Short examples illustrating AVX2 intrinsics for simple tasks

I was surprised by the lack of simple examples showing how to use AVX and AVX2 intrinsics. There doesn't seem to be a definitive book or even tutorial on the subject.

O Internet, if I am wrong, please correct me! I've learned that the best way to get information on the internet is not to ask a question, but to post the wrong answer.

Examples in Q&A format

So I'm setting myself homework problems and solving them as I go, producing a set of examples. This will be a long and painful process with many trips to the debugger and disassembler.

I will strive to keep the problems simple and the examples short, but I can't promise that a simple problem won't have a complex solution.

I hope I end up with a collection of self-contained code snippets that's useful to others. However, please remember that I am a beginner in the use of CPU vector instructions. I'm not claiming this code is exemplary.

Other resources

Unfortunately, most of these resources are old. Others are raw reference materials like Intel's instruction guides.

https://software.intel.com/en-us/articles/introduction-to-intel-advanced-vector-extensions/ https://software.intel.com/en-us/articles/benefits-of-intel-avx-for-small-matrices/ https://thinkingandcomputing.com/posts/using-avx-instructions-in-matrix-multiplication.html https://www.codeproject.com/Articles/874396/Crunching-Numbers-with-AVX-and-AVX https://software.intel.com/en-us/node/523876 https://www.cs.fsu.edu/~engelen/courses/HPC-adv/MMXandSSEexamples.txt http://sci.tuomastonteri.fi/programming/sse http://stackoverflow.com/questions/13577226/intel-sse-and-avx-examples-and-tutorials http://supercomputingblog.com/optimization/getting-started-with-sse-programming/ https://felix.abecassis.me/2011/09/cpp-getting-started-with-sse/ http://www.walkingrandomly.com/?p=3378 https://software.intel.com/sites/landingpage/IntrinsicsGuide/

Problem 0: get the code to compile and run

Does my computer support AVX2?

Most CPUs sold in the last 3-4 years should support AVX2. To find out, run

cat /proc/cpuinfo | grep avx2

on Linux, or

sysctl -a | grep AVX2

on Mac. Sorry Windows friends, I don't write programs on Windows but I would certainly appreciate a pull request that explains what to do.

What header files do I need?

It seems that the right header to include is immintrin.h, which then goes and includes individual headers like avx2intrin.h. On my installation, I get errors about unknown types if I include avx2intrin.h directly, as well as an error message saying "Never use <avx2intrin.h> directly; include <immintrin.h> instead."

Okay, seems pretty clear.

What compiler flags do I need?

The two platform combinations I develop on are clang+Mac and gcc+Linux. On both of these, -mavx2 does the right thing and I get no errors or non-AVX2 instructions in the output assembly.

Complete example

A complete compilation command looks like

gcc foo.c -mavx2

See examples/00-compile.c for a complete test program that should compile if you have everything set up.

Problem 1: add two 256-bit registers

So it begins! We will construct a vector value from 4 64-bit literals and then add it to itself.

Problem 2: add two (properly aligned) arrays of floats

Not only will we assume the input is correctly aligned, but also that their lengths are multiples of 256 bits.

Problem 3: add two arbitrary arrays of floats

Are there necessary restrictions on alignment with respect to each other, or can we take any two arrays of float anywhere in memory?

Problem 4: dot product

Let's add a reduction to the mix

Problem 5: linear search through an array

I have seen it asserted online that brute force linear search can beat binary search for arrays of size up to 10K. The calculations people give to support this claim involve vector instructions. Let's try writing a vectorized linear search.

Problem 6: 4 or 8 square roots at a time

Problem 7: scalar-vector multiplication

Problem 8: matrix-vector multiplication