[issue6493] Can not set value for structure members larger than 32 bits

[Meador Inge]

Meador Inge <meadori at gmail.com> added the comment:

The problem is with the 'BIT_MASK' macro, which is currently defined as the following for Windows:

   #define BIT_MASK(size) ((1 << NUM_BITS(size))-1)

The C standard says that any attempt to shift left by exactly the bit width is undefined behavior.  For the given test case 'NUM_BITS(size) == 32' and the '1' constant is of type 'int'.  Thus the behavior is undefined.

For x86 Windows with the MSVC++ compiler this ends up as 'SAL edx, CL' and the Intel manuals say that the shift count is masked by 0x1F.  Thus the bit mask ends up as '(1 << 0) - 1 == 0'.  Plug the zero mask into the 'SET' macro and you will see why the reported behavior is being observed.

I tested Hirokazu's last patch on Windows 7 64-bit and OS X 10.7;  I saw no regression.  I will apply the patch shortly.

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assignee:  -> meador.inge

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<http://bugs.python.org/issue6493>
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