Porting fixes for merged specpdl and backtrace stacks.

In particular this ports to 32-bit sparc Sun cc.
* eval.c (init_eval_once, grow_specpdl): Allocate a specbinding
array with a dummy element at specpdl[-1], so that its address can
be taken portably.
(unbind_to): Do not copy the binding; not needed, now that we
copy old_value in the one place where the copy is needed.
* fileio.c (Fwrite_region): Use ptrdiff_t, not int, for specpdl count.
* lisp.h (BITS_PER_PTRDIFF_T): Remove; no longer needed.
(union specbinding): Rename from struct specbinding.  Redo layout
to avoid the need for 'ptrdiff_t nargs : BITS_PER_PTRDIFF_T - 1;',
which is not portable.  With Sun C 5.12 32-bit sparc, the
declaration causes nargs to be an unsigned bitfield, a behavior
that the C standard allows; but Emacs wants nargs to be signed.
The overall type is now a union of structures rather than a
structure of union of structures, and the 'kind' member is now a
bitfield, so that the overall type doesn't grow.  All uses changed.
* process.c (Fmake_serial_process): Remove unnecessary initialization.

Fixes: debbugs:14643

5 files changed, 154 insertions, 115 deletions

diff --git a/src/ChangeLog b/src/ChangeLog
index 9a23c144e82..e1609356403 100644
--- a/src/ChangeLog
+++ b/src/ChangeLog
@@ -1,3 +1,24 @@
+2013-06-18  Paul Eggert  <eggert@cs.ucla.edu>
+
+        Porting fixes for merged specpdl and backtrace stacks (Bug#14643).
+        In particular this ports to 32-bit sparc Sun cc.
+        * eval.c (init_eval_once, grow_specpdl): Allocate a specbinding
+        array with a dummy element at specpdl[-1], so that its address can
+        be taken portably.
+        (unbind_to): Do not copy the binding; not needed, now that we
+        copy old_value in the one place where the copy is needed.
+        * fileio.c (Fwrite_region): Use ptrdiff_t, not int, for specpdl count.
+        * lisp.h (BITS_PER_PTRDIFF_T): Remove; no longer needed.
+        (union specbinding): Rename from struct specbinding.  Redo layout
+        to avoid the need for 'ptrdiff_t nargs : BITS_PER_PTRDIFF_T - 1;',
+        which is not portable.  With Sun C 5.12 32-bit sparc, the
+        declaration causes nargs to be an unsigned bitfield, a behavior
+        that the C standard allows; but Emacs wants nargs to be signed.
+        The overall type is now a union of structures rather than a
+        structure of union of structures, and the 'kind' member is now a
+        bitfield, so that the overall type doesn't grow.  All uses changed.
+        * process.c (Fmake_serial_process): Remove unnecessary initialization.
+
 2013-06-17  Paul Eggert  <eggert@cs.ucla.edu>
 
         * frame.c (x_report_frame_params): Cast parent_desc to uintptr_t.
diff --git a/src/eval.c b/src/eval.c
index 1b2f3bdc048..d3545add21d 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -76,17 +76,19 @@ Lisp_Object Vrun_hooks;
 
 Lisp_Object Vautoload_queue;
 
-/* Current number of specbindings allocated in specpdl.  */
+/* Current number of specbindings allocated in specpdl, not counting
+   the dummy entry specpdl[-1].  */
 
 ptrdiff_t specpdl_size;
 
-/* Pointer to beginning of specpdl.  */
+/* Pointer to beginning of specpdl.  A dummy entry specpdl[-1] exists
+   only so that its address can be taken.  */
 
-struct specbinding *specpdl;
+union specbinding *specpdl;
 
 /* Pointer to first unused element in specpdl.  */
 
-struct specbinding *specpdl_ptr;
+union specbinding *specpdl_ptr;
 
 /* Depth in Lisp evaluations and function calls.  */
 
@@ -116,102 +118,112 @@ static Lisp_Object funcall_lambda (Lisp_Object, ptrdiff_t, Lisp_Object *);
 static Lisp_Object apply_lambda (Lisp_Object fun, Lisp_Object args);
 
 static Lisp_Object
-specpdl_symbol (struct specbinding *pdl)
+specpdl_symbol (union specbinding *pdl)
 {
   eassert (pdl->kind >= SPECPDL_LET);
-  return pdl->v.let.symbol;
+  return pdl->let.symbol;
 }
 
 static Lisp_Object
-specpdl_old_value (struct specbinding *pdl)
+specpdl_old_value (union specbinding *pdl)
 {
   eassert (pdl->kind >= SPECPDL_LET);
-  return pdl->v.let.old_value;
+  return pdl->let.old_value;
 }
 
 static Lisp_Object
-specpdl_where (struct specbinding *pdl)
+specpdl_where (union specbinding *pdl)
 {
   eassert (pdl->kind > SPECPDL_LET);
-  return pdl->v.let.where;
+  return pdl->let.where;
 }
 
 static Lisp_Object
-specpdl_arg (struct specbinding *pdl)
+specpdl_arg (union specbinding *pdl)
 {
   eassert (pdl->kind == SPECPDL_UNWIND);
-  return pdl->v.unwind.arg;
+  return pdl->unwind.arg;
 }
 
 static specbinding_func
-specpdl_func (struct specbinding *pdl)
+specpdl_func (union specbinding *pdl)
 {
   eassert (pdl->kind == SPECPDL_UNWIND);
-  return pdl->v.unwind.func;
+  return pdl->unwind.func;
 }
 
 static Lisp_Object
-backtrace_function (struct specbinding *pdl)
+backtrace_function (union specbinding *pdl)
 {
   eassert (pdl->kind == SPECPDL_BACKTRACE);
-  return pdl->v.bt.function;
+  return pdl->bt.function;
 }
 
 static ptrdiff_t
-backtrace_nargs (struct specbinding *pdl)
+backtrace_nargs (union specbinding *pdl)
 {
   eassert (pdl->kind == SPECPDL_BACKTRACE);
-  return pdl->v.bt.nargs;
+  return pdl->bt.nargs;
 }
 
 static Lisp_Object *
-backtrace_args (struct specbinding *pdl)
+backtrace_args (union specbinding *pdl)
 {
   eassert (pdl->kind == SPECPDL_BACKTRACE);
-  return pdl->v.bt.args;
+  return pdl->bt.args;
 }
 
 static bool
-backtrace_debug_on_exit (struct specbinding *pdl)
+backtrace_debug_on_exit (union specbinding *pdl)
 {
   eassert (pdl->kind == SPECPDL_BACKTRACE);
-  return pdl->v.bt.debug_on_exit;
+  return pdl->bt.debug_on_exit;
 }
 
 /* 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; }
+set_backtrace_args (union specbinding *pdl, Lisp_Object *args)
+{
+  eassert (pdl->kind == SPECPDL_BACKTRACE);
+  pdl->bt.args = args;
+}
 
 static void
-set_backtrace_nargs (struct specbinding *pdl, ptrdiff_t n)
-{ eassert (pdl->kind == SPECPDL_BACKTRACE); pdl->v.bt.nargs = n; }
+set_backtrace_nargs (union specbinding *pdl, ptrdiff_t n)
+{
+  eassert (pdl->kind == SPECPDL_BACKTRACE);
+  pdl->bt.nargs = n;
+}
 
 static void
-set_backtrace_debug_on_exit (struct specbinding *pdl, bool doe)
-{ eassert (pdl->kind == SPECPDL_BACKTRACE); pdl->v.bt.debug_on_exit = doe; }
+set_backtrace_debug_on_exit (union specbinding *pdl, bool doe)
+{
+  eassert (pdl->kind == SPECPDL_BACKTRACE);
+  pdl->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 (union specbinding *) EXTERNALLY_VISIBLE;
+union specbinding *backtrace_top (void) EXTERNALLY_VISIBLE;
+union specbinding *backtrace_next (union specbinding *pdl) EXTERNALLY_VISIBLE;
 
-bool backtrace_p (struct specbinding *pdl)
+bool
+backtrace_p (union specbinding *pdl)
 { return pdl >= specpdl; }
 
-struct specbinding *
+union specbinding *
 backtrace_top (void)
 {
-  struct specbinding *pdl = specpdl_ptr - 1;
+  union specbinding *pdl = specpdl_ptr - 1;
   while (backtrace_p (pdl) && pdl->kind != SPECPDL_BACKTRACE)
     pdl--;
   return pdl;
 }
 
-struct specbinding *
-backtrace_next (struct specbinding *pdl)
+union specbinding *
+backtrace_next (union specbinding *pdl)
 {
   pdl--;
   while (backtrace_p (pdl) && pdl->kind != SPECPDL_BACKTRACE)
@@ -224,9 +236,9 @@ void
 init_eval_once (void)
 {
   enum { size = 50 };
-  specpdl = xmalloc (size * sizeof *specpdl);
+  union specbinding *pdlvec = xmalloc ((size + 1) * sizeof *specpdl);
   specpdl_size = size;
-  specpdl_ptr = specpdl;
+  specpdl = specpdl_ptr = pdlvec + 1;
   /* Don't forget to update docs (lispref node "Local Variables").  */
   max_specpdl_size = 1300; /* 1000 is not enough for CEDET's c-by.el.  */
   max_lisp_eval_depth = 600;
@@ -615,7 +627,7 @@ The return value is BASE-VARIABLE.  */)
     set_internal (base_variable, find_symbol_value (new_alias), Qnil, 1);
 
   {
-    struct specbinding *p;
+    union specbinding *p;
 
     for (p = specpdl_ptr; p > specpdl; )
       if ((--p)->kind >= SPECPDL_LET
@@ -681,7 +693,7 @@ usage: (defvar SYMBOL &optional INITVALUE DOCSTRING)  */)
       else
         { /* Check if there is really a global binding rather than just a let
              binding that shadows the global unboundness of the var.  */
-          struct specbinding *pdl = specpdl_ptr;
+          union specbinding *pdl = specpdl_ptr;
           while (pdl > specpdl)
             {
               if ((--pdl)->kind >= SPECPDL_LET
@@ -1480,7 +1492,7 @@ See also the function `condition-case'.  */)
   Vsignaling_function = Qnil;
   if (!NILP (error_symbol))
     {
-      struct specbinding *pdl = backtrace_next (backtrace_top ());
+      union specbinding *pdl = backtrace_next (backtrace_top ());
       if (backtrace_p (pdl) && EQ (backtrace_function (pdl), Qerror))
         pdl = backtrace_next (pdl);
       if (backtrace_p (pdl))
@@ -1984,8 +1996,10 @@ If LEXICAL is t, evaluate using lexical scoping.  */)
 static void
 grow_specpdl (void)
 {
-  register ptrdiff_t count = SPECPDL_INDEX ();
-  ptrdiff_t max_size = min (max_specpdl_size, PTRDIFF_MAX);
+  ptrdiff_t count = SPECPDL_INDEX ();
+  ptrdiff_t max_size = min (max_specpdl_size, PTRDIFF_MAX - 1000);
+  union specbinding *pdlvec = specpdl - 1;
+  ptrdiff_t pdlvecsize = specpdl_size + 1;
   if (max_size <= specpdl_size)
     {
       if (max_specpdl_size < 400)
@@ -1993,7 +2007,9 @@ grow_specpdl (void)
       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);
+  pdlvec = xpalloc (pdlvec, &pdlvecsize, 1, max_size + 1, sizeof *specpdl);
+  specpdl = pdlvec + 1;
+  specpdl_size = pdlvecsize - 1;
   specpdl_ptr = specpdl + count;
 }
 
@@ -2003,11 +2019,11 @@ 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->bt.kind = SPECPDL_BACKTRACE;
+  specpdl_ptr->bt.debug_on_exit = false;
+  specpdl_ptr->bt.function = function;
+  specpdl_ptr->bt.args = args;
+  specpdl_ptr->bt.nargs = nargs;
   specpdl_ptr++;
 }
 
@@ -3044,7 +3060,7 @@ DEFUN ("fetch-bytecode", Ffetch_bytecode, Sfetch_bytecode,
 bool
 let_shadows_buffer_binding_p (struct Lisp_Symbol *symbol)
 {
-  struct specbinding *p;
+  union specbinding *p;
   Lisp_Object buf = Fcurrent_buffer ();
 
   for (p = specpdl_ptr; p > specpdl; )
@@ -3063,7 +3079,7 @@ let_shadows_buffer_binding_p (struct Lisp_Symbol *symbol)
 bool
 let_shadows_global_binding_p (Lisp_Object symbol)
 {
-  struct specbinding *p;
+  union specbinding *p;
 
   for (p = specpdl_ptr; p > specpdl; )
     if ((--p)->kind >= SPECPDL_LET && EQ (specpdl_symbol (p), symbol))
@@ -3105,9 +3121,9 @@ 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.  */
-      specpdl_ptr->kind = SPECPDL_LET;
-      specpdl_ptr->v.let.symbol = symbol;
-      specpdl_ptr->v.let.old_value = SYMBOL_VAL (sym);
+      specpdl_ptr->let.kind = SPECPDL_LET;
+      specpdl_ptr->let.symbol = symbol;
+      specpdl_ptr->let.old_value = SYMBOL_VAL (sym);
       ++specpdl_ptr;
       if (!sym->constant)
         SET_SYMBOL_VAL (sym, value);
@@ -3120,10 +3136,10 @@ specbind (Lisp_Object symbol, Lisp_Object value)
     case SYMBOL_FORWARDED:
       {
         Lisp_Object ovalue = find_symbol_value (symbol);
-        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 ();
+        specpdl_ptr->let.kind = SPECPDL_LET_LOCAL;
+        specpdl_ptr->let.symbol = symbol;
+        specpdl_ptr->let.old_value = ovalue;
+        specpdl_ptr->let.where = Fcurrent_buffer ();
 
         eassert (sym->redirect != SYMBOL_LOCALIZED
                  || (EQ (SYMBOL_BLV (sym)->where, Fcurrent_buffer ())));
@@ -3131,7 +3147,7 @@ specbind (Lisp_Object symbol, Lisp_Object value)
         if (sym->redirect == SYMBOL_LOCALIZED)
           {
             if (!blv_found (SYMBOL_BLV (sym)))
-              specpdl_ptr->kind = SPECPDL_LET_DEFAULT;
+              specpdl_ptr->let.kind = SPECPDL_LET_DEFAULT;
           }
         else if (BUFFER_OBJFWDP (SYMBOL_FWD (sym)))
           {
@@ -3142,14 +3158,14 @@ specbind (Lisp_Object symbol, Lisp_Object value)
                happens with other buffer-local variables.  */
             if (NILP (Flocal_variable_p (symbol, Qnil)))
               {
-                specpdl_ptr->kind = SPECPDL_LET_DEFAULT;
+                specpdl_ptr->let.kind = SPECPDL_LET_DEFAULT;
                 ++specpdl_ptr;
                 Fset_default (symbol, value);
                 return;
               }
           }
         else
-          specpdl_ptr->kind = SPECPDL_LET;
+          specpdl_ptr->let.kind = SPECPDL_LET;
 
         specpdl_ptr++;
         set_internal (symbol, value, Qnil, 1);
@@ -3164,9 +3180,9 @@ record_unwind_protect (Lisp_Object (*function) (Lisp_Object), Lisp_Object arg)
 {
   if (specpdl_ptr == specpdl + specpdl_size)
     grow_specpdl ();
-  specpdl_ptr->kind = SPECPDL_UNWIND;
-  specpdl_ptr->v.unwind.func = function;
-  specpdl_ptr->v.unwind.arg = arg;
+  specpdl_ptr->unwind.kind = SPECPDL_UNWIND;
+  specpdl_ptr->unwind.func = function;
+  specpdl_ptr->unwind.arg = arg;
   specpdl_ptr++;
 }
 
@@ -3181,33 +3197,31 @@ unbind_to (ptrdiff_t count, Lisp_Object value)
 
   while (specpdl_ptr != specpdl + count)
     {
-      /* Copy the binding, and decrement specpdl_ptr, before we do
-         the work to unbind it.  We decrement first
-         so that an error in unbinding won't try to unbind
-         the same entry again, and we copy the binding first
-         in case more bindings are made during some of the code we run.  */
+      /* Decrement specpdl_ptr before we do the work to unbind it, so
+         that an error in unbinding won't try to unbind the same entry
+         again.  Take care to copy any parts of the binding needed
+         before invoking any code that can make more bindings.  */
 
-      struct specbinding this_binding;
-      this_binding = *--specpdl_ptr;
+      specpdl_ptr--;
 
-      switch (this_binding.kind)
+      switch (specpdl_ptr->kind)
         {
         case SPECPDL_UNWIND:
-          (*specpdl_func (&this_binding)) (specpdl_arg (&this_binding));
+          specpdl_func (specpdl_ptr) (specpdl_arg (specpdl_ptr));
           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
+          if (XSYMBOL (specpdl_symbol (specpdl_ptr))->redirect
               == SYMBOL_PLAINVAL)
-            SET_SYMBOL_VAL (XSYMBOL (specpdl_symbol (&this_binding)),
-                            specpdl_old_value (&this_binding));
+            SET_SYMBOL_VAL (XSYMBOL (specpdl_symbol (specpdl_ptr)),
+                            specpdl_old_value (specpdl_ptr));
           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));
+            Fset_default (specpdl_symbol (specpdl_ptr),
+                          specpdl_old_value (specpdl_ptr));
           break;
         case SPECPDL_BACKTRACE:
           break;
@@ -3220,17 +3234,17 @@ unbind_to (ptrdiff_t count, Lisp_Object value)
              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);
+            Lisp_Object symbol = specpdl_symbol (specpdl_ptr);
+            Lisp_Object where = specpdl_where (specpdl_ptr);
+            Lisp_Object old_value = specpdl_old_value (specpdl_ptr);
             eassert (BUFFERP (where));
 
-            if (this_binding.kind == SPECPDL_LET_DEFAULT)
-              Fset_default (symbol, specpdl_old_value (&this_binding));
+            if (specpdl_ptr->kind == SPECPDL_LET_DEFAULT)
+              Fset_default (symbol, old_value);
             /* 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);
+              set_internal (symbol, old_value, where, 1);
           }
           break;
         }
@@ -3259,7 +3273,7 @@ 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)
 {
-  struct specbinding *pdl = backtrace_top ();
+  union specbinding *pdl = backtrace_top ();
   register EMACS_INT i;
 
   CHECK_NUMBER (level);
@@ -3278,7 +3292,7 @@ DEFUN ("backtrace", Fbacktrace, Sbacktrace, 0, 0, "",
 Output stream used is value of `standard-output'.  */)
   (void)
 {
-  struct specbinding *pdl = backtrace_top ();
+  union specbinding *pdl = backtrace_top ();
   Lisp_Object tem;
   Lisp_Object old_print_level = Vprint_level;
 
@@ -3328,7 +3342,7 @@ or a lambda expression for macro calls.
 If NFRAMES is more than the number of frames, the value is nil.  */)
   (Lisp_Object nframes)
 {
-  struct specbinding *pdl = backtrace_top ();
+  union specbinding *pdl = backtrace_top ();
   register EMACS_INT i;
 
   CHECK_NATNUM (nframes);
@@ -3354,7 +3368,7 @@ If NFRAMES is more than the number of frames, the value is nil.  */)
 void
 mark_specpdl (void)
 {
-  struct specbinding *pdl;
+  union specbinding *pdl;
   for (pdl = specpdl; pdl != specpdl_ptr; pdl++)
     {
       switch (pdl->kind)
@@ -3362,6 +3376,7 @@ mark_specpdl (void)
         case SPECPDL_UNWIND:
           mark_object (specpdl_arg (pdl));
           break;
+
         case SPECPDL_BACKTRACE:
           {
             ptrdiff_t nargs = backtrace_nargs (pdl);
@@ -3372,12 +3387,15 @@ mark_specpdl (void)
               mark_object (backtrace_args (pdl)[nargs]);
           }
           break;
+
         case SPECPDL_LET_DEFAULT:
         case SPECPDL_LET_LOCAL:
           mark_object (specpdl_where (pdl));
+          /* Fall through.  */
         case SPECPDL_LET:
           mark_object (specpdl_symbol (pdl));
           mark_object (specpdl_old_value (pdl));
+          break;
         }
     }
 }
@@ -3385,7 +3403,7 @@ mark_specpdl (void)
 void
 get_backtrace (Lisp_Object array)
 {
-  struct specbinding *pdl = backtrace_next (backtrace_top ());
+  union specbinding *pdl = backtrace_next (backtrace_top ());
   ptrdiff_t i = 0, asize = ASIZE (array);
 
   /* Copy the backtrace contents into working memory.  */
@@ -3403,7 +3421,7 @@ get_backtrace (Lisp_Object array)
 
 Lisp_Object backtrace_top_function (void)
 {
-  struct specbinding *pdl = backtrace_top ();
+  union specbinding *pdl = backtrace_top ();
   return (backtrace_p (pdl) ? backtrace_function (pdl) : Qnil);
 }
 
diff --git a/src/fileio.c b/src/fileio.c
index 6a60186a84f..4a14f5a5911 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -4764,7 +4764,7 @@ This calls `write-region-annotate-functions' at the start, and
   struct stat st;
   EMACS_TIME modtime;
   ptrdiff_t count = SPECPDL_INDEX ();
-  int count1;
+  ptrdiff_t count1;
   Lisp_Object handler;
   Lisp_Object visit_file;
   Lisp_Object annotations;
diff --git a/src/lisp.h b/src/lisp.h
index f76bbfb9ead..e4033a2f45b 100644
--- a/src/lisp.h
+++ b/src/lisp.h
@@ -73,7 +73,6 @@ enum
     BITS_PER_SHORT     = CHAR_BIT * sizeof (short),
     BITS_PER_INT       = CHAR_BIT * sizeof (int),
     BITS_PER_LONG      = CHAR_BIT * sizeof (long int),
-    BITS_PER_PTRDIFF_T = CHAR_BIT * sizeof (ptrdiff_t),
     BITS_PER_EMACS_INT = CHAR_BIT * sizeof (EMACS_INT)
   };
 
@@ -2679,9 +2678,9 @@ typedef jmp_buf sys_jmp_buf;
    WHERE being a buffer or frame means we saw a buffer-local or frame-local
    value.  Other values of WHERE mean an internal error.
 
-   NOTE: The specbinding struct is defined here, because SPECPDL_INDEX is
+   NOTE: The specbinding union is defined here, because SPECPDL_INDEX is
    used all over the place, needs to be fast, and needs to know the size of
-   struct specbinding.  But only eval.c should access it.  */
+   union specbinding.  But only eval.c should access it.  */
 
 typedef Lisp_Object (*specbinding_func) (Lisp_Object);
 
@@ -2694,29 +2693,30 @@ enum specbind_tag {
   SPECPDL_LET_DEFAULT           /* A global binding for a localized var.  */
 };
 
-struct specbinding
+union specbinding
   {
-    enum specbind_tag kind;
-    union {
-      struct {
-        Lisp_Object arg;
-        specbinding_func func;
-      } unwind;
-      struct {
-        /* `where' is not used in the case of SPECPDL_LET.  */
-        Lisp_Object symbol, old_value, where;
-      } let;
-      struct {
-        Lisp_Object function;
-        Lisp_Object *args;
-        ptrdiff_t nargs : BITS_PER_PTRDIFF_T - 1;
-        bool debug_on_exit : 1;
-      } bt;
-    } v;
+    ENUM_BF (specbind_tag) kind : CHAR_BIT;
+    struct {
+      ENUM_BF (specbind_tag) kind : CHAR_BIT;
+      Lisp_Object arg;
+      specbinding_func func;
+    } unwind;
+    struct {
+      ENUM_BF (specbind_tag) kind : CHAR_BIT;
+      /* `where' is not used in the case of SPECPDL_LET.  */
+      Lisp_Object symbol, old_value, where;
+    } let;
+    struct {
+      ENUM_BF (specbind_tag) kind : CHAR_BIT;
+      bool debug_on_exit : 1;
+      Lisp_Object function;
+      Lisp_Object *args;
+      ptrdiff_t nargs;
+    } bt;
   };
 
-extern struct specbinding *specpdl;
-extern struct specbinding *specpdl_ptr;
+extern union specbinding *specpdl;
+extern union specbinding *specpdl_ptr;
 extern ptrdiff_t specpdl_size;
 
 LISP_INLINE ptrdiff_t
diff --git a/src/process.c b/src/process.c
index a873dd0cdb2..0afd8b5d2ac 100644
--- a/src/process.c
+++ b/src/process.c
@@ -2524,7 +2524,7 @@ usage:  (make-serial-process &rest ARGS)  */)
   struct gcpro gcpro1;
   Lisp_Object name, buffer;
   Lisp_Object tem, val;
-  ptrdiff_t specpdl_count = -1;
+  ptrdiff_t specpdl_count;
 
   if (nargs == 0)
     return Qnil;