[cfe-dev] c++11's alignas() ignored inside #pragma pack

[David Majnemer]

A fix is out for review here: http://reviews.llvm.org/D4649


On Wed, Jul 23, 2014 at 3:28 PM, Erik Schwiebert <eriksc at microsoft.com>
wrote:

>  Sounds good to me.  What’s the best way to track this?  Should I file a
> bug somewhere?  (Usually I work directly with Fariborz and Argyrios and
> other Apple folks in Radar, but this isn’t for an Apple-specific issue so
> it should go towards the open clang community instead…)
>
>
>
> Thanks,
>
> Schwieb
>
>
>
> *From:* metafoo at gmail.com [mailto:metafoo at gmail.com] *On Behalf Of *Richard
> Smith
> *Sent:* Wednesday, July 23, 2014 12:49 PM
> *To:* Erik Schwiebert
> *Cc:* jahanian; Reid Kleckner; cfe-dev Developers; Argyrios Kyrtzidis
> *Subject:* Re: [cfe-dev] c++11's alignas() ignored inside #pragma pack
>
>
>
> On Wed, Jul 23, 2014 at 10:18 AM, Erik Schwiebert <eriksc at microsoft.com>
> wrote:
>
>  Would it be possible to have clang support the alignas inside a pragma
> pack directive, if the structure in question is also inside a
> pragma ms_struct directive?  There are some teams at Microsoft using clang
> to target non-Windows platforms but sharing code that expects MSVC
> structure layout.  Using the Microsoft C++ ABI isn’t feasible as the
> platform in question is not a Microsoft Windows platform.
>
>
>
> Our documentation says:
>
>
>
> "clang supports the Microsoft #pragma pack feature for controlling record
> layout. GCC also contains support for this feature, however where MSVC and
> GCC are incompatible clang follows the MSVC definition."
>
>
>
> ... so I think we should do the obviously sane thing here in all cases
> (that is, alignas on a field should beat #pragma pack).
>
>
>
>  Thanks,
>
> Schwieb
>
> Microsoft Mac Team
>
>
>
> On Jul 23, 2014, at 9:48 AM, jahanian <fjahanian at apple.com> wrote:
>
>
>
>
> On Jul 23, 2014, at 9:45 AM, jahanian <fjahanian at apple.com> wrote:
>
>
>
>
>
> On Jul 22, 2014, at 5:26 PM, Reid Kleckner <rnk at google.com> wrote:
>
>
>
>
>
>
>
> Presumably they handle alignas the same way they handle
> __declspec(align(N)), which overrides packing.  MSVC appears to have two
> notions of alignment, required and advisory, while the C++ standard, Clang,
> and GCC only have one.  One way to observe this is to see that they will
> align an int64 to 8 bytes in a struct, but they will not realign the stack
> to 8 bytes when that struct is stack allocated.
>
>
>
> Also, Clang does respect alignas in this case when targeting the Microsoft
> C++ ABI.
>
>
>
> I tried the test case with these options for MS ABI:
>
>
>
> clang -cc1 -triple x86_64-windows-msvc -std=c++11 -S t.cpp
>
>
>
> Assembly indicates that it does not seem to respect alignas. Should I be
> using other options?
>
>
>
> main:
>
> movl $28, %eax
>
> movl $0, -4(%rsp)
>
> retq
>
>
>
>
>
>
>
> Oops. I was using an old clang. Sorry for the noise.
>
>
>
> main:
>
> movl $64, %eax
>
> retq
>
>
>
>   - Fariborz
>
>
>
>
>
>
>
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