From 3f715e17010549d3d07eaba66ea3db8b8961fc4c Mon Sep 17 00:00:00 2001
From: Gael Guennebaud <g.gael@free.fr>
Date: Tue, 24 May 2016 22:34:59 +0200
Subject: update doc wrt to unaligned vectorization

---
 doc/UnalignedArrayAssert.dox | 21 +++++++++++----------
 1 file changed, 11 insertions(+), 10 deletions(-)

(limited to 'doc/UnalignedArrayAssert.dox')

diff --git a/doc/UnalignedArrayAssert.dox b/doc/UnalignedArrayAssert.dox
index f0f84d25f..95d95a2d5 100644
--- a/doc/UnalignedArrayAssert.dox
+++ b/doc/UnalignedArrayAssert.dox
@@ -92,27 +92,28 @@ Note that here, Eigen::Quaternionf is only used as an example, more generally th
 
 \section explanation General explanation of this assertion
 
-\ref TopicFixedSizeVectorizable "fixed-size vectorizable Eigen objects" must absolutely be created at 16-byte-aligned locations, otherwise SIMD instructions adressing them will crash.
+\ref TopicFixedSizeVectorizable "fixed-size vectorizable Eigen objects" must absolutely be created at 16-byte-aligned locations, otherwise SIMD instructions addressing them will crash.
 
 Eigen normally takes care of these alignment issues for you, by setting an alignment attribute on them and by overloading their "operator new".
 
 However there are a few corner cases where these alignment settings get overridden: they are the possible causes for this assertion.
 
-\section getrid I don't care about vectorization, how do I get rid of that stuff?
+\section getrid I don't care about optimal vectorization, how do I get rid of that stuff?
 
-Two possibilities:
+Three possibilities:
 <ul>
-  <li>Define EIGEN_DONT_ALIGN_STATICALLY. That disables all 128-bit static alignment code, while keeping 128-bit heap alignment. This has the effect of
-      disabling vectorization for fixed-size objects (like Matrix4d) while keeping vectorization of dynamic-size objects
-      (like MatrixXd). But do note that this breaks ABI compatibility with the default behavior of 128-bit static alignment.</li>
-  <li>Or define both EIGEN_DONT_VECTORIZE and EIGEN_DISABLE_UNALIGNED_ARRAY_ASSERT. This keeps the
-      128-bit alignment code and thus preserves ABI compatibility, but completely disables vectorization.</li>
+  <li>Use the \c DontAlign option to Matrix, Array, Quaternion, etc. objects that gives you trouble. This way Eigen won't try to align them, and thus won"t assume any special alignment. On the down side, you will pay the cost of unaligned loads/stores for them, but on modern CPUs, the overhead is either null or marginal. See \link StructHavingEigenMembers_othersolutions here \endlink for an example.</li>
+  <li>Define \link TopicPreprocessorDirectivesPerformance EIGEN_DONT_ALIGN_STATICALLY \endlink. That disables all 16-byte (and above) static alignment code, while keeping 16-byte (or above) heap alignment. This has the effect of
+      vectorizing fixed-size objects (like Matrix4d) through unaligned stores (as controlled by \link TopicPreprocessorDirectivesPerformance EIGEN_UNALIGNED_VECTORIZE \endlink), while keeping unchanged the vectorization of dynamic-size objects
+      (like MatrixXd). But do note that this breaks ABI compatibility with the default behavior of static alignment.</li>
+  <li>Or define both \link TopicPreprocessorDirectivesPerformance  EIGEN_DONT_VECTORIZE \endlink and EIGEN_DISABLE_UNALIGNED_ARRAY_ASSERT. This keeps the
+      16-byte alignment code and thus preserves ABI compatibility, but completely disables vectorization.</li>
 </ul>
 
-If you want to know why defining EIGEN_DONT_VECTORIZE does not by itself disable 128-bit alignment and the assertion, here's the explanation:
+If you want to know why defining EIGEN_DONT_VECTORIZE does not by itself disable 16-byte alignment and the assertion, here's the explanation:
 
 It doesn't disable the assertion, because otherwise code that runs fine without vectorization would suddenly crash when enabling vectorization.
-It doesn't disable 128bit alignment, because that would mean that vectorized and non-vectorized code are not mutually ABI-compatible. This ABI compatibility is very important, even for people who develop only an in-house application, as for instance one may want to have in the same application a vectorized path and a non-vectorized path.
+It doesn't disable 16-byte alignment, because that would mean that vectorized and non-vectorized code are not mutually ABI-compatible. This ABI compatibility is very important, even for people who develop only an in-house application, as for instance one may want to have in the same application a vectorized path and a non-vectorized path.
 
 */
 
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