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Achieving warp speed with Rust

Contents:


If you're looking to write fast code in Rust, good news! Rust makes it really easy to write really fast code. The focus on zero-cost abstractions, the lack of implicit boxing and the static memory management means that even naïve code is often faster than the equivalent in other languages, and certainly faster than naïve code in any equally-safe language. Maybe, though, like most programmers you've spent your whole programming career safely insulated from having to think about any of the details of the machine, and now you want to dig a little deeper and find out the real reason that Python script you rewrote in Rust runs 100x faster and uses a 10th of the memory. After all, they both do the same thing and run on the same CPU, right?

So, here's an optimization guide, aimed at those who know how to program but maybe don't know how it maps to real ones and zeroes on the bare metal of your CPU. I'll try to weave practical tips about optimizing Rust code with explanations of the reason why it's faster than the alternative, and we'll end with a case study from the Rust standard library.

This post assumes decent familiarity with programming, a beginner's familiarity with Rust and almost no familiarity with CPU architecture.

Number one optimization tip: don't

Ok, I'll start with a few disclaimers before I get into the meat. Firstly, unless you're running into performance problems in real-life usage, optimize your code for readability before you optimize it for runtime performance. Compilers and humans alike are better at understanding boring, straightforward code, so if you write that first you'll be likely to have "good enough" performance with the added benefit of being maintainable. If you write nice, refactorable code it'll be easy to change if you later realize that it's being wasteful.

That's not to say that performance doesn't matter. You shouldn't only optimize slow programs. A long-running program using high amounts of CPU but not causing visible slowness is just as bad as a program that takes 30s instead of 3s to process some data, it's just that the former is wasting battery and the latter is wasting time. Weigh up the time it would take to optimize the code against the benefit you would get from the optimization.

The reason maintainability is so important is that lot of optimizations are "try it and see" - if you're unable to make sweeping changes to your code without breaking it you're going to have a really bad time. Now, speaking of "try it and see"...

Never optimize blindly

There are lots of performance-tracing tools out there. A famous tool/set of tools for C and other systems languages is valgrind, which are extremely powerful but can be scary to get started with, so if you just want to have a quick overview of what your program is doing from a performance standpoint check out this article on analyzing Rust with perf, a fantastic and easy-to-use performance tool for Linux. Unless there's a glaring flaw, like pervasive cloneing or a blatantly sub-optimal algorithm, perf will likely give better results than simply optimizing stuff that "looks slow".

Another tool that's good to help you avoid gotchas of all kinds (not just performance) is clippy, but you already knew that because you're using it on all your code to begin with, right?

perf also shows the cumulative cost of each function over the course of the program's runtime, which leads me to my next point:

Don't bother optimizing one-time costs

Your config-parsing code can be as slow as you like and it's unlikely to matter. Don't just optimize the slowest function in your program, optimize the one that takes up the most of your runtime. Those may be the same function, but if you get a 2 millisecond improvement on a function that's called 1000 times, that's better than a 1 second improvement on a function that's called once.

Improve your algorithms

Now, as with every article on performance, this is where I add in the requisite disclaimer of use better algorithms first. Don't invent new algorithms unless you do that for a living, but in all likelihood if you're running into performance problems it's more likely to be due to poor algorithms than to poor implementations. Most programmers test their code on small datasets, but if you have O(n²) complexity that won't appear until you've tried it on a larger dataset. If you don't know what your algorithm is, which is likely since most code is written without a specific algorithm in mind, just try to have as few loops as possible and remember that every use of collect has to iterate over the entire collection at least once, and so the more work you can do using less loops the better. This is the same as optimization in any language though, so this is all I'll say on algorithmic complexity for now. If you want to find out more, there are some excellent resources out there.

CPU architecture primer

So maybe the complexity of your algorithm can't be improved, and to make any improvements you need to get into the down-to-the-metal stuff. This is where the difference comes in between languages like Rust and C and languages like Python and Ruby. It's entirely possible that you know this already, but it's worth going over to make sure we're all on the same page.

There are two parts to any computation, the stuff it does and the stuff it does it on. The instructions, and the data.

Instructions are stored in the instruction cache - a chunk of really, really fast memory that's directly readable by the CPU. Each instruction can put and/or take data from the CPU's registers, which is a small number of small pieces of memory, either 32 or 64 bits depending on your computer's word size. Only a small amount of data can be in registers at any one time, however, and you can't take a pointer to a register, so sometimes the CPU must access the computer's RAM. Since RAM is slow, the CPU tries to read in bulk and then store the result in increasingly small, increasingly fast caches. If it tries to access data that isn't in the smallest cache, it has to read the slightly larger cache, continuing up until it reaches RAM. The upshot is: you want to keep your data as small as possible, and for data that is accessed together to be close to each other so the CPU loads as much of it at once as possible. This should be enough information to get you through the rest of this article, but if you want to dive deeper into it you can check out the structure and implementation section in the Wikipedia page for the CPU.

Keep as much as possible in cache

The further away your data is from your CPU the slower your program will be. The very worst place for data to be is on a different computer. A less awful - but still very awful - place for data to be is on your hard drive. Better still is in your RAM but as mentioned before, RAM is slow. Almost the best possible place for your data is in CPU cache. You may have heard some folklore that allocating is bad, and this is the main reason why. Accessing two different locations one after another on the stack is fine, since they're likely to be on the same cache line. Accessing two different locations one after another on the heap is significantly slower, since it's much less likely they they're directly next to each other. We'll go into exactly why this is in a moment.

If you have to allocate, because you need variable-size containers, shared ownership or owned trait objects (see below for why you probably don't need trait objects), try to put data that will be accessed in sequence in order in RAM, so that when the CPU reads one element it necessarily has to read the next few elements too, meaning it doesn't need to stall waiting for RAM in order to operate on them.

As a rule of thumb for whether something has to allocate: if you can tell me the amount of space the value will use up without running the program, it's stored on the stack. If you don't know something's size until runtime, it's allocated on the heap.

This means that String, Vec, HashMap and Box<Trait>/Box<[T]> all allocate, but any user-defined struct does not (it may contain something that does allocate, but it doesn't require any extra allocation to construct if you already have an instance of the allocating type). Box<T> where T has a statically-known size also allocates, so be careful of recursive enums. If you're creating a tree structure that then becomes immutable, like with an AST, you might want to consider using a TypedArena to get tighter control over memory use. TypedArena is still unstable though, and it increases complexity, so it's not suitable for all use-cases.

This is why you may have heard some complaints about Haskell's use of a linked list of characters to represent a string. I'm not going to beat gankro's wonderful rant on the subject, but suffice to say that this is more-or-less the worst possible data structure to choose when each individual element is small and the number of elements is large, because it needs to store a pointer for every element, instead of a single integer for the entire array.

Not only that, but without some truly crazy compiler optimizations this means that each element may not be directly after the element before it in memory. We'll get to how to calculate this in a moment, but essentially that means that Haskell's String type can cause a cache miss up to twice per element, whereas if you had a vector of Chars (assuming 32-bit chars) it could only cause a maximum of 1 cache miss for every 16 elements. This is why the performance-savvy in languages such as Haskell and Lisp know to use vector-like constructs when possible.

Back in the world of Rust, this means that you should avoid indirection-heavy representations Vec<Vec<_>> to represent a matrix, since this means that each sub-vec will likely be in a different location. Use a data structure that uses a flat Vec as a backing array and builds on top of it, unless you really do need to have the inner Vecs be both jagged (each sub-vec is of different size) and growable (you can change the size of each sub-vec independently at runtime). You probably don't need either, let alone both. If you need them to be a uniform size, store a Vec and a dimension tuple, if you need them to be jagged but don't need them to be growable, store a list of indices and return slices of the flat Vec using those. For an example of why this is good, let's dive into some basic math. Let's assume a matrix backed by flat vector and a number of columns (the number of rows can be inferred from columns + data length):

// This isn't how Vec is defined in the standard library but it's a simplified
// version with the same memory layout.
struct Vec<T> {
    pointer: *mut T,
    capacity: usize,
    len: usize,
}

struct Matrix<T> {
    data: Vec<T>,
    num_columns: usize,
}

So a matrix with N rows and M columns needs N * M * size_of::<T>() space for the elements, plus size_of::<*mut T>() + 3 * size_of::<usize>() for the "metadata" (the vector's pointer and the capacity, length and num_columns fields). If we're on a 64-bit CPU with 64 byte cache lines like the i7, we end up with both *mut T and usize being 4 bytes each. If we had a 4x4 matrix of f32 (also 4 bytes in size) this would mean:

Metadata size = 4 + 3 * 4 = 16
Maximum 2 cache misses

Data size = 4 * 4 * 4 = 64
Maximum 2 cache misses

Since the metadata and the data could be in separate parts of memory, we have to calculate maximum cache misses separately. Both the metadata and the data could cross a cache line and require two cache misses to load. This means that the whole matrix would miss the cache 4 times in the worst case. If we had a Vec<Vec<f32>> representation that would mean the size of:

Matrix metadata size = 4 + 2 * 4 = 12
Maximum 2 cache misses

Inner vector metadata size = 4 * (4 + 2 * 4) = 48
Maximum 2 cache misses

Data size = 4 * 4 = 16
Maximum 2 cache misses per row (8 cache misses total)

This means that the Vec<Vec<f32>> representation could miss the cache up to 12 times to read the whole array, much worse than the flat representation.

Even better, if you statically know the matrix's size you can use statically-sized arrays like [[T; N]; N]. These are even cheaper than flat vectors, although you obviously can't use them for data with a variable size at runtime. The 4x4 array in the previous example would be [[f32; 4]; 4] and take up 64 bytes, meaning that it would only take 2 cache misses to load in the worst case.

Keep as much as possible in registers

Now, the absolute best place for your data - registers. The more work you can do without non-local writes the more that rustc and LLVM can assume about your data's access patterns. This is good because it means that data can be mapped to the CPU's physical registers, which are the fastest memory on your entire computer, but even better, if you make your data suitable for registers then anything that happens to that data can be aggressively optimized. Writes and reads through pointers have certain ordering restrictions on them that prevent optimization, but there are no such restrictions on register-allocated data.

It's worth noting that since Rust restricts pointers more than C does, the ordering restrictions on pointers could be relaxed. This hasn't been implemented in LLVM yet since most of the optimization work is based on leveraging the rules of C and C-family languages. Even if they did implement relaxations on the reordering rules, however, storing data in registers will still be easier to optimize.

So how do you get rustc to allocate things to registers? Essentially, the less pointers you have to write at runtime the better. Writing to local variables is better than writing through a mutable pointer. As much as possible, you should try to constrain mutable writes to the data that you have ownership over. So a mutable loop counter is fine, but passing a mutable reference to a loop counter through multiple layers of functions is not (unless they end up getting inlined, of course). This is really just an extension of one of my first points: clean, boring code is easier to optimize than spaghetti.

Avoid Box<Trait>

The canonical way to create trait objects is Box<Trait>, but the majority of code can get away with &mut Trait, which also has dynamic dispatch but saves an allocation. If you absolutely need ownership then use a Box, but most use-cases can use an &Trait or &mut Trait. Even better is to avoid using a trait object all together. impl Trait is the obvious way to avoid them, but that doesn't allow you to store a heterogenous collection of elements that implement a single trait since it's basically type inference in a fancy hat. A good trick for when you want to allow a variable but finite number of implementors of a type because you want to choose between them or iterate over them, use either a tuple or a recursive generic struct like this:

struct Cons<Head, Tail>(Head, Tail);

Since data structures in Rust don't add any indirection or space overhead, you can implement a trait for this structure recursively and have a function that can take any number of parameters that runs as fast as an equivalent function that takes a fixed number of parameters. Here's an example of how this could look for a function that takes a list of functions and calls them:

Allocating version:

fn call_all_fns(fns: Vec<Box<FnBox() -> ()>>) {
    for f in fns {
        f();
    }
}

Allocation-free version:

struct Cons<First, Second>(First, Second);

trait HCons: Sized {
    fn cons<T>(self, other: T) -> Cons<Self, T> {
        Cons(self, other)
    }
}

impl<T: Sized> HCons for T {}

// This is a hack to get around the fact that manually implementing the `Fn`
// traits is currently unstable.
trait Callable {
    fn call(self);
}

impl<F: Fn() -> ()> Callable for F {
    fn call(self) { self() }
}

impl<First: Callable, Second: Callable> Callable for Cons<First, Second> {
    fn call(self) {
        self.0.call();
        self.1.call();
    }
}

fn call_all_fns_no_alloc<T: Callable>(fns: T) {
    fns.call();
}

Here's what they both look like in use:

fn main() {
    let first_fn = || { println!("Hello!"); };
    let second_fn = || { println!("World!"); };
    
    call_all_fns(vec![Box::new(first_fn), Box::new(second_fn)]);
    
    let first_fn = || { println!("Hello!"); };
    let second_fn = || { println!("World!"); };
    
    call_all_fns_no_alloc(first_fn.cons(second_fn));
}

The functions passed to call_all_fns_no_alloc are eligible for inlining, they require no space overhead, and their instructions and data are directly next to each other in memory and are therefore much faster to access than if each of them were boxed. For example, in combine there's a choice function that takes an array that could contain trait objects, but it also supplies a .or() combinator (and a choice! macro that expands to recursive .or calls) that returns an Or<A, B> that in turn implements Parser. This means that dispatch is static and the objects are all stored in order in memory (because it's just a set of recursive structs). You will still need dynamic dispatch for some cases, but using this method means that the number of cases where this is necessary is very small.

Use stack-based variable-length datatypes

Fixed-length datatypes are trivially storable on the stack, but for dynamically-sized data it's not so simple. However, smallvec, smallstring and tendril are all variable-length datatypes that allow you to store small numbers of elements on the stack (shameless plug: smallstring was written by me). Due to the law of small numbers, you are very likely to have more of these small strings than larger ones. This is good because it reduces allocation, but it's great if you're storing these in a Vec or HashMap, since you will have less indirection and therefore better cache use. A good rule of thumb is to never have more than one layer of pointers to dereference before you reach your value (NASA enforces this rule in their C code, albeit for reliability and not performance).

Libraries like smallvec are great for cache locality, since an array of SmallVec<[T; 4]> will have exactly the same cache-locality as an array of just T - as long as the length of each SmallVec is below 8 it just gets stored in order. Going back to the cache-miss calculations from earlier:

// This is a gross oversimplification of how this type is implemented in the
// crate, but it's enough to explain how it works.
enum SmallVec<T> {
    Small([T; 4]),
    Big(Vec<T>),
}

type Matrix<T> = SmallVec<SmallVec<T>>;

As long as there are less than or equal to 4 elements in the SmallVec, the size of each instance is the size of the data plus the size of the tag, which is:

let size_of_data = size_of::<T>() * 4;
let size_of_tag  = max(size_of::<u8>(), align_of::<T>());
size_of_data + size_of_tag

The obvious question is why the size of the tag isn't just size_of::<u8>(). This is because if T was more than 1 byte in size, this would mean that all of the elements would all be unaligned by 1 byte, which is bad. CPUs work much slower on unaligned data, but unless you write a compiler you will never have to think about that. The size of the data and its alignment don't have to be the same. For structs, for example, the alignment is typically the largest alignment of any of its members. For primitive types like pointers, integers and floats the alignment is the same as its size. The alignment and size of an f32 are both 4. The alignment of a SmallVec<f32> is the largest alignment of its members, which is same as the alignment of [f32; 4], which is the same as the alignment of f32: 4.

Consider we had a 4x4 matrix of f32, this would mean that the size of the matrix would be:

Inner SmallVec size = 4 * 4 + 4
Matrix size = 4 * (4 * 4 + 4) + 4 = 84
Maximum 3 cache misses

We don't need to calculate the inner and outer cache misses seperately because they are guaranteed to be next to each other in memory.

From a cache standpoint this is as good as the flat vector representation, but there's nothing stopping you from accidentally making the inner vectors different lengths and breaking the invariant that an array's rows should be the same length.

I want to make something clear: you will never do these calculations in the process of optimizing your program. This is merely some mathematical justification for the voodoo folklore that "allocation is bad", since that is often countered by "malloc is fast". Both statements are true - the actual process of allocating and deallocating memory is fast, but data structures that allocate are worse for use-cases that require maximum speed.

Loop unrolling is still cool

Duff's device is fun, but array-length-generic unrolled loops are unlikely to be faster than the equivalent optimized naïve code nowadays, since any optimizing compiler worth its bits will do this kind of optimization without having to mangle your code and ruining future-you's day.

Having said that, if you know that an array is likely to be a multiple of N size, try making it a &[[T; N]] and operating on a [T; N] in each iteration. This reduces the number of iterations (and therefore, the number of times you need to recalculate the loop variables) and allows the compiler to operate more aggressively on the loop body.

You can also use more classical loop unrolling if it allows you to reduce the "strength" of your operations. This means that if you have to calculate some value for each iteration of the loop and calculating this value takes longer than the body itself, manually unroll the body so you can calculate it less. Example: you can implement an integer logarithm function like so:

fn log_base(mut n: usize, base: usize) -> usize {
    let mut out = 1;

    loop {
        if n < base { return out; }

        out += 1;
        n /= base;
    }
}

However, n /= base; out += 1; is slower to calculate than n < base. To take advantage of this fact, you can unroll the loop like so:

fn log_base_unrolled(mut n: usize, base: usize) -> usize {
    const UNROLL_COUNT: usize = 4;

    // We use a fixed-size array to ensure that we don't get the array count and
    // the `out` skip value out of sync.
    let premultiplied_base: [_; UNROLL_COUNT] = [
        base,
        base * base,
        base * base * base,
        base * base * base * base,
    ];

    let mut out = 1;
    
    loop {
        if n < premultiplied_base[0] { return out; }
        if n < premultiplied_base[1] { return out + 1; }
        if n < premultiplied_base[2] { return out + 2; }
        if n < premultiplied_base[3] { return out + 3; }
        
        n /= precalculated_base[UNROLL_COUNT - 1];
        out += UNROLL_COUNT;
    }
}

Here are the benchmarks I used:

#[bench]
fn bench_log_base(b: &mut Bencher) {
    b.iter(|| {
        let input = black_box(5000000120510250);

        assert_eq!(log_base(input, 10), 16);
    });
}

#[bench]
fn bench_log_base_unrolled(b: &mut Bencher) {
    b.iter(|| {
        let input = black_box(5000000120510250);

        assert_eq!(log_base(input, 10), 16);
    });
}

test::black_box is a magic function that prevents rustc and LLVM calculating those function calls at compile-time and converting them into a constant, which usually they would (actually, it's not magic, it's just some inline assembly that doesn't do anything, since neither rustc nor LLVM will try to optimize anything that's been accessed by inline assembly).

This gives the following results:

test bench_log_base          ... bench:  18 ns/iter (+/- 0)
test bench_log_base_unrolled ... bench:   5 ns/iter (+/- 0)

Wait a minute, though, what happens when we give a non-constant value for base?

#[bench]
fn bench_log_base_nonconstbase(b: &mut Bencher) {
    b.iter(|| {
        let input = black_box(5000000120510250);
        let base = black_box(10);

        assert_eq!(log_base(input, base), 16);
    });
}

#[bench]
fn bench_log_base_unrolled_nonconstbase(b: &mut Bencher) {
    b.iter(|| {
        let input = black_box(5000000120510250);
        let base = black_box(10);

        assert_eq!(log_base_unrolled(input, base), 16);
    });
}
test bench_log_base_unrolled_nonconstbase ... bench:  37 ns/iter (+/- 1)
test bench_log_base_nonconstbase          ... bench: 199 ns/iter (+/- 5)

They're both much slower! Can we do better? Turns out yes, we can:

fn log_base_increasing(n: usize, base: usize) -> usize {
    const UNROLL_COUNT: usize = 4;

    let premultiplied_base: [_; UNROLL_COUNT] = [
        base,
        base * base,
        base * base * base,
        base * base * base * base,
    ];

    if n < premultiplied_base[0] { return 1; }
    if n < premultiplied_base[1] { return 2; }
    if n < premultiplied_base[2] { return 3; }
    if n < premultiplied_base[3] { return 4; }

    let mut out = UNROLL_COUNT + 1;
    let mut mul = premultiplied_base[UNROLL_COUNT - 1];

    loop {
        if n < premultiplied_base[0] * mul { return out; }
        if n < premultiplied_base[1] * mul { return out + 1; }
        if n < premultiplied_base[2] * mul { return out + 2; }
        if n < premultiplied_base[3] * mul { return out + 3; }

        mul *= premultiplied_base[UNROLL_COUNT - 1];
        out += UNROLL_COUNT;
    }
}

#[bench]
fn bench_log_base_increasing(b: &mut Bencher) {
    b.iter(|| {
        let input = black_box(5000000120510250);

        assert_eq!(log_base_increasing(input, 10), 16);
    });
}

#[bench]
fn bench_log_base_increasing_nonconstbase(b: &mut Bencher) {
    b.iter(|| {
        let input = black_box(5000000120510250);
        let base = black_box(10);

        assert_eq!(log_base_increasing(input, base), 16);
    });
}

Let's check out the results now:

test bench_log_base                         ... bench:  18 ns/iter (+/- 0)
test bench_log_base_nonconstbase            ... bench: 199 ns/iter (+/- 5)

test bench_log_base_unrolled                ... bench:   5 ns/iter (+/- 0)
test bench_log_base_unrolled_nonconstbase   ... bench:  37 ns/iter (+/- 1)

test bench_log_base_increasing              ... bench:   6 ns/iter (+/- 0)
test bench_log_base_increasing_nonconstbase ... bench:   8 ns/iter (+/- 1)

Turns out the compiler was doing something sneaky: it can optimize integer division by a constant into a multiplication combined with a shift. When it could no longer fold the constant into the function it slowed down considerably. It's ok to rely on const-folding if it allows you to gain considerable speedups and you know that the function will usually be called with constant arguments, but be careful. The things to look out for are if statements and integer division, both of which can be much slower with non-constant values compared to constants.

The fastest method by far converts to an f64, calls .log(base) on that, and then converts back. It doesn't work for large numbers, however, because of loss of precision. This is probably a good time to note that although adding and multiplying integers is faster than doing the same for floats, for code that does division by a non-constant value or something more complex like trigonometry, you should definitely use floats. The compiler can't do the conversion for you - it won't apply optimizations that make your code less precise - but you can check for areas where this would be an improvement and make the change manually.

assert! conditions beforehand

If you want to reduce the number of implicit asserts that get compiled into the code, then instead of this:

fn do_something_with_array(array: &[u8]) -> u8 {
    array[0] + array[1] + array[2] + array[3] + array[4] + array[5]
}

Do this:

fn do_something_with_array(array: &[u8]) -> u8 {
    assert!(array.len >= 5);
    array[0] + array[1] + array[2] + array[3] + array[4] + array[5]
}

This allows LLVM to realize that the later asserts are unreachable and elides them. This is useful for any code that may assert multiple different qualities about the same data, but is especially useful for indexing since we know that if array[n] succeeds then array[n - 1] will succeed too. This is similar to the point about fixed-length arrays in the previous section.

Essentially, try to consolidate checks into a single assert!. This means that the later checks become statically unreachable. If LLVM/Rust still don't optimize it away you can switch to using the unsafe indexing methods while ensuring that they're still safe. This tip is shamelessly stolen from a comment on /r/rust.

Use link-time optimization

Normally, Rust can only inline functions that are either defined in-crate or, in the case of functions in other libraries, have #[inline] specified. LTO allows the compiler to inline cross-crate, at the cost of a compile-time speed penalty. I am of the opinion that compile times only matter for debug builds, so that's a tradeoff I'm willing to make. As with everything else here, profile and check that the tradeoff is worthwhile.

Don't use #[inline(always)]

#[inline(always)] feels good as a performance hint, but the truth is that optimizing compilers are really good at working out when a function would benefit from being inlined, and Rust isn't constrained to the slower standardized C calling convention and can use fastcc, making function calls extremely cheap. You're more likely to cause the size of your executable to bloat. This takes up more space on your hard drive, of course, but that's not too much of a problem. If you have even a single bundled asset like images or audio they will likely dwarf the size of your executable.

The real issue here is that it can make your program no longer fit in the CPU's instruction cache. The CPU will only have to go to RAM for its instructions when functions are called with instructions outside of the current cache line. The larger your binary is, the more likely this is, and the more functions are inlined, the larger your binary is. It's not the end of the world to have a large binary, but unless you're really certain that something will be improved by manually marking it as inline, and you have benchmarks to back that up, it's just as likely that you'll slow a program down with careless inlining than to speed it up.

So now I've scared you off inlining, let's talk about when you should explicitly add inlining annotations. Small functions that are called often are a good target for inlining. Iterator::next, for example, or Deref::deref. The overhead from calling these functions may be larger than the time it takes to run the function itself. These are likely to be automatically inlined when called internally, but marking these as #[inline] will allow users of your library to inline them too, even if they don't use LTO. Only functions marked #[inline] will be considered for cross-crate inlining, but that means the definition has to be stored in the compiled library, causing bloat and increasing compile times. #[inline(always)] is even more niche, but it's sometimes nice to ensure that a tiny function will be inlined, or as a kind of documentation that the function call is free for if someone comes along and tries to manually inline it to improve performance. It really is very rare that you would want to do this, though, and it's best to just trust the compiler.

The other class of functions that are good targets for annotating inlining are ones that you know to often be called with constant parameters. We go into this later on, but {integer}::from_str_radix is an exellent example of this. Most uses of this function will have a constant as the second parameter, and so by judicious use of #[inline] we can prevent branching and expensive operations like division for the consumers of our library. It's not worth losing sleep over though, since they could just use link-time optimization if they need to squeeze out every last drop of performance.

Also, the compiler does really get it wrong sometimes, and can miss out on inlining opportunities that would improve code speed. However, only add #[inline(always)] annotation if you can prove with benchmarks that it improves the speed, and adding these annotations is a bit of a dark art. You effort is probably better spent elsewhere.

If you want to reduce the size of your code, you can try using panic = "abort". This removes the "landing pads" that allow Rust to show a nice stack trace after a panic, and causes any panic to end the program instantly. I have legitimately seen non-trivial speedups on benchmarks for the ion shell after adding this option to the release build, and I can only attribute it to making more code fit in the instruction cache. I have not tried it with many other programs, but it would probably only affect medium to large projects. Try it out on your codebase, it's as easy as adding one line to the Cargo.toml and it may improve your code's speed.

Parallelize, but not how you think

There's an absolutely amazing library for Haskell called Haxl, that automatically tracks the data dependencies of your network requests and batches them and runs them asynchronously as long as they don't overlap. It's something that shows the power of computational abstractions like monads and it's not something that has a, ahem, parallel in any other language, as far as I know. At least, not for IO. We've had this exact ability in the CPU for a long, long time. The CPU tracks the data dependencies of computations and will parallelize them wherever possible.

The reason data dependencies matter is that the CPU doesn't just execute one instruction at a time. As long as two instructions don't share a register they can safely be run simultaneously, so the CPU does so. This is essentially free parallelism without the need for locks, work queues or anything that affects your architecture at all, so you would be crazy not to take advantage of it.

Parallelizable computation also lends itself well to autovectorization, which is the process where the compiler realizes that you're doing the same thing to multiple different values and converts it to a special instruction that, well, does the same thing to multiple different values.

For example, the compiler could translate the following numerical code:

(a1 + a2) + (b1 + b2) + (c1 + c2) + (d1 + d2)

into just one instruction that executes all four subexpressions as fast as a single addition.

%intermediate-value = add-vectors [%a1 %b1 %c1 %d1] [%a2 %b2 %c2 %d2]
sum-parts %intermediate-value

A case study

Let's write a version of usize::from_str_radix that's about 30% faster than the one in the standard library.

// We're redefining these here since they're private in the stdlib
#[derive(Debug, Clone, PartialEq, Eq)]
struct ParseIntError {
    kind: IntErrorKind,
}

#[derive(Debug, Clone, PartialEq, Eq)]
enum IntErrorKind {
    Empty,
    InvalidDigit,
    Overflow,
    Underflow,
}

#[inline]
fn from_str_radix(input: &str, radix: usize) -> Result<usize, ParseIntError> {
    fn to_digit_ascii(ascii: u8, radix: usize) -> Result<usize, ParseIntError> {
        let decimal_digit = ascii.wrapping_sub(b'0');

        if radix > 10 && decimal_digit > 9 {
            let out = (ascii | 32).wrapping_sub(b'a') as usize;

            if out > radix - 10 {
                Err(ParseIntError { kind: IntErrorKind::InvalidDigit })
            } else {
                Ok(out + 10)
            }
        } else {
            let decimal_digit = decimal_digit as usize;
            if decimal_digit > radix {
                Err(ParseIntError { kind: IntErrorKind::InvalidDigit })
            } else {
                Ok(decimal_digit)
            }
        }
    }

    if radix > 36 {
        panic!("from_str_radix: radix is too high (maximum 36)");
    }

    let bytes = input.as_bytes();

    if bytes.len() == 0 {
        return Err(
            ParseIntError { kind: IntErrorKind::Empty }
        );
    }

    let bytes = match bytes[0] {
        b'+' => { &bytes[1..] },
        b'-' => { return Err(ParseIntError { kind: IntErrorKind::Underflow }) },
        _ => bytes,
    };

    let mut mul = radix;
    let mut index = bytes.len() - 1;

    let mut output = to_digit_ascii(bytes[index], radix)?;

    for &byte in bytes[..index].iter().rev() {
        let digit = to_digit_ascii(byte, radix)?;

        let next_output = output.wrapping_add(digit * mul);

        if output > next_output {
            return Err(
                ParseIntError { kind: IntErrorKind::Overflow }
            );
        }

        mul *= radix;
        output = next_output;
    }

    Ok(output)
}

I explicitly use wrapping_* functions not for optimization purposes (because overflow checks are removed at runtime), but because overflow is required for correct behaviour. You'll notice some optimizations here:

  • We start at the end and work backwards, keeping a "mul" counter. Originally I wrote a version of this that works forwards and multiplies output by radix each loop but the backwards method is 10% faster. This seems to be due to better instruction-level parallelism. The multiplications can be parallelized and only the addition relies on the previous iteration's value for output, and addition is much faster.

    Any folding operations can be improved in this way by exploiting the algebraic laws (in this case, the distributive law) to improve the number of operations that can be done in parallel.

  • We rely on overflow to test A < x < B comparisons. This is only useful here because we already need the x - A value, so we're saving an extra comparison and a bitwise and. In most code A < x && x < B is as cheap or cheaper than x - A < B with overflow.

  • We use | 32 to unify the codepaths for upper- and lowercase letters, reducing the number of comparisons we need to do.

  • We don't do output + digit * mul when output == 0 and mul == 1. This seems to be consistently 1ns faster, but it's possible that this doesn't make a difference and the 1ns speedup I'm seeing is pure luck. I reran the benchmarks both with and without the change multiple times and saw a consistent difference but this doesn't rule out luck. This is the problem with microbenchmarks, when the difference becomes small enough you can't tell whether you're really making it faster.

  • We use Rust's safe iterator interface, which is as fast as the C idiom of storing a "start" and "end" pointer so you can check whether the loop is finished with a simple ==. If you ever hear someone say "Rust's safety guarantees are useless because you need to drop down to unsafe to get any real speed" (I've seen this almost verbatim on Hacker News before) you can show them this.

  • We don't rely on const-folding in order to make our code fast, but it does run faster with a constant value for radix. Therefore, we add #[inline] to allow downstream crates to apply const-folding too.

The method I used for this is the basic method you should use for any optimization work: write a representative benchmark and then progressively tweak and rerun benchmarks until you can't shave off any more cycles. Doing this for pure functions is much easier, so one of the first things you should do to optimize any function that's called in a tight loop is to make it pure. This avoids indirect writes and reads (reads and writes to places in memory that are likely to be outside cache lines) and makes benchmarking much, much easier. If you use test-driven development for reliability, this is the equivalent for performance.

Extending this to work on signed integer types is an exercise for the reader. Tip: unlike C, you can rely on signed integers overflowing with 2's complement arithmetic.

Here are the functions I used to benchmark the code:

#[bench]
fn bench_from_str(b: &mut Bencher) {
    b.iter(|| {
        let input = black_box("1235112512");
        assert_eq!(from_str_radix(input, 10), Ok(1235112512));
        let input = black_box("FFaf125A");
        assert_eq!(from_str_radix(input, 16), Ok(0xFFaf125A));
    });
}

#[bench]
fn bench_from_str_native(b: &mut Bencher) {
    b.iter(|| {
        let input = black_box("1235112512");
        assert_eq!(usize::from_str_radix(input, 10), Ok(1235112512));
        let input = black_box("FFaf125A");
        assert_eq!(usize::from_str_radix(input, 16), Ok(0xFFaf125A));
    });
}

#[bench]
fn bench_from_str_nonconstradix(b: &mut Bencher) {
    b.iter(|| {
        let input = black_box("1235112512");
        let radix = black_box(10);
        assert_eq!(from_str_radix(input, radix), Ok(1235112512));
        let input = black_box("FFaf125A");
        let radix = black_box(16);
        assert_eq!(from_str_radix(input, radix), Ok(0xFFaf125A));
    });
}

#[bench]
fn bench_from_str_native_nonconstradix(b: &mut Bencher) {
    b.iter(|| {
        let input = black_box("1235112512");
        let radix = black_box(10);
        assert_eq!(usize::from_str_radix(input, radix), Ok(1235112512));
        let input = black_box("FFaf125A");
        let radix = black_box(16);
        assert_eq!(usize::from_str_radix(input, radix), Ok(0xFFaf125A));
    });
}

#[bench]
fn bench_from_str_1char(b: &mut Bencher) {
    b.iter(|| {
        let input = black_box("1");
        assert_eq!(from_str_radix(input, 10), Ok(1));
        let input = black_box("F");
        assert_eq!(from_str_radix(input, 16), Ok(0xF));
    });
}

#[bench]
fn bench_from_str_native_1char(b: &mut Bencher) {
    b.iter(|| {
        let input = black_box("1");
        assert_eq!(usize::from_str_radix(input, 10), Ok(1));
        let input = black_box("F");
        assert_eq!(usize::from_str_radix(input, 16), Ok(0xF));
    });
}

Results:

test bench_from_str                      ... bench:          22 ns/iter (+/- 7)
test bench_from_str_native               ... bench:          36 ns/iter (+/- 0)
test bench_from_str_nonconstradix        ... bench:          26 ns/iter (+/- 0)
test bench_from_str_native_nonconstradix ... bench:          39 ns/iter (+/- 0)
test bench_from_str_1char                ... bench:           5 ns/iter (+/- 0)
test bench_from_str_native_1char         ... bench:          13 ns/iter (+/- 0)

Something I've noticed with benchmarks below 1ms is that it can take some time to "spin up" the CPU. Occasionally the first benchmark in a set will take 20-30ns longer than the ones after it. If you duplicate the benchmark verbatim and take the number with the lowest variance this avoids the issue. I think this is due to the CPU needing to gather information in order to do proper branch prediction. Ideally you'd just not do micro-benchmarks, but some functions do legitimately call for it. Don't trust a benchmark, especially a microbenchmark, until you've rerun it multiple times.

When I reran this particular benchmark (at least 10 times in total, not including the benchmarks I ran while editing the code) to ensure that the numbers were stable, and although the averages are extremely stable (the native one sometimes was slightly slower, the 36ns value above is what I see most of the time), the variances are mostly 0-3ns with spikes of 13-26ns. I don't have a good explanation for this, expect a follow-up post with tips on writing better benchmarks.

This is a perfect example of why low-level optimization is important, since this is exactly the kind of function that could be used hundreds of thousands of times in parsers of textual data and a 10ns speedup here could lead to meaningful improvements over the life of the program. It's also an example of why you should avoid low-level optimization when possible. The original stdlib implementation of this function isn't the most idiomatic code, but it's significantly more readable than this. Having said that, though, it's a testament to Rust's commitment to zero-cost abstractions that you can write mostly-idiomatic, safe code and have it perform as well as equivalent C/C++ code that would require use of unsafe pointer arithmetic.

Wrapping up

If I had to sum the trick to optimization up in a pithy QotW-ready snippet it would be this:

The fastest code is code that doesn't run at all, the second-fastest code is code that never stops running.

Ideally, you want to do less work, and if you're doing the minimum amount of work you want to reduce the amount of time the CPU spends waiting around.

If you want more Rustic performance tips with more numbers I would consider BurntSushi's excellent post-mortem of ripgrep required reading for anyone wanting to write fast software (it was the thing that originally sent me down this deep, deep rabbit hole). For more general systems language-y points, check out Andrei Alexandrescu's talk "Fastware", from which the from_str_radix and log_base code was adapted. A lot of the points in this article are expansions upon points behind one of those two links.

I hope that whether you're a soft-shell Rustacean or a grizzled veteran, this has given you a better sense of when some code may be poorly performing, and what to do about it. Go make things go vroom.

bjorn3 commented May 18, 2017

This removes the "landing pads" that allow Rust to show a nice stack trace after a panic, and causes any panic to end the program instantly.

The landing pads are not used for printing a stack trace, but to unwind the stack (call drop impl for all values on the stack). stack traces are still shown with panic=abort.

hluk commented May 20, 2017

on a 64-bit CPU ... we end up with both *mut T and usize being 4 bytes each

I think for a 64-bit CPU this should be 8 bytes (at least for the pointer).

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