From 294f5f16dd6286a0ac14c2d8194db3a95fdfb301 Mon Sep 17 00:00:00 2001 From: Benoit Jacob Date: Mon, 12 Jan 2009 14:41:12 +0000 Subject: muuuch improved documentation for the unaligned array assert. also split into several pages for better reusability (more generally useful than just for this assert) --- doc/FixedSizeVectorizable.dox | 36 ++++++++++++++++++++++++++++++++++++ 1 file changed, 36 insertions(+) create mode 100644 doc/FixedSizeVectorizable.dox (limited to 'doc/FixedSizeVectorizable.dox') diff --git a/doc/FixedSizeVectorizable.dox b/doc/FixedSizeVectorizable.dox new file mode 100644 index 000000000..f53a6ef02 --- /dev/null +++ b/doc/FixedSizeVectorizable.dox @@ -0,0 +1,36 @@ +namespace Eigen { + +/** \page FixedSizeVectorizable Fixed-size vectorizable Eigen objects + +The goal of this page is to explain what we mean by "fixed-size vectorizable". + +\section summary Executive Summary + +An Eigen object is called "fixed-size vectorizable" if it has fixed size and that size is a multiple of 16 bytes. + +Examples include: +\li Eigen::Vector2d +\li Eigen::Vector4d +\li Eigen::Vector4f +\li Eigen::Matrix2d +\li Eigen::Matrix2f +\li Eigen::Matrix4d +\li Eigen::Matrix4f +\li Eigen::Transform3d +\li Eigen::Transform3f + +\section explanation Explanation + +First, "fixed-size" should be clear: an Eigen object has fixed size if its number of rows and its number of columns are fixed at compile-time. So for example Matrix3f has fixed size, but MatrixXf doesn't (the opposite of fixed-size is dynamic-size). + +The array of coefficients of a fixed-size Eigen object is a plain "static array", it is not dynamically allocated. For example, the data behind a Matrix4f is just a "float array[16]". + +Fixed-size objects are typically very small, which means that we want to handle them with zero runtime overhead -- both in terms of memory usage and of speed. + +Now, vectorization (both SSE and AltiVec) works with 128-bit packets. Moreover, for performance reasons, these packets need to be have 128-bit alignment. + +So it turns out that the only way that fixed-size Eigen objects can be vectorized, is if their size is a multiple of 128 bits, or 16 bytes. Eigen will then request 16-byte alignment for these object, and henceforth rely on these objects being aligned so no runtime check for alignment is performed. + +*/ + +} -- cgit v1.2.3