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post	Allocations in Rust	An introduction to the memory model		rust

There's an alchemy of distilling complex technical topics into articles and videos that change the way programmers see the tools they interact with on a regular basis. I knew what a linker was, but there's a staggering amount of complexity in between main() and your executable. Rust programmers use the Box type all the time, but there's a rich history of the Rust language itself wrapped up in how special it is.

In a similar vein, I want you to look at code and understand how memory is used; the complex choreography of operating system, compiler, and program that frees you to focus on functionality far-flung from frivolous book-keeping. The Rust compiler relieves a great deal of the cognitive burden associated with memory management, but we're going to step into its world for a while.

Let's learn a bit about memory in Rust.

Table of Contents

This post is intended as both guide and reference material; we'll work to establish an understanding of the different memory types Rust makes use of, then summarize each section for easy citation in the future. To that end, a table of contents is provided to assist in easy navigation:

• Foreword
• Stacking Up: Non-Heap Memory Types
• Piling On: Rust and the Heap
• Compiler Optimizations Make Everything Complicated
• Summary: When Does Rust Allocate?
• Appendix and Further Reading

Foreword

There's a simple checklist to see if you can skip over reading this article. You must:

1. Only write #![no_std] crates
2. Never use unsafe
3. Never use #![feature(alloc)]

For some uses of Rust, typically embedded devices, these constraints make sense. They're working with very limited memory, and the program binary size itself may significantly affect what's available! There's no operating system able to manage this "virtual memory" junk, but that's not an issue because there's only one running application. The embedonomicon is ever in mind, and interacting with the "real world" through extra peripherals is accomplished by reading and writing to exact memory addresses.

Most Rust programs find these requirements overly burdensome though. C++ developers would struggle without access to std::vector (except those hardcore no-STL guys), and Rust developers would struggle without std::vec. But in this scenario, std::vec is actually part of the alloc crate, and thus off-limits (because the alloc crate requires #![feature(alloc)]). Also, Box is right out for the same reason.

Whether writing code for embedded devices or not, the important thing in both situations is how much you know before your application starts about what its memory usage will look like. In embedded devices, there's a small, fixed amount of memory to use. In a browser, you have no idea how large google.com's home page is until you start trying to download it. The compiler uses this information (or lack thereof) to optimize how memory is used; put simply, your code runs faster when the compiler can guarantee exactly how much memory your program needs while it's running. This post is all about understanding the optimization tricks the compiler uses, and how you can help the compiler and make your programs more efficient.

Now let's address some conditions and caveats before going much further:

• We'll focus on "safe" Rust only; unsafe lets you use platform-specific allocation API's (think the [libc] and [winapi] implementations of [malloc]) that we'll ignore.
• We'll assume a "debug" build of Rust code (what you get with cargo run and cargo test) and address (hehe) "release" mode at the end (cargo run --release and cargo test --release).
• Because of the nature of the content, some (very simple) assembly-level code is involved. We'll keep this to a minimum, but I needed a refresher on the push and pop instructions while writing this post.

And a final warning worth repeating:

Rust does not currently have a rigorously and formally defined memory model.

-- the docs

Stacking Up: Non-Heap Memory Types

We'll start with the "happy path": what happens when Rust is able to figure out at compile time how much memory will be used in your program.

This is important because of the extra optimizations Rust uses when it can predict how much memory is needed! Let's go over a quick example:

const MICROS_PER_MILLI: u32 = 1000;
const NANOS_PER_MICRO: u32 = 1000;

pub fn millis_to_nanos(millis: u32) -> u32 {
    let micros = millis * MICROS_PER_MILLI;
    let nanos = micros * NANOS_PER_MICRO;

    return nanos;
}

-- Compiler Explorer

Forgive the overly simple code, but this shows off what the compiler can figure out about your program:

1. There's one u32 passed to the function, and two u32's used in the function body. Each one is 4 bytes, for a total of 12 bytes. We can temporarily reserve space for all variables because we know exactly how much space is needed.
   • If you're looking at the assembly: millis is stored in edi, micros is stored in eax, and nanos is stored in ecx. The eax register is re-used to store the final result.
2. Because MICROS_PER_MILLI and NANOS_PER_MICRO are constants, the compiler never allocates memory, and just burns the constants into the final program.
   • Look for the instructions mov edi, 1000 and mov ecx, 1000.

Given this information, the compiler can efficiently lay out your memory usage so that the program never needs to ask the kernel/allocator for memory! This example was a bit silly though, so let's talk about the more interesting details.

static and const: Program Allocations

The first memory type we'll look at is pretty special: when Rust can prove that a reference is valid for the lifetime of the program (static, not specifically 'static), and when a value is the same for the lifetime of the program (const). Understanding the distinction between reference and value is important for reasons we'll go into below. The full specification for these two memory types is available, but I'd rather take a hands-on approach to the topic.

const

The quick summary is this: const declares a read-only block of memory that is loaded as part of your program binary (during the call to exec(3)). Any const value resulting from calling a const fn is guaranteed to be materialized at compile-time (meaning that access at runtime will not invoke the const fn), even though the function is available at run-time as well. The compiler can choose to copy the constant value wherever it is deemed practical. Getting the address of a const value is legal, but not guaranteed to be the same even when referring to the same named identifier.

The first point is a bit strange - "read-only memory". Typically in Rust you can use "inner mutability" to modify things that aren't declared mut. RefCell provides an API to guarantee at runtime that some consistency rules are enforced:

use std::cell::RefCell;

fn my_mutator(cell: &RefCell<u8>) {
    // Even though we're given an immutable reference,
    // the `replace` method allows us to modify the inner value.
    cell.replace(14);
}

fn main() {
    let cell = RefCell::new(25);
    // Prints out 25
    println!("Cell: {:?}", cell);
    my_mutator(&cell);
    // Prints out 14
    println!("Cell: {:?}", cell);
}

-- Rust Playground

When const is involved though, modifications are silently ignored:

use std::cell::RefCell;

const CELL: RefCell<u8> = RefCell::new(25);

fn my_mutator(cell: &RefCell<u8>) {
    cell.replace(14);
}

fn main() {
    // First line prints 25 as expected
    println!("Cell: {:?}", &CELL);
    my_mutator(&CELL);
    // Second line *still* prints 25
    println!("Cell: {:?}", &CELL);
}

-- Rust Playground

And a second example using Once:

use std::sync::Once;

const SURPRISE: Once = Once::new();

fn main() {
    // This is how `Once` is supposed to be used
    SURPRISE.call_once(|| println!("Initializing..."));
    // Because `Once` is a `const` value, we never record it
    // having been initialized the first time, and this closure
    // will also execute.
    SURPRISE.call_once(|| println!("Initializing again???"));
}

-- Rust Playground

Clippy will treat this behavior as an error if attempted, but it's still something to be aware of.

The next thing to mention is that const values are loaded into memory as part of your program binary. Because of this, any const values declared in your program will be "realized" at compile-time; accessing them may trigger a main-memory lookup, but that's it.

use std::cell::RefCell;

const CELL: RefCell<u32> = RefCell::new(24);

pub fn multiply(value: u32) -> u32 {
    value * (*CELL.get_mut())
}

-- Compiler Explorer

The compiler only creates one RefCell, and uses it everywhere. However, that value is fully realized at compile time, and is fully stored in the .L__unnamed_1 section.

If it's helpful though, the compiler can choose to copy const values.

const FACTOR: u32 = 1000;

pub fn multiply(value: u32) -> u32 {
    value * FACTOR
}

pub fn multiply_twice(value: u32) -> u32 {
    value * FACTOR * FACTOR
}

-- Compiler Explorer

In this example, the FACTOR value is turned into the mov edi, 1000 instruction in both the multiply and multiply_twice functions; the "1000" value is never "stored" anywhere, as it's small enough to use directly.

Finally, getting the address of a const value is possible but not guaranteed to be unique (given that the compiler can choose to copy values). In my testing I was never able to get the compiler to copy a const value and get differing pointers, but the specifications are clear enough: don't rely on pointers to const values being consistent. To be frank, I have no idea why you'd ever care about a pointer to const.

static

Final note: thread_local!() is always a heap allocation.

push and pop: Stack Allocations

Example: Why doesn't Vec::new() go to the allocator?

Questions:

1. What is the "Push" instruction? Why do we like the stack?
2. How does Rust allocate arguments to the function?
3. How does Rust allocate variables created in the function but never returned?
4. How does Rust allocate variables created in the function and returned?
5. How do Option<> or Result<> affect structs?
6. How are arrays allocated?
7. Legal to pass an array as an argument?

Piling On - Rust and the Heap

Example: How to trigger a heap allocation

Questions:

1. Where do collection types allocate memory?
2. Does a Box<> always allocate heap?
   • Yes, with exception of compiler optimizations
3. Passing Box vs. genericizing/monomorphization
   • If it uses dyn Trait, it's on the heap.
4. Other pointer types? Do Rc<>/Arc<> force heap allocation?
   • Maybe? Part of the alloc crate, but should use qadapt to check

Compiler Optimizations Make Everything Complicated

Example: Compiler stripping out allocations of Box<>, Vec::push()

Appendix and Further Reading

Embedonomicon:

[libc]: CRATES.IO LINK [winapi]: CRATES.IO LINK [malloc]: MANPAGE LINK