# Implement Your Own Automatic Differentiation with Julia in ONE day

By Roger Luo

I was playing with AutoGrad.jl and Zygote.jl, they both look awesome, and AutoGrad.jl is already applied to the machine learning framework in Julia: Knet.jl. However, when I tried to read the source code of AutoGrad.jl, it is not large indeed. And as a PyTorch contributor and user, I personally prefer some of PyTorch's interfaces, therefore, I tried to implemented my own automatic differentiation and it just took me one day to finished the core part (including broadcast!), although, I spent a few hours more in the next following days to polish the interface (a weekend to write a blog post). But it is actually quite easy to implement an automatic differentiation package in Julia.

In this post, I'll introduce how did I implemented my own automatic differentiation, and maybe, you can build one of your own as well!

## ¶Automatic Differentiation: A Brief Intro

There are generally two kinds of automatic differentiation: forward mode differentiation and reverse mode differentiation. What we need in deep learning (as well as tensor networks in physics) is the reverse mode differentiation, because the model we are going to optimize usually contains quite a lot parameters. This is also called as back-propagation and requires something called comput-graph.

### ¶Comput-Graph

To illustrate this, I stealed some nice picture and re-plot them in animation from cs5740, 2017sp, Cornell.

Say we are calculating the following expression:

$$y = \mathbf{x}^T \mathbf{A} \mathbf{x} + \mathbf{b}\cdot \mathbf{x} + c$$

We will need to call several functions in Julia to get the result $y$, which is

1. $\mathbf{z_1} = \mathbf{x}^T$: transpose function.
2. $\mathbf{z_2} = \mathbf{z_1} A$ matrix-vector multiplication, which can be gemv in LinearAlgebra.BLAS, or just *.
3. $y1 = \mathbf{z2} \mathbf{x}$ vector dot operation, which is LinearAlgebra.dot or the UTF-8 operator x ⋅ y
4. $y2 = \mathbf{b} \cdot \mathbf{x}$ another vector dot
5. $y_1 + y_2 + c$ a scalar add function, one can calculate it by simply calling + operator in Julia.

In fact, we can draw a graph of this expression, which illustrates the relationship between each variable in this expression. Each node in the graph with a output arrow represents a variable and each node with a input arrow represents a function/operator.

The evaluation of the math equation above can then be expressed as a process called forward evaluation, it starts from the leaf nodes, which represents the inputs of the whole expression, e.g they are $\mathbf{x}, \mathbf{A}, \mathbf{b}, c$ in our expression. Each time, we receive the value of a node in the graph, we mark the node with green.

Now, let's calculate the gradients with chain rule, the number of gradients returned by each function is the same with their inputs. We mark the node with red color if we receive a gradient, the gradient will be back propagated through the graph, which is called back propagation or backward evaluation.

### ¶Dynamic Comput Graph VS Static Comput Graph

Although, the way of forward evaluation and backward evaluation are actually the same, but for implementation, we can construct the graph on the fly (like PyTorch) or statically declaration (like TensorFlow).

Generally, the difference between them is that:

whether to define the graph before the forward evaluation happens or along with the forward evaluation.

I'm a PyTorch syntax lover, so I'm going to implement my AD as a dynamic constructed graph. But I'm also planning to write a macro in Julia that "freeze" a dynamic graph to static graph, because in principle, static graph is easier to optimize, since we will be able to access the whole graph before evaluation happens, which allows us to dispatch methods statically, but it can be hard to debug for static graphs.

## ¶Define the Nodes in Computational Graph

Well, before we start writing something concrete, we can first define an abstract type for all nodes we are going to define:

### ¶Leaf Nodes

Same, define an abstract type first.

In PyTorch, a Variable is a multi-dimensional array (tensor) with a gradient (also store in a multi-dimensional array of the same size and data type). And it will accumulate the gradient if we backward propagate the graph for multiple times.

Accumulating is useful sometimes, when you want to calculate the expectation of the gradient, or manipulating a batch of data, but not always useful. But anyway, we have an abstract type, we can define different flavored leaf nodes later.

Here, we use in-complete initialization, since we don't really need to allocate a memory for the gradient at the beginning, we can just use a temporary memory later.

### ¶Other Nodes

Well, now we have some leaf nodes, but we need to store operations and their output for later use, so firstly, I define something called Node

It is a subtype of AbstractNode, and it stores a function call's arguments and keywords. However, we will need to consider broadcast and normal function calls, they are actually different, therefore we should not directly store the function, thus, let's write some traits:

Now we change Function to Operator

And we may make some constructors for convenience, since most f will be a method call rather than broadcast or self-defined operator, and we usually don't need the keyword arguments either:

In fact, Node is actually just a trait for some object (some subtype of Operator), we haven't define any type that store the output of each node in the graph, so here let's define a CachedNode which will cache the forward evaluation result of Node:

So, what we want this CachedNode do, is that we want it to store the forward evaluation result of a Node:

## ¶Evaluations

evaluation is the most important part, because we want to define our rules of evaluation in an extensible way, and try to make it efficient. Luckily, in Julia, we have multiple dispatch! Let's make use of it!

### ¶Forward Evaluation

But how to forward evaluate a Node? this depends what kind of method is implemented for this generic function forward:

1. if input is a Node, we re-dispatch this method to its operator's forward method (while forward evaluates the args and kwargs):

This will allow us to tweak the forward evaluation by simply implement a method for generic function forward, e.g, if we don't want to directly calculate the result of a linear operator $\mathbf{W}\mathbf{x} + \mathbf{b}$ rather than store two nodes separately (a matrix-vector multiplication * and a add function +).

1. if input is a CachedNode, this means our user is evaluating this node for second time (since we calculate the result when construct it), we will update its output
1. However, for simple function calls, we don't want to write something like

each time, let's make it simpler, by re-dispatching an operator's forward method to a function call:

This means, as long as, the operator defines its own call method, it does not need to implement a method for forward, e.g

We can just define the call method for Linear rather than define method for forward:

1. There could be some constants in the Node, e.g when we call Variable(2.0) + 1.0, this 1.0 is actually a constant, therefore, we can just return it self, when the input is not part of the computational graph (not subtype of AbstractNode) and define a default method for AbstractNode for better error messages.
1. For leaf nodes, they should directly return their value, but we might use other kind of leaf node to make non-PyTorch lover happy in the future, so let's define a generic function value for get this property:

And leaf nodes' forward directly return its value:

Okay! we have defined all we need for forward evaluation, now let's try to implement backward evaluation.

### ¶Backward Evaluation

The backward evaluation is actually similar to forward evaluation, we will call backward recursively on each node and its args (no, I'm not going to support backward on kwargs here, XD).

Firstly, for LeafNode, this is simple, e.g Variable will just take the grad

We will check if this grad member is defined (it is in-complete initialized!), if it is not, we will just use the memory of this gradient, or we add it to current gradient, just like PyTorch's Variable (or Tensor after v0.4).

And now, we need to define how to backward evaluation a CachedNode:

1. we gather the gradients of inputs from a function called gradient
2. we put each corresponding gradient to sub-node of current node and call their backward

Oh, you might want to add some assertion to output better error message here, we will check the type of gradient and output and also their size here, in most cases, gradient should have the exact same type and size with output:

but for subtype of AbstractArray, we can just allow them to have the same static parameter (tensor rank and data type), because we will probably dealing with SubArray and Array for some operators, which does not really matters

Finally we check the size of gradients and output

In Julia, the bounds check can be turned off by compiler option, we sometimes don't actually need to check bounds in runtime so we put this assertion in @boundscheck, so it will looks like:

OK, now, let's think about how to return the gradient, I would prefer our AD be highly extensible by Julia's multiple dispatch, and I will only need to define the gradient by define different methods for gradient, e.g

This can be implemented in the same way of forward: re-dispatch the method to different syntax:

We here dispatch the gradient of a CachedNode directly to a method implemented for Operator, but the same with forward we don't want to write Method trait each time, re-dispatch it again

Finally, define a default error massage:

So in this way, when we implement a specific method of some types for gradient, Julia will auto dispatch gradient to that method, e.g

Umm, and finally, I would like to have an eye-candy function for construct a node (but this depends on you, it is not actually necessay):

Okay, let's try to register an operator now!

remember we assumed gradient returns several gradients, the return of gradient has to be an iteratable of gradients.

However, the above gradients for scalars, this will just work. It won't work for arrays, we will need to re-dispatch broadcast in Julia.

Let me introduce some basic concepts of the interface of broadcast in Julia first, and then we will find a quite easy way to implement AD for broadcast:

All the broadcast mechanism is implemented in a module Broadcast in Base, each different type has its own BroadcastStyle (this is a trait). So what we need to do, is just to implement our own broadcast style and construct a CachedNode instead directly broadcasting the operation.

However, this is not enough, in Julia broadcast is lazy evaluated, which can fuse broadcast and provide better performance, we need to re-dispatch two interface: broadcasted and materialize

And we let materialize directly forward the gradient during backward evaluation:

Now, if you try to broadcast with this AD, you would find that the assertion we defined in backward is quite annoying (because lazy evaluation, its output is not actually the real output, but a middle type), let's mute them for broadcast:

There is a Julia package called DiffRules, it contains quite a lot differentiation rules defined as Julia Expr, so we can just use code generation to generate operators with it rather than define them ourselves:

The rules are in DiffRules.DEFINED_DIFFRULES, so we will just iterate through its key

the first argument mod is the module's name, like for sin, it is actually in Base, so the mod is Base and name is the function's name, nargs means the number of arguments, in DiffRules, there are only single argument functions and double arguments functions.

So the code generation will looks like

For how to use code generation in Julia, I would suggest the official documentation for understanding more about it: Code Generation. I escape abs here because the differentiation expression of abs generated by DiffRules can not be directly broadcasted by @. (this macro add a broadcast mark . to every function call), so I have to implement its gradient manually. But DiffRules will generate most of the math function's gradient for you!

## ¶Polish

We roughly implemented the core functionality of an AD, but there's still quite a lot to do to make it looks and feels better.

I defined better printing later here: show.jl, the basic idea is to re-dispatch our nodes via several traits, so we can insert a type into other type tree, e.g as subtype of AbstractArray and then make use of existing printing methods.

Then, to implement unit test, I copied the gradcheck function from PyTorch, which will calculate the jacobian of an operator with the AD package and compare it with numerical jacobian.

## ¶Benchmark

Okay, it is done! With about only 200~300 lines Julia, what can we get? Actually, I thought it would be just a toy, but it is actually amazing, when I tried to use it for my own work:

So I need to calculate something called matrix product state, well, I'm not going to talk about quantum physics, so in short, it is just some rank-3 tensors (3 dimensional array), and we will need to calculate something like the following expression:

where x1, x2, x3 are just matrix.

So I implemented the gradient of tr and matrix multiplication:

Now let's benchmark tr(x1 * x2) on CPU with other packages, with the following function call

and in PyTorch (our interface is quite similar to PyTorch, isn't it?)

In Julia, we use BenchmarkTools to measure the time, and in Python we can use the magic command timeit in ipython.

The value is defined as follows

Before we benchmark other packages, I also write a baseline function, which calculate the gradient manually:

And then test it with @benchmark, which will run this function for multiple times

and for PyTorch (version v0.4.1)

Our implementation is not bad, huh? Only about 4~5 μs slower than the baseline due to the dynamic construction of our computational graph in runtime and for this expression it is the fastest! It is about 5x faster than other packages in either Julia or Python/C++.

So, as you see, writing an AD package can be super sweet in Julia with multiple dispatch. You can actually write your own AD with a reasonable performance in Julia like a pro!

## ¶Acknowledgement

Thanks for Keno for benchmarking advice on Zygote, I was actually quite confusing about the performance and submitted an issue here: Zygote.jl/issues/28

And thanks for the Luxor.jl, I use this package for ploting the animation in the blog post. You might want to check my ugly plotting script here: plot.jl