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Re: compound assignment to atomic type

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On 11/10/2016 7:34 PM, spacewar@gmail.com wrote:
> I'm confused about C11 compound assignments with E1 (left hand side) being an atomic type.
>
> §6.5.16.2 ¶3 states:
>     If E1 has an atomic type, compound assignment is a
>     read-modify-write operation with memory_order_seq_cst memory order
>     semantics.
>
> Footnote 113 says
>     Where a pointer to an atomic object can be formed and E1 and E2 have
>     integer type, this is equivalent to the following code sequence [...]"
> with the code sequence using atomic_compare_exchange_strong().
>
> On the other hand, §7.17.7.5 ¶5 states:
>     NOTE The operation of the atomic_fetch and modify generic functions are
>     nearly equivalent to the operation of the corresponding op= compound
>     assignment operators. The only differences are that the compound
>     assignment operators are not guaranteed to operate atomically, [...]
>
> Doesn't that directly contradict §6.5.16.2 ¶3 and footnote 113?
>
> GCC 6.2.1 generates atomic RMW code for compound assignments with atomic LHS, but the NOTE in §7.17.7.5 makes me worry about that being non-portable.

FWIW, just to be safe, its much better to use the actual atomic 
functions to perform operations on atomic types. They let you 
specifically define exactly what you want, and what type of memory order 
semantics that particular operation requires for your specific program's 
needs.

If you use normal C to, say, increment something like foo = foo + 1 
where foo is an atomic type, the compiler will use strong CAS, and 
memory_order_seq_cst for a mem order. This is just, usually, way to 
strong for most work dealing with atomics.

Just stick to the atomic functions, and remove any possible ambiguities.

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compound assignment to atomic type spacewar@gmail.com - 2016-11-10 19:34 -0800
  Re: compound assignment to atomic type Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-11-11 16:04 +0000
    Re: compound assignment to atomic type supercat@casperkitty.com - 2016-11-11 09:01 -0800
      Re: compound assignment to atomic type Tim Rentsch <txr@alumni.caltech.edu> - 2016-11-15 10:54 -0800
    Re: compound assignment to atomic type spacewar@gmail.com - 2016-11-12 00:48 -0800
      Re: compound assignment to atomic type Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-11-12 11:06 +0000
        Re: compound assignment to atomic type supercat <flatfinger@casperkitty.com> - 2016-11-12 11:03 -0800
        Re: compound assignment to atomic type Tim Rentsch <txr@alumni.caltech.edu> - 2016-11-13 10:57 -0800
      Re: compound assignment to atomic type Tim Rentsch <txr@alumni.caltech.edu> - 2016-11-13 10:31 -0800
  Re: compound assignment to atomic type "Chris M. Thomasson" <invalid@invalid.invalid> - 2016-11-12 11:24 -0800
    Re: compound assignment to atomic type Tim Rentsch <txr@alumni.caltech.edu> - 2016-11-13 10:59 -0800
      Re: compound assignment to atomic type "Chris M. Thomasson" <invalid@invalid.invalid> - 2016-11-13 17:34 -0800
        Re: compound assignment to atomic type "Chris M. Thomasson" <invalid@invalid.invalid> - 2016-11-13 17:48 -0800
        Re: compound assignment to atomic type Tim Rentsch <txr@alumni.caltech.edu> - 2016-11-15 10:42 -0800

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