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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
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Re: forcing the compiler to reload from memory with c++0x Andy Venikov <swojchelowek@gmail.com> - 2011-02-03 23:19 -0500
Re: forcing the compiler to reload from memory with c++0x Andy Venikov <swojchelowek@gmail.com> - 2011-02-06 23:17 -0500
Re: forcing the compiler to reload from memory with c++0x Anthony Williams <anthony.ajw@gmail.com> - 2011-02-04 09:21 +0000
Re: forcing the compiler to reload from memory with c++0x Anthony Williams <anthony.ajw@gmail.com> - 2011-02-04 08:42 +0000
Re: forcing the compiler to reload from memory with c++0x Joshua Maurice <joshuamaurice@gmail.com> - 2011-02-03 22:45 -0800
Re: forcing the compiler to reload from memory with c++0x Joshua Maurice <joshuamaurice@gmail.com> - 2011-02-04 00:28 -0800
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