# FastPow

This package provides a macro `@fastpow`

that can speed up
the computation of *integer powers* in any Julia expression
by transforming them into *optimal sequences of multiplications*,
with a slight sacrifice in accuracy compared to Julia's built-in
`x^n`

function.

In particular, it uses optimal addition-chain exponentiation for (literal) integer powers up to 255, and for larger powers uses repeated squaring to first reduce the power to ≤ 255 and then use addition chains.

For example, `@fastpow z^25`

requires 6 multiplications, and for `z = 0.73`

it gives the correct answer to a relative error of `≈ 1.877e-15`

(about 8 ulps), vs. the default `z^25`

which gives the correct answer to a relative error of `≈ 6.03e-16`

(about 3 ulps) but is about 10× slower.

Note that you can apply the `@fastpow`

macro to a whole *block*
of Julia code at once. For example,

@fastpow function foo(x,y)
z = sin(x)^3 + sqrt(y)
return z^7 - 4x^5.3 + 3y^12
end

applies the `@fastpow`

transformation to *every* literal integer
exponent (`^3`

, `^7`

, and `^12`

) in the function `foo`

.

An alternative to `@fastpow`

is to use Julia's built-in
`@fastmath`

macro, which enables various LLVM optimizations
including, in some cases, faster integer powers using
repeated multiplication. The advantages of `@fastpow`

are that
it *guarantees* optimal addition-chain exponentiation
and that it works for exponentiating *any* Julia type
(e.g. complex numbers, matrices, …), whereas LLVM will only
optimize a small set of hardware numeric types.