From: David Laight <David.Laight@ACULAB.COM>
To: "'paulmck@linux.ibm.com'" <paulmck@linux.ibm.com>,
Peter Zijlstra <peterz@infradead.org>
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Subject: RE: [RFC PATCH] lib: Introduce generic __cmpxchg_u64() and use it where needed
Date: Fri, 2 Nov 2018 10:56:31 +0000 [thread overview]
Message-ID: <7d1ecd21c4c249138dfdd42b9aaa1cea@AcuMS.aculab.com> (raw)
In-Reply-To: <20181101170146.GQ4170@linux.ibm.com>
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
-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)
next prev parent reply other threads:[~2018-11-02 13:59 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-10-31 19:52 [RFC PATCH] lib: Introduce generic __cmpxchg_u64() and use it where needed Guenter Roeck
2018-10-31 21:32 ` Paul Burton
2018-10-31 22:02 ` Guenter Roeck
2018-10-31 23:32 ` Paul Burton
2018-11-01 0:17 ` Trond Myklebust
2018-11-01 13:18 ` Mark Rutland
2018-11-01 14:59 ` Peter Zijlstra
2018-11-01 15:22 ` Trond Myklebust
2018-11-01 16:32 ` Peter Zijlstra
2018-11-01 16:59 ` Eric Dumazet
2018-11-01 17:14 ` Peter Zijlstra
2018-11-01 17:27 ` Peter Zijlstra
2018-11-01 20:29 ` Paul E. McKenney
2018-11-01 21:38 ` Peter Zijlstra
2018-11-01 22:26 ` Paul E. McKenney
2018-11-01 17:43 ` Paul E. McKenney
2018-11-01 17:01 ` Paul E. McKenney
2018-11-01 17:18 ` Peter Zijlstra
2018-11-01 17:34 ` Paul E. McKenney
2018-11-01 17:46 ` Dmitry Vyukov
2018-11-01 21:45 ` Peter Zijlstra
2018-11-02 10:56 ` David Laight [this message]
2018-11-02 12:23 ` Peter Zijlstra
2018-11-02 13:38 ` Paul E. McKenney
2018-11-02 13:37 ` Paul E. McKenney
2018-11-02 16:19 ` Andrey Ryabinin
2018-11-05 10:38 ` Peter Zijlstra
2018-11-05 14:24 ` Peter Zijlstra
2018-11-01 17:54 ` Paul Burton
2018-11-01 1:18 ` Guenter Roeck
2018-11-01 6:30 ` Trond Myklebust
2018-11-01 15:28 ` Guenter Roeck
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