Use alignas to fix GCALIGN-related bugs

Use alignas and unions to specify alignments of objects needing
addresses that are at least a multiple of GCALIGNMENT.  Using
these standard C facilities should be safer than relying on ad hoc
and poorly-understood features like GCC’s __attribute__
((aligned (N))), the root cause for recent porting bugs like
Bug#29040.  The alignas macro was standardized by C11 and Gnulib
supports alignas for pre-C11 platforms.  I have tested this on Sun
Studio 12 sparc (2007) and GCC 4.4.7 x86-64 (2012) as well as on
more recent platforms like GCC 7.2.1 (2017) on Fedora 26 (both
x86-64 and x86).
* lib-src/make-docfile.c (close_emacs_globals): lispsym is now
just an array of struct Lisp_Symbol, since struct Lisp_Symbol is
now properly aligned.  All uses changed.
* src/alloc.c (NEXT_FREE_LISP_STRING): Just use the new u.next
member; this is simpler and safer than casting a pointer that
might not be aligned properly.
(aligned_Lisp_Symbol): Remove.  No longer needed, now that struct
Lisp_Symbol is aligned properly.  All uses replaced with struct
Lisp_Symbol.
* src/lisp.h (GCALIGNED): Remove, as it does not work as expected:
it can cause the natural alignment to be ignored.  All uses
replaced by unions with a ‘char alignas (GCALIGNMENT)’ member as
described below.
(struct Lisp_Symbol, struct Lisp_Cons, struct Lisp_String):
Change definition from ‘struct TAG { MEMBERS };’ to
‘struct TAG { union { struct { MEMBERS } s; char alignas
(GCALIGNMENT) gcaligned; } u; };’.  This guarantees ‘struct TAG’
to have an alignment that at least max (GCALIGNMENT, N) where N is
its old alignment.  All uses like ‘PTR->MEMBER’ changed to
‘PTR->u.s.MEMBER’; these uses were supposed to be mostly private
anyway.  Verify that the resulting ‘struct TAG’ is properly
aligned for Emacs.
(union vectorlike_header): New member ‘gcaligned’ to guarantee
that this type, and its containing types like ‘struct Lisp_Subr’,
‘struct buffer’ and ‘struct thread_state’, are all properly
aligned for Emacs.
(struct Lisp_String): New union member ‘next’, for the benefit
of NEXT_FREE_LISP_STRING.
(union Aligned_Cons, union Aligned_String): Remove.  All uses
replaced by struct Lisp_Cons and struct Lisp_String, since they
are now properly aligned.
(USE_STACK_CONS, USE_STACK_STRING): Simplify now that we can
assume struct Lisp_Cons and struct Lisp_String are properly
aligned.

1 files changed, 101 insertions, 110 deletions

diff --git a/src/alloc.c b/src/alloc.c
index 5a44d7a9fc2..3b87195b707 100644
--- a/src/alloc.c
+++ b/src/alloc.c
@@ -211,9 +211,9 @@ alloc_unexec_post (void)
 /* Mark, unmark, query mark bit of a Lisp string.  S must be a pointer
    to a struct Lisp_String.  */
 
-#define MARK_STRING(S)          ((S)->size |= ARRAY_MARK_FLAG)
-#define UNMARK_STRING(S)        ((S)->size &= ~ARRAY_MARK_FLAG)
-#define STRING_MARKED_P(S)      (((S)->size & ARRAY_MARK_FLAG) != 0)
+#define MARK_STRING(S)          ((S)->u.s.size |= ARRAY_MARK_FLAG)
+#define UNMARK_STRING(S)        ((S)->u.s.size &= ~ARRAY_MARK_FLAG)
+#define STRING_MARKED_P(S)      (((S)->u.s.size & ARRAY_MARK_FLAG) != 0)
 
 #define VECTOR_MARK(V)          ((V)->header.size |= ARRAY_MARK_FLAG)
 #define VECTOR_UNMARK(V)        ((V)->header.size &= ~ARRAY_MARK_FLAG)
@@ -1730,14 +1730,14 @@ static EMACS_INT total_string_bytes;
    string_free_list, return a pointer to its successor in the
    free-list.  */
 
-#define NEXT_FREE_LISP_STRING(S) (*(struct Lisp_String **) (S))
+#define NEXT_FREE_LISP_STRING(S) ((S)->u.next)
 
 /* Return a pointer to the sdata structure belonging to Lisp string S.
    S must be live, i.e. S->data must not be null.  S->data is actually
    a pointer to the `u.data' member of its sdata structure; the
    structure starts at a constant offset in front of that.  */
 
-#define SDATA_OF_STRING(S) ((sdata *) ((S)->data - SDATA_DATA_OFFSET))
+#define SDATA_OF_STRING(S) ((sdata *) ((S)->u.s.data - SDATA_DATA_OFFSET))
 
 
 #ifdef GC_CHECK_STRING_OVERRUN
@@ -1818,9 +1818,10 @@ ptrdiff_t
 string_bytes (struct Lisp_String *s)
 {
   ptrdiff_t nbytes =
-    (s->size_byte < 0 ? s->size & ~ARRAY_MARK_FLAG : s->size_byte);
+    (s->u.s.size_byte < 0 ? s->u.s.size & ~ARRAY_MARK_FLAG : s->u.s.size_byte);
 
-  if (!PURE_P (s) && s->data && nbytes != SDATA_NBYTES (SDATA_OF_STRING (s)))
+  if (!PURE_P (s) && s->u.s.data
+      && nbytes != SDATA_NBYTES (SDATA_OF_STRING (s)))
     emacs_abort ();
   return nbytes;
 }
@@ -1926,7 +1927,7 @@ allocate_string (void)
         {
           s = b->strings + i;
           /* Every string on a free list should have NULL data pointer.  */
-          s->data = NULL;
+          s->u.s.data = NULL;
           NEXT_FREE_LISP_STRING (s) = string_free_list;
           string_free_list = s;
         }
@@ -1965,10 +1966,10 @@ allocate_string (void)
 
 
 /* Set up Lisp_String S for holding NCHARS characters, NBYTES bytes,
-   plus a NUL byte at the end.  Allocate an sdata structure for S, and
-   set S->data to its `u.data' member.  Store a NUL byte at the end of
-   S->data.  Set S->size to NCHARS and S->size_byte to NBYTES.  Free
-   S->data if it was initially non-null.  */
+   plus a NUL byte at the end.  Allocate an sdata structure DATA for
+   S, and set S->u.s.data to SDATA->u.data.  Store a NUL byte at the
+   end of S->u.s.data.  Set S->u.s.size to NCHARS and S->u.s.size_byte
+   to NBYTES.  Free S->u.s.data if it was initially non-null.  */
 
 void
 allocate_string_data (struct Lisp_String *s,
@@ -1984,7 +1985,7 @@ allocate_string_data (struct Lisp_String *s,
   /* Determine the number of bytes needed to store NBYTES bytes
      of string data.  */
   needed = SDATA_SIZE (nbytes);
-  if (s->data)
+  if (s->u.s.data)
     {
       old_data = SDATA_OF_STRING (s);
       old_nbytes = STRING_BYTES (s);
@@ -2043,13 +2044,13 @@ allocate_string_data (struct Lisp_String *s,
 
   MALLOC_UNBLOCK_INPUT;
 
-  s->data = SDATA_DATA (data);
+  s->u.s.data = SDATA_DATA (data);
 #ifdef GC_CHECK_STRING_BYTES
   SDATA_NBYTES (data) = nbytes;
 #endif
-  s->size = nchars;
-  s->size_byte = nbytes;
-  s->data[nbytes] = '\0';
+  s->u.s.size = nchars;
+  s->u.s.size_byte = nbytes;
+  s->u.s.data[nbytes] = '\0';
 #ifdef GC_CHECK_STRING_OVERRUN
   memcpy ((char *) data + needed, string_overrun_cookie,
           GC_STRING_OVERRUN_COOKIE_SIZE);
@@ -2093,7 +2094,7 @@ sweep_strings (void)
         {
           struct Lisp_String *s = b->strings + i;
 
-          if (s->data)
+          if (s->u.s.data)
             {
               /* String was not on free-list before.  */
               if (STRING_MARKED_P (s))
@@ -2102,7 +2103,7 @@ sweep_strings (void)
                   UNMARK_STRING (s);
 
                   /* Do not use string_(set|get)_intervals here.  */
-                  s->intervals = balance_intervals (s->intervals);
+                  s->u.s.intervals = balance_intervals (s->u.s.intervals);
 
                   ++total_strings;
                   total_string_bytes += STRING_BYTES (s);
@@ -2125,7 +2126,7 @@ sweep_strings (void)
 
                   /* Reset the strings's `data' member so that we
                      know it's free.  */
-                  s->data = NULL;
+                  s->u.s.data = NULL;
 
                   /* Put the string on the free-list.  */
                   NEXT_FREE_LISP_STRING (s) = string_free_list;
@@ -2264,7 +2265,7 @@ compact_small_strings (void)
                     {
                       eassert (tb != b || to < from);
                       memmove (to, from, nbytes + GC_STRING_EXTRA);
-                      to->string->data = SDATA_DATA (to);
+                      to->string->u.s.data = SDATA_DATA (to);
                     }
 
                   /* Advance past the sdata we copied to.  */
@@ -2544,7 +2545,7 @@ make_uninit_multibyte_string (EMACS_INT nchars, EMACS_INT nbytes)
     return empty_multibyte_string;
 
   s = allocate_string ();
-  s->intervals = NULL;
+  s->u.s.intervals = NULL;
   allocate_string_data (s, nchars, nbytes);
   XSETSTRING (string, s);
   string_chars_consed += nbytes;
@@ -2729,8 +2730,8 @@ static struct Lisp_Cons *cons_free_list;
 void
 free_cons (struct Lisp_Cons *ptr)
 {
-  ptr->u.chain = cons_free_list;
-  ptr->car = Vdead;
+  ptr->u.s.u.chain = cons_free_list;
+  ptr->u.s.car = Vdead;
   cons_free_list = ptr;
   consing_since_gc -= sizeof *ptr;
   total_free_conses++;
@@ -2749,7 +2750,7 @@ DEFUN ("cons", Fcons, Scons, 2, 2, 0,
       /* We use the cdr for chaining the free list
          so that we won't use the same field that has the mark bit.  */
       XSETCONS (val, cons_free_list);
-      cons_free_list = cons_free_list->u.chain;
+      cons_free_list = cons_free_list->u.s.u.chain;
     }
   else
     {
@@ -2786,7 +2787,7 @@ check_cons_list (void)
   struct Lisp_Cons *tail = cons_free_list;
 
   while (tail)
-    tail = tail->u.chain;
+    tail = tail->u.s.u.chain;
 }
 #endif
 
@@ -3543,27 +3544,17 @@ usage: (make-byte-code ARGLIST BYTE-CODE CONSTANTS DEPTH &optional DOCSTRING INT
                            Symbol Allocation
  ***********************************************************************/
 
-/* Like struct Lisp_Symbol, but padded so that the size is a multiple
-   of the required alignment.  */
-
-union aligned_Lisp_Symbol
-{
-  struct Lisp_Symbol s;
-  unsigned char c[(sizeof (struct Lisp_Symbol) + GCALIGNMENT - 1)
-                  & -GCALIGNMENT];
-};
-
 /* Each symbol_block is just under 1020 bytes long, since malloc
    really allocates in units of powers of two and uses 4 bytes for its
    own overhead.  */
 
 #define SYMBOL_BLOCK_SIZE \
-  ((1020 - sizeof (struct symbol_block *)) / sizeof (union aligned_Lisp_Symbol))
+  ((1020 - sizeof (struct symbol_block *)) / sizeof (struct Lisp_Symbol))
 
 struct symbol_block
 {
   /* Place `symbols' first, to preserve alignment.  */
-  union aligned_Lisp_Symbol symbols[SYMBOL_BLOCK_SIZE];
+  struct Lisp_Symbol symbols[SYMBOL_BLOCK_SIZE];
   struct symbol_block *next;
 };
 
@@ -3587,7 +3578,7 @@ static struct Lisp_Symbol *symbol_free_list;
 static void
 set_symbol_name (Lisp_Object sym, Lisp_Object name)
 {
-  XSYMBOL (sym)->name = name;
+  XSYMBOL (sym)->u.s.name = name;
 }
 
 void
@@ -3596,15 +3587,15 @@ init_symbol (Lisp_Object val, Lisp_Object name)
   struct Lisp_Symbol *p = XSYMBOL (val);
   set_symbol_name (val, name);
   set_symbol_plist (val, Qnil);
-  p->redirect = SYMBOL_PLAINVAL;
+  p->u.s.redirect = SYMBOL_PLAINVAL;
   SET_SYMBOL_VAL (p, Qunbound);
   set_symbol_function (val, Qnil);
   set_symbol_next (val, NULL);
-  p->gcmarkbit = false;
-  p->interned = SYMBOL_UNINTERNED;
-  p->trapped_write = SYMBOL_UNTRAPPED_WRITE;
-  p->declared_special = false;
-  p->pinned = false;
+  p->u.s.gcmarkbit = false;
+  p->u.s.interned = SYMBOL_UNINTERNED;
+  p->u.s.trapped_write = SYMBOL_UNTRAPPED_WRITE;
+  p->u.s.declared_special = false;
+  p->u.s.pinned = false;
 }
 
 DEFUN ("make-symbol", Fmake_symbol, Smake_symbol, 1, 1, 0,
@@ -3621,7 +3612,7 @@ Its value is void, and its function definition and property list are nil.  */)
   if (symbol_free_list)
     {
       XSETSYMBOL (val, symbol_free_list);
-      symbol_free_list = symbol_free_list->next;
+      symbol_free_list = symbol_free_list->u.s.next;
     }
   else
     {
@@ -3634,7 +3625,7 @@ Its value is void, and its function definition and property list are nil.  */)
           symbol_block_index = 0;
           total_free_symbols += SYMBOL_BLOCK_SIZE;
         }
-      XSETSYMBOL (val, &symbol_block->symbols[symbol_block_index].s);
+      XSETSYMBOL (val, &symbol_block->symbols[symbol_block_index]);
       symbol_block_index++;
     }
 
@@ -4587,7 +4578,7 @@ live_string_holding (struct mem_node *m, void *p)
       if (0 <= offset && offset < STRING_BLOCK_SIZE * sizeof b->strings[0])
         {
           struct Lisp_String *s = p = cp -= offset % sizeof b->strings[0];
-          if (s->data)
+          if (s->u.s.data)
             return make_lisp_ptr (s, Lisp_String);
         }
     }
@@ -4621,7 +4612,7 @@ live_cons_holding (struct mem_node *m, void *p)
               || offset / sizeof b->conses[0] < cons_block_index))
         {
           struct Lisp_Cons *s = p = cp -= offset % sizeof b->conses[0];
-          if (!EQ (s->car, Vdead))
+          if (!EQ (s->u.s.car, Vdead))
             return make_lisp_ptr (s, Lisp_Cons);
         }
     }
@@ -4656,7 +4647,7 @@ live_symbol_holding (struct mem_node *m, void *p)
               || offset / sizeof b->symbols[0] < symbol_block_index))
         {
           struct Lisp_Symbol *s = p = cp -= offset % sizeof b->symbols[0];
-          if (!EQ (s->function, Vdead))
+          if (!EQ (s->u.s.function, Vdead))
             return make_lisp_symbol (s);
         }
     }
@@ -4984,7 +4975,7 @@ mark_memory (void *start, void *end)
        Lisp_Object obj = build_string ("test");
        struct Lisp_String *s = XSTRING (obj);
        Fgarbage_collect ();
-       fprintf (stderr, "test '%s'\n", s->data);
+       fprintf (stderr, "test '%s'\n", s->u.s.data);
        return Qnil;
      }
 
@@ -5484,16 +5475,16 @@ make_pure_string (const char *data,
 {
   Lisp_Object string;
   struct Lisp_String *s = pure_alloc (sizeof *s, Lisp_String);
-  s->data = (unsigned char *) find_string_data_in_pure (data, nbytes);
-  if (s->data == NULL)
+  s->u.s.data = (unsigned char *) find_string_data_in_pure (data, nbytes);
+  if (s->u.s.data == NULL)
     {
-      s->data = pure_alloc (nbytes + 1, -1);
-      memcpy (s->data, data, nbytes);
-      s->data[nbytes] = '\0';
+      s->u.s.data = pure_alloc (nbytes + 1, -1);
+      memcpy (s->u.s.data, data, nbytes);
+      s->u.s.data[nbytes] = '\0';
     }
-  s->size = nchars;
-  s->size_byte = multibyte ? nbytes : -1;
-  s->intervals = NULL;
+  s->u.s.size = nchars;
+  s->u.s.size_byte = multibyte ? nbytes : -1;
+  s->u.s.intervals = NULL;
   XSETSTRING (string, s);
   return string;
 }
@@ -5506,10 +5497,10 @@ make_pure_c_string (const char *data, ptrdiff_t nchars)
 {
   Lisp_Object string;
   struct Lisp_String *s = pure_alloc (sizeof *s, Lisp_String);
-  s->size = nchars;
-  s->size_byte = -1;
-  s->data = (unsigned char *) data;
-  s->intervals = NULL;
+  s->u.s.size = nchars;
+  s->u.s.size_byte = -1;
+  s->u.s.data = (unsigned char *) data;
+  s->u.s.intervals = NULL;
   XSETSTRING (string, s);
   return string;
 }
@@ -5620,7 +5611,7 @@ purecopy (Lisp_Object obj)
       || SUBRP (obj))
     return obj;    /* Already pure.  */
 
-  if (STRINGP (obj) && XSTRING (obj)->intervals)
+  if (STRINGP (obj) && XSTRING (obj)->u.s.intervals)
     message_with_string ("Dropping text-properties while making string `%s' pure",
                          obj, true);
 
@@ -5675,10 +5666,10 @@ purecopy (Lisp_Object obj)
     }
   else if (SYMBOLP (obj))
     {
-      if (!XSYMBOL (obj)->pinned && !c_symbol_p (XSYMBOL (obj)))
+      if (!XSYMBOL (obj)->u.s.pinned && !c_symbol_p (XSYMBOL (obj)))
         { /* We can't purify them, but they appear in many pure objects.
              Mark them as `pinned' so we know to mark them at every GC cycle.  */
-          XSYMBOL (obj)->pinned = true;
+          XSYMBOL (obj)->u.s.pinned = true;
           symbol_block_pinned = symbol_block;
         }
       /* Don't hash-cons it.  */
@@ -5891,10 +5882,10 @@ mark_pinned_symbols (void)
 
   for (sblk = symbol_block_pinned; sblk; sblk = sblk->next)
     {
-      union aligned_Lisp_Symbol *sym = sblk->symbols, *end = sym + lim;
+      struct Lisp_Symbol *sym = sblk->symbols, *end = sym + lim;
       for (; sym < end; ++sym)
-        if (sym->s.pinned)
-          mark_object (make_lisp_symbol (&sym->s));
+        if (sym->u.s.pinned)
+          mark_object (make_lisp_symbol (sym));
 
       lim = SYMBOL_BLOCK_SIZE;
     }
@@ -6256,7 +6247,7 @@ mark_char_table (struct Lisp_Vector *ptr, enum pvec_type pvectype)
     {
       Lisp_Object val = ptr->contents[i];
 
-      if (INTEGERP (val) || (SYMBOLP (val) && XSYMBOL (val)->gcmarkbit))
+      if (INTEGERP (val) || (SYMBOLP (val) && XSYMBOL (val)->u.s.gcmarkbit))
         continue;
       if (SUB_CHAR_TABLE_P (val))
         {
@@ -6499,7 +6490,7 @@ mark_object (Lisp_Object arg)
           break;
         CHECK_ALLOCATED_AND_LIVE (live_string_p);
         MARK_STRING (ptr);
-        MARK_INTERVAL_TREE (ptr->intervals);
+        MARK_INTERVAL_TREE (ptr->u.s.intervals);
 #ifdef GC_CHECK_STRING_BYTES
         /* Check that the string size recorded in the string is the
            same as the one recorded in the sdata structure.  */
@@ -6640,17 +6631,17 @@ mark_object (Lisp_Object arg)
 
     case Lisp_Symbol:
       {
-        register struct Lisp_Symbol *ptr = XSYMBOL (obj);
+        struct Lisp_Symbol *ptr = XSYMBOL (obj);
       nextsym:
-        if (ptr->gcmarkbit)
+        if (ptr->u.s.gcmarkbit)
           break;
         CHECK_ALLOCATED_AND_LIVE_SYMBOL ();
-        ptr->gcmarkbit = 1;
+        ptr->u.s.gcmarkbit = 1;
         /* Attempt to catch bogus objects.  */
-        eassert (valid_lisp_object_p (ptr->function));
-        mark_object (ptr->function);
-        mark_object (ptr->plist);
-        switch (ptr->redirect)
+        eassert (valid_lisp_object_p (ptr->u.s.function));
+        mark_object (ptr->u.s.function);
+        mark_object (ptr->u.s.plist);
+        switch (ptr->u.s.redirect)
           {
           case SYMBOL_PLAINVAL: mark_object (SYMBOL_VAL (ptr)); break;
           case SYMBOL_VARALIAS:
@@ -6671,11 +6662,11 @@ mark_object (Lisp_Object arg)
             break;
           default: emacs_abort ();
           }
-        if (!PURE_P (XSTRING (ptr->name)))
-          MARK_STRING (XSTRING (ptr->name));
-        MARK_INTERVAL_TREE (string_intervals (ptr->name));
+        if (!PURE_P (XSTRING (ptr->u.s.name)))
+          MARK_STRING (XSTRING (ptr->u.s.name));
+        MARK_INTERVAL_TREE (string_intervals (ptr->u.s.name));
         /* Inner loop to mark next symbol in this bucket, if any.  */
-        po = ptr = ptr->next;
+        po = ptr = ptr->u.s.next;
         if (ptr)
           goto nextsym;
       }
@@ -6729,14 +6720,14 @@ mark_object (Lisp_Object arg)
         CHECK_ALLOCATED_AND_LIVE (live_cons_p);
         CONS_MARK (ptr);
         /* If the cdr is nil, avoid recursion for the car.  */
-        if (EQ (ptr->u.cdr, Qnil))
+        if (EQ (ptr->u.s.u.cdr, Qnil))
           {
-            obj = ptr->car;
+            obj = ptr->u.s.car;
             cdr_count = 0;
             goto loop;
           }
-        mark_object (ptr->car);
-        obj = ptr->u.cdr;
+        mark_object (ptr->u.s.car);
+        obj = ptr->u.s.u.cdr;
         cdr_count++;
         if (cdr_count == mark_object_loop_halt)
           emacs_abort ();
@@ -6797,7 +6788,7 @@ survives_gc_p (Lisp_Object obj)
       break;
 
     case Lisp_Symbol:
-      survives_p = XSYMBOL (obj)->gcmarkbit;
+      survives_p = XSYMBOL (obj)->u.s.gcmarkbit;
       break;
 
     case Lisp_Misc:
@@ -6873,9 +6864,9 @@ sweep_conses (void)
                   if (!CONS_MARKED_P (&cblk->conses[pos]))
                     {
                       this_free++;
-                      cblk->conses[pos].u.chain = cons_free_list;
+                      cblk->conses[pos].u.s.u.chain = cons_free_list;
                       cons_free_list = &cblk->conses[pos];
-                      cons_free_list->car = Vdead;
+                      cons_free_list->u.s.car = Vdead;
                     }
                   else
                     {
@@ -6894,7 +6885,7 @@ sweep_conses (void)
         {
           *cprev = cblk->next;
           /* Unhook from the free list.  */
-          cons_free_list = cblk->conses[0].u.chain;
+          cons_free_list = cblk->conses[0].u.s.u.chain;
           lisp_align_free (cblk);
         }
       else
@@ -7018,39 +7009,39 @@ sweep_symbols (void)
   symbol_free_list = NULL;
 
   for (int i = 0; i < ARRAYELTS (lispsym); i++)
-    lispsym[i].s.gcmarkbit = 0;
+    lispsym[i].u.s.gcmarkbit = 0;
 
   for (sblk = symbol_block; sblk; sblk = *sprev)
     {
       int this_free = 0;
-      union aligned_Lisp_Symbol *sym = sblk->symbols;
-      union aligned_Lisp_Symbol *end = sym + lim;
+      struct Lisp_Symbol *sym = sblk->symbols;
+      struct Lisp_Symbol *end = sym + lim;
 
       for (; sym < end; ++sym)
         {
-          if (!sym->s.gcmarkbit)
+          if (!sym->u.s.gcmarkbit)
             {
-              if (sym->s.redirect == SYMBOL_LOCALIZED)
+              if (sym->u.s.redirect == SYMBOL_LOCALIZED)
                 {
-                  xfree (SYMBOL_BLV (&sym->s));
+                  xfree (SYMBOL_BLV (sym));
                   /* At every GC we sweep all symbol_blocks and rebuild the
                      symbol_free_list, so those symbols which stayed unused
                      between the two will be re-swept.
                      So we have to make sure we don't re-free this blv next
                      time we sweep this symbol_block (bug#29066).  */
-                  sym->s.redirect = SYMBOL_PLAINVAL;
+                  sym->u.s.redirect = SYMBOL_PLAINVAL;
                 }
-              sym->s.next = symbol_free_list;
-              symbol_free_list = &sym->s;
-              symbol_free_list->function = Vdead;
+              sym->u.s.next = symbol_free_list;
+              symbol_free_list = sym;
+              symbol_free_list->u.s.function = Vdead;
               ++this_free;
             }
           else
             {
               ++num_used;
-              sym->s.gcmarkbit = 0;
+              sym->u.s.gcmarkbit = 0;
               /* Attempt to catch bogus objects.  */
-              eassert (valid_lisp_object_p (sym->s.function));
+              eassert (valid_lisp_object_p (sym->u.s.function));
             }
         }
 
@@ -7062,7 +7053,7 @@ sweep_symbols (void)
         {
           *sprev = sblk->next;
           /* Unhook from the free list.  */
-          symbol_free_list = sblk->symbols[0].s.next;
+          symbol_free_list = sblk->symbols[0].u.s.next;
           lisp_free (sblk);
         }
       else
@@ -7289,10 +7280,10 @@ symbol_uses_obj (Lisp_Object symbol, Lisp_Object obj)
   struct Lisp_Symbol *sym = XSYMBOL (symbol);
   Lisp_Object val = find_symbol_value (symbol);
   return (EQ (val, obj)
-          || EQ (sym->function, obj)
-          || (!NILP (sym->function)
-              && COMPILEDP (sym->function)
-              && EQ (AREF (sym->function, COMPILED_BYTECODE), obj))
+          || EQ (sym->u.s.function, obj)
+          || (!NILP (sym->u.s.function)
+              && COMPILEDP (sym->u.s.function)
+              && EQ (AREF (sym->u.s.function, COMPILED_BYTECODE), obj))
           || (!NILP (val)
               && COMPILEDP (val)
               && EQ (AREF (val, COMPILED_BYTECODE), obj)));
@@ -7323,15 +7314,15 @@ which_symbols (Lisp_Object obj, EMACS_INT find_max)
 
        for (sblk = symbol_block; sblk; sblk = sblk->next)
          {
-           union aligned_Lisp_Symbol *aligned_sym = sblk->symbols;
+           struct Lisp_Symbol *asym = sblk->symbols;
            int bn;
 
-           for (bn = 0; bn < SYMBOL_BLOCK_SIZE; bn++, aligned_sym++)
+           for (bn = 0; bn < SYMBOL_BLOCK_SIZE; bn++, asym++)
              {
                if (sblk == symbol_block && bn >= symbol_block_index)
                  break;
 
-               Lisp_Object sym = make_lisp_symbol (&aligned_sym->s);
+               Lisp_Object sym = make_lisp_symbol (asym);
                if (symbol_uses_obj (sym, obj))
                  {
                    found = Fcons (sym, found);