You Won’t Believe This One Weird CPU Instruction!

Posted on 8 September 2019
Tags: programming

This is a pseudo-transcript of a presentation I did at !!Con 2019.

Most CPU architectures in use today have an instruction called popcount, short for “population count”. Here’s what it does: it counts the number of set bits in a machine word. For example (assuming 8-bit words for simplicity), popcount(00100110) is 3 and popcount(01100000) is 2.

You might be wondering, like I was, if there’s more to this instruction, but that’s all it does! This doesn’t seem very useful, right?

I thought this might be a recent addition for some hyperspecialised use case, but it has in fact been present in CPU architectures since at least 1961:

So what’s going on?

The NSA Instruction

popcount is also known as “The NSA Instruction”, and a very entertaining thread on comp.arch discusses its uses inside and outside cryptography. It is rumoured that it was originally added to CPU instructions at the behest of the NSA. As this archived email thread puts it:

It was almost a tradition that one of the first of any new faster CDC machine was delivered to a “good customer” - picked up at the factory by an anonymous truck, and never heard from again.

This makes for a great story, but what were they using it for?

One measure of information content is the Hamming weight, which is the number of symbols in a string that are different from the zero-symbol of the alphabet. For a binary string, this is exactly popcount!

As explained here, the NSA wanted to do cryptanalysis on intercepted messages, and since the CDC 6000 had 60-bit words, one word was enough to store most alphabets they were interested in. They were able to:

  1. Split a message into lines
  2. Set a bit for each unique character they encountered per line
  3. Use popcount to count the distinct characters
  4. Use the count as a hash for further cryptanalysis

Curiously, popcount seems to have disappeared from instruction sets between the mid-1970s and the mid-2000s, so there has to be more to it than cryptographic applications to explain its return. What else can it be used for?

Error Correction

Related to the concept of Hamming weight is Hamming distance, which is the number of differing positions between two strings of identical length. For two binary strings x and y, this is just the popcount of them XORed together. For example:

00100110
01100000 ^
--------
01000110

popcount(01000110) = 3

For telecommunications applications, this helps us calculate the signal distance, where a known word is sent over the wire and the number of flipped bits are counted to provide an estimate of the error introduced by transmission.

We can then design an error-correcting code accordingly, e.g. if we want to be robust against up to 2 flipped bits, our code words need to differ in Hamming distance by at least 5.

Binary Convolutional Neural Networks

And now for something completely different: binary convolutional neural networks! But first, what are they?

In summary, we have to do binary matrix multiplication. But what’s special about binary matrices?

Ordinary matrix multiplication on 32-bit values is a good fit on desktop computers with powerful CPUs and GPUs, but increasingly we also want to do useful work on smaller and simpler devices, such as smartphones, routers, smartwatches, etc. We can decompose these more complex matrices into layers of binary matrices, and these resulting matrices are so much easier to store and operate on that we are better off even though there are more layers.

Where does popcount come into play? It’s used to calculate the dot product of two binary matrices:

a = xnor(x, y)
b = popcount(a)
c = len(a)
dot(x, y) = 2 × b − c

More details are available here and here.

Chess Programming

Many chess programs store data using a bitboard representation, which conveniently fits into a 64-bit word. Population Count has been used to perform meaningful operations with this representation, such as calculating the mobility of a piece.

Molecular Fingerprinting

This is related to the notion of Hamming distance above: molecules are hashed in some way and compared (with popcount) to determine how similar they are. More details on that here.

Hash Array Mapped Tries

This is where I first learned about popcount! The HAMT is a data structure (pioneered by Phil Bagwell) that can store a very large number of values (usually 32 or 64) in an array at each node of the trie. However, allocating memory for a 32 or 64-element array every time can be incredibly wasteful, especially if the array only actually contains a handful of elements. The solution is to add a bitmask in which the number of bits that are set corresponds to the number of elements in the array, which allows the array to grow and shrink as required. Calculating the index for a given element efficiently can then be done using popcount. You can learn more about how they work from this blog post, where I implement them myself.

Succinct Data Structures

This is an exciting new area of research that focuses on how to store data in as little space as possible, without having to decompress it in order to do useful work. One technique is to think in terms of arrays of bits (bitvectors), which can be queried using two operations:

Making these operations efficient on large bitvectors requires constructing an index and using it effectively, both involving popcount. There’s a good overview of the RRR index here, and as far as I can tell the current state-of-the-art approach is described in Space-Efficient, High-Performance Rank & Select Structures on Uncompressed Bit Sequences.

Compiler Optimisations

popcount has become so pervasive that both GCC and Clang will detect an implementation of popcount and replace it with the built-in instruction. Imagine Clippy going “I see you are trying to implement popcount, let me go ahead and fix that for you”! The relevant LLVM code is here. Daniel Lemire points to this as an example of the surprising cleverness of modern compilers.

Conclusion

From beginnings shrouded in mystery, popcount has emerged as a generally useful, if slightly unusual, CPU instruction. I love how it ties together such different fields of computing, and I wonder how many other similarly weird instructions are out there. If you have a favourite, I’d love to hear about it!