Re: [PATCH] c++: implement C++17 hardware interference size

[Jonathan Wakely <jwakely@redhat.com>]

On Fri, 16 Jul 2021 at 20:26, Matthias Kretz <m.kretz@gsi.de> wrote:
>
> On Friday, 16 July 2021 18:54:30 CEST Jonathan Wakely wrote:
> > On Fri, 16 Jul 2021 at 16:33, Jason Merrill wrote:
> > > Adjusting them based on tuning would certainly simplify a significant use
> > > case, perhaps the only reasonable use.  Cases more concerned with ABI
> > > stability probably shouldn't use them at all. And that would mean not
> > > needing to worry about the impossible task of finding the right values for
> > > an entire architecture.
> >
> > But it would be quite a significant change in behaviour if -mtune
> > started affecting ABI, wouldn't it?
>
> For existing code -mtune still doesn't affect ABI.

True, because existing code isn't using the constants.

>The users who write
>
> struct keep_apart {
>   alignas(std::hardware_destructive_interference_size) std::atomic<int> cat;
>   alignas(std::hardware_destructive_interference_size) std::atomic<int> dog;
> };
>
> *want* to have different sizeof(keep_apart) depending on the CPU the code is
> compiled for. I.e. they *ask* for getting their ABI broken.

Right, but the person who wants that and the person who chooses the
-mtune option might be different people.

A distro might add -mtune=core2 to all package builds by default, not
expecting it to cause ABI changes. Some header in a package in the
distro might start using the constants. Now everybody who includes
that header needs to use the same -mtune option as the distro default.

That change in the behaviour and expected use of an existing option
seems scary to me. Even with a warning about using the constants
(because somebody's just going to use #pragma around their use of the
constants to disable the warning, and now the ABI impact of -mtune is
much less obvious).

It's much less scary in a world where the code is written and used by
the same group of people, but for something like a linux distro it
worries me.