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Re: compound assignment to atomic type

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"Chris M. Thomasson" <invalid@invalid.invalid> writes:

> 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 p3 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 p5 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 p3 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.

That is an interesting statement.  What do you recommend in place
of memory_order_seq_cst, perhaps under various scenarios?

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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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