SF bug [#467145] Python 2.2a4 build problem on HPUX 11.0.

The platform requires 8-byte alignment for doubles, but the GC header
was 12 bytes and that threw off the natural alignment of the double
members of a subtype of complex.  The fix puts the GC header into a
union with a double as the other member, to force no-looser-than
double alignment of GC headers.  On boxes that require 8-byte alignment
for doubles, this may add pad bytes to the GC header accordingly; ditto
for platforms that *prefer* 8-byte alignment for doubles.  On platforms
that don't care, it shouldn't change the memory layout (because the
size of the old GC header is certainly greater than the size of a double
on all platforms, so unioning with a double shouldn't change size or
alignment on such boxes).

1 files changed, 15 insertions, 12 deletions

diff --git a/Include/objimpl.h b/Include/objimpl.h
index db4cb6d..e8aad54 100644
--- a/Include/objimpl.h
+++ b/Include/objimpl.h
@@ -266,10 +266,13 @@ extern DL_IMPORT(void) _PyObject_GC_UnTrack(PyObject *);
 #ifdef WITH_CYCLE_GC
 
 /* GC information is stored BEFORE the object structure */
-typedef struct _gc_head {
-	struct _gc_head *gc_next; /* not NULL if object is tracked */
-	struct _gc_head *gc_prev;
-	int gc_refs;
+typedef union _gc_head {
+	struct {
+		union _gc_head *gc_next; /* not NULL if object is tracked */
+		union _gc_head *gc_prev;
+		int gc_refs;
+	} gc;
+	double dummy;  /* force worst-case alignment */
 } PyGC_Head;
 
 extern PyGC_Head _PyGC_generation0;
@@ -278,20 +281,20 @@ extern PyGC_Head _PyGC_generation0;
  * collector it must be safe to call the ob_traverse method. */
 #define _PyObject_GC_TRACK(o) do { \
 	PyGC_Head *g = (PyGC_Head *)(o)-1; \
-	if (g->gc_next != NULL) \
+	if (g->gc.gc_next != NULL) \
 		Py_FatalError("GC object already in linked list"); \
-	g->gc_next = &_PyGC_generation0; \
-	g->gc_prev = _PyGC_generation0.gc_prev; \
-	g->gc_prev->gc_next = g; \
-	_PyGC_generation0.gc_prev = g; \
+	g->gc.gc_next = &_PyGC_generation0; \
+	g->gc.gc_prev = _PyGC_generation0.gc.gc_prev; \
+	g->gc.gc_prev->gc.gc_next = g; \
+	_PyGC_generation0.gc.gc_prev = g; \
     } while (0);
 
 /* Tell the GC to stop tracking this object. */
 #define _PyObject_GC_UNTRACK(o) do { \
 	PyGC_Head *g = (PyGC_Head *)(o)-1; \
-	g->gc_prev->gc_next = g->gc_next; \
-	g->gc_next->gc_prev = g->gc_prev; \
-	g->gc_next = NULL; \
+	g->gc.gc_prev->gc.gc_next = g->gc.gc_next; \
+	g->gc.gc_next->gc.gc_prev = g->gc.gc_prev; \
+	g->gc.gc_next = NULL; \
     } while (0);
 
 #define PyObject_GC_Track(op) _PyObject_GC_Track((PyObject *)op)