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POConstructing Eigen expression templates with Boost.Proto
 

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  1. POConstructing Eigen expression templates with Boost.Proto
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    <p>I'd like to use Boost.Proto to transform an embedded domain-specific language into a series of matrix operations implemented with the Eigen library. Since efficiency is important, I want proto to generate Eigen expression templates and avoid premature evaluation.</p> <p>I've implemented a simple grammar that can generate matrix multiplication expressions. The code below compiles without warnings (on g++ 4.8.0 and Intel C++ 2013.3, with Boost 1.54.0 and Eigen 3.1.3) and works as long as my expression only has a single multiplication operation. As soon as I add more multiplications to the chain, it crashes. Valgrind tells me that this is because one of the Eigen::GeneralProduct expression template temporaries gets destroyed before the evaluation is completed.</p> <p>I don't understand why this happens, or what I can do to prevent it. All help is appreciated!</p> <pre><code>#include &lt;iostream&gt; #include &lt;boost/fusion/container.hpp&gt; #include &lt;boost/mpl/int.hpp&gt; #include &lt;boost/mpl/void.hpp&gt; #include &lt;boost/proto/proto.hpp&gt; #include &lt;boost/ref.hpp&gt; #include &lt;boost/type_traits/remove_const.hpp&gt; #include &lt;boost/type_traits/remove_reference.hpp&gt; #include &lt;boost/utility.hpp&gt; #include &lt;Eigen/Dense&gt; namespace fusion = boost::fusion; namespace mpl = boost::mpl; namespace proto = boost::proto; typedef Eigen::Matrix&lt;float, Eigen::Dynamic, Eigen::Dynamic&gt; matrix; // Placeholders const proto::terminal&lt;mpl::int_&lt;0&gt; &gt;::type I1 = {{}}; const proto::terminal&lt;mpl::int_&lt;1&gt; &gt;::type I2 = {{}}; const proto::terminal&lt;mpl::int_&lt;2&gt; &gt;::type I3 = {{}}; // Grammar template&lt;class Rule, class Callable = proto::callable&gt; struct External : proto::when&lt;Rule, proto::external_transform&gt; {}; struct matmul_transform : proto::callable { template&lt;class Sig&gt; struct result; template&lt;class This, class MatrixExpr1, class MatrixExpr2&gt; struct result&lt;This(MatrixExpr1, MatrixExpr2)&gt; { typedef typename Eigen::ProductReturnType&lt; typename boost::remove_const&lt;typename boost::remove_reference&lt;MatrixExpr1&gt;::type&gt;::type, typename boost::remove_const&lt;typename boost::remove_reference&lt;MatrixExpr2&gt;::type&gt;::type&gt;::Type type; }; template&lt;class MatrixExpr1, class MatrixExpr2&gt; typename result&lt;matmul_transform(MatrixExpr1, MatrixExpr2)&gt;::type operator()(const MatrixExpr1 &amp;a, const MatrixExpr2 &amp;b) const { return a * b; } }; struct MatmulGrammar; struct InputPlaceholder : proto::terminal&lt;proto::_&gt; {}; struct MatrixMultiplication : proto::multiplies&lt;MatmulGrammar, MatmulGrammar&gt; {}; struct MatmulGrammar : proto::or_&lt; External&lt;InputPlaceholder&gt;, External&lt;MatrixMultiplication&gt; &gt; {}; struct matmul_transforms : proto::external_transforms&lt; proto::when&lt;MatrixMultiplication, matmul_transform(MatmulGrammar(proto::_left), MatmulGrammar(proto::_right))&gt;, proto::when&lt;InputPlaceholder, proto::functional::at(proto::_data, proto::_value)&gt; &gt; {}; int main() { matrix mat1(2,2), mat2(2,2), mat3(2,2), result(2,2); mat1 &lt;&lt; 1, 2, 3, 4; mat2 &lt;&lt; 5, 6, 7, 8; mat3 &lt;&lt; 1, 3, 6, 9; MatmulGrammar mmg; // THIS WORKS: result = mmg(I1 * I2, mpl::void_(), (proto::data = fusion::make_vector(boost::cref(mat1), boost::cref(mat2), boost::cref(mat3)), proto::transforms = matmul_transforms())); std::cout &lt;&lt; result &lt;&lt; std::endl; // THIS CRASHES: result = mmg(I1 * I2 * I3, mpl::void_(), (proto::data = fusion::make_vector(boost::cref(mat1), boost::cref(mat2), boost::cref(mat3)), proto::transforms = matmul_transforms())); std::cout &lt;&lt; result &lt;&lt; std::endl; return 0; } </code></pre>
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    1. COThere was a talk in this years C++Now about using eigen with proto. If I remember correctly they talked explicitly about this problem and explained how they solved it. You can find the slides [here](https://github.com/boostcon/cppnow_presentations_2013/blob/master/fri/proto-eigen-fem.pdf?raw=true), the video of the talk [here](http://www.youtube.com/watch?v=pDTQlwXkjvU) and the source code [here](https://github.com/barche/eigen-proto) if you are interested. Unfortunately the audio is missing in the last minutes, but everything works fine until then.
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