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

[Matthias Kretz]

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. 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. If they wanted to 
specify the value themselves on the command line they'd written:

struct keep_apart {
  alignas(SOME_MACRO) std::atomic<int> cat;
  alignas(SOME_MACRO) std::atomic<int> dog;
};

I would be very disappointed if std::hardware_destructive_interference_size 
and std::hardware_constructive_interference_size turn into a glorified macro.

-- 
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 Dr. Matthias Kretz                           https://mattkretz.github.io
 GSI Helmholtz Centre for Heavy Ion Research               https://gsi.de
 std::experimental::simd              https://github.com/VcDevel/std-simd
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