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Diffstat (limited to 'doc/html/atomic/limitations.html')
| -rw-r--r-- | doc/html/atomic/limitations.html | 22 |
1 files changed, 15 insertions, 7 deletions
diff --git a/doc/html/atomic/limitations.html b/doc/html/atomic/limitations.html index 6a8858e539..f127667b65 100644 --- a/doc/html/atomic/limitations.html +++ b/doc/html/atomic/limitations.html @@ -29,8 +29,8 @@ </h2></div></div></div> <p> While <span class="bold"><strong>Boost.Atomic</strong></span> strives to implement the - atomic operations from C++11 as faithfully as possible, there are a few limitations - that cannot be lifted without compiler support: + atomic operations from C++11 and later as faithfully as possible, there are + a few limitations that cannot be lifted without compiler support: </p> <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> <li class="listitem"> @@ -45,10 +45,18 @@ Crucially, <code class="computeroutput"><span class="identifier">memory_order_consume</span></code> only affects computationally-dependent operations, but in general there is nothing preventing a compiler from transforming a computation dependency - into a control dependency. A C++11 compiler would be forbidden from such - a transformation. <span class="bold"><strong>Advise</strong></span>: Use <code class="computeroutput"><span class="identifier">memory_order_consume</span></code> only in conjunction - with pointer values, as the compiler cannot speculate and transform these - into control dependencies. + into a control dependency. A fully compliant C++11 compiler would be forbidden + from such a transformation, but in practice most if not all compilers have + chosen to promote <code class="computeroutput"><span class="identifier">memory_order_consume</span></code> + to <code class="computeroutput"><span class="identifier">memory_order_acquire</span></code> + instead (see <a href="https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59448" target="_top">this</a> + gcc bug for example). In the current implementation <span class="bold"><strong>Boost.Atomic</strong></span> + follows that trend, but this may change in the future. <span class="bold"><strong>Advice</strong></span>: + In general, avoid <code class="computeroutput"><span class="identifier">memory_order_consume</span></code> + and use <code class="computeroutput"><span class="identifier">memory_order_acquire</span></code> + instead. Use <code class="computeroutput"><span class="identifier">memory_order_consume</span></code> + only in conjunction with pointer values, and only if you can ensure that + the compiler cannot speculate and transform these into control dependencies. </li> <li class="listitem"> <span class="bold"><strong>Fence operations enforce "too strong" compiler @@ -66,7 +74,7 @@ </div> <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> <td align="left"></td> -<td align="right"><div class="copyright-footer">Copyright © 2011 Helge Bahmann<br>Copyright © 2012 Tim Blechmann<br>Copyright © 2013 Andrey Semashev<p> +<td align="right"><div class="copyright-footer">Copyright © 2011 Helge Bahmann<br>Copyright © 2012 Tim Blechmann<br>Copyright © 2013, 2017 Andrey Semashev<p> Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>) </p> |