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Re: Clarification on the applicability of compile-time optimization to a struct variable which encompasses a mixture of fields declared as volatile and non-volatile individually

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On Sat, Jul 07, 2012 at 11:29:02AM +1200, Ian Collins wrote:
> On 07/ 7/12 06:33 AM, Uncle Steve wrote:
> >On Fri, Jul 06, 2012 at 06:11:28PM -0400, Eric Sosman wrote:
> >>On 7/6/2012 2:01 PM, Uncle Steve wrote:
> >>>On Sat, Jul 07, 2012 at 09:20:57AM +1200, Ian Collins wrote:
> >>>>On 07/ 7/12 05:08 AM, Uncle Steve wrote:
> >>>>>[...]
> >>>>>This is a concern in my mind right now as I'm writing some posix
> >>>>>threads code, and so obviously there are a large number of variables
> >>>>>which ought to be declared volatile.
> >>>>o
> >>>>What leads you to that conclusion?  That line of though usually implies
> >>>>you are misunderstanding the relationship between volatile and threads.
> >>>>
> >>>>What are you trying to achieve with volatile?
> >>>
> >>>Well, I have some data structures that will be accessed from arbitrary
> >>>threads, so I want to make sure there's no funny business happening
> >>>behind the scenes where the compiler decides that certain accesses are
> >>>optional.
> >>
> >>     Although the latest "C11" Standard adds some multi-threading
> >>support, I'll ignore that because (1) C11 implementations are not
> >>exactly thick upon the ground as yet, (2) you mentioned you're
> >>using Pthreads anyhow, and (3) I haven't studied the C11 threading
> >>stuff closely enough to be a competent commentator. ;-)
> >>
> >>     So: As long as you're using an established non-C-specific
> >>threading framework, I suggest you visit comp.programming.threads,
> >>a newsgroup devoted to multi-threading issues.  There you will learn
> >>that `volatile' is neither necessary *nor* sufficient in connection
> >>with variables shared between threads.  (You needn't take my word on
> >>this: Just browse the comp.programming.threads archives and you'll
> >>see the local experts refuting this misunderstanding every couple
> >>weeks. It's comp.programming.threads' equivalent of `void main'.)
> >>
> >>     Or, as my first threading teacher put it, "Don't Do That."
> >
> >Is there are C standard threading framework?
> 
> Yes, read C11.  If you are familiar with pthreads, you'll see where most 
> of the new primitives originate.

Well, I don't follow gcc development, so I don't know when those
features will be available on my system.

> >Threaded code is a de-facto reality in today's multi-core environment.
> >Save for the embedded sector, most systems will run threads in
> >parallel, and that is that.
> 
> Only if you incorporate threads in your code either explicitly or 
> implicitly with something like OpenMP.

Well, sure, but I'd like to think that multithreaded applications will
be popular.  It helps to improve the interactivity of applications
with a GUI frontend for one thing, never mind what you can do with
message-passing interfaces or asynchronus network I/O threads.
 
> >C has to recognize this watershed moment
> >in the refinement of the language.
> 
> It has.

O.K., so do nested structure elements inherit the parent structure's
volatile attribute?
 
> >While the specifics of an implementation within a given operating
> >system are not C issues, certain common optimizations can be a problem
> >with certain algorithms, and so there ought to be a C solution to the
> >problem.  What is it about the volatile attribute that makes it
> >insufficient in your mind to the problem presented by threaded code?
> 
> Given you are the one going against conventional wisdom, what in your 
> mind makes volatile a solution to the problem presented by threaded code?
>
> In addition, what is "the problem presented by threaded code"?

Well, code optimization from global CSE might eliminate memory
accesses to variable that could be modified by other threads in ways
that affect the algorithm.  Depends of the code.


Regards,

Uncle Steve

-- 
The moon has never been closer.

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Re: Clarification on the applicability of compile-time optimization to a struct variable which encompasses a mixture of fields declared as volatile and non-volatile individually Uncle Steve <stevet810@gmail.com> - 2012-07-06 14:33 -0400
  Re: Clarification on the applicability of compile-time optimization to a struct variable which encompasses a mixture of fields declared as volatile and non-volatile individually Ian Collins <ian-news@hotmail.com> - 2012-07-07 11:29 +1200
    Re: Clarification on the applicability of compile-time optimization to a struct variable which encompasses a mixture of fields declared as volatile and non-volatile individually Uncle Steve <stevet810@gmail.com> - 2012-07-06 16:27 -0400
      Re: Clarification on the applicability of compile-time optimization to a struct variable which encompasses a mixture of fields declared as volatile and non-volatile individually Ian Collins <ian-news@hotmail.com> - 2012-07-07 12:37 +1200
        Re: Clarification on the applicability of compile-time optimization to a struct variable which encompasses a mixture of fields declared as volatile and non-volatile individually Uncle Steve <stevet810@gmail.com> - 2012-07-06 17:29 -0400
          Re: Clarification on the applicability of compile-time optimization to a struct variable which encompasses a mixture of fields declared as volatile and non-volatile individually Ian Collins <ian-news@hotmail.com> - 2012-07-07 15:50 +1200
            Re: Clarification on the applicability of compile-time optimization to a struct variable which encompasses a mixture of fields declared as volatile and non-volatile individually Uncle Steve <stevet810@gmail.com> - 2012-07-06 19:55 -0400
          Re: Clarification on the applicability of compile-time optimization to a struct variable which encompasses a mixture of fields declared as volatile and non-volatile individually Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2012-07-07 17:04 +0300
  Re: Clarification on the applicability of compile-time optimization to a struct variable which encompasses a mixture of fields declared as volatile and non-volatile individually Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-07-07 08:16 -0400
    Re: Clarification on the applicability of compile-time optimization to a struct variable which encompasses a mixture of fields declared as volatile and non-volatile individually Uncle Steve <stevet810@gmail.com> - 2012-07-07 05:09 -0400

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