VREV reverses the order of 8, 16 or 32-bit elements within a vector. For example, VSWP d0, d1 swaps the most and least-significant 64-bits of q0. VMOV and VSWP are the simplest permute instructions, copying the contents of an entire register to another, or swapping the values in a pair of registers.Īlthough you may not regard them as permute instructions, they can be used to change the values in the two D registers that make up a Q register.
Vector code group manual#
As always, benchmark or profile your code regularly, and check your processor's Technical Reference Manual ( Cortex-A8, Cortex-A9) for performance details.
Simple permutations can be achieved using instructions that take a single cycle to issue, whereas the more complex operations use multiple cycles, and may require additional registers to be set up. Neon provides a range of permutation instructions, from basic reversals to arbitrary vector reconstruction. If you have considered all of these, but none put your data in a more suitable format, try using the permutation instructions. Using more than one of these techniques can be still be more efficient than additional permutation instructions. Even if this doesn't completely eliminate the need to permute, it can reduce the number of additional instructions you need. As we've seen previously, load and store instructions have the ability to interleave and deinterleave. A small change to an earlier processing stage, adjusting the way in which data is stored to memory, may reduce or eliminate the need for permutation operations.
A different algorithm may be available that uses a similar number of processing steps, but can handle data in a different format. However, consider data locality, and its effect on cache performance before changing your data structures. It often costs nothing to store your data in a more appropriate format, avoiding the need to permute on load and store. How do you avoid unnecessary permutes? There are a number of options: Your code is not speed optimal until it uses the fewest number of cycles to complete a task move and permute instructions are often good areas to target optimization. Permutation instructions are similar to move instructions, in that they often represent CPU cycles consumed preparing data, rather than processing it. Before we beginīefore you dive into using the permutation instructions provided by Neon, consider whether you really need to use them. Permutation instructions rearrange individual elements, selected from single or multiple registers, to form a new vector. This reordering operation is called a permutation.
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. 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. This article describes the instructions provided by Neon for rearranging data within vectors. Coding for Neon - permutation - rearranging vectors.This blog has been updated and formalized into a guide on Arm developer.
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