[libstdc++/65033] Give alignment info to libatomic

[Jonathan Wakely]

On 25/03/15 11:36 -0700, Richard Henderson wrote:
>On 03/25/2015 09:22 AM, Jonathan Wakely wrote:
>>      private:
>> -      _Tp _M_i;
>> +      // Align 1/2/4/8/16-byte types the same as integer types of that size.
>> +      // This matches the alignment effects of the C11 _Atomic qualifier.
>> +      static constexpr int _S_alignment
>> +	= sizeof(_Tp) == sizeof(char)	   ? alignof(char)
>> +	: sizeof(_Tp) == sizeof(short)	   ? alignof(short)
>> +	: sizeof(_Tp) == sizeof(int)	   ? alignof(int)
>> +	: sizeof(_Tp) == sizeof(long)	   ? alignof(long)
>> +	: sizeof(_Tp) == sizeof(long long) ? alignof(long long)
>> +#ifdef _GLIBCXX_USE_INT128
>> +	: sizeof(_Tp) == sizeof(__int128)  ? alignof(__int128)
>> +#endif
>> +	: alignof(_Tp);
>> +
>> +      alignas(_S_alignment) _Tp _M_i;
>
>
>Surely not by reducing a larger alignment applied to _Tp.
>I.e.
>
>  static constexpr int _S_min_alignment
>	= sizeof(_Tp) == sizeof(char)	   ? alignof(char)
>	: sizeof(_Tp) == sizeof(short)	   ? alignof(short)
>	: sizeof(_Tp) == sizeof(int)	   ? alignof(int)
>	: sizeof(_Tp) == sizeof(long)	   ? alignof(long)
>	: sizeof(_Tp) == sizeof(long long) ? alignof(long long)
>#ifdef _GLIBCXX_USE_INT128
>	: sizeof(_Tp) == sizeof(__int128)  ? alignof(__int128)
>#endif
>	: 0;
>
>  static constexpr int _S_alignment
>	= _S_min_alignment > alignof(_Tp) ? _S_min_alignment : alignof(_Tp);

Doh, good catch. I'll make that change and add a test with a type that
has alignof(X) > sizeof(X).


On 25/03/15 11:39 -0700, Richard Henderson wrote:
>On 03/25/2015 09:22 AM, Jonathan Wakely wrote:
>> +static_assert( alignof(std::atomic<twoints>) > alignof(int),
>> +               "std::atomic not suitably aligned" );
>
>This is only true if int64_t has alignment larger than int32_t,
>which is unfortunately not always the case.

Huh, didn't realise that. I could change the tests to check it's
alignof(std::int64_t) as the next assertion does, but is it safe to
assume that struct twoints { int a; int b; } is exactly 64 bits
everywhere?

I'd prefer not to have the test say "if sizeof(twoints) ==
sizeof(long), test this, otherwise if sizeof(twoints) == ..."