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

[Jonathan Wakely]

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?

> 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. 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.