Fix test_atomic failure caused unaligned AO_double_t access on x86 (VC++)

Passing an unaligned AO_double_t pointer to AO double-wide primitives
results in an undefined behavior of the latter when running on x86 (or
violation of the corresponding assertion on the alignment).
MS VC++/x86 aligns AO_double_t values on a 4-byte boundary by default.
For the proper alignment, __declspec(align(8)) should be applied to
variables defined in the client code (which uses double-wide AO
primitives).  Unfortunately, the attribute cannot be added to
AO_double_t definition itself because the compiler does not allow the
attribute for function arguments.

This patch introduces AO_DOUBLE_ALIGN attribute for use by clients of
the double-wide AO primitives (and, thus, AO_stack clients).
Matters only Visual Studio compiler for X86.
The inner clients (atomic_ops_malloc, test_atomic, test_stack) are
updated to use this attribute.

* doc/README_win32.txt [x86] (AO_DOUBLE_ALIGN): Document.
* src/atomic_ops/sysdeps/generic_pthread.h (AO_DOUBLE_ALIGN): Define
(as empty).
* src/atomic_ops/sysdeps/standard_ao_double_t.h [!_WIN64 && _WIN32
&& !__GNUC__ && _MSC_VER] (AO_DOUBLE_ALIGN): Define as declspec
align(8); document it.
* src/atomic_ops/sysdeps/standard_ao_double_t.h [!AO_DOUBLE_ALIGN]
(AO_DOUBLE_ALIGN): Define as empty (otherwise).
* src/atomic_ops_stack.h [AO_USE_ALMOST_LOCK_FREE && !AO_DOUBLE_ALIGN]
(AO_DOUBLE_ALIGN): Likewise.
* src/atomic_ops_malloc.c (AO_free_list): Use AO_DOUBLE_ALIGN attribute.
* tests/test_stack.c (the_list): Likewise.
* src/atomic_ops_stack.h [!AO_USE_ALMOST_LOCK_FREE] (AO_stack_t):
Document AO_DOUBLE_ALIGN usage (by clients).
* tests/test_atomic_include.template (test_atomicXX): Use
AO_DOUBLE_ALIGN attribute for old_w and w double-wide local variables
(to avoid alignment assertion violation or AO primitives undefined
behavior on x86 if the test code is compiled by VC++).

1 files changed, 5 insertions, 0 deletions

diff --git a/doc/README_win32.txt b/doc/README_win32.txt
index e2b87ad..812006c 100644
--- a/doc/README_win32.txt
+++ b/doc/README_win32.txt
@@ -26,6 +26,11 @@ Most clients of atomic_ops.h will need to define AO_ASSUME_WINDOWS98 before
 including it.  Compare_and_swap is otherwise not available.
 Defining AO_ASSUME_VISTA will make compare_double_and_swap_double available
 as well.
+Please note that MS compiler for x86 does not align AO_double_t on an 8-byte
+boundary, thus to avoid an undefined behavior, an AO_double_t (volatile)
+variable should be declared with AO_DOUBLE_ALIGN attribute if the variable
+reference is passed to an AO primitive (the attribute is not applicable to
+arguments and pointers).
 
 Note that the library is covered by the GNU General Public License, while
 the top 2 of these pieces allow use in proprietary code.