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Re: compound assignment to atomic type

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spacewar@gmail.com writes:

> 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?

This came up in a DR about the use of various terms relating to atomic
types and atomic operations:

http://www.open-std.org/jtc1/sc22/wg14/www/docs/summary.htm#dr_486

> 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.

The DR is under review, but I get the feeling the committee agrees that
the wording is confusing.  I think you can rely on x <op>= y having the
RMW sematics described in 6.5.16.2 (along with the extensive footnote
113).

-- 
Ben.

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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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