---
title: "Rust: powf is slow"
author: "James Pace"
date: "2024/01/18"
---

I'm currently working on my motion planner on a microcontroller project and
was looking at switching from [libm][libm] to [micromath][micromath] as the
library I use for standard math operations.
For context, in Rust, normal math operations (like absolute value and square
root for floats) are provided as part of the standard library (called `std`).
The standard library is not available in environments that don't have a backing
operating system.
Crates than run in those environments thus can't use `std`, which is called being
`no_std`.
A lot of functions that are in `std` are actually in two other crates, `core` or `alloc`
which can be used in `no_std` environments.
The normal math functions aren't one of them, and therefore a different library has to be
used.

My intitial implementation of the planner used `libm` (largely because I found it first),
and when I later found `micromath` I wanted to do some profiling comparisons to see which
one was faster.
I'm going to make a separate blog post with statistics from the profiling, but I wanted to
write a quick article today talking about `powf`.

One of the operations my planner does a lot is find the distance between two things.
My initial implementation did something like
```rust
fn distance(point1: &Point, point2: &Point) -> f32 {
     let squared_dist = powf((point2.x - point1.x), 2.0) + powf((point2.y - point1.y), 2.0);
     sqrt(squared_dist)
}
```
which works fine.

When I was profiling I switched to:
```rust
fn distance(point1: &Point, point2: &Point) -> f32 {
     let squared_dist = powf((point2.x - point1.x), 2.0) + powf((point2.y - point1.y), 2.0);
     sqrt(squared_dist)
}
```
which was noticeably faster for both `libm` and `micromath` particularly on the `microbit`
that I was using for profiling.

## Does this replicate for `std::powf`?

Not when building with `--release`. [^release-rant]



## How much faster?

[^release-rant]: As an aside, I think I'm going to switch to using `--release` when building rust code
                 all the time. The optimizations help a ton, to the extent I would never "use" software
                 not built with `--release` and extrapolating performance, even experientially, from
                 binaries built with the `dev` profile is just not transferrable at all.

[libm]: https://github.com/rust-lang/libm
[micromath]: https://github.com/tarcieri/micromath