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

Gregory P. Smith webhook-mailer at python.org
Sun Jun 2 21:51:35 EDT 2019

commit: 8766cb74e186d3820db0a855ccd780d6d84461f7
branch: 3.7
author: Inada Naoki <songofacandy at gmail.com>
committer: Gregory P. Smith <greg at krypto.org>
date: 2019-06-02T18:51:31-07:00

[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.

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..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.

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