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, 10 insertions, 9 deletions

diff --git a/src/buffer.c b/src/buffer.c
index edeed55e8be..4ae5e811b07 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -61,7 +61,7 @@ struct buffer *all_buffers;
    Setting the default value also goes through the alist of buffers
    and stores into each buffer that does not say it has a local value.  */
 
-struct GCALIGNED buffer buffer_defaults;
+struct buffer buffer_defaults;
 
 /* This structure marks which slots in a buffer have corresponding
    default values in buffer_defaults.
@@ -84,7 +84,7 @@ struct buffer buffer_local_flags;
 /* This structure holds the names of symbols whose values may be
    buffer-local.  It is indexed and accessed in the same way as the above.  */
 
-struct GCALIGNED buffer buffer_local_symbols;
+struct buffer buffer_local_symbols;
 
 /* Return the symbol of the per-buffer variable at offset OFFSET in
    the buffer structure.  */
@@ -1021,7 +1021,8 @@ reset_buffer_local_variables (struct buffer *b, bool permanent_too)
                           newlist = Fcons (elt, newlist);
                       }
                   newlist = Fnreverse (newlist);
-                  if (XSYMBOL (local_var)->trapped_write == SYMBOL_TRAPPED_WRITE)
+                  if (XSYMBOL (local_var)->u.s.trapped_write
+                      == SYMBOL_TRAPPED_WRITE)
                     notify_variable_watchers (local_var, newlist,
                                               Qmakunbound, Fcurrent_buffer ());
                   XSETCDR (XCAR (tmp), newlist);
@@ -1034,7 +1035,7 @@ reset_buffer_local_variables (struct buffer *b, bool permanent_too)
           else
             XSETCDR (last, XCDR (tmp));
 
-          if (XSYMBOL (local_var)->trapped_write == SYMBOL_TRAPPED_WRITE)
+          if (XSYMBOL (local_var)->u.s.trapped_write == SYMBOL_TRAPPED_WRITE)
             notify_variable_watchers (local_var, Qnil,
                                       Qmakunbound, Fcurrent_buffer ());
         }
@@ -1166,7 +1167,7 @@ buffer_local_value (Lisp_Object variable, Lisp_Object buffer)
   sym = XSYMBOL (variable);
 
  start:
-  switch (sym->redirect)
+  switch (sym->u.s.redirect)
     {
     case SYMBOL_VARALIAS: sym = indirect_variable (sym); goto start;
     case SYMBOL_PLAINVAL: result = SYMBOL_VAL (sym); break;
@@ -2096,7 +2097,7 @@ void set_buffer_internal_2 (register struct buffer *b)
         {
           Lisp_Object var = XCAR (XCAR (tail));
           struct Lisp_Symbol *sym = XSYMBOL (var);
-          if (sym->redirect == SYMBOL_LOCALIZED /* Just to be sure.  */
+          if (sym->u.s.redirect == SYMBOL_LOCALIZED /* Just to be sure.  */
               && SYMBOL_BLV (sym)->fwd)
             /* Just reference the variable
                to cause it to become set for this buffer.  */
@@ -2752,7 +2753,7 @@ swap_out_buffer_local_variables (struct buffer *b)
   for (alist = oalist; CONSP (alist); alist = XCDR (alist))
     {
       Lisp_Object sym = XCAR (XCAR (alist));
-      eassert (XSYMBOL (sym)->redirect == SYMBOL_LOCALIZED);
+      eassert (XSYMBOL (sym)->u.s.redirect == SYMBOL_LOCALIZED);
       /* Need not do anything if some other buffer's binding is
          now cached.  */
       if (EQ (SYMBOL_BLV (XSYMBOL (sym))->where, buffer))
@@ -5423,8 +5424,8 @@ defvar_per_buffer (struct Lisp_Buffer_Objfwd *bo_fwd, const char *namestring,
   bo_fwd->type = Lisp_Fwd_Buffer_Obj;
   bo_fwd->offset = offset;
   bo_fwd->predicate = predicate;
-  sym->declared_special = 1;
-  sym->redirect = SYMBOL_FORWARDED;
+  sym->u.s.declared_special = true;
+  sym->u.s.redirect = SYMBOL_FORWARDED;
   SET_SYMBOL_FWD (sym, (union Lisp_Fwd *) bo_fwd);
   XSETSYMBOL (PER_BUFFER_SYMBOL (offset), sym);