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

[r4start]

On 27/09/2011 22:26, Eli Friedman wrote:
> On Mon, Sep 26, 2011 at 11:29 PM, r4start<r4start at gmail.com>  wrote:
>> 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?
> Just fixing the RecordLayoutBuilder to set the alignment correctly
> should be enough to start with.  It has wider implications, though,
> which might be a bit tricky to model: it means that we can't assume
> *any* struct is actually aligned.
>
> -Eli
Ok, I test my CGRecordLayoutBuilder for Microsoft and when it is ready 
I'll publish it.