nargs, Lisp_Object *args, char *used)
 Lisp_Object
 larger_vector (Lisp_Object vec, ptrdiff_t incr_min, ptrdiff_t nitems_max)
 {
+  eassert (VECTORP (vec));
   struct Lisp_Vector *v;
-  ptrdiff_t incr, incr_max, old_size, new_size;
+  ptrdiff_t old_size = ASIZE (vec);
   ptrdiff_t C_language_max = min (PTRDIFF_MAX, SIZE_MAX) / sizeof *v->contents;
-  ptrdiff_t n_max = (0 <= nitems_max && nitems_max < C_language_max
+  ptrdiff_t n_max = (nitems_max == 0 ? old_size + incr_min
+                     : 0 < nitems_max && nitems_max < C_language_max
 		     ? nitems_max : C_language_max);
-  eassert (VECTORP (vec));
   eassert (0 < incr_min && -1 <= nitems_max);
-  old_size = ASIZE (vec);
-  incr_max = n_max - old_size;
-  incr = max (incr_min, min (old_size >> 1, incr_max));
+  ptrdiff_t incr_max = n_max - old_size;
+  ptrdiff_t incr = max (incr_min, min (old_size >> 1, incr_max));
   if (incr_max < incr)
     memory_full (SIZE_MAX);
-  new_size = old_size + incr;
+  ptrdiff_t new_size = old_size + incr;
   v = allocate_vector (new_size);
   memcpy (v->contents, XVECTOR (vec)->contents, old_size * sizeof *v->contents);
   memclear (v->contents + old_size, incr * word_size);
@@ -3828,10 +3827,10 @@ maybe_resize_hash_table (struct Lisp_Hash_Table *h)
 	message ("Growing hash table to: %"pI"d", new_size);
 #endif
 
-      set_hash_key_and_value (h, larger_vector (h->key_and_value,
-						2 * (new_size - old_size), -1));
-      set_hash_next (h, larger_vector (h->next, new_size - old_size, -1));
-      set_hash_hash (h, larger_vector (h->hash, new_size - old_size, -1));
+      set_hash_key_and_value (h, larger_vector (h->key_and_value,
+						2 * (new_size - old_size), 0));
+      set_hash_next (h, larger_vector (h->next, new_size - old_size, 0));
+      set_hash_hash (h, larger_vector (h->hash, new_size - old_size, 0));
       set_hash_index (h, Fmake_vector (make_number (index_size), Qnil));
 
       /* Update the free list.  Do it so that new entries are added at

In GNU Emacs 26.0.50.1 (x86_64-unknown-linux-gnu, X toolkit, Xaw3d scroll bars)
 of 2017-01-30 built on lechazo
Repository revision: 5ddca31bf707686aafaf891c216702b64657c516
Windowing system distributor 'The X.Org Foundation', version 11.0.11900000
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