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<div class="title">Structures Having Eigen Members<div class="ingroups"><a class="el" href="group__DenseMatrixManipulation__Alignement.html">Alignment issues</a></div></div> </div>
</div><!--header-->
<div class="contents">
<h1><a class="anchor" id="summary"></a>
Executive Summary</h1>
<p>If you define a structure having members of <a class="el" href="group__TopicFixedSizeVectorizable.html">fixed-size vectorizable Eigen types</a>, you must overload its "operator new" so that it generates 16-bytes-aligned pointers. Fortunately, <a class="el" href="namespaceEigen.html" title="Namespace containing all symbols from the Eigen library. ">Eigen</a> provides you with a macro EIGEN_MAKE_ALIGNED_OPERATOR_NEW that does that for you.</p>
<h1><a class="anchor" id="what"></a>
What kind of code needs to be changed?</h1>
<p>The kind of code that needs to be changed is this:</p>
<div class="fragment"><div class="line"><span class="keyword">class </span>Foo</div>
<div class="line">{</div>
<div class="line"> ...</div>
<div class="line"> <a class="code" href="classEigen_1_1Matrix.html">Eigen::Vector2d</a> v;</div>
<div class="line"> ...</div>
<div class="line">};</div>
<div class="line"></div>
<div class="line">...</div>
<div class="line"></div>
<div class="line">Foo *foo = <span class="keyword">new</span> Foo;</div>
</div><!-- fragment --><p>In other words: you have a class that has as a member a <a class="el" href="group__TopicFixedSizeVectorizable.html">fixed-size vectorizable Eigen object</a>, and then you dynamically create an object of that class.</p>
<h1><a class="anchor" id="how"></a>
How should such code be modified?</h1>
<p>Very easy, you just need to put a EIGEN_MAKE_ALIGNED_OPERATOR_NEW macro in a public part of your class, like this:</p>
<div class="fragment"><div class="line"><span class="keyword">class </span>Foo</div>
<div class="line">{</div>
<div class="line"> ...</div>
<div class="line"> <a class="code" href="classEigen_1_1Matrix.html">Eigen::Vector2d</a> v;</div>
<div class="line"> ...</div>
<div class="line">public:</div>
<div class="line"> EIGEN_MAKE_ALIGNED_OPERATOR_NEW</div>
<div class="line">};</div>
<div class="line"></div>
<div class="line">...</div>
<div class="line"></div>
<div class="line">Foo *foo = <span class="keyword">new</span> Foo;</div>
</div><!-- fragment --><p>This macro makes "new Foo" always return an aligned pointer.</p>
<p>If this approach is too intrusive, see also the <a class="el" href="group__TopicStructHavingEigenMembers.html#othersolutions">Other solutions</a>.</p>
<h1><a class="anchor" id="why"></a>
Why is this needed?</h1>
<p>OK let's say that your code looks like this:</p>
<div class="fragment"><div class="line"><span class="keyword">class </span>Foo</div>
<div class="line">{</div>
<div class="line"> ...</div>
<div class="line"> <a class="code" href="classEigen_1_1Matrix.html">Eigen::Vector2d</a> v;</div>
<div class="line"> ...</div>
<div class="line">};</div>
<div class="line"></div>
<div class="line">...</div>
<div class="line"></div>
<div class="line">Foo *foo = <span class="keyword">new</span> Foo;</div>
</div><!-- fragment --><p>A <a class="el" href="group__matrixtypedefs.html#ga685d563d586f4820b4a2df9a07d98c23">Eigen::Vector2d</a> consists of 2 doubles, which is 128 bits. Which is exactly the size of a SSE packet, which makes it possible to use SSE for all sorts of operations on this vector. But SSE instructions (at least the ones that <a class="el" href="namespaceEigen.html" title="Namespace containing all symbols from the Eigen library. ">Eigen</a> uses, which are the fast ones) require 128-bit alignment. Otherwise you get a segmentation fault.</p>
<p>For this reason, <a class="el" href="namespaceEigen.html" title="Namespace containing all symbols from the Eigen library. ">Eigen</a> takes care by itself to require 128-bit alignment for <a class="el" href="group__matrixtypedefs.html#ga685d563d586f4820b4a2df9a07d98c23">Eigen::Vector2d</a>, by doing two things: </p>
<ul>
<li><a class="el" href="namespaceEigen.html" title="Namespace containing all symbols from the Eigen library. ">Eigen</a> requires 128-bit alignment for the <a class="el" href="group__matrixtypedefs.html#ga685d563d586f4820b4a2df9a07d98c23">Eigen::Vector2d</a>'s array (of 2 doubles). With GCC, this is done with a <b>attribute</b> ((aligned(16))). </li>
<li><a class="el" href="namespaceEigen.html" title="Namespace containing all symbols from the Eigen library. ">Eigen</a> overloads the "operator new" of <a class="el" href="group__matrixtypedefs.html#ga685d563d586f4820b4a2df9a07d98c23">Eigen::Vector2d</a> so it will always return 128-bit aligned pointers.</li>
</ul>
<p>Thus, normally, you don't have to worry about anything, <a class="el" href="namespaceEigen.html" title="Namespace containing all symbols from the Eigen library. ">Eigen</a> handles alignment for you...</p>
<p>... except in one case. When you have a class Foo like above, and you dynamically allocate a new Foo as above, then, since Foo doesn't have aligned "operator new", the returned pointer foo is not necessarily 128-bit aligned.</p>
<p>The alignment attribute of the member v is then relative to the start of the class, foo. If the foo pointer wasn't aligned, then foo->v won't be aligned either!</p>
<p>The solution is to let class Foo have an aligned "operator new", as we showed in the previous section.</p>
<h1><a class="anchor" id="movetotop"></a>
Should I then put all the members of Eigen types at the beginning of my class?</h1>
<p>That's not required. Since <a class="el" href="namespaceEigen.html" title="Namespace containing all symbols from the Eigen library. ">Eigen</a> takes care of declaring 128-bit alignment, all members that need it are automatically 128-bit aligned relatively to the class. So code like this works fine:</p>
<div class="fragment"><div class="line"><span class="keyword">class </span>Foo</div>
<div class="line">{</div>
<div class="line"> <span class="keywordtype">double</span> x;</div>
<div class="line"> <a class="code" href="classEigen_1_1Matrix.html">Eigen::Vector2d</a> v;</div>
<div class="line"><span class="keyword">public</span>:</div>
<div class="line"> EIGEN_MAKE_ALIGNED_OPERATOR_NEW</div>
<div class="line">};</div>
</div><!-- fragment --><h1><a class="anchor" id="dynamicsize"></a>
What about dynamic-size matrices and vectors?</h1>
<p>Dynamic-size matrices and vectors, such as <a class="el" href="group__matrixtypedefs.html#ga3da45e59796fbacf67fa568297927bd1">Eigen::VectorXd</a>, allocate dynamically their own array of coefficients, so they take care of requiring absolute alignment automatically. So they don't cause this issue. The issue discussed here is only with <a class="el" href="group__TopicFixedSizeVectorizable.html">fixed-size vectorizable matrices and vectors</a>.</p>
<h1><a class="anchor" id="bugineigen"></a>
So is this a bug in Eigen?</h1>
<p>No, it's not our bug. It's more like an inherent problem of the C++98 language specification, and seems to be taken care of in the upcoming language revision: <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2341.pdf">see this document</a>.</p>
<h1><a class="anchor" id="conditional"></a>
What if I want to do this conditionnally (depending on template parameters) ?</h1>
<p>For this situation, we offer the macro EIGEN_MAKE_ALIGNED_OPERATOR_NEW_IF(NeedsToAlign). It will generate aligned operators like EIGEN_MAKE_ALIGNED_OPERATOR_NEW if NeedsToAlign is true. It will generate operators with the default alignment if NeedsToAlign is false.</p>
<p>Example:</p>
<div class="fragment"><div class="line"><span class="keyword">template</span><<span class="keywordtype">int</span> n> <span class="keyword">class </span>Foo</div>
<div class="line">{</div>
<div class="line"> <span class="keyword">typedef</span> <a class="code" href="classEigen_1_1Matrix.html">Eigen::Matrix<float,n,1></a> Vector;</div>
<div class="line"> <span class="keyword">enum</span> { NeedsToAlign = (<span class="keyword">sizeof</span>(Vector)%16)==0 };</div>
<div class="line"> ...</div>
<div class="line"> Vector v;</div>
<div class="line"> ...</div>
<div class="line">public:</div>
<div class="line"> EIGEN_MAKE_ALIGNED_OPERATOR_NEW_IF(NeedsToAlign)</div>
<div class="line">};</div>
<div class="line"></div>
<div class="line">...</div>
<div class="line"></div>
<div class="line">Foo<4> *foo4 = <span class="keyword">new</span> Foo<4>; <span class="comment">// foo4 is guaranteed to be 128bit-aligned</span></div>
<div class="line">Foo<3> *foo3 = <span class="keyword">new</span> Foo<3>; <span class="comment">// foo3 has only the system default alignment guarantee</span></div>
</div><!-- fragment --><h1><a class="anchor" id="othersolutions"></a>
Other solutions</h1>
<p>In case putting the EIGEN_MAKE_ALIGNED_OPERATOR_NEW macro everywhere is too intrusive, there exists at least two other solutions.</p>
<h2><a class="anchor" id="othersolutions1"></a>
Disabling alignment</h2>
<p>The first is to disable alignment requirement for the fixed size members: </p>
<div class="fragment"><div class="line"><span class="keyword">class </span>Foo</div>
<div class="line">{</div>
<div class="line"> ...</div>
<div class="line"> <a class="code" href="classEigen_1_1Matrix.html">Eigen::Matrix<double,2,1,Eigen::DontAlign></a> v;</div>
<div class="line"> ...</div>
<div class="line">};</div>
</div><!-- fragment --><p> This has for effect to disable vectorization when using <code>v</code>. If a function of Foo uses it several times, then it still possible to re-enable vectorization by copying it into an aligned temporary vector: </p>
<div class="fragment"><div class="line"><span class="keywordtype">void</span> Foo::bar()</div>
<div class="line">{</div>
<div class="line"> <a class="code" href="classEigen_1_1Matrix.html">Eigen::Vector2d</a> av(v);</div>
<div class="line"> <span class="comment">// use av instead of v</span></div>
<div class="line"> ...</div>
<div class="line"> <span class="comment">// if av changed, then do:</span></div>
<div class="line"> v = av;</div>
<div class="line">}</div>
</div><!-- fragment --><h2><a class="anchor" id="othersolutions2"></a>
Private structure</h2>
<p>The second consist in storing the fixed-size objects into a private struct which will be dynamically allocated at the construction time of the main object:</p>
<div class="fragment"><div class="line"><span class="keyword">struct </span>Foo_d</div>
<div class="line">{</div>
<div class="line"> EIGEN_MAKE_ALIGNED_OPERATOR_NEW</div>
<div class="line"> <a class="code" href="group__matrixtypedefs.html#ga685d563d586f4820b4a2df9a07d98c23">Vector2d</a> v;</div>
<div class="line"> ...</div>
<div class="line">};</div>
<div class="line"></div>
<div class="line"></div>
<div class="line"><span class="keyword">struct </span>Foo {</div>
<div class="line"> Foo() { init_d(); }</div>
<div class="line"> ~Foo() { <span class="keyword">delete</span> d; }</div>
<div class="line"> <span class="keywordtype">void</span> bar()</div>
<div class="line"> {</div>
<div class="line"> <span class="comment">// use d->v instead of v</span></div>
<div class="line"> ...</div>
<div class="line"> }</div>
<div class="line"><span class="keyword">private</span>:</div>
<div class="line"> <span class="keywordtype">void</span> init_d() { d = <span class="keyword">new</span> Foo_d; }</div>
<div class="line"> Foo_d* d;</div>
<div class="line">};</div>
</div><!-- fragment --><p>The clear advantage here is that the class Foo remains unchanged regarding alignment issues. The drawback is that a heap allocation will be required whatsoever. </p>
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