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Date:   Fri, 2 Nov 2018 06:38:37 -0700
From:   "Paul E. McKenney" <paulmck@linux.ibm.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     David Laight <David.Laight@ACULAB.COM>,
        Trond Myklebust <trondmy@hammerspace.com>,
        "mark.rutland@arm.com" <mark.rutland@arm.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "ralf@linux-mips.org" <ralf@linux-mips.org>,
        "jlayton@kernel.org" <jlayton@kernel.org>,
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        "will.deacon@arm.com" <will.deacon@arm.com>,
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        "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
        "davem@davemloft.net" <davem@davemloft.net>,
        "arnd@arndb.de" <arnd@arndb.de>,
        "paulus@samba.org" <paulus@samba.org>,
        "mpe@ellerman.id.au" <mpe@ellerman.id.au>,
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        "aryabinin@virtuozzo.com" <aryabinin@virtuozzo.com>,
        "dvyukov@google.com" <dvyukov@google.com>
Subject: Re: [RFC PATCH] lib: Introduce generic __cmpxchg_u64() and use it
 where needed
Reply-To: paulmck@linux.ibm.com
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On Fri, Nov 02, 2018 at 01:23:28PM +0100, Peter Zijlstra wrote:
> 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.

There has been an effort to reduce UB, but not sure how far they got.

							Thanx, Paul