62 lines
2.4 KiB
Markdown
62 lines
2.4 KiB
Markdown
---
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title: "Rust: powf is slow"
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author: "James Pace"
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date: "2024/01/18"
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---
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I'm currently working on my motion planner on a microcontroller project and
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was looking at switching from [libm][libm] to [micromath][micromath] as the
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library I use for standard math operations.
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For context, in Rust, normal math operations (like absolute value and square
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root for floats) are provided as part of the standard library (called `std`).
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The standard library is not available in environments that don't have a backing
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operating system.
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Crates than run in those environments thus can't use `std`, which is called being
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`no_std`.
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A lot of functions that are in `std` are actually in two other crates, `core` or `alloc`
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which can be used in `no_std` environments.
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The normal math functions aren't one of them, and therefore a different library has to be
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used.
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My intitial implementation of the planner used `libm` (largely because I found it first),
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and when I later found `micromath` I wanted to do some profiling comparisons to see which
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one was faster.
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I'm going to make a separate blog post with statistics from the profiling, but I wanted to
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write a quick article today talking about `powf`.
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One of the operations my planner does a lot is find the distance between two things.
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My initial implementation did something like
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```rust
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fn distance(point1: &Point, point2: &Point) -> f32 {
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let squared_dist = powf((point2.x - point1.x), 2.0) + powf((point2.y - point1.y), 2.0);
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sqrt(squared_dist)
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}
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```
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which works fine.
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When I was profiling I switched to:
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```rust
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fn distance(point1: &Point, point2: &Point) -> f32 {
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let squared_dist = powf((point2.x - point1.x), 2.0) + powf((point2.y - point1.y), 2.0);
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sqrt(squared_dist)
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}
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```
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which was noticeably faster for both `libm` and `micromath` particularly on the `microbit`
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that I was using for profiling.
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## Does this replicate for `std::powf`?
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Not when building with `--release`. [^release-rant]
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## How much faster?
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[^release-rant]: As an aside, I think I'm going to switch to using `--release` when building rust code
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all the time. The optimizations help a ton, to the extent I would never "use" software
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not built with `--release` and extrapolating performance, even experientially, from
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binaries built with the `dev` profile is just not transferrable at all.
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[libm]: https://github.com/rust-lang/libm
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[micromath]: https://github.com/tarcieri/micromath
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