From: "Paul E. McKenney" <paulmck@linux.ibm.com>
To: David Laight <David.Laight@ACULAB.COM>
Cc: Peter Zijlstra <peterz@infradead.org>,
Trond Myklebust <trondmy@hammerspace.com>,
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Subject: Re: [RFC PATCH] lib: Introduce generic __cmpxchg_u64() and use it where needed
Date: Fri, 2 Nov 2018 06:37:28 -0700 [thread overview]
Message-ID: <20181102133728.GR4170@linux.ibm.com> (raw)
In-Reply-To: <7d1ecd21c4c249138dfdd42b9aaa1cea@AcuMS.aculab.com>
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.
There are no C++ compilers for those DSPs, correct? (Some of the
C++ standards commmittee members believe that they have fully checked,
but they might well have missed something.)
> 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.
Heh! COBOL and FORTRAN also helped fund my first pass through university.
> > > 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.
The usual response is something like this:
for (i = 1; i < n; i++)
where the compiler has no idea what range of values "n" might take on.
Can't say that I am convinced by that example, but at least we do have
-fno-strict-overflow.
Thanx, Paul
next prev parent reply other threads:[~2018-11-02 13:37 UTC|newest]
Thread overview: 34+ 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
2018-11-02 12:23 ` Peter Zijlstra
2018-11-02 13:38 ` Paul E. McKenney
2018-11-02 13:37 ` Paul E. McKenney [this message]
2018-11-02 16:19 ` Andrey Ryabinin
2018-11-05 10:38 ` Peter Zijlstra
2018-11-05 14:24 ` Peter Zijlstra
2018-11-01 17:51 ` [PATCH] SUNRPC: Use atomic(64)_t for seq_send(64) Paul Burton
2018-11-01 17:57 ` Trond Myklebust
2018-11-01 17:54 ` [RFC PATCH] lib: Introduce generic __cmpxchg_u64() and use it where needed 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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