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Re: "volatile" on variables and current processors

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red floyd <dont.bother@its.invalid> writes:
> On 8/19/2019 1:56 PM, Vir Campestris wrote:
>> In the old days volatile on a variable meant putting it in memory, not 
>> holding it in a register. That was easy.
>> 
>
> No.  That's not what volatile means.  ISO/IEC 14882:2003 7.1.5.1/8 says
> that volatile is an instruction to the optimizer.
>
> "volatile is a hint to the implementation to avoid aggressive
> optimization involving the object because the value of the object
> might be changed by means undetectable by an implementation."
>
> DISCLAIMER:  C++03 is the latest copy of the standard that I have.
> Can someone check a newer version and see if that's still what is
> indicated?

ISO/IEC 14882:2011(E) 7.1.6.1 [dcl.type.cv] p6-7:

    6 If an attempt is made to refer to an object defined with a
    volatile-qualified type through the use of a glvalue with a
    non-volatile-qualified type, the program behavior is undefined.

    7 [ Note: volatile is a hint to the implementation to avoid
    aggressive optimization involving the object because the value of
    the object might be changed by means undetectable by an
    implementation. See 1.9 for detailed semantics. In general, the
    semantics of volatile are intended to be the same in C++ as they
    are in C. — end note ]

1.9 [intro.execution] paragraph 8 says:

    8 The least requirements on a conforming implementation are:
      — Access to volatile objects are evaluated strictly according to
        the rules of the abstract machine.
    [...]

I don't have a copy of the C++17 standard, but N4700 (a draft of the
C++17 standard) 10.1.7.1 [dcl.type.cv] p5-6 says:

    5 The semantics of an access through a volatile glvalue are
    implementation-defined. If an attempt is made to access an object
    defined with a volatile-qualified type through the use of a
    non-volatile glvalue, the behavior is undefined.

    6 [ Note: volatile is a hint to the implementation to avoid
    aggressive optimization involving the object because the value of
    the object might be changed by means undetectable by an
    implementation. Furthermore, for some implementations, volatile
    might indicate that special hardware instructions are required to
    access the object. See 4.6 for detailed semantics. In general, the
    semantics of volatile are intended to be the same in C++ as they
    are in C. — end note ]

4.6 [intro.execution] p7 says:

    7 The least requirements on a conforming implementation are:
      (7.1) — Accesses through volatile glvalues are evaluated strictly
              according to the rules of the abstract machine.

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Will write code for food.
void Void(void) { Void(); } /* The recursive call of the void */

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"volatile" on variables and current processors Vir Campestris <vir.campestris@invalid.invalid> - 2019-08-19 21:56 +0100
  Re: "volatile" on variables and current processors red floyd <dont.bother@its.invalid> - 2019-08-19 15:20 -0700
    Re: "volatile" on variables and current processors red floyd <dont.bother@its.invalid> - 2019-08-19 15:21 -0700
      Re: "volatile" on variables and current processors "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-08-19 16:37 -0700
        Re: "volatile" on variables and current processors Vir Campestris <vir.campestris@invalid.invalid> - 2019-08-20 21:58 +0100
          Re: "volatile" on variables and current processors scott@slp53.sl.home (Scott Lurndal) - 2019-08-20 22:18 +0000
            Re: "volatile" on variables and current processors Vir Campestris <vir.campestris@invalid.invalid> - 2019-08-22 21:39 +0100
          Re: "volatile" on variables and current processors Paavo Helde <myfirstname@osa.pri.ee> - 2019-08-21 09:02 +0300
            Re: "volatile" on variables and current processors "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-08-20 23:10 -0700
          Re: "volatile" on variables and current processors David Brown <david.brown@hesbynett.no> - 2019-08-21 08:34 +0200
            Re: "volatile" on variables and current processors Chris Vine <chris@cvine--nospam--.freeserve.co.uk> - 2019-08-21 20:39 +0100
              Re: "volatile" on variables and current processors Chris Vine <chris@cvine--nospam--.freeserve.co.uk> - 2019-08-21 21:02 +0100
              Re: "volatile" on variables and current processors David Brown <david.brown@hesbynett.no> - 2019-08-22 13:30 +0200
        Re: "volatile" on variables and current processors Bonita Montero <Bonita.Montero@gmail.com> - 2019-08-21 08:43 +0200
      Re: "volatile" on variables and current processors "Alf P. Steinbach" <alf.p.steinbach+usenet@gmail.com> - 2019-08-20 02:22 +0200
        Re: "volatile" on variables and current processors David Brown <david.brown@hesbynett.no> - 2019-08-20 10:05 +0200
    Re: "volatile" on variables and current processors Keith Thompson <kst-u@mib.org> - 2019-08-19 17:08 -0700
  Re: "volatile" on variables and current processors Bonita Montero <Bonita.Montero@gmail.com> - 2019-08-20 06:08 +0200
  Re: "volatile" on variables and current processors David Brown <david.brown@hesbynett.no> - 2019-08-20 10:14 +0200
  Re: "volatile" on variables and current processors Juha Nieminen <nospam@thanks.invalid> - 2019-08-20 13:03 +0000
    Re: "volatile" on variables and current processors David Brown <david.brown@hesbynett.no> - 2019-08-20 16:18 +0200

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