How Can AVX2 Instructions Optimize Left-Based Packing with a Mask?

How to Efficiently Pack Left Based on a Mask Using AVX2?

Problem Overview:

Given an input array and an output array, the goal is to write only those elements that pass a specific condition into the output array. This operation is crucial in various applications, including data filtering and image manipulation.

SSE Approach:

In SSE, this process was traditionally accomplished using a shuffle control data approach, as described in the provided code snippet. However, this method becomes cumbersome for AVX, which has 8-wide vectors, requiring a large lookup table.

AVX2 Solution:

To address this issue, AVX2 offers two options:

1. Using BMI2 Instructions:

   • Utilize the vpermd instruction for variable-width permutations.
   • Employ the pext instruction from BMI2 to generate masks on the fly, extracting bits and assembling them in the desired order.

2. Lut Approach:

   • Create a compressed LUT for the shuffle control data, saving memory space compared to SSE.
   • Use set1(), vpsrlvd(), and vpand() to unpack the LUT entries, maintaining 8-wide vectors.

Best Method:

The optimal approach depends on the specific requirements of the application. For large data sets, the LUT approach may be preferred due to its lower overhead and improved cache efficiency. However, for smaller data sets or applications that prioritize speed, the BMI2-based solution can provide better performance.

The above is the detailed content of How Can AVX2 Instructions Optimize Left-Based Packing with a Mask?. For more information, please follow other related articles on the PHP Chinese website!