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

[Jonathan Wakely]

On 31/03/15 07:54 -0700, Richard Henderson wrote:
>On 03/31/2015 06:41 AM, Jonathan Wakely wrote:
>> This is the best I've come up with, does anyone have any better ideas
>> than the #else branch to hardcode alignment of 16-byte types to 16?
>
>If there's no 16 byte type, are we convinced this matters?  I mean, there isn't
>a 16-byte atomic instruction for 32-bit x86 (or any other 32-bit cpu of which I
>am aware).  So we're forced to use a locking path anyway.

The C front end gives struct S { char s[16]; } 16 byte alignment, and
I'd like std::atomic<S> and _Atomic struct S to be layout compatible,
although it's not essential (or required by any standard).

And it matters most for the integral types, not arbitrary structs.

>And if we do want the alignment, do we stop pretending with all the sizeof's
>and alignof's and just use power-of-two size alignment for all of them, e.g.
>
>  min_align = ((size & (size - 1)) || size > 16 ? 0 : size)

Yeah, I wondered about that too. Joseph indicated there are targets
where C gives alignof(_Atomic X) != sizeof(X), which is why the target
hook exists, but maybe we can just not worry about those targets for
now.  For GCC 6 we can look into the new attribute Andrew did in the
atomics branch so that we can make std::atomic use the target hook
directly instead of trying to simulate its effects in C++ code.