On Thu, 1 Jan 2026, 13:37 Frederick Virchanza Gotham via Std-Proposals, <
std-proposals_at_[hidden]> wrote:

>
>
> On Thursday, January 1, 2026, Simon Schröder wrote:
>
>>
>>
>> You seem to forget that you are linking against the C and C++ standard
>> libraries.These have to be compiled either with backwards compatibility or
>> with cmpxchg16b. Also, there are parts of them which are typically dynamic
>> libraries. Are you suggesting that Linux distributions should ship two
>> libraries and do some magic to select the right one? I think it is quite
>> obvious why Linux distributions choose backwards compatibility.
>>
>
>
>
> Backwards-compatibility is a noble goal . . . I mean it was vital that the
> Playstation 2 could play all the Playstation 1 games, and it's handy that
> you can put a Mach 3 Turbo razor into a Mach 3 handle.
>
> But there comes a point when the backwards-compatibility just isn't worth
> it. I've been trying to dig up info and here's what I found:
>
> Intel: All Intel x86_64 CPUs include CMPXCHG16B
>

Nocona doesn't, as far as I know


> AMD: The holdouts were early K8 parts before Revision F. Those pre-F
> (2006 and earlier) chips lacked CMPXCHG16B.
>
> So we're talking about computers from about 17 or 18 years ago. That's a
> long time. That's like making sure the Playstation 5 can still play
> Playstation 2 games. I'll admit it would be cool to be able to play PS2
> games on a PS5, but not at any substantial cost.
>
> I'm on holidays right now but when I get home next week I'm patching the
> GNU compiler so that CMPXCHG16B will be placed inline.
>
> And worst case scenario if you run one of my programs on an AMD computer
> from 2006, you'll just see:
>
> SIGILL: Illegal Instruction
>
> So it will be a hard failure rather than an intermittent bug. Plus, just
> to be safe, I'll edit the libatomic function so that it calls 'abort' --
> just in case there's a translation unit or shared library somewhere that
> tries to use it.
>
>
>
>
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