speice.io/_drafts/understanding-allocations-in-rust.md

layout	title	description	category	tags
post	Allocations in Rust	An introduction to the memory model		rust understanding-allocations

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 at the end for easy future citation. To that end, a table of contents is in order:

• Foreword
• The Whole World: Global Memory Usage
• Stacking Up: Fixed Memory
• A Heaping Helping: Dynamic Memory
• Compiler Optimizations: What It's Done For You Lately
• Summary: When Does Rust Allocate?

Foreword

Rust's three defining features of Performance, Reliability, and Productivity are all driven to a great degree by the how the Rust compiler understands memory ownership. Unlike managed memory languages (Java, Python), Rust doesn't really garbage collect, leading to fast code when dynamic (heap) memory isn't necessary. When heap memory is necessary, Rust ensures you can't accidentally mis-manage it. And because the compiler handles memory "ownership" for you, developers never need to worry about accidentally deleting data that was needed somewhere else.

That said, there are situations where you won't benefit from work the Rust compiler is doing. If you:

1. Never use unsafe
2. Never use #![feature(alloc)] or the alloc crate

...then it's not possible for you to use dynamic memory!

For some uses of Rust, typically embedded devices, these constraints make sense. They have 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 specific 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 people), and Rust developers would struggle without std::vec. But in this scenario, std::vec is actually aliased to a part of the alloc crate, and thus off-limits. Box, Rc, etc., are also unusable 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 how the compiler reasons about your program, with an emphasis on how to design your programs for performance.

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 (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 (pun intended) release mode at the end (cargo run --release and cargo test --release).
• All content will be run using Rust 1.32, as that's the highest currently supported in the Compiler Exporer. As such, we'll avoid upcoming innovations like compile-time evaluation of static that are available in nightly.
• Because of the nature of the content, some (very simple) assembly-level code is involved. We'll keep this simple, but I found a refresher on the push and pop instructions was helpful while writing this post.

Finally, I'll do what I can to flag potential future changes but the Rust docs have a notice worth repeating:

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

-- the docs

Compiler Optimizations: What It's Done For You Lately

1. Box<> getting inlined into stack allocations
2. Vec::push() === Vec::with_capacity() for fixed/predictable capacities
3. Inlining statics that don't change value