Messages in this thread | | | Date | Fri, 2 Nov 2018 06:37:28 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [RFC PATCH] lib: Introduce generic __cmpxchg_u64() and use it where needed |
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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
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