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Almost-final-draft of primitives post

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_posts/2018-08-21-rusts-primitives-are-weird.md → _posts/2018-09-01-rusts-primitives-are-weird.md
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@ -1,12 +1,13 @@
---
layout: post
title: "Rust's primitives are Weird (and cool)"
title: "Rust's Primitives are Weird (and Cool)"
description: "but mostly weird."
category:
tags: [rust, c, java, python, x86]
---
I wrote a really small Rust program a while back that I was 100% convinced couldn't possibly run:
I wrote a really small Rust program a while back because I was curious. I was 100% convinced it
couldn't possibly run:
```rust
fn main() {
@ -14,7 +15,7 @@ fn main() {
}
```
And to my complete befuddlement, it compiled, it ran, and it produced a completely sensible output.
And to my complete befuddlement, it compiled, ran, and produced a completely sensible output.
The reason I was so surprised has to do with how Rust treats a special category of things
I'm going to call *primitives*. In the current version of the Rust book, you'll see them
referred to as [scalars](rust_scalar), and in older versions they'll be called [primitives](rust_primitive).
@ -23,24 +24,13 @@ why this program is so cool requires talking about a number of other programming
and keeping a consistent terminology makes things easier.
**You've been warned:** this is going to be a tedious post about a relatively minor issue that involves
a quick jaunt all the way through Java, Python, C, and x86 Assembly, but demonstrates a really cool
way that Rust thinks differently about the world.
But because I'm not a monster, here's someone else who's just as excited as you are to learn about
primitives:
![Excited dog](/assets/images/rust-primitives/excited.jpg)
> [Unreasonably excited doggo][excited_doggo]
Java, Python, C, and x86 Assembly. And also me pretending like I know what I'm talking about with assembly.
# Defining primitives (Java)
My day job is in Java. I'm continually amazed by how much of the world runs on Java,
and somehow manages to continue functioning. Like, it can't be that good, because nothing
in Computer Science functions that well. And yet, Java is maybe one of the few things
CS people can high-five and say "you know what, we did a good thing."
But that's not what this post is about. In Java, there's a special name for
some specific types of values:
The reason I'm using the name *primitive* comes from how much of my life is Java right now.
Spoiler alert: a lot of it. And for the most part I like Java, but I digress. In Java, there's a
special name for some specific types of values:
> ```
bool char byte
@ -72,12 +62,14 @@ Main.java:5: error: int cannot be dereferenced
1 error
```
The reason for this error is that only things inheriting from
[`Object`](https://docs.oracle.com/javase/9/docs/api/java/lang/Object.html)
can have instance methods, and the primitive types do not in fact inherit this.
Specifically, Java considers [`Object`](https://docs.oracle.com/javase/10/docs/api/java/lang/Object.html)
and things that inherit from it as pointers, and thus we have to dereference the pointer
before the fields and methods it defines can be used. In contrast, *primitive types are just values* -
there's nothing to be dereferenced. In memory, they're just a sequence of bits.
If we really want, we can turn the `int` into an
[`Integer`](https://docs.oracle.com/javase/9/docs/api/java/lang/Integer.html) and then
turn that into a `String` and print it, but that seems like a lot of work:
[`Integer`](https://docs.oracle.com/javase/10/docs/api/java/lang/Integer.html) and then
dereference it, but it's a bit wasteful:
```java
class Main {
@ -89,23 +81,15 @@ class Main {
}
```
This allows us to create the variable `y` of type `Integer`, and at run time peek into `y`
to locate the `toString()` function and call it.
So why do we have to jump through the extra hoops for this? The reason is partially that Java
treats the primitive values as just a "bag of bits"; there are no functions to call, no references
to maintain, it's just a set number of bits to represent a value. If you call a function using
`int` or `long` as an argument, internally Java will copy the bits across and your original value
can't be modified.
And if Rust has a similar "bag of bits" representation for its primitives (spoiler alert: it does),
that gives us our first question: how does Rust get away with calling the equivalent of instance methods?
This creates the variable `y` of type `Integer` (which inherits `Object`), and at run time we
dereference `y` to locate the `toString()` function and call it. Rust obviously handles things a bit
differently, but we have to look at some low-level details to see how differently it actually is.
# Low Level Handling of Primitives (C)
Now, I still want to show off the "bag of bits" representation of primitives in Rust. But to do that,
we have to expose a bit of how your computer thinks about those values. Let's consider the following
code in C:
We first need to build a foundation for reading and understanding the assembly code the
final answer involves. Let's begin with showing how the `C` language (and your computer)
thinks about "primitive" values in memory:
```c
void my_function(int num) {}
@ -116,21 +100,26 @@ int main() {
}
```
And to drive the point home (and pretend like I understand assembly), let's take a look at the result
using the [compiler explorer](https://godbolt.org/z/lgNYcc): <span style="font-size:.6em">whose output has been lightly edited</span>
The [compiler explorer](https://godbolt.org/z/lgNYcc) gives us an easy way of showing off
the assembly-level code that's generated: <span style="font-size:.6em">whose output has been lightly edited</span>
```
```nasm
main:
push rbp
mov rbp, rsp
sub rsp, 16
; We assign the value `8` to `x` here
mov DWORD PTR [rbp-4], 8
; And copy the bits making up `x` to a location
; `my_function` can access
; `my_function` can access (`edi`)
mov eax, DWORD PTR [rbp-4]
mov edi, eax
; Call `my_function` and give it control
call my_function
mov eax, 0
leave
ret
@ -138,17 +127,18 @@ main:
my_function:
push rbp
mov rbp, rsp
; Copy the bits out of the pre-determined location
; Copy the bits out of the pre-determined location (`edi`)
; to somewhere we can use
mov DWORD PTR [rbp-4], edi
nop
pop rbp
ret
```
At a really low level of memory, we're copying bits around; nothing crazy. That's what the `mov` instruction
is intended to do (use [this][x86_guide] as a reference). But to show how similar Rust is, let's take a look at the equivalent
Rust code in the [compiler explorer](https://godbolt.org/z/cAlmk0): <span style="font-size:.6em">again, lightly edited</span>
At a really low level of memory, we're copying bits around using the [`mov`][x86_guide] instruction; nothing crazy.
But to show how similar Rust is, let's take a look at our program translated from C to Rust:
```rust
fn my_function(x: i32) {}
@ -159,38 +149,48 @@ fn main() {
}
```
```
And the assembly generated when we stick it in the [compiler explorer](https://godbolt.org/z/cAlmk0):
<span style="font-size:.6em">again, lightly edited</span>
```nasm
example::main:
push rax
; Look familiar? We're copying bits to a location for `my_function`
; The compiler just optimizes out holding `x` in memory
mov edi, 8
; Call `my_function` and give it control
call example::my_function
pop rax
ret
example::my_function:
sub rsp, 4
; And copying those bits again, just like in C
mov dword ptr [rsp], edi
add rsp, 4
ret
```
The generated Rust looks almost identical to C, and is the same as how Java thinks of primitives: just bits in memory.
The generated Rust assembly is functionally pretty close to the C assembly (and Java as well):
*When working with primitives, we're just dealing with bits in memory*.
And now that we're a bit more familiar with the low-level representation of primitives, it's time to answer:
how exactly does Rust manage to compile `8.to_string()`?
In Java we have to dereference a pointer to call its functions; in Rust, there's no pointer to dereference. So what
exactly is going on with this `.to_string()` function call?
# impl primitive (and Python)
Now it's time to reveal my <strike>trap card</strike> <strike>dirty secret</strike> revelation: *Rust has
Now it's time to <strike>reveal my trap card</strike> show the revelation that tied all this together: *Rust has
implementations for its primitive types.* That's right, `impl` blocks aren't only for `structs` and `traits`,
primitives get them too. Don't believe me? Check out [u32](https://doc.rust-lang.org/std/primitive.u32.html),
[f64](https://doc.rust-lang.org/std/primitive.f64.html) and [char](https://doc.rust-lang.org/std/primitive.char.html)
as examples.
But the really interesting bit is how Rust turns the code we started with into assembly. Let's break out the
But the really interesting bit is how Rust turns those `impl` blocks into assembly. Let's break out the
[compiler explorer](https://godbolt.org/z/6LBEwq) once again:
```rust
@ -199,31 +199,32 @@ pub fn main() {
}
```
And the interesting bits in the assembly:
And the interesting bits in the assembly: <span style="font-size:.6em">heavily trimmed down</span>
```
```nasm
example::main:
sub rsp, 24
mov rdi, rsp
lea rax, [rip + .Lbyte_str.u]
mov rsi, rax
; Bombshell right here
call <T as alloc::string::ToString>::to_string@PLT
mov rdi, rsp
call core::ptr::drop_in_place
add rsp, 24
ret
```
Now, this assembly is far more complicated, but here's the big revelation: **we're calling
`to_string()` as a function that isn't bound to the instance of `8`**. Instead of thinking
of the value 8 as an instance of `u32` and then peeking in to find the location of the function
we want to call, we have a function that exists outside of the instance and just give
that function the value `8`.
Now, this assembly is a bit more complicated, but here's the big revelation: **we're calling
`to_string()` as a function that exists all on its own, and giving it the instance of `8`**.
Instead of thinking of the value 8 as an instance of `u32` and then peeking in to find
the location of the function we want to call (like Java), we have a function that exists
outside of the instance and just give that function the value `8`.
This is an incredibly technical detail, but the interesting idea I had was this:
*if `to_string()` is a static function, can I refer to the unbound function and give
it an instance?*
*if `to_string()` is a static function, can I refer to the unbound function and give it an instance?*
Better explained in code (and a [compiler explorer](https://godbolt.org/z/fJY-gA) link
because I seriously love this thing):
@ -242,7 +243,7 @@ impl MyVal {
pub fn main() {
let my_val = MyVal { x: 8 };
// THESE ARE THE SAME
// THESE ARE TOTALLY EQUIVALENT
my_val.to_string();
MyVal::to_string(&my_val);
}
@ -252,7 +253,7 @@ Rust is totally fine "binding" the function call to the instance, and also as a
MIND == BLOWN.
Python does something equivalent where I can both call functions bound to their instances
Python does the same thing where I can both call functions bound to their instances
and also call as an unbound function where I give it the instance:
```python
@ -268,9 +269,9 @@ m.my_function()
MyClass.my_function(m)
```
That said, Python still doesn't treat "primitives" as things that can have instance methods:
And Python tries to make you *think* that primitives can have instance methods...
```
```python
>>> dir(8)
['__abs__', '__add__', '__and__', '__class__', '__cmp__', '__coerce__',
'__delattr__', '__div__', '__divmod__', '__doc__', '__float__', '__floordiv__',
@ -285,27 +286,31 @@ That said, Python still doesn't treat "primitives" as things that can have insta
8.__str__()
^
SyntaxError: invalid syntax
>>> # It will run if we assign it first though:
>>> x = 8
>>> x.__str__()
'8'
```
...but in practice it's a bit complicated.
So while Python handles binding instance methods in a way similar to Rust, it's still not able
to run the example we started with.
# Conclusion
This was a super-roundabout way of demonstrating it, but the way Rust handles incredibly minor details
like primitives is one of the reasons I enjoy the language. It's optimized like C in how it lays out
memory and is efficient ("bag of bits" representation). And it still has a lot of
the nice features I like in Python that make it easy to work with the language (late/static binding).
like primitives leads to really cool effects. Primitives are optimized like C in how they have a
space-efficient memory layout, yet the language still has a lot of features I enjoy in Python
(like both instance and late binding).
And even given that, there are still areas where Rust shines that none of the other languages discussed do;
as a kinda quirky feature of Rust's type system, `8.to_string()` is actually valid code.
And when you put it together, there are areas where Rust does cool things nobody else can;
as a quirky feature of Rust's type system, `8.to_string()` is actually valid code.
There aren't too many grand lessons to be learned from this, the behavior I'm talking about is
a relatively minor detail in the grand picture. But it's still something I learned where Rust
just gets the details right, and I love it.
Now go forth and fool your friends into thinking you know assembly. This is all I've got.
[x86_guide]: http://www.cs.virginia.edu/~evans/cs216/guides/x86.html
[excited_doggo]: https://flic.kr/p/2jr8Zp
[java_primitive]: https://docs.oracle.com/javase/tutorial/java/nutsandbolts/datatypes.html
[compiler_explorer]: https://godbolt.org/
[rust_scalar]: https://doc.rust-lang.org/book/second-edition/ch03-02-data-types.html#scalar-types

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@ -93,7 +93,7 @@ a {
position: relative;
display: inline-block;
padding: 5px 1px;
padding: 1px 1px;
transition: color ease 0.3s;
/* Hover animation effect for all buttons */
@ -166,9 +166,6 @@ hr {
pre { overflow: auto; }
code, pre {
}
small {
color: gray;
}