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This article describes the instructions provided by NEON for rearranging data within vectors. Previous articles in this series: Part 1: Loads and Stores, Part 2: Dealing with Leftovers, Part 3: Matrix Multiplication and Part 4: Shifting Left and Right.

When writing code for NEON, you may find that sometimes, the data in your registers are not quite in the correct format for your algorithm. You may need to rearrange the elements in your vectors so that subsequent arithmetic can add the correct parts together, or perhaps the data passed to your function is in a strange format, and must be reordered before your speedy SIMD code can handle it.

This reordering operation is called a permutation. Permutation instructions rearrange individual elements, selected from single or multiple registers, to form a new vector.

Before you dive into using the permutation instructions provided by NEON, consider whether you rea...

本文将介绍NEON提供的移位运算，并显示如何利用移位运算在常用颜色深度之间转换影像数据。本系列前期已发布的文章包括：第1部分：加载与存储，第2部分：余数的处理，第3部分：矩阵乘法。

向量移位 NEON上的移位与标量ARM编码中可能用到的移位非常相似，即每个向量元素的位数均向左或向右移位，出现在每个元素左侧或右侧的位将被删除；它们不能移位至相邻的元素。

We have seen how to load and store data with NEON, and how to handle the leftovers resulting from vector processing. Let us move on to doing some useful data processing – multiplying matrices.

In this post, we will look at how to efficiently multiply four-by-four matrices together, an operation frequently used in the world of 3D graphics. We will assume that the matrices are stored in memory in column-major order – this is the format used by OpenGL-ES.

We start by examining the matrix mutiply operation in detail, by expanding the calculation, and identifying sub-operations that can be implemented using NEON instructions.

Notice that in the diagram, we multiply each column of the first matrix (in red) by a corresponding single value in the second matrix (blue) then add together the results for each element to give a column of results. This operation is repeated for each of the four columns in the result matrix.

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In the first post on NEON about loads and stores we looked at transferring data between the NEON processing unit and memory. In this post, we deal with an often encountered problem: input data that is not a multiple of the length of the vectors you want to process. You need to handle the leftover elements at the start or end of the array - what is the best way to do this on NEON?

Using NEON typically involves operating on vectors of data from four to sixteen elements in length. Frequently, you will find that your array is not a multiple of that length, and you have to process those leftover elements separately.

For example, you want to load, process and store eight elements per iteration using NEON, but your array is 21 elements long. The first two iterations go well, but for the third, there are only five elements remaining to be processed. What do you do?

There are three ways to handle these leftovers. The methods vary in requirements, performance, and code size. They are listed below in order, with the fastest approach first.

If you can change the size of the arrays that you are processing, increase the length of the array to the next multiple of the vector size using padding elements. This allows you to read and write beyond the end of your data without corrupting ad...