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, 6 insertions, 6 deletions

diff --git a/src/minibuf.c b/src/minibuf.c
index a2f3324f99f..913c93001ef 100644
--- a/src/minibuf.c
+++ b/src/minibuf.c
@@ -1280,8 +1280,8 @@ is used to further constrain the set of candidates.  */)
 		error ("Bad data in guts of obarray");
 	      elt = bucket;
 	      eltstring = elt;
-	      if (XSYMBOL (bucket)->next)
-		XSETSYMBOL (bucket, XSYMBOL (bucket)->next);
+	      if (XSYMBOL (bucket)->u.s.next)
+		XSETSYMBOL (bucket, XSYMBOL (bucket)->u.s.next);
 	      else
 		XSETFASTINT (bucket, 0);
 	    }
@@ -1533,8 +1533,8 @@ with a space are ignored unless STRING itself starts with a space.  */)
 		error ("Bad data in guts of obarray");
 	      elt = bucket;
 	      eltstring = elt;
-	      if (XSYMBOL (bucket)->next)
-		XSETSYMBOL (bucket, XSYMBOL (bucket)->next);
+	      if (XSYMBOL (bucket)->u.s.next)
+		XSETSYMBOL (bucket, XSYMBOL (bucket)->u.s.next);
 	      else
 		XSETFASTINT (bucket, 0);
 	    }
@@ -1754,9 +1754,9 @@ the values STRING, PREDICATE and `lambda'.  */)
 			tem = tail;
 			break;
 		      }
-		    if (XSYMBOL (tail)->next == 0)
+		    if (XSYMBOL (tail)->u.s.next == 0)
 		      break;
-		    XSETSYMBOL (tail, XSYMBOL (tail)->next);
+		    XSETSYMBOL (tail, XSYMBOL (tail)->u.s.next);
 		  }
 	    }
 	}