diff -uNr Python-3.6.15.orig/Include/objimpl.h Python-3.6.15/Include/objimpl.h
--- Python-3.6.15.orig/Include/objimpl.h	2024-01-22 20:40:32.190635318 +0100
+++ Python-3.6.15/Include/objimpl.h	2024-01-22 20:41:00.330865534 +0100
@@ -255,7 +255,11 @@
         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 -uNr "Python-3.6.15.orig/Misc/NEWS.d/next/Core and Builtins/2019-04-16-11-52-21.bpo-27987.n2_DcQ.rst" "Python-3.6.15/Misc/NEWS.d/next/Core and Builtins/2019-04-16-11-52-21.bpo-27987.n2_DcQ.rst"
--- "Python-3.6.15.orig/Misc/NEWS.d/next/Core and Builtins/2019-04-16-11-52-21.bpo-27987.n2_DcQ.rst"	1970-01-01 01:00:00.000000000 +0100
+++ "Python-3.6.15/Misc/NEWS.d/next/Core and Builtins/2019-04-16-11-52-21.bpo-27987.n2_DcQ.rst"	2024-01-22 20:43:37.405496340 +0100
@@ -0,0 +1,3 @@
+pymalloc returns memory blocks aligned by 16 bytes, instead of 8 bytes, on
+64-bit platforms to conform x86-64 ABI. Recent compilers assume this alignment
+more often. Patch by Inada Naoki.
\ No newline at end of file
diff -uNr "Python-3.6.15.orig/Misc/NEWS.d/next/Core and Builtins/2019-05-15-18-28-43.bpo-27987.FaxuLy.rst" "Python-3.6.15/Misc/NEWS.d/next/Core and Builtins/2019-05-15-18-28-43.bpo-27987.FaxuLy.rst"
--- "Python-3.6.15.orig/Misc/NEWS.d/next/Core and Builtins/2019-05-15-18-28-43.bpo-27987.FaxuLy.rst"	1970-01-01 01:00:00.000000000 +0100
+++ "Python-3.6.15/Misc/NEWS.d/next/Core and Builtins/2019-05-15-18-28-43.bpo-27987.FaxuLy.rst"	2024-01-22 20:44:05.622397005 +0100
@@ -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.
\ No newline at end of file
diff -uNr Python-3.6.15.orig/Objects/obmalloc.c Python-3.6.15/Objects/obmalloc.c
--- Python-3.6.15.orig/Objects/obmalloc.c	2024-01-22 20:40:32.287302775 +0100
+++ Python-3.6.15/Objects/obmalloc.c	2024-01-22 20:42:22.008204376 +0100
@@ -650,8 +650,14 @@
  *
  * You shouldn't change this unless you know what you are doing.
  */
+
+#if SIZEOF_VOID_P > 4
+#define ALIGNMENT              16               /* must be 2^N */
+#define ALIGNMENT_SHIFT         4
+#else
 #define ALIGNMENT               8               /* must be 2^N */
 #define ALIGNMENT_SHIFT         3
+#endif
 
 /* Return the number of bytes in size class I, as a uint. */
 #define INDEX2SIZE(I) (((uint)(I) + 1) << ALIGNMENT_SHIFT)