[cfe-dev] Implementation of the CGRecordLayoutBuilder for Microsoft ABI.

[r4start]

On 27/09/2011 04:07, Eli Friedman wrote:
> On Mon, Sep 26, 2011 at 4:49 PM, David Blaikie<dblaikie at gmail.com>  wrote:
>>
>> On Mon, Sep 26, 2011 at 4:36 PM, Francois Pichet<pichet2000 at gmail.com>
>> wrote:
>>> On Mon, Sep 26, 2011 at 6:52 PM, Eli Friedman<eli.friedman at gmail.com>
>>> wrote:
>>>> On Mon, Sep 26, 2011 at 6:09 AM, r4start<r4start at gmail.com>  wrote:
>>>>> I really don't understand why align=4 isn't correct.
>>>>> I always thought that #pragma pack only affects members of the
>>>>> structure.
>>>>> And __declspec(align(n)) specifies the alignment for objects of type.
>>>>> In MSVS align for B is 4 and for A is 1.
>>>> The only answer I can come up with is the model MSVC uses is simply
>>>> insane.
>>>>
>>>> One more attempt to get some sanity: does the following compile with
>>>> MSVC?  If so, what is the output?
>>>>
>>>> #include<cstdio>
>>>> class A {
>>>>   char c;
>>>> };
>>>> class B : public virtual A {
>>>>   public:
>>>>   void *p;
>>>> };
>>>> B* x = new B[10];
>>>> int main() { std::printf("%d\n", (int)(x[1].p)); }
>>>>
>>> it compiles and the output is 0.
>>> not sure what you are trying to test here.
>> I'll guess that Eli meant to print&x[1].p (to see whether the void* pointer
>> was aligned or not), perhaps. I modified the program to print:
>>      std::printf("%d\n", (int)(&x[0].p));
>>      std::printf("%d\n", (int)(&x[1].p));
>> &  the result I got (in debug) was:
>> 1077836
>> 1077845
> Right... so the alignment is in fact 1.  So to be compliant with the
> C++ standard on x86-32 Windows, we can't assume any alignment on loads
> and stores.  Fun.
>
Without a specify __declspec(align(#)) is true.
So, what approach is the best to resolve this problem?
Patch StructLayout or smth else?

  - Dmitry.