From: Inada Naoki <songofacandy@gmail.com>
Date: Mon, 3 Jun 2019 01:51:32 +0000 (+0900)
Subject: [3.7] bpo-27987: align PyGC_Head to alignof(long double) (GH-13335) (GH-13581)
X-Git-Tag: v3.7.4rc1~65
X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=8766cb74e186d3820db0a855ccd780d6d84461f7;p=thirdparty%2FPython%2Fcpython.git

[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.
---

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..98073471ca7e
--- /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
+ensure GC-ed objects are aligned properly.  Patch by Inada Naoki.