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

[Jason Merrill <jason@redhat.com>]

On Fri, Jul 16, 2021, 12:54 PM Jonathan Wakely <jwakely@redhat.com> 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?
>

Absolutely, though with the below warning, which could mention this issue,
it would only affect the ABI of code that ignores the warning. Code that
silences it by specifying values would not be affected.

> I'm thinking about warning by default for any use of the variables without
> > explicitly specifying their values on the command line. Users could
> disable
> > the warning if they're happy using whatever the defaults happen to be.
>
> I like that suggestion.
>
> Maybe the warning could suggest optimal values based on the current
> -mtune flag.


Sounds good.

That way -mtune wouldn't need to alter ABI, but by
> combining -mtune with explicit values for the variables you get the
> best performance. And -mtune without overriding the default values
> preserves ABI.
>
>