[3.7] bpo-27987: align PyGC_Head to alignof(long double) (GH-13335) (GH-13581)

This reverts commit 2156fec1f7a8f9972e90cdbaf404e3fd9eaccb35.

Now that https://github.com/python/cpython/commit/1b85f4ec45a5d63188ee3866bd55eb29fdec7fbf is in, this change makes sense.

1 files changed, 5 insertions, 1 deletions

diff --git a/Include/objimpl.h b/Include/objimpl.h
index 057bb50cbd..0436ba7899 100644
--- a/Include/objimpl.h
+++ b/Include/objimpl.h
@@ -255,7 +255,11 @@ typedef union _gc_head {
         union _gc_head *gc_prev;
         Py_ssize_t gc_refs;
     } gc;
-    double dummy;  /* force worst-case alignment */
+    long double dummy;  /* force worst-case alignment */
+    // malloc returns memory block aligned for any built-in types and
+    // long double is the largest standard C type.
+    // On amd64 linux, long double requires 16 byte alignment.
+    // See bpo-27987 for more discussion.
 } PyGC_Head;
 
 extern PyGC_Head *_PyGC_generation0;