Reinstall recent GC-related changes

The report that they broke macOS was a false alarm, as the
previous commit was also broken (Bug#43152#62).
* src/alloc.c (live_string_holding, live_cons_holding)
(live_symbol_holding):
Count only pointers that point to a struct component,
or are a tagged pointer to the start of the struct.
Exception: for non-bool-vector pseudovectors,
count any pointer past the header, since it’s too much
of a pain to write code for every pseudovector.
(live_float_holding, live_vector_pointer):
New functions, which are similar about counting pointers.
(live_float_p, live_large_vector_holding)
(live_small_vector_pointer, mark_maybe_pointer): Use them.
(mark_maybe_object, mark_maybe_objects): Remove,
and remove all callers; mark_maybe_pointer now suffices.
(mark_objects): New function.
* src/alloc.c (mark_vectorlike, mark_face_cache):
* src/eval.c (mark_specpdl):
* src/fringe.c (mark_fringe_data):
* src/keyboard.c (mark_kboards):
Simplify by using mark_objects.
* src/lisp.h (SAFE_ALLOCA_LISP_EXTRA):
Clear any Lisp_Object arrays large enough to not fit into the stack,
so that GC need not worry about whether they contain objects.

5 files changed, 129 insertions, 144 deletions

diff --git a/src/alloc.c b/src/alloc.c
index b16b2f8b93e..b12922b5858 100644
--- a/src/alloc.c
+++ b/src/alloc.c
@@ -4457,9 +4457,17 @@ live_string_holding (struct mem_node *m, void *p)
      must not be on the free-list.  */
   if (0 <= offset && offset < sizeof b->strings)
     {
-      struct Lisp_String *s = p = cp -= offset % sizeof b->strings[0];
-      if (s->u.s.data)
-	return s;
+      ptrdiff_t off = offset % sizeof b->strings[0];
+      if (off == Lisp_String
+	  || off == 0
+	  || off == offsetof (struct Lisp_String, u.s.size_byte)
+	  || off == offsetof (struct Lisp_String, u.s.intervals)
+	  || off == offsetof (struct Lisp_String, u.s.data))
+	{
+	  struct Lisp_String *s = p = cp -= off;
+	  if (s->u.s.data)
+	    return s;
+	}
     }
   return NULL;
 }
@@ -4489,9 +4497,15 @@ live_cons_holding (struct mem_node *m, void *p)
       && (b != cons_block
 	  || offset / sizeof b->conses[0] < cons_block_index))
     {
-      struct Lisp_Cons *s = p = cp -= offset % sizeof b->conses[0];
-      if (!deadp (s->u.s.car))
-	return s;
+      ptrdiff_t off = offset % sizeof b->conses[0];
+      if (off == Lisp_Cons
+	  || off == 0
+	  || off == offsetof (struct Lisp_Cons, u.s.u.cdr))
+	{
+	  struct Lisp_Cons *s = p = cp -= off;
+	  if (!deadp (s->u.s.car))
+	    return s;
+	}
     }
   return NULL;
 }
@@ -4522,9 +4536,23 @@ live_symbol_holding (struct mem_node *m, void *p)
       && (b != symbol_block
 	  || offset / sizeof b->symbols[0] < symbol_block_index))
     {
-      struct Lisp_Symbol *s = p = cp -= offset % sizeof b->symbols[0];
-      if (!deadp (s->u.s.function))
-	return s;
+      ptrdiff_t off = offset % sizeof b->symbols[0];
+      if (off == Lisp_Symbol
+
+	  /* Plain '|| off == 0' would run afoul of GCC 10.2
+	     -Wlogical-op, as Lisp_Symbol happens to be zero.  */
+	  || (Lisp_Symbol != 0 && off == 0)
+
+	  || off == offsetof (struct Lisp_Symbol, u.s.name)
+	  || off == offsetof (struct Lisp_Symbol, u.s.val)
+	  || off == offsetof (struct Lisp_Symbol, u.s.function)
+	  || off == offsetof (struct Lisp_Symbol, u.s.plist)
+	  || off == offsetof (struct Lisp_Symbol, u.s.next))
+	{
+	  struct Lisp_Symbol *s = p = cp -= off;
+	  if (!deadp (s->u.s.function))
+	    return s;
+	}
     }
   return NULL;
 }
@@ -4536,23 +4564,70 @@ live_symbol_p (struct mem_node *m, void *p)
 }
 
 
-/* Return true if P is a pointer to a live Lisp float on
-   the heap.  M is a pointer to the mem_block for P.  */
+/* If P is a (possibly-tagged) pointer to a live Lisp_Float on the
+   heap, return the address of the Lisp_Float.  Otherwise, return NULL.
+   M is a pointer to the mem_block for P.  */
 
-static bool
-live_float_p (struct mem_node *m, void *p)
+static struct Lisp_Float *
+live_float_holding (struct mem_node *m, void *p)
 {
   eassert (m->type == MEM_TYPE_FLOAT);
   struct float_block *b = m->start;
   char *cp = p;
   ptrdiff_t offset = cp - (char *) &b->floats[0];
 
-  /* P must point to the start of a Lisp_Float and not be
-     one of the unused cells in the current float block.  */
-  return (0 <= offset && offset < sizeof b->floats
-	  && offset % sizeof b->floats[0] == 0
+  /* P must point to (or be a tagged pointer to) the start of a
+     Lisp_Float and not be one of the unused cells in the current
+     float block.  */
+  if (0 <= offset && offset < sizeof b->floats)
+    {
+      int off = offset % sizeof b->floats[0];
+      if ((off == Lisp_Float || off == 0)
 	  && (b != float_block
-	      || offset / sizeof b->floats[0] < float_block_index));
+	      || offset / sizeof b->floats[0] < float_block_index))
+	{
+	  p = cp - off;
+	  return p;
+	}
+    }
+  return NULL;
+}
+
+static bool
+live_float_p (struct mem_node *m, void *p)
+{
+  return live_float_holding (m, p) == p;
+}
+
+/* Return VECTOR if P points within it, NULL otherwise.  */
+
+static struct Lisp_Vector *
+live_vector_pointer (struct Lisp_Vector *vector, void *p)
+{
+  void *vvector = vector;
+  char *cvector = vvector;
+  char *cp = p;
+  ptrdiff_t offset = cp - cvector;
+  return ((offset == Lisp_Vectorlike
+	   || offset == 0
+	   || (sizeof vector->header <= offset
+	       && offset < vector_nbytes (vector)
+	       && (! (vector->header.size & PSEUDOVECTOR_FLAG)
+		   ? (offsetof (struct Lisp_Vector, contents) <= offset
+		      && (((offset - offsetof (struct Lisp_Vector, contents))
+			   % word_size)
+			  == 0))
+		   /* For non-bool-vector pseudovectors, treat any pointer
+		      past the header as valid since it's too much of a pain
+		      to write special-case code for every pseudovector.  */
+		   : (! PSEUDOVECTOR_TYPEP (&vector->header, PVEC_BOOL_VECTOR)
+		      || offset == offsetof (struct Lisp_Bool_Vector, size)
+		      || (offsetof (struct Lisp_Bool_Vector, data) <= offset
+			  && (((offset
+				- offsetof (struct Lisp_Bool_Vector, data))
+			       % sizeof (bits_word))
+			      == 0))))))
+	  ? vector : NULL);
 }
 
 /* If P is a pointer to a live, large vector-like object, return the object.
@@ -4563,10 +4638,7 @@ static struct Lisp_Vector *
 live_large_vector_holding (struct mem_node *m, void *p)
 {
   eassert (m->type == MEM_TYPE_VECTORLIKE);
-  struct Lisp_Vector *vp = p;
-  struct Lisp_Vector *vector = large_vector_vec (m->start);
-  struct Lisp_Vector *next = ADVANCE (vector, vector_nbytes (vector));
-  return vector <= vp && vp < next ? vector : NULL;
+  return live_vector_pointer (large_vector_vec (m->start), p);
 }
 
 static bool
@@ -4596,7 +4668,7 @@ live_small_vector_holding (struct mem_node *m, void *p)
     {
       struct Lisp_Vector *next = ADVANCE (vector, vector_nbytes (vector));
       if (vp < next && !PSEUDOVECTOR_TYPEP (&vector->header, PVEC_FREE))
-	return vector;
+	return live_vector_pointer (vector, vp);
       vector = next;
     }
   return NULL;
@@ -4608,117 +4680,33 @@ live_small_vector_p (struct mem_node *m, void *p)
   return live_small_vector_holding (m, p) == p;
 }
 
-/* Mark OBJ if we can prove it's a Lisp_Object.  */
+/* If P points to Lisp data, mark that as live if it isn't already
+   marked.  */
 
 static void
-mark_maybe_object (Lisp_Object obj)
+mark_maybe_pointer (void *p)
 {
+  struct mem_node *m;
+
 #if USE_VALGRIND
-  VALGRIND_MAKE_MEM_DEFINED (&obj, sizeof (obj));
+  VALGRIND_MAKE_MEM_DEFINED (&p, sizeof (p));
 #endif
 
-  int type_tag = XTYPE (obj);
-  intptr_t pointer_word_tag = LISP_WORD_TAG (type_tag), offset, ipo;
-
-  switch (type_tag)
-    {
-    case_Lisp_Int: case Lisp_Type_Unused0:
-      return;
-
-    case Lisp_Symbol:
-      offset = (intptr_t) lispsym;
-      break;
-
-    default:
-      offset = 0;
-      break;
-    }
-
-  INT_ADD_WRAPV ((intptr_t) XLP (obj), offset - pointer_word_tag, &ipo);
-  void *po = (void *) ipo;
-
   /* If the pointer is in the dump image and the dump has a record
      of the object starting at the place where the pointer points, we
      definitely have an object.  If the pointer is in the dump image
      and the dump has no idea what the pointer is pointing at, we
      definitely _don't_ have an object.  */
-  if (pdumper_object_p (po))
+  if (pdumper_object_p (p))
     {
       /* Don't use pdumper_object_p_precise here! It doesn't check the
          tag bits. OBJ here might be complete garbage, so we need to
          verify both the pointer and the tag.  */
-      if (pdumper_find_object_type (po) == type_tag)
-        mark_object (obj);
-      return;
-    }
-
-  struct mem_node *m = mem_find (po);
-
-  if (m != MEM_NIL)
-    {
-      bool mark_p = false;
-
-      switch (type_tag)
-	{
-	case Lisp_String:
-	  mark_p = m->type == MEM_TYPE_STRING && live_string_p (m, po);
-	  break;
-
-	case Lisp_Cons:
-	  mark_p = m->type == MEM_TYPE_CONS && live_cons_p (m, po);
-	  break;
-
-	case Lisp_Symbol:
-	  mark_p = m->type == MEM_TYPE_SYMBOL && live_symbol_p (m, po);
-	  break;
-
-	case Lisp_Float:
-	  mark_p = m->type == MEM_TYPE_FLOAT && live_float_p (m, po);
-	  break;
-
-	case Lisp_Vectorlike:
-	  mark_p = (m->type == MEM_TYPE_VECTOR_BLOCK
-		    ? live_small_vector_p (m, po)
-		    : (m->type == MEM_TYPE_VECTORLIKE
-		       && live_large_vector_p (m, po)));
-	  break;
-
-	default:
-	  eassume (false);
-	}
-
-      if (mark_p)
-	mark_object (obj);
-    }
-}
-
-void
-mark_maybe_objects (Lisp_Object const *array, ptrdiff_t nelts)
-{
-  for (Lisp_Object const *lim = array + nelts; array < lim; array++)
-    mark_maybe_object (*array);
-}
-
-/* If P points to Lisp data, mark that as live if it isn't already
-   marked.  */
-
-static void
-mark_maybe_pointer (void *p)
-{
-  struct mem_node *m;
-
-#if USE_VALGRIND
-  VALGRIND_MAKE_MEM_DEFINED (&p, sizeof (p));
-#endif
-
-  if (pdumper_object_p (p))
-    {
       int type = pdumper_find_object_type (p);
       if (pdumper_valid_object_type_p (type))
         mark_object (type == Lisp_Symbol
                      ? make_lisp_symbol (p)
                      : make_lisp_ptr (p, type));
-      /* See mark_maybe_object for why we can confidently return.  */
       return;
     }
 
@@ -4762,9 +4750,12 @@ mark_maybe_pointer (void *p)
 	  break;
 
 	case MEM_TYPE_FLOAT:
-	  if (! live_float_p (m, p))
-	    return;
-	  obj = make_lisp_ptr (p, Lisp_Float);
+	  {
+	    struct Lisp_Float *h = live_float_holding (m, p);
+	    if (!h)
+	      return;
+	    obj = make_lisp_ptr (h, Lisp_Float);
+	  }
 	  break;
 
 	case MEM_TYPE_VECTORLIKE:
@@ -4849,11 +4840,6 @@ mark_memory (void const *start, void const *end)
       intptr_t ip;
       INT_ADD_WRAPV ((intptr_t) p, (intptr_t) lispsym, &ip);
       mark_maybe_pointer ((void *) ip);
-
-      verify (alignof (Lisp_Object) % GC_POINTER_ALIGNMENT == 0);
-      if (alignof (Lisp_Object) == GC_POINTER_ALIGNMENT
-	  || (uintptr_t) pp % alignof (Lisp_Object) == 0)
-	mark_maybe_object (*(Lisp_Object const *) pp);
     }
 }
 
@@ -6261,7 +6247,6 @@ mark_vectorlike (union vectorlike_header *header)
 {
   struct Lisp_Vector *ptr = (struct Lisp_Vector *) header;
   ptrdiff_t size = ptr->header.size;
-  ptrdiff_t i;
 
   eassert (!vector_marked_p (ptr));
 
@@ -6276,8 +6261,7 @@ mark_vectorlike (union vectorlike_header *header)
      the number of Lisp_Object fields that we should trace.
      The distinction is used e.g. by Lisp_Process which places extra
      non-Lisp_Object fields at the end of the structure...  */
-  for (i = 0; i < size; i++) /* ...and then mark its elements.  */
-    mark_object (ptr->contents[i]);
+  mark_objects (ptr->contents, size);
 }
 
 /* Like mark_vectorlike but optimized for char-tables (and
@@ -6376,8 +6360,7 @@ mark_face_cache (struct face_cache *c)
 {
   if (c)
     {
-      int i, j;
-      for (i = 0; i < c->used; ++i)
+      for (int i = 0; i < c->used; i++)
 	{
 	  struct face *face = FACE_FROM_ID_OR_NULL (c->f, i);
 
@@ -6386,8 +6369,7 @@ mark_face_cache (struct face_cache *c)
 	      if (face->font && !vectorlike_marked_p (&face->font->header))
 		mark_vectorlike (&face->font->header);
 
-	      for (j = 0; j < LFACE_VECTOR_SIZE; ++j)
-		mark_object (face->lface[j]);
+	      mark_objects (face->lface, LFACE_VECTOR_SIZE);
 	    }
 	}
     }
@@ -6500,6 +6482,13 @@ mark_hash_table (struct Lisp_Vector *ptr)
     }
 }
 
+void
+mark_objects (Lisp_Object *obj, ptrdiff_t n)
+{
+  for (ptrdiff_t i = 0; i < n; i++)
+    mark_object (obj[i]);
+}
+
 /* Determine type of generic Lisp_Object and mark it accordingly.
 
    This function implements a straightforward depth-first marking
diff --git a/src/eval.c b/src/eval.c
index 9daae92e55a..126ee2e9555 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -3960,7 +3960,7 @@ mark_specpdl (union specbinding *first, union specbinding *ptr)
 	  break;
 
 	case SPECPDL_UNWIND_ARRAY:
-	  mark_maybe_objects (pdl->unwind_array.array, pdl->unwind_array.nelts);
+	  mark_objects (pdl->unwind_array.array, pdl->unwind_array.nelts);
 	  break;
 
 	case SPECPDL_UNWIND_EXCURSION:
@@ -3974,8 +3974,7 @@ mark_specpdl (union specbinding *first, union specbinding *ptr)
 	    mark_object (backtrace_function (pdl));
 	    if (nargs == UNEVALLED)
 	      nargs = 1;
-	    while (nargs--)
-	      mark_object (backtrace_args (pdl)[nargs]);
+	    mark_objects (backtrace_args (pdl), nargs);
 	  }
 	  break;
 
diff --git a/src/fringe.c b/src/fringe.c
index c3d64fefc82..75496692d53 100644
--- a/src/fringe.c
+++ b/src/fringe.c
@@ -1733,11 +1733,7 @@ If nil, also continue lines which are exactly as wide as the window.  */);
 void
 mark_fringe_data (void)
 {
-  int i;
-
-  for (i = 0; i < max_fringe_bitmaps; i++)
-    if (!NILP (fringe_faces[i]))
-      mark_object (fringe_faces[i]);
+  mark_objects (fringe_faces, max_fringe_bitmaps);
 }
 
 /* Initialize this module when Emacs starts.  */
diff --git a/src/keyboard.c b/src/keyboard.c
index 5fa58abce1d..590d183c4c6 100644
--- a/src/keyboard.c
+++ b/src/keyboard.c
@@ -12475,13 +12475,11 @@ keys_of_keyboard (void)
 void
 mark_kboards (void)
 {
-  KBOARD *kb;
-  Lisp_Object *p;
-  for (kb = all_kboards; kb; kb = kb->next_kboard)
+  for (KBOARD *kb = all_kboards; kb; kb = kb->next_kboard)
     {
       if (kb->kbd_macro_buffer)
-        for (p = kb->kbd_macro_buffer; p < kb->kbd_macro_ptr; p++)
-          mark_object (*p);
+	mark_objects (kb->kbd_macro_buffer,
+		      kb->kbd_macro_ptr - kb->kbd_macro_buffer);
       mark_object (KVAR (kb, Voverriding_terminal_local_map));
       mark_object (KVAR (kb, Vlast_command));
       mark_object (KVAR (kb, Vreal_last_command));
diff --git a/src/lisp.h b/src/lisp.h
index bc069ef2774..88e69b9061d 100644
--- a/src/lisp.h
+++ b/src/lisp.h
@@ -3756,12 +3756,12 @@ extern AVOID memory_full (size_t);
 extern AVOID buffer_memory_full (ptrdiff_t);
 extern bool survives_gc_p (Lisp_Object);
 extern void mark_object (Lisp_Object);
+extern void mark_objects (Lisp_Object *, ptrdiff_t);
 #if defined REL_ALLOC && !defined SYSTEM_MALLOC && !defined HYBRID_MALLOC
 extern void refill_memory_reserve (void);
 #endif
 extern void alloc_unexec_pre (void);
 extern void alloc_unexec_post (void);
-extern void mark_maybe_objects (Lisp_Object const *, ptrdiff_t);
 extern void mark_stack (char const *, char const *);
 extern void flush_stack_call_func1 (void (*func) (void *arg), void *arg);
 
@@ -4873,7 +4873,10 @@ safe_free_unbind_to (ptrdiff_t count, ptrdiff_t sa_count, Lisp_Object val)
       (buf) = AVAIL_ALLOCA (alloca_nbytes);		       \
     else						       \
       {							       \
-	(buf) = xmalloc (alloca_nbytes);		       \
+	/* Although only the first nelt words need clearing,   \
+	   typically EXTRA is 0 or small so just use xzalloc;  \
+	   this is simpler and often faster.  */	       \
+	(buf) = xzalloc (alloca_nbytes);		       \
 	record_unwind_protect_array (buf, nelt);	       \
       }							       \
   } while (false)