On Fri, Apr 12, 2019 at 05:50:30PM +0100, David Howells wrote:
> Linus Torvalds <torvalds@linux-foundation.org> wrote:
> 
> > We should never have stack alignment bigger than 16 bytes.  And
> > preferably not even that.
> 
> At least one arch I know of (FRV) had instructions that could atomically
> load/store register pairs or register quads, but they had to be pair- or
> quad-aligned (ie. 8- or 16-byte), which made for more efficient code if you
> could use them.
> 
> I don't know whether any arch we currently support has features like this (I
> know some have multi-reg load/stores, but they seem to require only
> word-alignment).

ARC (iirc) has u64 atomics with natural alignment requirements but
alignof(u64)=4 due it being a 32bit arch. Which is awkward.

ARMv7 can also do u64 ops when aligned right, but I forgot if they have
proper alignment or not.