Re: single copy atomicity for double load/stores on 32-bit systems

[Paul E. McKenney]

On Mon, Jun 03, 2019 at 06:08:35PM +0000, Vineet Gupta wrote:
> On 5/31/19 1:21 AM, Peter Zijlstra wrote:
> >> I'm not sure how to interpret "natural alignment" for the case of double
> >> load/stores on 32-bit systems where the hardware and ABI allow for 4 byte
> >> alignment (ARCv2 LDD/STD, ARM LDRD/STRD ....)
> > Natural alignment: !((uintptr_t)ptr % sizeof(*ptr))
> >
> > For any u64 type, that would give 8 byte alignment. the problem
> > otherwise being that your data spans two lines/pages etc..
> 
> Sure, but as Paul said, if the software doesn't expect them to be atomic by
> default, they could span 2 hardware lines to keep the implementation simpler/sane.

I could imagine 8-byte types being only four-byte aligned on 32-bit systems,
but it would be quite a surprise on 64-bit systems.

							Thanx, Paul