| From 0b91f8a668201fc58fa732b8acc496caedfdbae0 Mon Sep 17 00:00:00 2001 |
| From: Florian Weimer <fw@deneb.enyo.de> |
| Date: Sun, 29 Apr 2018 12:18:33 -0700 |
| Subject: [PATCH] Indicate that _PyGC_Head is only 8-byte aligned. (closes |
| bpo-33374) |
| |
| By spec, the "long double" in _PyGC_Head requires the union to always be 16-byte |
| aligned. However, obmalloc only yields 8-byte alignment. Compilers including GCC |
| 8 are starting to use alignment information to do store-merging. So, the "long |
| double" needs to be changed to a simple "double" as was long ago done in Python |
| 3 by e348c8d154cf6342c79d627ebfe89dfe9de23817. For 2.7, we need to add some |
| dummy padding to make sure _PyGC_Head stays the same size. |
| |
| Upstream: https://bugs.python.org/issue33374 |
| Signed-off-by: Stefan Becker <chemobejk@gmail.com> |
| --- |
| Include/objimpl.h | 17 ++++++++++++++++- |
| .../2018-04-29-12-07-00.bpo-33374.-xegL6.rst | 3 +++ |
| 2 files changed, 19 insertions(+), 1 deletion(-) |
| create mode 100644 Misc/NEWS.d/next/Core and Builtins/2018-04-29-12-07-00.bpo-33374.-xegL6.rst |
| |
| diff --git a/Include/objimpl.h b/Include/objimpl.h |
| index 5f2868332955..cbf6bc3f8763 100644 |
| --- a/Include/objimpl.h |
| +++ b/Include/objimpl.h |
| @@ -248,6 +248,20 @@ 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_old *gc_next; |
| + union _gc_head_old *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 +269,8 @@ 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 at least 8-byte alignment. */ |
| + char dummy_padding[sizeof(union _gc_head_old)]; |
| } PyGC_Head; |
| |
| extern PyGC_Head *_PyGC_generation0; |
| diff --git a/Misc/NEWS.d/next/Core and Builtins/2018-04-29-12-07-00.bpo-33374.-xegL6.rst b/Misc/NEWS.d/next/Core and Builtins/2018-04-29-12-07-00.bpo-33374.-xegL6.rst |
| new file mode 100644 |
| index 000000000000..9ec1a605c8f2 |
| --- /dev/null |
| +++ b/Misc/NEWS.d/next/Core and Builtins/2018-04-29-12-07-00.bpo-33374.-xegL6.rst |
| @@ -0,0 +1,3 @@ |
| +Tweak the definition of PyGC_Head, so compilers do not believe it is always |
| +16-byte aligned on x86. This prevents crashes with more aggressive |
| +optimizations present in GCC 8. |