Allocations in Rust: Global memory

February 5, 2019 · 8 min read

Bradlee Speice

The first memory type we'll look at is pretty special: when Rust can prove that a value is fixed for the life of a program (const), and when a reference is unique for the life of a program (static as a declaration, not 'static as a lifetime), we can make use of global memory. This special section of data is embedded directly in the program binary so that variables are ready to go once the program loads; no additional computation is necessary.

Understanding the value/reference distinction is important for reasons we'll go into below, and while the full specification for these two keywords is available, we'll take a hands-on approach to the topic.

`const` values

When a value is guaranteed to be unchanging in your program (where "value" may be scalars, structs, etc.), you can declare it const. This tells the compiler that it's safe to treat the value as never changing, and enables some interesting optimizations; not only is there no initialization cost to creating the value (it is loaded at the same time as the executable parts of your program), but the compiler can also copy the value around if it speeds up the code.

The points we need to address when talking about const are:

Const values are stored in read-only memory - it's impossible to modify.
Values resulting from calling a const fn are materialized at compile-time.
The compiler may (or may not) copy const values wherever it chooses.

Read-Only

The first point is a bit strange - "read-only memory." The Rust book mentions in a couple places that using mut with constants is illegal, but it's also important to demonstrate just how immutable they are. Typically in Rust you can use interior mutability to modify things that aren't declared mut. RefCell provides an example of this pattern in action:

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, interior mutability is impossible:

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

When the const specification refers to "rvalues", this behavior is what they refer to. Clippy will treat this as an error, but it's still something to be aware of.

Initialization

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 (with a fixed address, so your CPU may be able to prefetch the value), but that's it.

use std::cell::RefCell;

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

pub fn multiply(value: u32) -> u32 {
    // CELL is stored at `.L__unnamed_1`
    value * (*CELL.get_mut())
}

-- Compiler Explorer

The compiler creates one RefCell, uses it everywhere, and never needs to call the RefCell::new function.

Copying

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

const FACTOR: u32 = 1000;

pub fn multiply(value: u32) -> u32 {
    // See assembly line 4 for the `mov edi, 1000` instruction
    value * FACTOR
}

pub fn multiply_twice(value: u32) -> u32 {
    // See assembly lines 22 and 29 for `mov edi, 1000` instructions
    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 inline into the assembly instructions.

Finally, getting the address of a const value is possible, but not guaranteed to be unique (because the compiler can choose to copy values). I was unable to get non-unique pointers in my testing (even using different crates), but the specifications are clear enough: don't rely on pointers to const values being consistent. To be frank, caring about locations for const values is almost certainly a code smell.

`static` values

Static variables are related to const variables, but take a slightly different approach. When we declare that a reference is unique for the life of a program, you have a static variable (unrelated to the 'static lifetime). Because of the reference/value distinction with const/static, static variables behave much more like typical "global" variables.

But to understand static, here's what we'll look at:

static variables are globally unique locations in memory.
Like const, static variables are loaded at the same time as your program being read into memory.
All static variables must implement the Sync marker trait.
Interior mutability is safe and acceptable when using static variables.

Memory Uniqueness

The single biggest difference between const and static is the guarantees provided about uniqueness. Where const variables may or may not be copied in code, static variables are guarantee to be unique. If we take a previous const example and change it to static, the difference should be clear:

static FACTOR: u32 = 1000;

pub fn multiply(value: u32) -> u32 {
    // The assembly to `mul dword ptr [rip + example::FACTOR]` is how FACTOR gets used
    value * FACTOR
}

pub fn multiply_twice(value: u32) -> u32 {
    // The assembly to `mul dword ptr [rip + example::FACTOR]` is how FACTOR gets used
    value * FACTOR * FACTOR
}

-- Compiler Explorer

Where previously there were plenty of references to multiplying by 1000, the new assembly refers to FACTOR as a named memory location instead. No initialization work needs to be done, but the compiler can no longer prove the value never changes during execution.

Initialization

Next, let's talk about initialization. The simplest case is initializing static variables with either scalar or struct notation:

#[derive(Debug)]
struct MyStruct {
    x: u32
}

static MY_STRUCT: MyStruct = MyStruct {
    // You can even reference other statics
    // declared later
    x: MY_VAL
};

static MY_VAL: u32 = 24;

fn main() {
    println!("Static MyStruct: {:?}", MY_STRUCT);
}

-- Rust Playground

Things can get a bit weirder when using const fn though. In most cases, it just works:

#[derive(Debug)]
struct MyStruct {
    x: u32
}

impl MyStruct {
    const fn new() -> MyStruct {
        MyStruct { x: 24 }
    }
}

static MY_STRUCT: MyStruct = MyStruct::new();

fn main() {
    println!("const fn Static MyStruct: {:?}", MY_STRUCT);
}

-- Rust Playground

However, there's a caveat: you're currently not allowed to use const fn to initialize static variables of types that aren't marked Sync. For example, RefCell::new() is a const fn, but because RefCell isn't Sync, you'll get an error at compile time:

use std::cell::RefCell;

// error[E0277]: `std::cell::RefCell<u8>` cannot be shared between threads safely
static MY_LOCK: RefCell<u8> = RefCell::new(0);

-- Rust Playground

It's likely that this will change in the future though.

The `Sync` marker

Which leads well to the next point: static variable types must implement the Sync marker. Because they're globally unique, it must be safe for you to access static variables from any thread at any time. Most struct definitions automatically implement the Sync trait because they contain only elements which themselves implement Sync (read more in the Nomicon). This is why earlier examples could get away with initializing statics, even though we never included an impl Sync for MyStruct in the code. To demonstrate this property, Rust refuses to compile our earlier example if we add a non-Sync element to the struct definition:

use std::cell::RefCell;

struct MyStruct {
    x: u32,
    y: RefCell<u8>,
}

// error[E0277]: `std::cell::RefCell<u8>` cannot be shared between threads safely
static MY_STRUCT: MyStruct = MyStruct {
    x: 8,
    y: RefCell::new(8)
};

-- Rust Playground

Interior mutability

Finally, while static mut variables are allowed, mutating them is an unsafe operation. If we want to stay in safe Rust, we can use interior mutability to accomplish similar goals:

use std::sync::Once;

// This example adapted from https://doc.rust-lang.org/std/sync/struct.Once.html#method.call_once
static INIT: Once = Once::new();

fn main() {
    // Note that while `INIT` is declared immutable, we're still allowed
    // to mutate its interior
    INIT.call_once(|| println!("Initializing..."));
    // This code won't panic, as the interior of INIT was modified
    // as part of the previous `call_once`
    INIT.call_once(|| panic!("INIT was called twice!"));
}

-- Rust Playground

const values​

Read-Only​

Initialization​

Copying​

static values​

Memory Uniqueness​

Initialization​

The Sync marker​

Interior mutability​

`const` values

Read-Only

Initialization

Copying

`static` values

Memory Uniqueness

Initialization

The `Sync` marker

Interior mutability