Remove `all_buffers` and the associated `next` field of buffers

* src/alloc.c (enum mem_type): Remove MEM_TYPE_BUFFER.
(allocate_buffer): Allocate like any other pseudovector.
Don't register on `all_buffers` any more.
(live_buffer_holding, live_buffer_p): Delete functions.
(mark_maybe_object, valid_lisp_object_p): Don't pay attention to
MEM_TYPE_BUFFER any more.
(garbage_collect): Only compact the live buffers.
(mark_buffer): Mark the undo_list of dead buffers here.
(mark_object): Buffers are normal pseudovectors now.
(sweep_buffers): Don't do the actual sweep here, just cleanup the
markers and only for live buffers.

* src/buffer.c (all_buffers): Remove variable.
(Fkill_buffer): Don't check indirect dead buffers.
Set the undo_list before we remove ourselves from the list of live buffers.
(Fbuffer_swap_text, Fset_buffer_multibyte): Don't check indirect dead
buffers.
(init_buffer_once): Don't set `all_buffers`.
(init_buffer): Don't map new memory for dead buffers.

* src/buffer.h (struct buffer): Remove `next` field.
(FOR_EACH_BUFFER): Remove macro.

* src/pdumper.c (dump_buffer): Don't dump the `next` field.

4 files changed, 49 insertions, 126 deletions

diff --git a/src/alloc.c b/src/alloc.c
index eed73bcdc4e..cc9ba8dbf50 100644
--- a/src/alloc.c
+++ b/src/alloc.c
@@ -432,7 +432,6 @@ inline static void set_interval_marked (INTERVAL i);
 enum mem_type
 {
   MEM_TYPE_NON_LISP,
-  MEM_TYPE_BUFFER,
   MEM_TYPE_CONS,
   MEM_TYPE_STRING,
   MEM_TYPE_SYMBOL,
@@ -3331,12 +3330,10 @@ allocate_pseudovector (int memlen, int lisplen,
 struct buffer *
 allocate_buffer (void)
 {
-  struct buffer *b = lisp_malloc (sizeof *b, false, MEM_TYPE_BUFFER);
-
+  struct buffer *b
+    = ALLOCATE_PSEUDOVECTOR (struct buffer, cursor_in_non_selected_windows_,
+                             PVEC_BUFFER);
   BUFFER_PVEC_INIT (b);
-  /* Put B on the chain of all buffers including killed ones.  */
-  b->next = all_buffers;
-  all_buffers = b;
   /* Note that the rest fields of B are not initialized.  */
   return b;
 }
@@ -4596,40 +4593,6 @@ live_vector_p (struct mem_node *m, void *p)
   return !NILP (live_vector_holding (m, p));
 }
 
-/* If P is a pointer into a valid buffer object, return the buffer.
-   Otherwise, return nil.  M is a pointer to the mem_block for P.
-   If IGNORE_KILLED is non-zero, treat killed buffers as invalid.  */
-
-static Lisp_Object
-live_buffer_holding (struct mem_node *m, void *p, bool ignore_killed)
-{
-  /* P must point into the block, and the buffer must not
-     have been killed unless ALL-BUFFERS is true.  */
-  if (m->type == MEM_TYPE_BUFFER)
-    {
-      struct buffer *b = m->start;
-      char *cb = m->start;
-      char *cp = p;
-      ptrdiff_t offset = cp - cb;
-      if (0 <= offset && offset < sizeof *b
-          && !(ignore_killed && NILP (b->name_)))
-        {
-          Lisp_Object obj;
-          XSETBUFFER (obj, b);
-          return obj;
-        }
-    }
-  return Qnil;
-}
-
-/* If P is a pointer into a live (valid and not killed) buffer object,
-   return non-zero.  */
-static bool
-live_buffer_p (struct mem_node *m, void *p)
-{
-  return !NILP (live_buffer_holding (m, p, true));
-}
-
 /* Mark OBJ if we can prove it's a Lisp_Object.  */
 
 static void
@@ -4684,8 +4647,7 @@ mark_maybe_object (Lisp_Object obj)
           break;
 
         case Lisp_Vectorlike:
-          mark_p = (EQ (obj, live_vector_holding (m, po))
-                    || EQ (obj, live_buffer_holding (m, po, false)));
+          mark_p = (EQ (obj, live_vector_holding (m, po)));
           break;
 
         default:
@@ -4754,10 +4716,6 @@ mark_maybe_pointer (void *p)
           /* Nothing to do; not a pointer to Lisp memory.  */
           break;
 
-        case MEM_TYPE_BUFFER:
-          obj = live_buffer_holding (m, p, false);
-          break;
-
         case MEM_TYPE_CONS:
           obj = live_cons_holding (m, p);
           break;
@@ -5157,9 +5115,6 @@ valid_lisp_object_p (Lisp_Object obj)
     case MEM_TYPE_SPARE:
       return 0;
 
-    case MEM_TYPE_BUFFER:
-      return live_buffer_p (m, p) ? 1 : 2;
-
     case MEM_TYPE_CONS:
       return live_cons_p (m, p);
 
@@ -5976,7 +5931,7 @@ maybe_garbage_collect (void)
 void
 garbage_collect (void)
 {
-  struct buffer *nextb;
+  Lisp_Object tail, buffer;
   char stack_top_variable;
   bool message_p;
   ptrdiff_t count = SPECPDL_INDEX ();
@@ -5992,8 +5947,8 @@ garbage_collect (void)
 
   /* Don't keep undo information around forever.
      Do this early on, so it is no problem if the user quits.  */
-  FOR_EACH_BUFFER (nextb)
-    compact_buffer (nextb);
+  FOR_EACH_LIVE_BUFFER (tail, buffer)
+    compact_buffer (XBUFFER (buffer));
 
   byte_ct tot_before = (profiler_memory_running
                         ? total_bytes_of_live_objects ()
@@ -6083,8 +6038,9 @@ garbage_collect (void)
 
   compact_font_caches ();
 
-  FOR_EACH_BUFFER (nextb)
+  FOR_EACH_LIVE_BUFFER (tail, buffer)
     {
+      struct buffer *nextb = XBUFFER (buffer);
       if (!EQ (BVAR (nextb, undo_list), Qt))
         bset_undo_list (nextb, compact_undo_list (BVAR (nextb, undo_list)));
       /* Now that we have stripped the elements that need not be
@@ -6349,7 +6305,12 @@ mark_buffer (struct buffer *buffer)
 
   /* For now, we just don't mark the undo_list.  It's done later in
      a special way just before the sweep phase, and after stripping
-     some of its elements that are not needed any more.  */
+     some of its elements that are not needed any more.
+     Note: this later processing is only done for live buffers, so
+     for dead buffers, the undo_list should be nil (set by Fkill_buffer),
+     but just to be on the safe side, we mark it here.  */
+  if (!BUFFER_LIVE_P (buffer))
+      mark_object (BVAR (buffer, undo_list));
 
   mark_overlay (buffer->overlays_before);
   mark_overlay (buffer->overlays_after);
@@ -6613,23 +6574,12 @@ mark_object (Lisp_Object arg)
           = PSEUDOVECTOR_TYPE (ptr);
 
         if (pvectype != PVEC_SUBR &&
-            pvectype != PVEC_BUFFER &&
             !main_thread_p (po))
           CHECK_LIVE (live_vector_p);
 
         switch (pvectype)
           {
           case PVEC_BUFFER:
-#if GC_CHECK_MARKED_OBJECTS
-            {
-              struct buffer *b;
-              FOR_EACH_BUFFER (b)
-                if (b == po)
-                  break;
-              if (b == NULL)
-                emacs_abort ();
-            }
-#endif /* GC_CHECK_MARKED_OBJECTS */
             mark_buffer ((struct buffer *) ptr);
             break;
 
@@ -7108,25 +7058,17 @@ NO_INLINE /* For better stack traces */
 static void
 sweep_buffers (void)
 {
-  struct buffer *buffer, **bprev = &all_buffers;
+  Lisp_Object tail, buf;
 
   gcstat.total_buffers = 0;
-  for (buffer = all_buffers; buffer; buffer = *bprev)
-    if (!vectorlike_marked_p (&buffer->header))
-      {
-        *bprev = buffer->next;
-        lisp_free (buffer);
-      }
-    else
-      {
-        if (!pdumper_object_p (buffer))
-          XUNMARK_VECTOR (buffer);
-        /* Do not use buffer_(set|get)_intervals here.  */
-        buffer->text->intervals = balance_intervals (buffer->text->intervals);
-        unchain_dead_markers (buffer);
-        gcstat.total_buffers++;
-        bprev = &buffer->next;
-      }
+  FOR_EACH_LIVE_BUFFER (tail, buf)
+    {
+      struct buffer *buffer = XBUFFER (buf);
+      /* Do not use buffer_(set|get)_intervals here.  */
+      buffer->text->intervals = balance_intervals (buffer->text->intervals);
+      unchain_dead_markers (buffer);
+      gcstat.total_buffers++;
+    }
 }
 
 /* Sweep: find all structures not marked, and free them.  */
diff --git a/src/buffer.c b/src/buffer.c
index 4e121ca4cab..d8842a6d770 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -51,11 +51,6 @@ along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.  */
 #include "w32heap.h"            /* for mmap_* */
 #endif
 
-/* First buffer in chain of all buffers (in reverse order of creation).
-   Threaded through ->header.next.buffer.  */
-
-struct buffer *all_buffers;
-
 /* This structure holds the default values of the buffer-local variables
    defined with DEFVAR_PER_BUFFER, that have special slots in each buffer.
    The default value occupies the same slot in this structure
@@ -1786,15 +1781,11 @@ cleaning up all windows currently displaying the buffer to be killed. */)
      ask questions or their hooks get errors.  */
   if (!b->base_buffer && b->indirections > 0)
     {
-      struct buffer *other;
+      Lisp_Object tail, other;
 
-      FOR_EACH_BUFFER (other)
-        if (other->base_buffer == b)
-          {
-            Lisp_Object buf;
-            XSETBUFFER (buf, other);
-            Fkill_buffer (buf);
-          }
+      FOR_EACH_LIVE_BUFFER (tail, other)
+        if (XBUFFER (other)->base_buffer == b)
+          Fkill_buffer (other);
 
       /* Exit if we now have killed the base buffer (Bug#11665).  */
       if (!BUFFER_LIVE_P (b))
@@ -1849,6 +1840,9 @@ cleaning up all windows currently displaying the buffer to be killed. */)
 
   tem = Vinhibit_quit;
   Vinhibit_quit = Qt;
+  /* Once the buffer is removed from Vbuffer_alist, its undo_list field is
+     not traced by the GC in the same way.  So set it to nil early.  */
+  bset_undo_list (b, Qnil);
   /* Remove the buffer from the list of all buffers.  */
   Vbuffer_alist = Fdelq (Frassq (buffer, Vbuffer_alist), Vbuffer_alist);
   /* If replace_buffer_in_windows didn't do its job fix that now.  */
@@ -1963,7 +1957,6 @@ cleaning up all windows currently displaying the buffer to be killed. */)
     }
   bset_width_table (b, Qnil);
   unblock_input ();
-  bset_undo_list (b, Qnil);
 
   /* Run buffer-list-update-hook.  */
   if (!NILP (Vrun_hooks) && !b->inhibit_buffer_hooks)
@@ -2351,10 +2344,10 @@ results, see Info node `(elisp)Swapping Text'.  */)
     error ("Cannot swap indirect buffers's text");
 
   { /* This is probably harder to make work.  */
-    struct buffer *other;
-    FOR_EACH_BUFFER (other)
-      if (other->base_buffer == other_buffer
-          || other->base_buffer == current_buffer)
+    Lisp_Object tail, other;
+    FOR_EACH_LIVE_BUFFER (tail, other)
+      if (XBUFFER (other)->base_buffer == other_buffer
+          || XBUFFER (other)->base_buffer == current_buffer)
         error ("One of the buffers to swap has indirect buffers");
   }
 
@@ -2502,7 +2495,7 @@ current buffer is cleared.  */)
   (Lisp_Object flag)
 {
   struct Lisp_Marker *tail, *markers;
-  struct buffer *other;
+  Lisp_Object btail, other;
   ptrdiff_t begv, zv;
   bool narrowed = (BEG != BEGV || Z != ZV);
   bool modified_p = !NILP (Fbuffer_modified_p (Qnil));
@@ -2755,13 +2748,16 @@ current buffer is cleared.  */)
 
   /* Copy this buffer's new multibyte status
      into all of its indirect buffers.  */
-  FOR_EACH_BUFFER (other)
-    if (other->base_buffer == current_buffer && BUFFER_LIVE_P (other))
-      {
-        BVAR (other, enable_multibyte_characters)
-          = BVAR (current_buffer, enable_multibyte_characters);
-        other->prevent_redisplay_optimizations_p = 1;
-      }
+  FOR_EACH_LIVE_BUFFER (btail, other)
+    {
+      struct buffer *o = XBUFFER (other);
+      if (o->base_buffer == current_buffer && BUFFER_LIVE_P (o))
+        {
+          BVAR (o, enable_multibyte_characters)
+            = BVAR (current_buffer, enable_multibyte_characters);
+          o->prevent_redisplay_optimizations_p = true;
+        }
+    }
 
   /* Restore the modifiedness of the buffer.  */
   if (!modified_p && !NILP (Fbuffer_modified_p (Qnil)))
@@ -5327,8 +5323,6 @@ init_buffer_once (void)
   Vbuffer_alist = Qnil;
   current_buffer = 0;
   pdumper_remember_lv_ptr_raw (&current_buffer, Lisp_Vectorlike);
-  all_buffers = 0;
-  pdumper_remember_lv_ptr_raw (&all_buffers, Lisp_Vectorlike);
 
   QSFundamental = build_pure_c_string ("Fundamental");
 
@@ -5359,7 +5353,7 @@ init_buffer (void)
 #ifdef USE_MMAP_FOR_BUFFERS
   if (dumped_with_unexec_p ())
     {
-      struct buffer *b;
+      Lisp_Object tail, buffer;
 
 #ifndef WINDOWSNT
       /* These must be reset in the dumped Emacs, to avoid stale
@@ -5381,8 +5375,9 @@ init_buffer (void)
          " *code-conversion-work*".  They are created by
          init_buffer_once and init_window_once (which are not called
          in the dumped Emacs), and by the first call to coding.c routines.  */
-      FOR_EACH_BUFFER (b)
+      FOR_EACH_LIVE_BUFFER (tail, buffer)
         {
+          struct buffer *b = XBUFFER (buffer);
           b->text->beg = NULL;
           enlarge_buffer_text (b, 0);
         }
diff --git a/src/buffer.h b/src/buffer.h
index 31f497ea40a..abb1294d038 100644
--- a/src/buffer.h
+++ b/src/buffer.h
@@ -570,9 +570,6 @@ struct buffer
      In an indirect buffer, this is the own_text field of another buffer.  */
   struct buffer_text *text;
 
-  /* Next buffer, in chain of all buffers, including killed ones.  */
-  struct buffer *next;
-
   /* Char position of point in buffer.  */
   ptrdiff_t pt;
 
@@ -1104,15 +1101,6 @@ BUFFER_CHECK_INDIRECTION (struct buffer *b)
     }
 }
 
-/* Chain of all buffers, including killed ones.  */
-
-extern struct buffer *all_buffers;
-
-/* Used to iterate over the chain above.  */
-
-#define FOR_EACH_BUFFER(b) \
-  for ((b) = all_buffers; (b); (b) = (b)->next)
-
 /* This structure holds the default values of the buffer-local variables
    that have special slots in each buffer.
    The default value occupies the same slot in this structure
diff --git a/src/pdumper.c b/src/pdumper.c
index e52163cdae2..b5861950c92 100644
--- a/src/pdumper.c
+++ b/src/pdumper.c
@@ -2845,8 +2845,6 @@ dump_buffer (struct dump_context *ctx, const struct buffer *in_buffer)
      ctx->obj_offset + dump_offsetof (struct buffer, text),
      base_offset + dump_offsetof (struct buffer, own_text));
 
-  dump_field_lv_rawptr (ctx, out, buffer, &buffer->next,
-                        Lisp_Vectorlike, WEIGHT_NORMAL);
   DUMP_FIELD_COPY (out, buffer, pt);
   DUMP_FIELD_COPY (out, buffer, pt_byte);
   DUMP_FIELD_COPY (out, buffer, begv);