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

[Inada Naoki]

https://github.com/python/cpython/commit/ea2b76bdc5f97f49701213d105b8ec2387ea2fa5
commit: ea2b76bdc5f97f49701213d105b8ec2387ea2fa5
branch: 3.7
author: Inada Naoki <songofacandy at gmail.com>
committer: GitHub <noreply at github.com>
date: 2019-05-25T21:13:33+09:00
summary:

bpo-27987: align PyGC_Head to alignof(long double) (GH-13335)

files:
A Misc/NEWS.d/next/Core and Builtins/2019-05-15-18-28-43.bpo-27987.FaxuLy.rst
M Include/objimpl.h

diff --git a/Include/objimpl.h b/Include/objimpl.h
index 057bb50cbda9..0436ba7899d9 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;
diff --git a/Misc/NEWS.d/next/Core and Builtins/2019-05-15-18-28-43.bpo-27987.FaxuLy.rst b/Misc/NEWS.d/next/Core and Builtins/2019-05-15-18-28-43.bpo-27987.FaxuLy.rst
new file mode 100644
index 000000000000..97ca37b262a1
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2019-05-15-18-28-43.bpo-27987.FaxuLy.rst	
@@ -0,0 +1,2 @@
+``PyGC_Head`` structure is aligned to ``long double``.  This is needed to
+GC-ed objects are aligned properly.  Patch by Inada Naoki.