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).
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 @@
#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 @@
* 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)