Fix GC bugs --with-wide-int and Qnil == 0

Use the same alignment for the !USE_LSB_TAG case as for the
more-typical USE_LSB_TAG case.  The attempt to support arbitrary
alignments with !USE_LSB_TAG had subtle bugs in garbage collection
once we changed the representation of symbols so that Qnil == 0.
Problem reported by Eli Zaretskii (Bug#20862).
* src/alloc.c (XMALLOC_HEADER_ALIGNMENT) [XMALLOC_OVERRUN_CHECK]:
* src/alloc.c (vector_alignment, union aligned_Lisp_Symbol)
(union aligned_Lisp_Misc, maybe_lisp_pointer, pure_alloc):
Use same alignment for !USE_LSB_TAG as for USE_LSB_TAG.
* src/alloc.c (POINTERS_MIGHT_HIDE_IN_OBJECTS): Remove.
This optimization in the !USE_LSB_TAG case is no longer valid when
symbols are represented via offsets.  Change the only use to
assume that pointers might hide in objects.
* src/lisp.h (alignas) [!USE_LSB_TAG]:
Require support in this case, too.
(TAG_SYMOFFSET, XSYMBOL) [!USE_LSB_TAG]: Do not shift the offset.
This is OK, because the !USE_LSB_TAG case now applies only when
Lisp_Object is wider than void *, so there's no longer any need
to shift the offset.  Not shifting the offset means that
symbol representations have the same alignment as pointers,
which the GC assumes.

1 files changed, 2 insertions, 9 deletions

diff --git a/src/lisp.h b/src/lisp.h
index 198f116fe02..c3289c9d700 100644
--- a/src/lisp.h
+++ b/src/lisp.h
@@ -278,10 +278,7 @@ error !;
 #endif
 
 #ifndef alignas
-# define alignas(alignment) /* empty */
-# if USE_LSB_TAG
-#  error "USE_LSB_TAG requires alignas"
-# endif
+# error "alignas not defined"
 #endif
 
 #ifdef HAVE_STRUCT_ATTRIBUTE_ALIGNED
@@ -731,9 +728,7 @@ struct Lisp_Symbol
 
 /* Yield an integer that contains a symbol tag along with OFFSET.
    OFFSET should be the offset in bytes from 'lispsym' to the symbol.  */
-#define TAG_SYMOFFSET(offset)                               \
-  TAG_PTR (Lisp_Symbol,                                     \
-           ((uintptr_t) (offset) >> (USE_LSB_TAG ? 0 : GCTYPEBITS)))
+#define TAG_SYMOFFSET(offset) TAG_PTR (Lisp_Symbol, offset)
 
 /* XLI_BUILTIN_LISPSYM (iQwhatever) is equivalent to
    XLI (builtin_lisp_symbol (Qwhatever)),
@@ -899,8 +894,6 @@ INLINE struct Lisp_Symbol *
 XSYMBOL (Lisp_Object a)
 {
   uintptr_t i = (uintptr_t) XUNTAG (a, Lisp_Symbol);
-  if (! USE_LSB_TAG)
-    i <<= GCTYPEBITS;
   void *p = (char *) lispsym + i;
   return p;
 }