Avoid accessing uninitialized bool_vector words

Although loading uninitialized works from memory and then ignoring
the result works fine on conventional architectures, it
technically has undefined behavior in C, so redo bool_vector
allocation so that the code never does that.  This can improve
performance when allocating large vectors of nil, since calloc can
clear the memory lazily.
* src/alloc.c (make_clear_bool_vector): New function,
a generalization of make_uninit_bool_vector.
(make_uninit_bool_vector): Use it.
(Fmake_bool_vector): If !INIT, rely on make_clear_bool_vector.
* src/alloc.c (Fbool_vector):
* src/fns.c (Freverse): Don’t access uninitialized bool_vector words.

src/alloc.c

@@ -453,6 +453,7 @@ no_sanitize_memcpy (void *dest, void const *src, size_t size)
 
 #endif /* MAX_SAVE_STACK > 0 */
 
+static struct Lisp_Vector *allocate_clear_vector (ptrdiff_t, bool);
 static void unchain_finalizer (struct Lisp_Finalizer *);
 static void mark_terminals (void);
 static void gc_sweep (void);
@@ -2406,10 +2407,11 @@ bool_vector_fill (Lisp_Object a, Lisp_Object init)
   return a;
 }
 
-/* Return a newly allocated, uninitialized bool vector of size NBITS.  */
+/* Return a newly allocated, bool vector of size NBITS.  If CLEARIT,
+   clear its slots; otherwise the vector's slots are uninitialized.  */
 
 Lisp_Object
-make_uninit_bool_vector (EMACS_INT nbits)
+make_clear_bool_vector (EMACS_INT nbits, bool clearit)
 {
   Lisp_Object val;
   EMACS_INT words = bool_vector_words (nbits);
@@ -2420,28 +2422,35 @@ make_uninit_bool_vector (EMACS_INT nbits)
   if (PTRDIFF_MAX < needed_elements)
     memory_full (SIZE_MAX);
   struct Lisp_Bool_Vector *p
-    = (struct Lisp_Bool_Vector *) allocate_vector (needed_elements);
+    = (struct Lisp_Bool_Vector *) allocate_clear_vector (needed_elements,
+							 clearit);
+  /* Clear padding at end; but only if necessary, to avoid polluting the
+     data cache.  */
+  if (!clearit && nbits % BITS_PER_BITS_WORD != 0)
+    p->data[words - 1] = 0;
+
   XSETVECTOR (val, p);
   XSETPVECTYPESIZE (XVECTOR (val), PVEC_BOOL_VECTOR, 0, 0);
   p->size = nbits;
-
-  /* Clear padding at the end.  */
-  if (words)
-    p->data[words - 1] = 0;
-
   return val;
 }
 
+/* Return a newly allocated, uninitialized bool vector of size NBITS.  */
+
+Lisp_Object
+make_uninit_bool_vector (EMACS_INT nbits)
+{
+  return make_clear_bool_vector (nbits, false);
+}
+
 DEFUN ("make-bool-vector", Fmake_bool_vector, Smake_bool_vector, 2, 2, 0,
        doc: /* Return a new bool-vector of length LENGTH, using INIT for each element.
 LENGTH must be a number.  INIT matters only in whether it is t or nil.  */)
   (Lisp_Object length, Lisp_Object init)
 {
-  Lisp_Object val;
-
   CHECK_FIXNAT (length);
-  val = make_uninit_bool_vector (XFIXNAT (length));
-  return bool_vector_fill (val, init);
+  Lisp_Object val = make_clear_bool_vector (XFIXNAT (length), NILP (init));
+  return NILP (init) ? val : bool_vector_fill (val, init);
 }
 
 DEFUN ("bool-vector", Fbool_vector, Sbool_vector, 0, MANY, 0,
@@ -2450,13 +2459,10 @@ Allows any number of arguments, including zero.
 usage: (bool-vector &rest OBJECTS)  */)
   (ptrdiff_t nargs, Lisp_Object *args)
 {
-  ptrdiff_t i;
-  Lisp_Object vector;
-
-  vector = make_uninit_bool_vector (nargs);
-  for (i = 0; i < nargs; i++)
-    bool_vector_set (vector, i, !NILP (args[i]));
-
+  Lisp_Object vector = make_clear_bool_vector (nargs, true);
+  for (ptrdiff_t i = 0; i < nargs; i++)
+    if (!NILP (args[i]))
+      bool_vector_set (vector, i, true);
   return vector;
 }
 

src/fns.c

@@ -2309,12 +2309,12 @@ See also the function `nreverse', which is used more often.  */)
     }
   else if (BOOL_VECTOR_P (seq))
     {
-      ptrdiff_t i;
       EMACS_INT nbits = bool_vector_size (seq);
 
-      new = make_uninit_bool_vector (nbits);
-      for (i = 0; i < nbits; i++)
-	bool_vector_set (new, i, bool_vector_bitref (seq, nbits - i - 1));
+      new = make_clear_bool_vector (nbits, true);
+      for (ptrdiff_t i = 0; i < nbits; i++)
+	if (bool_vector_bitref (seq, nbits - i - 1))
+	  bool_vector_set (new, i, true);
     }
   else if (STRINGP (seq))
     {

src/lisp.h

@@ -4578,6 +4578,7 @@ list4i (intmax_t a, intmax_t b, intmax_t c, intmax_t d)
   return list4 (make_int (a), make_int (b), make_int (c), make_int (d));
 }
 
+extern Lisp_Object make_clear_bool_vector (EMACS_INT, bool);
 extern Lisp_Object make_uninit_bool_vector (EMACS_INT);
 extern Lisp_Object bool_vector_fill (Lisp_Object, Lisp_Object);
 extern AVOID string_overflow (void);