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From: Anthony Williams <anthony.ajw@gmail.com>
Newsgroups: comp.programming.threads
Subject: Re: forcing the compiler to reload from memory with c++0x
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Date: Fri, 04 Feb 2011 09:21:52 +0000
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Joshua Maurice <joshuamaurice@gmail.com> writes:

> Well, specifically, the compiler may optimize this:
>   atomic<int> a_int;
>   //...
>   int tmp = a_int.get(relaxed);
>   do_something(tmp);
>   for ( ; ! a_int.get(relaxed) ; )
>   {
>   }
>
> to this:
>   atomic<int> a_int;
>   //...
>   int tmp = a_int.get(relaxed);
>   do_something(tmp);
>   for ( ; ! tmp ; tmp = a_int.get(relaxed))
>   {
>   }

Correct (assuming that do_something doesn't introduce any memory
ordering constraints). Then, if it can prove that !tmp is false
(i.e. tmp is non-zero) then the loop will never execute, so it can omit
the whole loop.

> But it cannot optimize it to this:
>   atomic<int> a_int;
>   //...
>   int tmp = a_int.get(relaxed);
>   do_something(tmp);
>   for ( ; ! tmp ; )
>   {
>   }

Correct.

> It's perfectly fine to remove a finite number of loads (assuming it
> doesn't violate other visibility requirements) as the first rewrite
> does, but it cannot remove /all/ loads from a loop like the second
> rewrite, because that would violate the "become visible in a
> 'reasonable' [read: finite] amount of time" guarantee.

Correct.

Anthony
-- 
Author of C++ Concurrency in Action     http://www.stdthread.co.uk/book/
just::thread C++0x thread library             http://www.stdthread.co.uk
Just Software Solutions Ltd       http://www.justsoftwaresolutions.co.uk
15 Carrallack Mews, St Just, Cornwall, TR19 7UL, UK. Company No. 5478976