linux-kernel - RE: [RFC PATCH] lib: Introduce generic __cmpxchg

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Date:   Fri, 2 Nov 2018 10:56:31 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     "'paulmck@...ux.ibm.com'" <paulmck@...ux.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>
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        "mark.rutland@....com" <mark.rutland@....com>,
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Subject: RE: [RFC PATCH] lib: Introduce generic __cmpxchg_u64() and use it
 where needed

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.

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.

> > 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.

	David

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