# Which functions need rules?

In principle, a perfect AD system only needs rules for basic operations and can infer the rules for more complicated functions automatically. In practice, performance needs to be considered as well.

Some functions use ccall internally, for example ^. These functions cannot be differentiated through by AD systems, and need custom rules.

Other functions can in principle be differentiated through by an AD system, but there exists a mathematical insight that can dramatically improve the computation of the derivative. An example is numerical integration, where writing a rule implementing the fundamental theorem of calculus removes the need to perform AD through numerical integration.

Furthermore, AD systems make different trade-offs in performance due to their design. This means that a certain rule will help one AD system, but not improve (and also not harm) another. Below, we list some patterns relevant for the Zygote.jl AD system.

Rules for functions which mutate its arguments, e.g. sort!, should not be written at the moment. While technically they are supported, they would break Zygote.jl such that it would sometimes quietly return the wrong answer. This may be resolved in the future by allowing AD systems to opt-in or opt-out of certain types of rules.

### Patterns that need rules in Zygote.jl

There are a few classes of functions that Zygote cannot differentiate through. Custom rules will need to be written for these to make AD work.

Other patterns can be AD'ed through, but the backward pass performance can be greatly improved by writing a rule.

#### Functions which mutate arrays

For example,

function addone(a::AbstractArray)
b = similar(a)
b .= a .+ 1
return sum(b)
end

complains that

julia> using Zygote
ERROR: Mutating arrays is not supported

However, upon adding the rrule (restart the REPL after calling gradient)

function ChainRules.rrule(::typeof(addone), a)
return NoTangent(), ones(length(a))
end
end

julia> gradient(addone, a)
([1.0, 1.0, 1.0],)

Notice that addone(a) mutates another array b internally, but not its input. This is commonly done in less trivial functions, and is often what Zygote's Mutating arrays is not supported error is telling you, even though you did not intend to mutate anything. Functions which mutate their own input are much more problematic. These are the ones named (by convention) with an exclamation mark, such as fill!(a, x) or push!(a, x). It is not possible to write rules which handle all uses of such a function correctly, on current Zygote.

Why restarting REPL after calling gradient?

When gradient is called in Zygote for a function with no rrule defined, a backward pass for the function call is generated and cached. When gradient is called for the second time on the same function signature, the backward pass is reused without checking whether an an rrule has been defined between the two calls to gradient.

If an rrule is defined before the first call to gradient it should register the rule and use it, but that prevents comparing what happens before and after the rrule is defined. To compare both versions with and without an rrule in the REPL simultaneously, define a function f(x) = <body> (no rrule), another function f_cr(x) = f(x), and an rrule for f_cr.

Calling Zygote.refresh() will often have the same effect as restarting the REPL.

#### Exception handling

Zygote does not support differentiating through try/catch statements. For example, differentiating through

function exception(x)
try
return x^2
catch e
println("could not square input")
throw(e)
end
end

does not work

julia> gradient(exception, 3.0)
ERROR: Compiling Tuple{typeof(exception),Int64}: try/catch is not supported.

without an rrule defined (restart the REPL after calling gradient)

function ChainRulesCore.rrule(::typeof(exception), x)
y = exception(x)
function exception_pullback(ȳ)
return NoTangent(), 2*x
end
return y, exception_pullback
end
julia> gradient(exception, 3.0)
(6.0,)

#### Loops

Julia runs loops fast. Unfortunately Zygote differentiates through loops slowly. So, for example, computing the mean squared error by using a loop

function mse(y, ŷ)
N = length(y)
s = 0.0
for i in 1:N
s +=  (y[i] - ŷ[i])^2.0
end
return s/N
end

takes a lot longer to AD through

julia> y = rand(30)
julia> ŷ = rand(30)
julia> @btime gradient(mse, $y,$ŷ)
38.180 μs (993 allocations: 65.00 KiB)

than if we supply an rrule, (restart the REPL after calling gradient)

function ChainRules.rrule(::typeof(mse), x, x̂)
output = mse(x, x̂)
function mse_pullback(ȳ)
N = length(x)
g = (2 ./ N) .* (x .- x̂) .* ȳ
return NoTangent(), g, -g
end
return output, mse_pullback
end

which is much faster

julia> @btime gradient(mse, $y,$ŷ)
143.697 ns (2 allocations: 672 bytes)

#### Inplace accumulation

Inplace accumulation of gradients is slow in Zygote. The issue, demonstrated in the folowing example, is that the gradient of getindex allocates an array of zeros with a single non-zero element.

function sum3(array)
x = array[1]
y = array[2]
z = array[3]
return x+y+z
end
julia> @btime gradient(sum3, rand(30))
424.510 ns (9 allocations: 2.06 KiB)

Computing the gradient with only a single array allocation using an rrule (restart the REPL after calling gradient)

function ChainRulesCore.rrule(::typeof(sum3), a)
y = sum3(a)
function sum3_pullback(ȳ)
end
julia> @btime gradient(sum3, rand(30))
192.818 ns (3 allocations: 784 bytes)