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[Python-Dev] Python 2.7, long double vs allocator alignment, GCC 8 on x86-64


* Gregory P. Smith:

> The proper fix for this in the code would likely break ABI compatibility
> (ie: not possible in python 2.7 or any other stable release).
>
> Clang's UBSAN (undefined behavior sanitizer) has been flagging this one for
> a long time.
>
> In Python 3 a double is used instead of long double since 2012 as I did
> some digging at the time:
> https://github.com/python/cpython/commit/e348c8d154cf6342c79d627ebfe89dfe9de23817

A slightly more ABI-safe version of that change looks like this:

diff --git a/Include/objimpl.h b/Include/objimpl.h
index 55e83eced6..aa906144dc 100644
--- a/Include/objimpl.h
+++ b/Include/objimpl.h
@@ -248,6 +248,18 @@ PyAPI_FUNC(PyVarObject *) _PyObject_GC_Resize(PyVarObject *, Py_ssize_t);
 /* for source compatibility with 2.2 */
 #define _PyObject_GC_Del PyObject_GC_Del
 
+/* Former over-aligned definition of PyGC_Head, used to compute the
+   size of the padding for the new version below. */
+union _gc_head;
+union _gc_head_old {
+    struct {
+        union _gc_head *gc_next;
+        union _gc_head *gc_prev;
+        Py_ssize_t gc_refs;
+    } gc;
+    long double dummy;
+};
+
 /* GC information is stored BEFORE the object structure. */
 typedef union _gc_head {
     struct {
@@ -255,7 +267,8 @@ typedef union _gc_head {
         union _gc_head *gc_prev;
         Py_ssize_t gc_refs;
     } gc;
-    long double dummy;  /* force worst-case alignment */
+    double dummy;  /* force worst-case alignment */
+    char dummy_padding[sizeof(union _gc_head_old)];
 } PyGC_Head;
 
 extern PyGC_Head *_PyGC_generation0;

This preserves the offset used by _Py_AS_GC in case it has been built
into existing binaries.  It may be more appropriate to do it this way
for Python 2.7.  I think it's also more conservative than the
allocator changes.