1 files changed, 270 insertions, 215 deletions

diff --git a/src/eval.c b/src/eval.c
index 69483a9b205..d1d074df777 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -32,8 +32,6 @@ along with GNU Emacs.  If not, see <http://www.gnu.org/licenses/>.  */
 #include "xterm.h"
 #endif
 
-struct backtrace *backtrace_list;
-
 #if !BYTE_MARK_STACK
 static
 #endif
@@ -105,7 +103,7 @@ static EMACS_INT when_entered_debugger;
 
 /* The function from which the last `signal' was called.  Set in
    Fsignal.  */
-
+/* FIXME: We should probably get rid of this!  */
 Lisp_Object Vsignaling_function;
 
 /* If non-nil, Lisp code must not be run since some part of Emacs is
@@ -117,20 +115,48 @@ Lisp_Object inhibit_lisp_code;
 static Lisp_Object funcall_lambda (Lisp_Object, ptrdiff_t, Lisp_Object *);
 static Lisp_Object apply_lambda (Lisp_Object fun, Lisp_Object args);
 
-/* Functions to set Lisp_Object slots of struct specbinding.  */
+/* Functions to modify slots of backtrace records.  */
+
+static void
+set_backtrace_args (struct specbinding *pdl, Lisp_Object *args)
+{ eassert (pdl->kind == SPECPDL_BACKTRACE); pdl->v.bt.args = args; }
+
+static void
+set_backtrace_nargs (struct specbinding *pdl, ptrdiff_t n)
+{ eassert (pdl->kind == SPECPDL_BACKTRACE); pdl->v.bt.nargs = n; }
 
 static void
-set_specpdl_symbol (Lisp_Object symbol)
+set_backtrace_debug_on_exit (struct specbinding *pdl, bool doe)
+{ eassert (pdl->kind == SPECPDL_BACKTRACE); pdl->v.bt.debug_on_exit = doe; }
+
+/* Helper functions to scan the backtrace.  */
+
+bool backtrace_p (struct specbinding *) EXTERNALLY_VISIBLE;
+struct specbinding *backtrace_top (void) EXTERNALLY_VISIBLE;
+struct specbinding *backtrace_next (struct specbinding *pdl) EXTERNALLY_VISIBLE;
+
+bool backtrace_p (struct specbinding *pdl)
+{ return pdl >= specpdl; }
+
+struct specbinding *
+backtrace_top (void)
 {
-  specpdl_ptr->symbol = symbol;
+  struct specbinding *pdl = specpdl_ptr - 1;
+  while (backtrace_p (pdl) && pdl->kind != SPECPDL_BACKTRACE)
+    pdl--;
+  return pdl;
 }
 
-static void
-set_specpdl_old_value (Lisp_Object oldval)
+struct specbinding *
+backtrace_next (struct specbinding *pdl)
 {
-  specpdl_ptr->old_value = oldval;
+  pdl--;
+  while (backtrace_p (pdl) && pdl->kind != SPECPDL_BACKTRACE)
+    pdl--;
+  return pdl;
 }
 
+
 void
 init_eval_once (void)
 {
@@ -151,7 +177,6 @@ init_eval (void)
   specpdl_ptr = specpdl;
   catchlist = 0;
   handlerlist = 0;
-  backtrace_list = 0;
   Vquit_flag = Qnil;
   debug_on_next_call = 0;
   lisp_eval_depth = 0;
@@ -234,7 +259,7 @@ static void
 do_debug_on_call (Lisp_Object code)
 {
   debug_on_next_call = 0;
-  backtrace_list->debug_on_exit = 1;
+  set_backtrace_debug_on_exit (specpdl_ptr - 1, true);
   call_debugger (Fcons (code, Qnil));
 }
 
@@ -530,9 +555,8 @@ The return value is BASE-VARIABLE.  */)
     struct specbinding *p;
 
     for (p = specpdl_ptr; p > specpdl; )
-      if ((--p)->func == NULL
-          && (EQ (new_alias,
-                  CONSP (p->symbol) ? XCAR (p->symbol) : p->symbol)))
+      if ((--p)->kind >= SPECPDL_LET
+          && (EQ (new_alias, specpdl_symbol (p))))
         error ("Don't know how to make a let-bound variable an alias");
   }
 
@@ -597,8 +621,9 @@ usage: (defvar SYMBOL &optional INITVALUE DOCSTRING)  */)
           struct specbinding *pdl = specpdl_ptr;
           while (pdl > specpdl)
             {
-              if (EQ ((--pdl)->symbol, sym) && !pdl->func
-                  && EQ (pdl->old_value, Qunbound))
+              if ((--pdl)->kind >= SPECPDL_LET
+                  && EQ (specpdl_symbol (pdl), sym)
+                  && EQ (specpdl_old_value (pdl), Qunbound))
                 {
                   message_with_string
                     ("Warning: defvar ignored because %s is let-bound",
@@ -937,7 +962,7 @@ usage: (catch TAG BODY...)  */)
 
 /* Set up a catch, then call C function FUNC on argument ARG.
    FUNC should return a Lisp_Object.
-   This is how catches are done from within C code. */
+   This is how catches are done from within C code.  */
 
 Lisp_Object
 internal_catch (Lisp_Object tag, Lisp_Object (*func) (Lisp_Object), Lisp_Object arg)
@@ -949,7 +974,6 @@ internal_catch (Lisp_Object tag, Lisp_Object (*func) (Lisp_Object), Lisp_Object
   c.next = catchlist;
   c.tag = tag;
   c.val = Qnil;
-  c.backlist = backtrace_list;
   c.handlerlist = handlerlist;
   c.lisp_eval_depth = lisp_eval_depth;
   c.pdlcount = SPECPDL_INDEX ();
@@ -1014,7 +1038,6 @@ unwind_to_catch (struct catchtag *catch, Lisp_Object value)
 #ifdef DEBUG_GCPRO
   gcpro_level = gcprolist ? gcprolist->level + 1 : 0;
 #endif
-  backtrace_list = catch->backlist;
   lisp_eval_depth = catch->lisp_eval_depth;
 
   sys_longjmp (catch->jmp, 1);
@@ -1115,7 +1138,6 @@ internal_lisp_condition_case (volatile Lisp_Object var, Lisp_Object bodyform,
 
   c.tag = Qnil;
   c.val = Qnil;
-  c.backlist = backtrace_list;
   c.handlerlist = handlerlist;
   c.lisp_eval_depth = lisp_eval_depth;
   c.pdlcount = SPECPDL_INDEX ();
@@ -1131,7 +1153,7 @@ internal_lisp_condition_case (volatile Lisp_Object var, Lisp_Object bodyform,
 
       /* Note that this just undoes the binding of h.var; whoever
          longjumped to us unwound the stack to c.pdlcount before
-         throwing. */
+         throwing.  */
       unbind_to (c.pdlcount, Qnil);
       return val;
     }
@@ -1170,7 +1192,6 @@ internal_condition_case (Lisp_Object (*bfun) (void), Lisp_Object handlers,
 
   c.tag = Qnil;
   c.val = Qnil;
-  c.backlist = backtrace_list;
   c.handlerlist = handlerlist;
   c.lisp_eval_depth = lisp_eval_depth;
   c.pdlcount = SPECPDL_INDEX ();
@@ -1208,7 +1229,6 @@ internal_condition_case_1 (Lisp_Object (*bfun) (Lisp_Object), Lisp_Object arg,
 
   c.tag = Qnil;
   c.val = Qnil;
-  c.backlist = backtrace_list;
   c.handlerlist = handlerlist;
   c.lisp_eval_depth = lisp_eval_depth;
   c.pdlcount = SPECPDL_INDEX ();
@@ -1250,7 +1270,6 @@ internal_condition_case_2 (Lisp_Object (*bfun) (Lisp_Object, Lisp_Object),
 
   c.tag = Qnil;
   c.val = Qnil;
-  c.backlist = backtrace_list;
   c.handlerlist = handlerlist;
   c.lisp_eval_depth = lisp_eval_depth;
   c.pdlcount = SPECPDL_INDEX ();
@@ -1294,7 +1313,6 @@ internal_condition_case_n (Lisp_Object (*bfun) (ptrdiff_t, Lisp_Object *),
 
   c.tag = Qnil;
   c.val = Qnil;
-  c.backlist = backtrace_list;
   c.handlerlist = handlerlist;
   c.lisp_eval_depth = lisp_eval_depth;
   c.pdlcount = SPECPDL_INDEX ();
@@ -1362,7 +1380,6 @@ See also the function `condition-case'.  */)
     = (NILP (error_symbol) ? Fcar (data) : error_symbol);
   register Lisp_Object clause = Qnil;
   struct handler *h;
-  struct backtrace *bp;
 
   immediate_quit = 0;
   abort_on_gc = 0;
@@ -1398,13 +1415,13 @@ See also the function `condition-case'.  */)
      too.  Don't do this when ERROR_SYMBOL is nil, because that
      is a memory-full error.  */
   Vsignaling_function = Qnil;
-  if (backtrace_list && !NILP (error_symbol))
+  if (!NILP (error_symbol))
     {
-      bp = backtrace_list->next;
-      if (bp && EQ (bp->function, Qerror))
-        bp = bp->next;
-      if (bp)
-        Vsignaling_function = bp->function;
+      struct specbinding *pdl = backtrace_next (backtrace_top ());
+      if (backtrace_p (pdl) && EQ (backtrace_function (pdl), Qerror))
+        pdl = backtrace_next (pdl);
+      if (backtrace_p (pdl))
+        Vsignaling_function = backtrace_function (pdl);
     }
 
   for (h = handlerlist; h; h = h->next)
@@ -1901,6 +1918,36 @@ If LEXICAL is t, evaluate using lexical scoping.  */)
   return unbind_to (count, eval_sub (form));
 }
 
+static void
+grow_specpdl (void)
+{
+  register ptrdiff_t count = SPECPDL_INDEX ();
+  ptrdiff_t max_size = min (max_specpdl_size, PTRDIFF_MAX);
+  if (max_size <= specpdl_size)
+    {
+      if (max_specpdl_size < 400)
+        max_size = max_specpdl_size = 400;
+      if (max_size <= specpdl_size)
+        signal_error ("Variable binding depth exceeds max-specpdl-size", Qnil);
+    }
+  specpdl = xpalloc (specpdl, &specpdl_size, 1, max_size, sizeof *specpdl);
+  specpdl_ptr = specpdl + count;
+}
+
+void
+record_in_backtrace (Lisp_Object function, Lisp_Object *args, ptrdiff_t nargs)
+{
+  eassert (nargs >= UNEVALLED);
+  if (specpdl_ptr == specpdl + specpdl_size)
+    grow_specpdl ();
+  specpdl_ptr->kind = SPECPDL_BACKTRACE;
+  specpdl_ptr->v.bt.function = function;
+  specpdl_ptr->v.bt.args = args;
+  specpdl_ptr->v.bt.nargs = nargs;
+  specpdl_ptr->v.bt.debug_on_exit = false;
+  specpdl_ptr++;
+}
+
 /* Eval a sub-expression of the current expression (i.e. in the same
    lexical scope).  */
 Lisp_Object
@@ -1908,7 +1955,6 @@ eval_sub (Lisp_Object form)
 {
   Lisp_Object fun, val, original_fun, original_args;
   Lisp_Object funcar;
-  struct backtrace backtrace;
   struct gcpro gcpro1, gcpro2, gcpro3;
 
   if (SYMBOLP (form))
@@ -1946,12 +1992,8 @@ eval_sub (Lisp_Object form)
   original_fun = XCAR (form);
   original_args = XCDR (form);
 
-  backtrace.next = backtrace_list;
-  backtrace.function = original_fun; /* This also protects them from gc.  */
-  backtrace.args = &original_args;
-  backtrace.nargs = UNEVALLED;
-  backtrace.debug_on_exit = 0;
-  backtrace_list = &backtrace;
+  /* This also protects them from gc.  */
+  record_in_backtrace (original_fun, &original_args, UNEVALLED);
 
   if (debug_on_next_call)
     do_debug_on_call (Qt);
@@ -2005,8 +2047,8 @@ eval_sub (Lisp_Object form)
               gcpro3.nvars = argnum;
             }
 
-          backtrace.args = vals;
-          backtrace.nargs = XINT (numargs);
+          set_backtrace_args (specpdl_ptr - 1, vals);
+          set_backtrace_nargs (specpdl_ptr - 1, XINT (numargs));
 
           val = (XSUBR (fun)->function.aMANY) (XINT (numargs), vals);
           UNGCPRO;
@@ -2027,8 +2069,8 @@ eval_sub (Lisp_Object form)
 
           UNGCPRO;
 
-          backtrace.args = argvals;
-          backtrace.nargs = XINT (numargs);
+          set_backtrace_args (specpdl_ptr - 1, argvals);
+          set_backtrace_nargs (specpdl_ptr - 1, XINT (numargs));
 
           switch (i)
             {
@@ -2118,9 +2160,9 @@ eval_sub (Lisp_Object form)
   check_cons_list ();
 
   lisp_eval_depth--;
-  if (backtrace.debug_on_exit)
+  if (backtrace_debug_on_exit (specpdl_ptr - 1))
     val = call_debugger (Fcons (Qexit, Fcons (val, Qnil)));
-  backtrace_list = backtrace.next;
+  specpdl_ptr--;
 
   return val;
 }
@@ -2600,7 +2642,6 @@ usage: (funcall FUNCTION &rest ARGUMENTS)  */)
   ptrdiff_t numargs = nargs - 1;
   Lisp_Object lisp_numargs;
   Lisp_Object val;
-  struct backtrace backtrace;
   register Lisp_Object *internal_args;
   ptrdiff_t i;
 
@@ -2614,12 +2655,8 @@ usage: (funcall FUNCTION &rest ARGUMENTS)  */)
         error ("Lisp nesting exceeds `max-lisp-eval-depth'");
     }
 
-  backtrace.next = backtrace_list;
-  backtrace.function = args[0];
-  backtrace.args = &args[1];    /* This also GCPROs them.  */
-  backtrace.nargs = nargs - 1;
-  backtrace.debug_on_exit = 0;
-  backtrace_list = &backtrace;
+  /* This also GCPROs them.  */
+  record_in_backtrace (args[0], &args[1], nargs - 1);
 
   /* Call GC after setting up the backtrace, so the latter GCPROs the args.  */
   maybe_gc ();
@@ -2744,9 +2781,9 @@ usage: (funcall FUNCTION &rest ARGUMENTS)  */)
     }
   check_cons_list ();
   lisp_eval_depth--;
-  if (backtrace.debug_on_exit)
+  if (backtrace_debug_on_exit (specpdl_ptr - 1))
     val = call_debugger (Fcons (Qexit, Fcons (val, Qnil)));
-  backtrace_list = backtrace.next;
+  specpdl_ptr--;
   return val;
 }
 
@@ -2778,15 +2815,17 @@ apply_lambda (Lisp_Object fun, Lisp_Object args)
 
   UNGCPRO;
 
-  backtrace_list->args = arg_vector;
-  backtrace_list->nargs = i;
+  set_backtrace_args (specpdl_ptr - 1, arg_vector);
+  set_backtrace_nargs (specpdl_ptr - 1, i);
   tem = funcall_lambda (fun, numargs, arg_vector);
 
   /* Do the debug-on-exit now, while arg_vector still exists.  */
-  if (backtrace_list->debug_on_exit)
-    tem = call_debugger (Fcons (Qexit, Fcons (tem, Qnil)));
-  /* Don't do it again when we return to eval.  */
-  backtrace_list->debug_on_exit = 0;
+  if (backtrace_debug_on_exit (specpdl_ptr - 1))
+    {
+      /* Don't do it again when we return to eval.  */
+      set_backtrace_debug_on_exit (specpdl_ptr - 1, false);
+      tem = call_debugger (Fcons (Qexit, Fcons (tem, Qnil)));
+    }
   SAFE_FREE ();
   return tem;
 }
@@ -2936,20 +2975,38 @@ DEFUN ("fetch-bytecode", Ffetch_bytecode, Sfetch_bytecode,
   return object;
 }
 
-static void
-grow_specpdl (void)
+/* Return true if SYMBOL currently has a let-binding
+   which was made in the buffer that is now current.  */
+
+bool
+let_shadows_buffer_binding_p (struct Lisp_Symbol *symbol)
 {
-  register ptrdiff_t count = SPECPDL_INDEX ();
-  ptrdiff_t max_size = min (max_specpdl_size, PTRDIFF_MAX);
-  if (max_size <= specpdl_size)
-    {
-      if (max_specpdl_size < 400)
-        max_size = max_specpdl_size = 400;
-      if (max_size <= specpdl_size)
-        signal_error ("Variable binding depth exceeds max-specpdl-size", Qnil);
-    }
-  specpdl = xpalloc (specpdl, &specpdl_size, 1, max_size, sizeof *specpdl);
-  specpdl_ptr = specpdl + count;
+  struct specbinding *p;
+  Lisp_Object buf = Fcurrent_buffer ();
+
+  for (p = specpdl_ptr; p > specpdl; )
+    if ((--p)->kind > SPECPDL_LET)
+      {
+        struct Lisp_Symbol *let_bound_symbol = XSYMBOL (specpdl_symbol (p));
+        eassert (let_bound_symbol->redirect != SYMBOL_VARALIAS);
+        if (symbol == let_bound_symbol
+            && EQ (specpdl_where (p), buf))
+          return 1;
+      }
+
+  return 0;
+}
+
+bool
+let_shadows_global_binding_p (Lisp_Object symbol)
+{
+  struct specbinding *p;
+
+  for (p = specpdl_ptr; p > specpdl; )
+    if ((--p)->kind >= SPECPDL_LET && EQ (specpdl_symbol (p), symbol))
+      return 1;
+
+  return 0;
 }
 
 /* `specpdl_ptr->symbol' is a field which describes which variable is
@@ -2985,9 +3042,9 @@ specbind (Lisp_Object symbol, Lisp_Object value)
     case SYMBOL_PLAINVAL:
       /* The most common case is that of a non-constant symbol with a
          trivial value.  Make that as fast as we can.  */
-      set_specpdl_symbol (symbol);
-      set_specpdl_old_value (SYMBOL_VAL (sym));
-      specpdl_ptr->func = NULL;
+      specpdl_ptr->kind = SPECPDL_LET;
+      specpdl_ptr->v.let.symbol = symbol;
+      specpdl_ptr->v.let.old_value = SYMBOL_VAL (sym);
       ++specpdl_ptr;
       if (!sym->constant)
         SET_SYMBOL_VAL (sym, value);
@@ -3000,59 +3057,36 @@ specbind (Lisp_Object symbol, Lisp_Object value)
     case SYMBOL_FORWARDED:
       {
         Lisp_Object ovalue = find_symbol_value (symbol);
-        specpdl_ptr->func = 0;
-        set_specpdl_old_value (ovalue);
+        specpdl_ptr->kind = SPECPDL_LET_LOCAL;
+        specpdl_ptr->v.let.symbol = symbol;
+        specpdl_ptr->v.let.old_value = ovalue;
+        specpdl_ptr->v.let.where = Fcurrent_buffer ();
 
         eassert (sym->redirect != SYMBOL_LOCALIZED
-                 || (EQ (SYMBOL_BLV (sym)->where,
-                         SYMBOL_BLV (sym)->frame_local ?
-                         Fselected_frame () : Fcurrent_buffer ())));
+                 || (EQ (SYMBOL_BLV (sym)->where, Fcurrent_buffer ())));
 
-        if (sym->redirect == SYMBOL_LOCALIZED
-            || BUFFER_OBJFWDP (SYMBOL_FWD (sym)))
+        if (sym->redirect == SYMBOL_LOCALIZED)
+          {
+            if (!blv_found (SYMBOL_BLV (sym)))
+              specpdl_ptr->kind = SPECPDL_LET_DEFAULT;
+          }
+        else if (BUFFER_OBJFWDP (SYMBOL_FWD (sym)))
           {
-            Lisp_Object where, cur_buf = Fcurrent_buffer ();
-
-            /* For a local variable, record both the symbol and which
-               buffer's or frame's value we are saving.  */
-            if (!NILP (Flocal_variable_p (symbol, Qnil)))
-              {
-                eassert (sym->redirect != SYMBOL_LOCALIZED
-                         || (blv_found (SYMBOL_BLV (sym))
-                             && EQ (cur_buf, SYMBOL_BLV (sym)->where)));
-                where = cur_buf;
-              }
-            else if (sym->redirect == SYMBOL_LOCALIZED
-                     && blv_found (SYMBOL_BLV (sym)))
-              where = SYMBOL_BLV (sym)->where;
-            else
-              where = Qnil;
-
-            /* We're not using the `unused' slot in the specbinding
-               structure because this would mean we have to do more
-               work for simple variables.  */
-            /* FIXME: The third value `current_buffer' is only used in
-               let_shadows_buffer_binding_p which is itself only used
-               in set_internal for local_if_set.  */
-            eassert (NILP (where) || EQ (where, cur_buf));
-            set_specpdl_symbol (Fcons (symbol, Fcons (where, cur_buf)));
-
             /* If SYMBOL is a per-buffer variable which doesn't have a
                buffer-local value here, make the `let' change the global
                value by changing the value of SYMBOL in all buffers not
                having their own value.  This is consistent with what
                happens with other buffer-local variables.  */
-            if (NILP (where)
-                && sym->redirect == SYMBOL_FORWARDED)
+            if (NILP (Flocal_variable_p (symbol, Qnil)))
               {
-                eassert (BUFFER_OBJFWDP (SYMBOL_FWD (sym)));
+                specpdl_ptr->kind = SPECPDL_LET_DEFAULT;
                 ++specpdl_ptr;
                 Fset_default (symbol, value);
                 return;
               }
           }
         else
-          set_specpdl_symbol (symbol);
+          specpdl_ptr->kind = SPECPDL_LET;
 
         specpdl_ptr++;
         set_internal (symbol, value, Qnil, 1);
@@ -3067,9 +3101,9 @@ record_unwind_protect (Lisp_Object (*function) (Lisp_Object), Lisp_Object arg)
 {
   if (specpdl_ptr == specpdl + specpdl_size)
     grow_specpdl ();
-  specpdl_ptr->func = function;
-  set_specpdl_symbol (Qnil);
-  set_specpdl_old_value (arg);
+  specpdl_ptr->kind = SPECPDL_UNWIND;
+  specpdl_ptr->v.unwind.func = function;
+  specpdl_ptr->v.unwind.arg = arg;
   specpdl_ptr++;
 }
 
@@ -3093,41 +3127,50 @@ unbind_to (ptrdiff_t count, Lisp_Object value)
       struct specbinding this_binding;
       this_binding = *--specpdl_ptr;
 
-      if (this_binding.func != 0)
-        (*this_binding.func) (this_binding.old_value);
-      /* If the symbol is a list, it is really (SYMBOL WHERE
-         . CURRENT-BUFFER) where WHERE is either nil, a buffer, or a
-         frame.  If WHERE is a buffer or frame, this indicates we
-         bound a variable that had a buffer-local or frame-local
-         binding.  WHERE nil means that the variable had the default
-         value when it was bound.  CURRENT-BUFFER is the buffer that
-         was current when the variable was bound.  */
-      else if (CONSP (this_binding.symbol))
+      switch (this_binding.kind)
         {
-          Lisp_Object symbol, where;
-
-          symbol = XCAR (this_binding.symbol);
-          where = XCAR (XCDR (this_binding.symbol));
-
-          if (NILP (where))
-            Fset_default (symbol, this_binding.old_value);
-          /* If `where' is non-nil, reset the value in the appropriate
-             local binding, but only if that binding still exists.  */
-          else if (BUFFERP (where)
-                   ? !NILP (Flocal_variable_p (symbol, where))
-                   : !NILP (Fassq (symbol, XFRAME (where)->param_alist)))
-            set_internal (symbol, this_binding.old_value, where, 1);
+        case SPECPDL_UNWIND:
+          (*specpdl_func (&this_binding)) (specpdl_arg (&this_binding));
+          break;
+        case SPECPDL_LET:
+          /* If variable has a trivial value (no forwarding), we can
+             just set it.  No need to check for constant symbols here,
+             since that was already done by specbind.  */
+          if (XSYMBOL (specpdl_symbol (&this_binding))->redirect
+              == SYMBOL_PLAINVAL)
+            SET_SYMBOL_VAL (XSYMBOL (specpdl_symbol (&this_binding)),
+                            specpdl_old_value (&this_binding));
+          else
+            /* NOTE: we only ever come here if make_local_foo was used for
+               the first time on this var within this let.  */
+            Fset_default (specpdl_symbol (&this_binding),
+                          specpdl_old_value (&this_binding));
+          break;
+        case SPECPDL_BACKTRACE:
+          break;
+        case SPECPDL_LET_LOCAL:
+        case SPECPDL_LET_DEFAULT:
+          { /* If the symbol is a list, it is really (SYMBOL WHERE
+             . CURRENT-BUFFER) where WHERE is either nil, a buffer, or a
+             frame.  If WHERE is a buffer or frame, this indicates we
+             bound a variable that had a buffer-local or frame-local
+             binding.  WHERE nil means that the variable had the default
+             value when it was bound.  CURRENT-BUFFER is the buffer that
+             was current when the variable was bound.  */
+            Lisp_Object symbol = specpdl_symbol (&this_binding);
+            Lisp_Object where = specpdl_where (&this_binding);
+            eassert (BUFFERP (where));
+
+            if (this_binding.kind == SPECPDL_LET_DEFAULT)
+              Fset_default (symbol, specpdl_old_value (&this_binding));
+            /* If this was a local binding, reset the value in the appropriate
+               buffer, but only if that buffer's binding still exists.  */
+            else if (!NILP (Flocal_variable_p (symbol, where)))
+              set_internal (symbol, specpdl_old_value (&this_binding),
+                            where, 1);
+          }
+          break;
         }
-      /* If variable has a trivial value (no forwarding), we can
-         just set it.  No need to check for constant symbols here,
-         since that was already done by specbind.  */
-      else if (XSYMBOL (this_binding.symbol)->redirect == SYMBOL_PLAINVAL)
-        SET_SYMBOL_VAL (XSYMBOL (this_binding.symbol),
-                        this_binding.old_value);
-      else
-        /* NOTE: we only ever come here if make_local_foo was used for
-           the first time on this var within this let.  */
-        Fset_default (this_binding.symbol, this_binding.old_value);
     }
 
   if (NILP (Vquit_flag) && !NILP (quitf))
@@ -3153,18 +3196,16 @@ DEFUN ("backtrace-debug", Fbacktrace_debug, Sbacktrace_debug, 2, 2, 0,
 The debugger is entered when that frame exits, if the flag is non-nil.  */)
   (Lisp_Object level, Lisp_Object flag)
 {
-  register struct backtrace *backlist = backtrace_list;
+  struct specbinding *pdl = backtrace_top ();
   register EMACS_INT i;
 
   CHECK_NUMBER (level);
 
-  for (i = 0; backlist && i < XINT (level); i++)
-    {
-      backlist = backlist->next;
-    }
+  for (i = 0; backtrace_p (pdl) && i < XINT (level); i++)
+    pdl = backtrace_next (pdl);
 
-  if (backlist)
-    backlist->debug_on_exit = !NILP (flag);
+  if (backtrace_p (pdl))
+    set_backtrace_debug_on_exit (pdl, !NILP (flag));
 
   return flag;
 }
@@ -3174,58 +3215,41 @@ DEFUN ("backtrace", Fbacktrace, Sbacktrace, 0, 0, "",
 Output stream used is value of `standard-output'.  */)
   (void)
 {
-  register struct backtrace *backlist = backtrace_list;
-  Lisp_Object tail;
+  struct specbinding *pdl = backtrace_top ();
   Lisp_Object tem;
-  struct gcpro gcpro1;
   Lisp_Object old_print_level = Vprint_level;
 
   if (NILP (Vprint_level))
     XSETFASTINT (Vprint_level, 8);
 
-  tail = Qnil;
-  GCPRO1 (tail);
-
-  while (backlist)
+  while (backtrace_p (pdl))
     {
-      write_string (backlist->debug_on_exit ? "* " : "  ", 2);
-      if (backlist->nargs == UNEVALLED)
+      write_string (backtrace_debug_on_exit (pdl) ? "* " : "  ", 2);
+      if (backtrace_nargs (pdl) == UNEVALLED)
         {
-          Fprin1 (Fcons (backlist->function, *backlist->args), Qnil);
+          Fprin1 (Fcons (backtrace_function (pdl), *backtrace_args (pdl)),
+                  Qnil);
           write_string ("\n", -1);
         }
       else
         {
-          tem = backlist->function;
+          tem = backtrace_function (pdl);
           Fprin1 (tem, Qnil);   /* This can QUIT.  */
           write_string ("(", -1);
-          if (backlist->nargs == MANY)
-            {                   /* FIXME: Can this happen?  */
-              bool later_arg = 0;
-              for (tail = *backlist->args; !NILP (tail); tail = Fcdr (tail))
-                {
-                  if (later_arg)
-                    write_string (" ", -1);
-                  Fprin1 (Fcar (tail), Qnil);
-                  later_arg = 1;
-                }
-            }
-          else
-            {
-              ptrdiff_t i;
-              for (i = 0; i < backlist->nargs; i++)
-                {
-                  if (i) write_string (" ", -1);
-                  Fprin1 (backlist->args[i], Qnil);
-                }
-            }
+          {
+            ptrdiff_t i;
+            for (i = 0; i < backtrace_nargs (pdl); i++)
+              {
+                if (i) write_string (" ", -1);
+                Fprin1 (backtrace_args (pdl)[i], Qnil);
+              }
+          }
           write_string (")\n", -1);
         }
-      backlist = backlist->next;
+      pdl = backtrace_next (pdl);
     }
 
   Vprint_level = old_print_level;
-  UNGCPRO;
   return Qnil;
 }
 
@@ -3241,53 +3265,84 @@ or a lambda expression for macro calls.
 If NFRAMES is more than the number of frames, the value is nil.  */)
   (Lisp_Object nframes)
 {
-  register struct backtrace *backlist = backtrace_list;
+  struct specbinding *pdl = backtrace_top ();
   register EMACS_INT i;
-  Lisp_Object tem;
 
   CHECK_NATNUM (nframes);
 
   /* Find the frame requested.  */
-  for (i = 0; backlist && i < XFASTINT (nframes); i++)
-    backlist = backlist->next;
+  for (i = 0; backtrace_p (pdl) && i < XFASTINT (nframes); i++)
+    pdl = backtrace_next (pdl);
 
-  if (!backlist)
+  if (!backtrace_p (pdl))
     return Qnil;
-  if (backlist->nargs == UNEVALLED)
-    return Fcons (Qnil, Fcons (backlist->function, *backlist->args));
+  if (backtrace_nargs (pdl) == UNEVALLED)
+    return Fcons (Qnil,
+                  Fcons (backtrace_function (pdl), *backtrace_args (pdl)));
   else
     {
-      if (backlist->nargs == MANY) /* FIXME: Can this happen?  */
-        tem = *backlist->args;
-      else
-        tem = Flist (backlist->nargs, backlist->args);
+      Lisp_Object tem = Flist (backtrace_nargs (pdl), backtrace_args (pdl));
 
-      return Fcons (Qt, Fcons (backlist->function, tem));
+      return Fcons (Qt, Fcons (backtrace_function (pdl), tem));
     }
 }
 
 
-#if BYTE_MARK_STACK
 void
-mark_backtrace (void)
+mark_specpdl (void)
 {
-  register struct backtrace *backlist;
-  ptrdiff_t i;
-
-  for (backlist = backtrace_list; backlist; backlist = backlist->next)
+  struct specbinding *pdl;
+  for (pdl = specpdl; pdl != specpdl_ptr; pdl++)
     {
-      mark_object (backlist->function);
+      switch (pdl->kind)
+        {
+        case SPECPDL_UNWIND:
+          mark_object (specpdl_arg (pdl));
+          break;
+        case SPECPDL_BACKTRACE:
+          {
+            ptrdiff_t nargs = backtrace_nargs (pdl);
+            mark_object (backtrace_function (pdl));
+            if (nargs == UNEVALLED)
+              nargs = 1;
+            while (nargs--)
+              mark_object (backtrace_args (pdl)[nargs]);
+          }
+          break;
+        case SPECPDL_LET_DEFAULT:
+        case SPECPDL_LET_LOCAL:
+          mark_object (specpdl_where (pdl));
+        case SPECPDL_LET:
+          mark_object (specpdl_symbol (pdl));
+          mark_object (specpdl_old_value (pdl));
+        }
+    }
+}
+
+void
+get_backtrace (Lisp_Object array)
+{
+  struct specbinding *pdl = backtrace_next (backtrace_top ());
+  ptrdiff_t i = 0, asize = ASIZE (array);
 
-      if (backlist->nargs == UNEVALLED
-          || backlist->nargs == MANY) /* FIXME: Can this happen?  */
-        i = 1;
+  /* Copy the backtrace contents into working memory.  */
+  for (; i < asize; i++)
+    {
+      if (backtrace_p (pdl))
+        {
+          ASET (array, i, backtrace_function (pdl));
+          pdl = backtrace_next (pdl);
+        }
       else
-        i = backlist->nargs;
-      while (i--)
-        mark_object (backlist->args[i]);
+        ASET (array, i, Qnil);
     }
 }
-#endif
+
+Lisp_Object backtrace_top_function (void)
+{
+  struct specbinding *pdl = backtrace_top ();
+  return (backtrace_p (pdl) ? backtrace_function (pdl) : Qnil);
+}
 
 void
 syms_of_eval (void)