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

[jahanian]

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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