– forwards and reverse

This article demonstrates how to perform source transformations on a program to generate forward mode and reverse mode derivative programs (automatic differentiation, or “AD”). My aim is to write the shortest possible article that communicates all the essential features of a source-to-source AD system with a particular focus on making the reverse mode transformation clear.

The goal of brevity means that a lot of possible commentary has been omitted. If you find this makes some part of the article hard to understand then please contact me and I’ll do my best to clarify. In particular this article contains hardly any mathematical content at all. I hope that the reader who is familiar with multivariate calculus will be able to obtain an intuitive understanding of how AD relates to mathematical techniques he or she is already familiar with. A more in-depth description of the relationship will have to wait for another article.

Let’s consider the following pseudocode program that performs some elementary arithmetic through a sequence of assignment statements.

```
p = 7 * x
r = 1 / y
q = p * x * 5
v = 2 * p * q + 3 * r
```

`x`

and `y`

are not defined in the program so I’m going to informally consider them to be “inputs”; `v`

is not used anywhere so I’m going to consider it to be the “output”. (I won’t burden the article by formalising these notions here.)

We’ll do a small amount of preparation to our original program which will preserve its behaviour and get it into a form in which it is straightforward to apply the automatic differentiation (AD) algorithms. It is possible to apply AD algorithms without doing these transformations first but then the AD algorithms would have to do equivalent operations implicitly. Doing these transformations first is a kind of separation of concerns.

Let’s use prefix functions instead of infix operators. Infix operators are more familiar for arithmetic but the AD algorithms will be clearer to present if we use prefix functions. Additionally I want every function to have exactly one argument (although that argument may be a tuple). Single-argument style will make the reverse mode transformation much clearer (although it does not make any difference for forward mode). For example, `x1 + x2`

would become `add (x1, x2)`

. Our program becomes

```
p = mul (7, x)
r = div (1, y)
q = mul (mul (p, x), 5)
v = add (mul (mul (2, p), q), mul (3, r))
```

Next let’s convert to a form where every function is applied to (tuples of) variables and constants only, i.e. where there are no nested sub-expressions (besides potentially nested tuples). We assign each nested sub-expression to an intermediate variable. For example

`a = add (add (b, c), d)`

would become

```
i = add (b, c)
a = add (i, d)
```

The choice of `i`

is arbitrary; it just has to be a variable that’s not used elsewhere in our program. This form without nested subexpressions is a lot like ANF from the field of functional compiler construction. It’s also a lot like the SSA form of assembly language. After removing nested subexpressions, our program becomes

```
p = mul (7, x)
r = div (1, y)
i1 = mul (p, x)
q = mul (i1, 5)
i2 = mul (2, p)
i3 = mul (i2, q)
i4 = mul (3, r)
v = add (i3, i4)
```

We have performed all the transformations needed to prepare our program and we are ready to proceed to differentiation. We will differentiate the program line-by-line, that is, both the forward mode and reverse mode differentiation algorithms will generate one line of derivative code for each line of input code. But what *is* the derivative of an assignment statement? For forward mode, the derivatives correspond quite closely to what you might be familiar with from a first multivariate calculus course..

If a line of our pseudocode program is

`y = add (x1, x2)`

then the derivative line is

`dy = add (dx1, dx2)`

If a line of our pseudocode program is

`y = mul (x1, x2)`

then the derivative line is

`dy = add (mul (x2, dx1), mul (x1, dx2))`

If a line of our pseudocode program is

`y = div (x1, x2)`

then the derivative line is

`dy = add (div (dx1, x2), negate (mul (div (x1, mul (x2, x2)), dx2)))`

The forward mode transformation applies the appropriate differentiation rule to each line in the input program (listed on the left) to obtain a derivative line (listed on the right). To each line we apply exactly one rule and the form of the rule does not depend on any of the other lines.

```
p = mul (7, x) | dp = mul (7, dx)
r = div (1, y) | dr = negate (div (dy, mul (y, y)))
i1 = mul (p, x) | di1 = add (mul (dp, x), mul (p, dx))
q = mul (i1, 5) | dq = mul (di1, 5)
i2 = mul (2, p) | di2 = mul (2, dp)
i3 = mul (i2, q) | di3 = add (mul (di2, q), mul (i2, dq))
i4 = mul (3, r) | di4 = mul (3, dr)
v = add (i3, i4) | dv = add (di3, di4)
```

If we form a new program consisting of the sequence of assignments on the left followed by the sequence of assignments on the right then we have a program that calculates the forward derivative! The “inputs” of this program are `x`

, `y`

, `dx`

and `dy`

. The “outputs” are `v`

and `dv`

.

(The derivatives of constants are zero and I’ve left terms that are zero out for simplicity.)

In fact we can be a little more clever. We can interleave the assignments, so an assignment from the left is immediately followed by its corresponding assignment from the right, that is

```
p = mul (7, x)
dp = mul (7, dx)
r = div (1, y)
dr = negate (div (dy, mul (y, y)))
i1 = mul (p, x)
di1 = add (mul (dp, x), mul (p, dx))
q = mul (i1, 5)
dq = mul (di1, 5)
i2 = mul (2, p)
di2 = mul (2, dp)
i3 = mul (i2, q)
di3 = add (mul (di2, q), mul (i2, dq))
i4 = mul (3, r)
di4 = mul (3, dr)
v = add (i3, i4)
dv = add (di3, di4)
```

This interleaving demonstrates an important property of the automatic derivative: that it uses space proportional to the space usage of the original program. Specifically, as soon as we no longer need a variable that was assigned in the original program we no longer need the corresponding `d`

version either.

We can also see another important property of the forward derivative: it runs in time proportional to the run time of the original program (assuming that the derivative of every primitive runs in time proportional to the run time of the primitive itself).

Now that we’ve shown how to generate the forward mode derivative we can move on to the reverse mode derivative. Reverse mode requires two additional ideas:

We need to convert our original program to “explicit duplication” form: if a variable is used more than once then we make that explicit in the structure of the program. This is unusual but straightforward.

We need to use a form of the derivative that will be unfamiliar to most readers. It will appear quite bizarre when seeing it for the first time but it is crucial to implementing the reverse mode derivative.

Before applying the reverse mode AD transformation we will convert to “explicit duplication” form. Again, the transformation is not strictly required but if we omit it then the differentiation pass will have to do it implicitly. We take the ANF form of the program and insert explicit duplications (`dup`

) for any variable that is used more that once. Recall that after removing nested subexpressions our program was

```
p = mul (7, x)
r = div (1, y)
i1 = mul (p, x)
q = mul (i1, 5)
i2 = mul (2, p)
i3 = mul (i2, q)
i4 = mul (3, r)
v = add (i3, i4)
```

We can see that `x`

and `p`

appear on the right hand side (i.e. are consumed) twice each. Therefore, they will need explicit duplication, so that each variable in the resulting program is used only once. With explicit duplication the program looks like

```
(x1, x2) = dup x
p = mul (7, x1)
(p1, p2) = dup p
r = div (1, y)
i1 = mul (p1, x2)
q = mul (i1, 5)
i2 = mul (2, p2)
i3 = mul (i2, q)
i4 = mul (3, r)
v = add (i3, i4)
```

(If a variable were used *n* times then we would have to insert *n* − 1 `dup`

s for it. In our example no variable is used more than twice.)

Notice that now not only is every variable defined exactly once, but every variable is also *used* exactly once (except the inputs and outputs, `x`

, `y`

and `v`

– I won’t say more here about how exactly these seemingly special cases fit into the story). This property is important for a reason which will be explained when we come to generate the reverse mode program.

The line-by-line differentiation rules for generating the reverse mode need another article to explain thoroughly, but in this article I will hope to provide some basic intuition via examples and the informal notion that the reverse mode program calculates how sensitive the output is to different variables. For example, if the variable `y`

appears in the original program then the variable `d_dy`

will appear in the reverse mode program and measures “how sensitive the output is to small changes in `y`

”. (I’ll abbreviate this to “`d_dy`

is the sensitivity to `y`

”.)

If a line of our pseudocode program is

`y = add (x1, x2)`

then the derivatives are

```
d_dx1 = d_dy
d_dx2 = d_dy
```

because the sensitivity to `x1`

is the same as the sensitivity to `y`

(and likewise for `x2`

). This is written on a single line as

`(d_dx1, d_dx2) = dup (d_dy)`

If a line of our program was

`y = mul (x1, x2)`

then the derivative line is

`(d_dx1, d_dx2) = (mul (x2, d_dy), mul (x1, d_dy))`

because the sensitivity to `x1`

is `x2`

times the sensitivity to `y`

(and similarly for `x2`

).

If a line of our program was

`(x1, x2) = dup (x)`

then the derivative line is

`d_dx = add (d_dx1, d_dx2)`

because the sensitivity to `x`

is the sensitivity to `x1`

plus the sensitivity to `x2`

.

Like forward mode before it, the reverse mode transformation applies the appropriate differentiation rule to each line in the input program (listed on the left) to obtain a derivative line (listed on the right). To each line we apply exactly one rule and the form of the rule does not depend on any of the other lines.

```
(x1, x2) = dup x | d_dx = add (d_x1, d_dx2)
p = mul (7, x1) | (_, d_dx) = mul (d_dp, (x1, 7))
(p1, p2) = dup p | d_dp = add (d_dp1, d_dp2)
r = div (1, y) | d_dy = negate (div (d_dr, mul (y, y)))
i1 = mul (p1, x2) | (d_dp1, d_dx1) = mul (d_di1, (x, p1))
q = mul (i1, 5) | (d_di1, _) = mul (d_dq, (5, di1)
i2 = mul (2, p2) | (_, d_dp2) = mul (d_di2, (p2, 2))
i3 = mul (i2, q) | (d_di2, d_dq) = mul (d_di3, (q, i2))
i4 = mul (3, r) | (_, d_dr) = mul (d_di4, (r, 3))
v = add (i3, i4) | (d_di3, d_di4) = dup(d_dv)
```

If we form a new program consisting of the sequence of assignments on the left followed by the sequence of assignments on the right *in reverse order* then we have a program that calculates the reverse derivative! The “inputs” of this program are `x`

, `y`

and `d_dv`

. The “outputs” are `v`

, `d_dx`

and `d_dy`

.

(Note that, similar to how in forward mode we omitted derivatives of constants because they are zero, in reverse mode we omit the calculation of derivatives *with respect to* constants because they have no effect on the rest of the program.)

The explicit duplication property is important because in the code generated by the reverse mode transformation, usages in the original program become definitions in the reverse mode program; correspondingly, definitions in the original program become usages in the reverse mode program. Therefore it is important that there is exactly one of each: a variable cannot have two definitions! For example, consider the original source assignment `(x1, x2) = dup x`

. `x`

is “used” in this line, and `x1`

and `x2`

are “defined”. It gives rise to the assignment `d_dx = add (d_x1, d_dx2)`

in the reverse mode program. `d_dx`

is “defined” in this line and `d_dx1`

and `d_dx2`

are “used”.

Again we see an important property of the automatic derivative: it runs in time proportional to the run time of the original program (assuming that the derivative of every primitive runs in time proportional to the run time of the primitive itself).

If we look carefully we can also see another property of the reverse derivative: it might use space proportional to the run time of the original program! Notice that the value of `x1`

needs to be kept around throughout the lifetime of the program so that we can calculate `d_dx`

. Once we’ve calculated a value we can’t just use it and throw it away, like we could in forward mode. (There’s a technique called “checkpointing” to address this which prefers to rerun computations rather than store their results, decreasing space usage but increasing run time. Siskind and Pearlmutter have a useful introduction to checkpointing.)

This is a description of how to differentiate what are called “straight-line” programs, that is it does not cover recursion or loops, or conditionals. Arrays are not explicitly treated here either, although they fit naturally into this framework. Dealing properly with those concepts requires extending the presentation given here into something which would require a significantly longer article.

An in-depth analysis of the sense in which the resulting programs are the “derivative” of the input program will have to wait for another article.

Source-to-source forward and reverse mode automatic differentiation can be expressed as follows

Apply simple transformations to get your program into a form where it can be differentiated line-by-line.

Apply the differentiation rule for each line separately.

The forward mode differentiation rules are quite close to what you might already be familiar with. The reverse mode rules are probably not, which might go some way to explaining why the reverse mode derivative has a reputation for being very mysterious.

If you have any questions then please contact me.

Thanks to Mark Saroufim and Pashmina Cameron for helpful feedback.