First cleanup pass on the foreword

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Bradlee Speice 2019-02-10 12:18:36 -05:00
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@ -9,12 +9,12 @@ tags: [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](https://www.youtube.com/watch?v=dOfucXtyEsU).
[the OS and `main()`](https://www.youtube.com/watch?v=dOfucXtyEsU).
Rust programmers use the [`Box`](https://doc.rust-lang.org/stable/std/boxed/struct.Box.html)
type all the time, but there's a rich history of the Rust language itself wrapped up in
[how special it is](https://manishearth.github.io/blog/2017/01/10/rust-tidbits-box-is-special/).
In a similar vein, I want you to look at code and understand how memory is used;
In a similar vein, this series attempts 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
@ -24,9 +24,10 @@ 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:
This series is intended as both learning and reference material; we'll work through the
different memory types Rust uses, and explain the implications of each. Ultimately,
a summary will be provided as a cheat sheet for easy future reference. To that end,
a table of contents is in order:
- Foreword
- [Global Memory Usage: The Whole World](/2019/02/the-whole-world)
@ -38,26 +39,28 @@ section at the end for easy future citation. To that end, a table of contents is
# Foreword
Rust's three defining features of [Performance, Reliability, and Productivity](https://www.rust-lang.org/)
are all driven to a great degree by the how the Rust compiler understands
[memory ownership](https://doc.rust-lang.org/book/ch04-01-what-is-ownership.html). Unlike managed memory
languages (Java, Python), Rust [doesn't really](https://words.steveklabnik.com/borrow-checking-escape-analysis-and-the-generational-hypothesis)
garbage collect, leading to fast code when [dynamic (heap) memory](https://en.wikipedia.org/wiki/Memory_management#Dynamic_memory_allocation)
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.
are all driven to a great degree by the how the Rust compiler understands memory usage.
Unlike managed memory languages (Java, Python), Rust
[doesn't really](https://words.steveklabnik.com/borrow-checking-escape-analysis-and-the-generational-hypothesis)
garbage collect; instead, it uses an [ownership](https://doc.rust-lang.org/book/ch04-01-what-is-ownership.html)
system to reason about how long objects will last in your program. In some cases, if the life of an object
is fairly transient, Rust can make use of a very fast region called the "stack." When that's not possible,
Rust uses [dynamic (heap) memory](https://en.wikipedia.org/wiki/Memory_management#Dynamic_memory_allocation)
and the ownership system to ensure you can't accidentally corrupt memory. It's not as fast, but it is
important to have available.
That said, there are situations where you won't benefit from work the Rust compiler is doing.
If you:
That said, there are specific situations in Rust where you'd never need to worry about the stack/heap
distinction! If you:
1. Never use `unsafe`
2. Never use `#![feature(alloc)]` or the [`alloc` crate](https://doc.rust-lang.org/alloc/index.html)
...then it's not possible for you to use dynamic memory!
For some uses of Rust, typically embedded devices, these constraints make sense.
For some uses of Rust, typically embedded devices, these constraints are OK.
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"](https://en.wikipedia.org/wiki/Virtual_memory) junk, but that's
this ["virtual memory"](https://en.wikipedia.org/wiki/Virtual_memory) thing, but that's
not an issue because there's only one running application. The
[embedonomicon](https://docs.rust-embedded.org/embedonomicon/preface.html) is ever in mind,
and interacting with the "real world" through extra peripherals is accomplished by
@ -66,8 +69,8 @@ reading and writing to [specific memory addresses](https://bob.cs.sonoma.edu/Int
Most Rust programs find these requirements overly burdensome though. C++ developers
would struggle without access to [`std::vector`](https://en.cppreference.com/w/cpp/container/vector)
(except those hardcore no-STL people), and Rust developers would struggle without
[`std::vec`](https://doc.rust-lang.org/std/vec/struct.Vec.html). But in this scenario,
`std::vec` is actually aliased to a part of the
[`std::vec`](https://doc.rust-lang.org/std/vec/struct.Vec.html). But with the constraints above,
`std::vec` is actually a part of the
[`alloc` crate](https://doc.rust-lang.org/alloc/vec/struct.Vec.html), and thus off-limits.
`Box`, `Rc`, etc., are also unusable for the same reason.
@ -75,11 +78,10 @@ Whether writing code for embedded devices or not, the important thing in both si
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](https://www.google.com)'s home page is until you start
trying to download it. The compiler uses this information (or lack thereof) to optimize
trying to download it. The compiler uses this knowledge (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.
how much memory your program needs while it's running. This series is all about understanding
how the compiler reasons about your program, with an emphasis on the implications for performance.
Now let's address some conditions and caveats before going much further:
@ -91,14 +93,14 @@ Now let's address some conditions and caveats before going much further:
[Compiler Exporer](https://godbolt.org/). As such, we'll avoid upcoming innovations like
[compile-time evaluation of `static`](https://github.com/rust-lang/rfcs/blob/master/text/0911-const-fn.md)
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](https://stackoverflow.com/a/4584131/1454178)
- Because of the nature of the content, being able to read assembly is helpful.
We'll keep it simple, but I [found](https://stackoverflow.com/a/4584131/1454178)
a [refresher](https://stackoverflow.com/a/26026278/1454178) on the `push` and `pop`
[instructions](http://www.cs.virginia.edu/~evans/cs216/guides/x86.html)
was helpful while writing this post.
was helpful while writing thi[Raph Levien](https://docs.google.com/presentation/d/1q-c7UAyrUlM-eZyTo1pd8SZ0qwA_wYxmPZVOQkoDmH4/edit?usp=sharing)s.
- I've tried to be precise in saying only what I can prove using the tools (ASM, docs)
that are available. That said, if there's something said in error, please reach out
and let me know - [bradlee@speice.io](mailto:bradlee@speice.io)
that are available, but if there's something said in error it will be corrected
expeditiously. Please let me know at [bradlee@speice.io](mailto:bradlee@speice.io)
Finally, I'll do what I can to flag potential future changes but the Rust docs
have a notice worth repeating:

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@ -37,9 +37,14 @@ the memory model in Rust:
- `const` is a fixed value; the compiler is allowed to copy it wherever useful.
- `static` is a fixed reference; the compiler will guarantee it is unique.
And a nice visualizaton of the rules, courtesy of
[Raph Levien](https://docs.google.com/presentation/d/1q-c7UAyrUlM-eZyTo1pd8SZ0qwA_wYxmPZVOQkoDmH4/edit?usp=sharing):
![Container Sizes in Rust](/assets/images/2019-02-04-container-size.svg)
---
And if you've read through this series: thanks. I've enjoyed the process that went
into writing this, both in building new tools and forcing myself to understand
the content well enough to explain it. I hope this is valuable as a reference to you
as well.
If you've taken the time to read through this series: thanks. I've enjoyed the
process that went into writing this, both in building new tools and learning
the material well enough to explain it. I hope this is valuable as a reference
to you as well.

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