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

[Erik Schwiebert]

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<mailto: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<mailto:fjahanian at apple.com>> wrote:


On Jul 23, 2014, at 9:45 AM, jahanian <fjahanian at apple.com<mailto:fjahanian at apple.com>> wrote:



On Jul 22, 2014, at 5:26 PM, Reid Kleckner <rnk at google.com<mailto: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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