Re: [RFC PATCH] lib: Introduce generic __cmpxchg_u64() and use it where needed

[Peter Zijlstra]

On Fri, Nov 02, 2018 at 10:56:31AM +0000, David Laight wrote:
> From: Paul E. McKenney
> > Sent: 01 November 2018 17:02
> ...
> > And there is a push to define C++ signed arithmetic as 2s complement,
> > but there are still 1s complement systems with C compilers.  Just not
> > C++ compilers.  Legacy...
> 
> Hmmm... I've used C compilers for DSPs where signed integer arithmetic
> used the 'data registers' and would saturate, unsigned used the 'address
> registers' and wrapped.
> That was deliberate because it is much better to clip analogue values.

Seems a dodgy heuristic if you ask me.

> Then there was the annoying cobol run time that didn't update the
> result variable if the result wouldn't fit.
> Took a while to notice that the sum of a list of values was even wrong!
> That would be perfectly valid for C - if unexpected.

That's just insane ;-)

> > > But for us using -fno-strict-overflow which actually defines signed
> > > overflow
> 
> I wonder how much real code 'strict-overflow' gets rid of?
> IIRC gcc silently turns loops like:
> 	int i; for (i = 1; i != 0; i *= 2) ...
> into infinite ones.
> Which is never what is required.

Nobody said C was a 'safe' language. But less UB makes a better language
IMO. Ideally we'd get all UBs filled in -- but I realise C has a few
very 'interesting' ones that might be hard to get rid of.