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

[Eli Friedman]

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.

-Eli