From bb9a7cd4ad048d7368e6bfa4f96a49f4b4367f7d Mon Sep 17 00:00:00 2001 From: Bradlee Speice Date: Sat, 1 Sep 2018 14:54:43 -0400 Subject: [PATCH] Almost-final-draft of primitives post --- ... 2018-09-01-rusts-primitives-are-weird.md} | 151 +++++++++--------- _sass/base/_reset.scss | 7 +- 2 files changed, 80 insertions(+), 78 deletions(-) rename _posts/{2018-08-21-rusts-primitives-are-weird.md => 2018-09-01-rusts-primitives-are-weird.md} (52%) diff --git a/_posts/2018-08-21-rusts-primitives-are-weird.md b/_posts/2018-09-01-rusts-primitives-are-weird.md similarity index 52% rename from _posts/2018-08-21-rusts-primitives-are-weird.md rename to _posts/2018-09-01-rusts-primitives-are-weird.md index f9b8566..ca73f84 100644 --- a/_posts/2018-08-21-rusts-primitives-are-weird.md +++ b/_posts/2018-09-01-rusts-primitives-are-weird.md @@ -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): whose output has been lightly edited +The [compiler explorer](https://godbolt.org/z/lgNYcc) gives us an easy way of showing off +the assembly-level code that's generated: whose output has been lightly edited -``` +```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): again, lightly edited +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): +again, lightly edited + +```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 trap card dirty secret revelation: *Rust has +Now it's time to reveal my trap card 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: heavily trimmed down -``` +```nasm example::main: sub rsp, 24 mov rdi, rsp lea rax, [rip + .Lbyte_str.u] mov rsi, rax + ; Bombshell right here call ::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 diff --git a/_sass/base/_reset.scss b/_sass/base/_reset.scss index 09d8c98..39fe6c3 100644 --- a/_sass/base/_reset.scss +++ b/_sass/base/_reset.scss @@ -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 */ @@ -166,10 +166,7 @@ hr { pre { overflow: auto; } -code, pre { - -} small { - color: gray; + color: gray; }