<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <meta http-equiv="Content-Type" content="text/xhtml;charset=UTF-8"/> <meta http-equiv="X-UA-Compatible" content="IE=9"/> <meta name="generator" content="Doxygen 1.8.5"/> <title>Eigen: Compiler making a wrong assumption on stack alignment</title> <link href="tabs.css" rel="stylesheet" type="text/css"/> <script type="text/javascript" src="jquery.js"></script> <script type="text/javascript" src="dynsections.js"></script> <link href="navtree.css" rel="stylesheet" type="text/css"/> <script type="text/javascript" src="resize.js"></script> <script type="text/javascript" src="navtree.js"></script> <script type="text/javascript"> $(document).ready(initResizable); $(window).load(resizeHeight); </script> <link href="search/search.css" rel="stylesheet" type="text/css"/> <script type="text/javascript" src="search/search.js"></script> <script type="text/javascript"> $(document).ready(function() { searchBox.OnSelectItem(0); }); </script> <link href="doxygen.css" rel="stylesheet" type="text/css" /> <link href="eigendoxy.css" rel="stylesheet" type="text/css"> <!-- --> <script type="text/javascript" src="eigen_navtree_hacks.js"></script> <!-- <script type="text/javascript"> --> <!-- </script> --> </head> <body> <div id="top"><!-- do not remove this div, it is closed by doxygen! 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If you hit this issue, please upgrade to GCC 4.5 and report to us, so we can update this page.</h4> <p>This is an issue that, so far, we met only with GCC on Windows: for instance, MinGW and TDM-GCC.</p> <p>By default, in a function like this,</p> <div class="fragment"><div class="line"><span class="keywordtype">void</span> foo()</div> <div class="line">{</div> <div class="line"> <a class="code" href="classEigen_1_1Quaternion.html">Eigen::Quaternionf</a> q;</div> <div class="line"> <span class="comment">//...</span></div> <div class="line">}</div> </div><!-- fragment --><p>GCC assumes that the stack is already 16-byte-aligned so that the object <em>q</em> will be created at a 16-byte-aligned location. For this reason, it doesn't take any special care to explicitly align the object <em>q</em>, as <a class="el" href="namespaceEigen.html" title="Namespace containing all symbols from the Eigen library. ">Eigen</a> requires.</p> <p>The problem is that, in some particular cases, this assumption can be wrong on Windows, where the stack is only guaranteed to have 4-byte alignment. Indeed, even though GCC takes care of aligning the stack in the main function and does its best to keep it aligned, when a function is called from another thread or from a binary compiled with another compiler, the stack alignment can be corrupted. This results in the object 'q' being created at an unaligned location, making your program crash with the <a class="el" href="group__TopicUnalignedArrayAssert.html">assertion on unaligned arrays</a>. So far we found the three following solutions.</p> <h1><a class="anchor" id="sec_sol1"></a> Local solution</h1> <p>A local solution is to mark such a function with this attribute: </p> <div class="fragment"><div class="line">__attribute__((force_align_arg_pointer)) void foo()</div> <div class="line">{</div> <div class="line"> <a class="code" href="classEigen_1_1Quaternion.html">Eigen::Quaternionf</a> q;</div> <div class="line"> <span class="comment">//...</span></div> <div class="line">}</div> </div><!-- fragment --><p> Read <a href="http://gcc.gnu.org/onlinedocs/gcc-4.4.0/gcc/Function-Attributes.html#Function-Attributes">this GCC documentation</a> to understand what this does. Of course this should only be done on GCC on Windows, so for portability you'll have to encapsulate this in a macro which you leave empty on other platforms. The advantage of this solution is that you can finely select which function might have a corrupted stack alignment. Of course on the downside this has to be done for every such function, so you may prefer one of the following two global solutions.</p> <h1><a class="anchor" id="sec_sol2"></a> Global solutions</h1> <p>A global solution is to edit your project so that when compiling with GCC on Windows, you pass this option to GCC: </p> <div class="fragment"><div class="line">-mincoming-stack-boundary=2</div> </div><!-- fragment --><p> Explanation: this tells GCC that the stack is only required to be aligned to 2^2=4 bytes, so that GCC now knows that it really must take extra care to honor the 16 byte alignment of <a class="el" href="group__TopicFixedSizeVectorizable.html">fixed-size vectorizable Eigen types</a> when needed.</p> <p>Another global solution is to pass this option to gcc: </p> <div class="fragment"><div class="line">-mstackrealign</div> </div><!-- fragment --><p> which has the same effect than adding the <code>force_align_arg_pointer</code> attribute to all functions.</p> <p>These global solutions are easy to use, but note that they may slowdown your program because they lead to extra prologue/epilogue instructions for every function. </p> </div><!-- contents --> </div><!-- doc-content --> <!-- start footer part --> <div id="nav-path" class="navpath"><!-- id is needed for treeview function! --> <ul> <li class="footer">Generated on Mon Oct 28 2013 11:04:27 for Eigen by <a href="http://www.doxygen.org/index.html"> <img class="footer" src="doxygen.png" alt="doxygen"/></a> 1.8.5 </li> </ul> </div> <!-- Piwik --> <!-- <script type="text/javascript"> var pkBaseURL = (("https:" == document.location.protocol) ? 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