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author | 2010-08-23 17:23:30 +0100 | |
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committer | 2010-08-23 17:23:30 +0100 | |
commit | 474c2996bd7da42663bdc7696c2143f1ab1f1af5 (patch) | |
tree | 2a4f6a7e243b5c6549624a0b250dce0d96ff2ad4 /doc/I11_Aliasing.dox | |
parent | d1111d625ccfc29a4b4a0db0192cbf6e2fdedf3c (diff) |
Docs: add section on resolving the aliasing issue.
Diffstat (limited to 'doc/I11_Aliasing.dox')
-rw-r--r-- | doc/I11_Aliasing.dox | 56 |
1 files changed, 48 insertions, 8 deletions
diff --git a/doc/I11_Aliasing.dox b/doc/I11_Aliasing.dox index e026d02ce..9c6c2ebba 100644 --- a/doc/I11_Aliasing.dox +++ b/doc/I11_Aliasing.dox @@ -31,19 +31,19 @@ The output is not what one would expect. The problem is the assignment \code mat.bottomRightCorner(2,2) = mat.topLeftCorner(2,2); \endcode -After the assignment, the (2,2) entry in the bottom right corner should have the value of \c mat(1,1) before -the assignment, which is 5. However, the output shows that \c mat(2,2) is actually 1. The problem is that -Eigen uses lazy evaluation (see \ref TopicEigenExpressionTemplates) for <tt>mat.topLeftCorner(2,2)</tt>. The -result is similar to +This assignment exhibits aliasing: the coefficient \c mat(1,1) appears both in the block +<tt>mat.bottomRightCorner(2,2)</tt> on the left-hand side of the assignment and the block +<tt>mat.topLeftCorner(2,2)</tt> on the right-hand side. After the assignment, the (2,2) entry in the bottom +right corner should have the value of \c mat(1,1) before the assignment, which is 5. However, the output shows +that \c mat(2,2) is actually 1. The problem is that Eigen uses lazy evaluation (see +\ref TopicEigenExpressionTemplates) for <tt>mat.topLeftCorner(2,2)</tt>. The result is similar to \code mat(1,1) = mat(0,0); mat(1,2) = mat(0,1); mat(2,1) = mat(1,0); mat(2,2) = mat(1,1); \endcode -Thus, \c mat(2,2) is assigned the \e new value of \c mat(1,1) instead of the old value. The issue here is that -the blocks <tt>mat.bottomRightCorner(2,2)</tt> and <tt>mat.topLeftCorner(2,2)</tt> overlap, because both -contain the coefficient <tt>mat(1,1)</tt> at the centre of the 3-by-3 matrix \c mat . The next section +Thus, \c mat(2,2) is assigned the \e new value of \c mat(1,1) instead of the old value. The next section explains how to solve this problem by calling \link DenseBase::eval() eval()\endlink. Note that if \c mat were a bigger, then the blocks would not overlap, and there would be no aliasing @@ -75,7 +75,47 @@ aliasing problem. See \ref TopicAssertions for more information about Eigen's ru \section TopicAliasingSolution Resolving aliasing issues -Synopsis: xxxInPlace(), eval(). +If you understand the cause of the aliasing issue, then it is obvious what must happen to solve it: Eigen has +to evaluate the right-hand side fully into a temporary matrix/array and then assign it to the left-hand +side. The function \link DenseBase::eval() eval() \endlink does precisely that. + +For example, here is the corrected version of the first example above: + +<table class="tutorial_code"><tr><td> +Example: \include TopicAliasing_block_correct.cpp +</td> +<td> +Output: \verbinclude TopicAliasing_block_correct.out +</td></tr></table> + +Now, \c mat(2,2) equals 5 after the assignment, as it should be. + +The same solution also works for the second example, with the transpose: simply replace the line +<tt>a = a.transpose();</tt> with <tt>a = a.transpose().eval();</tt>. However, in this common case there is a +better solution. Eigen provides the special-purpose function +\link DenseBase::transposeInPlace() transposeInPlace() \endlink which replaces a matrix by its transpose. +This is shown below: + +<table class="tutorial_code"><tr><td> +Example: \include tut_arithmetic_transpose_inplace.cpp +</td> +<td> +Output: \verbinclude tut_arithmetic_transpose_inplace.out +</td></tr></table> + +If an xxxInPlace() function is available, then it is best to use it, because it indicate more clearly what you +are doing. This may also allow Eigen to optimize more aggressively. These are some of the xxxInPlace() +functions provided: + +<table class="tutorial_code" align="center"> +<tr> <td> <b>Original function</b> </td> <td> <b>In-place function</b> </td> </tr> +<tr> <td> MatrixBase::adjoint() </td> <td> MatrixBase::adjointInPlace() </td> </tr> +<tr> <td> DenseBase::reverse() </td> <td> DenseBase::reverseInPlace() </td> </tr> +<tr> <td> LDLT::solve() </td> <td> LDLT::solveInPlace() </td> </tr> +<tr> <td> LLT::solve() </td> <td> LLT::solveInPlace() </td> </tr> +<tr> <td> TriangularView::solve() </td> <td> TriangularView::solveInPlace() </td> </tr> +<tr> <td> DenseBase::transpose() </td> <td> DenseBase::transposeInPlace() </td> </tr> +</table> \section TopicAliasingCwise Aliasing and component-wise operations |