speice.io/blog/2019-02-05-the-whole-world/index.mdx

---
slug: 2019/02/the-whole-world
title: "Allocations in Rust: Global memory"
date: 2019-02-05 12:00:00
authors: [bspeice]
tags: []
---

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`](https://doc.rust-lang.org/book/ch10-03-lifetime-syntax.html#the-static-lifetime) 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](https://github.com/rust-lang/rfcs/blob/master/text/0246-const-vs-static.md) for
these two keywords is available, we'll take a hands-on approach to the topic.

<!-- truncate -->

## `const` values

When a _value_ is guaranteed to be unchanging in your program (where "value" may be scalars,
`struct`s, 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](https://doc.rust-lang.org/book/ch03-01-variables-and-mutability.html#differences-between-variables-and-constants)
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](https://doc.rust-lang.org/book/ch15-05-interior-mutability.html) to modify
things that aren't declared `mut`.
[`RefCell`](https://doc.rust-lang.org/std/cell/struct.RefCell.html) provides an example of this
pattern in action:

```rust
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](https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=8e4bea1a718edaff4507944e825a54b2)

When `const` is involved though, interior mutability is impossible:

```rust
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](https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=88fe98110c33c1b3a51e341f48b8ae00)

And a second example using [`Once`](https://doc.rust-lang.org/std/sync/struct.Once.html):

```rust
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](https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=c3cc5979b5e5434eca0f9ec4a06ee0ed)

When the
[`const` specification](https://github.com/rust-lang/rfcs/blob/26197104b7bb9a5a35db243d639aee6e46d35d75/text/0246-const-vs-static.md)
refers to ["rvalues"](http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2010/n3055.pdf), this
behavior is what they refer to. [Clippy](https://github.com/rust-lang/rust-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.

```rust
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](https://godbolt.org/z/Th8boO)

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.

```rust
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](https://godbolt.org/z/ZtS54X)

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`](https://doc.rust-lang.org/std/marker/trait.Sync.html) 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:

```rust
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](https://godbolt.org/z/uxmiRQ)

Where [previously](#copying) 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:

```rust
#[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](https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=b538dbc46076f12db047af4f4403ee6e)

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

```rust
#[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](https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=8c796a6e7fc273c12115091b707b0255)

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()`](https://doc.rust-lang.org/std/cell/struct.RefCell.html#method.new) is a
`const fn`, but because
[`RefCell` isn't `Sync`](https://doc.rust-lang.org/std/cell/struct.RefCell.html#impl-Sync), you'll
get an error at compile time:

```rust
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](https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=c76ef86e473d07117a1700e21fd45560)

It's likely that this will
[change in the future](https://github.com/rust-lang/rfcs/blob/master/text/0911-const-fn.md) though.

### The `Sync` marker

Which leads well to the next point: static variable types must implement the
[`Sync` marker](https://doc.rust-lang.org/std/marker/trait.Sync.html). 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](https://doc.rust-lang.org/nomicon/send-and-sync.html)). 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:

```rust
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](https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=40074d0248f056c296b662dbbff97cfc)

### 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:

```rust
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](https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=3ba003a981a7ed7400240caadd384d59)