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Finish up static and start on stack

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Bradlee Speice
2019-01-21 17:14:58 -05:00
parent 00b8d05387
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_drafts/understanding-allocations-in-rust.md
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@ -75,7 +75,7 @@ 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
(think the [libc] and [winapi] implementations of [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 (hehe) "release" mode at the end (`cargo run --release` and `cargo test --release`).
and address (pun intended) "release" mode at the end (`cargo run --release` and `cargo test --release`).
- All content will be run using Rust 1.31, as that's the highest currently supported in the
[Compiler Exporer](https://godbolt.org/). As such, we'll avoid talking about things like
[compile-time evaluation of `static`](https://github.com/rust-lang/rfcs/blob/master/text/0911-const-fn.md)
@ -257,7 +257,7 @@ pub fn multiply_twice(value: u32) -> u32 {
value * FACTOR * FACTOR
}
```
-- [Compiler Explorer](https://godbolt.org/z/Qc7tHM)
-- [Compiler Explorer](https://odbolt.org/z/Qc7tHM)
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
@ -282,15 +282,215 @@ and `thread_local!()` macros later.
More generally, `static` variables are globally unique locations in memory,
the contents of which are loaded as part of your program being read into main memory.
They allow initialization with both raw values and (most) `const fn` calls, and must
implement the [`Sync`](https://doc.rust-lang.org/std/marker/trait.Sync.html)
marker trait. The initial value is loaded along with the program/library binary,
though it can change during the time your program is running. While `static mut`
variables are allowed, mutating a static is considered an `unsafe` operation.
Unlike `const` though, interior mutability is accepted, and can be done with `safe` code.
They allow initialization with both raw values and `const fn` calls, and the initial
value is loaded along with the program/library binary. All static variables must
be of a type that implements the [`Sync`](https://doc.rust-lang.org/std/marker/trait.Sync.html)
marker trait. And while `static mut` variables are allowed, mutating a static is considered
an `unsafe` operation.
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 {
value * FACTOR
}
pub fn multiply_twice(value: u32) -> u32 {
value * FACTOR * FACTOR
}
```
-- [Compiler Explorer](https://godbolt.org/z/MGBr5Y)
Where [previously](https://godbolt.org/z/MGBr5Y) 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.
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 get a bit weirder when using `const fn`. In most cases, things just work:
```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`. As an example, even though
[`RefCell::new()`](https://doc.rust-lang.org/std/cell/struct.RefCell.html#method.new)
is `const fn`, 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,
so be on the lookout.
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`. This is why earlier examples could get away with initializing
statics, even though we never included an `impl Sync for MyStruct` in the code.
For more on the `Sync` trait, the [Nomicon](https://doc.rust-lang.org/nomicon/send-and-sync.html)
has a much more thorough treatment. But as an example, 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)
Finally, while `static mut` variables are allowed, mutating them is an `unsafe` operation.
Unlike `const` however, interior mutability is acceptable. To demonstrate:
```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)
## **push** and **pop**: Stack Allocations
**const** and **static** are perfectly fine, but it's very rare that we know
at compile-time about either references or values that will be the same for the entire
time our program is running. Put another way, it's not often the case that either you
or your compiler know how much memory your entire program will need.
However, there are still some optimizations the compiler can do if it knows how much
memory individual functions will need. Specifically, the compiler can make use of
"stack" memory (as opposed to "heap" memory) which can be managed far faster in
both the short- and long-term. When requesting memory, the
[`push` instruction](http://www.cs.virginia.edu/~evans/cs216/guides/x86.html)
can typically complete in [1 or 2 cycles](https://agner.org/optimize/instruction_tables.ods)
(<1 nanosecond on modern CPUs). Heap memory instead requires using an allocator
(specialized software to track what memory is in use) to reserve space.
And when you're finished with memory, the `pop` instruction likewise runs in
1-3 cycles, as opposed to an allocator needing to worry about memory fragmentation
and other issues. All sorts of incredibly sophisticated techniques have been used
to design allocators:
- [Garbage Collection](https://en.wikipedia.org/wiki/Garbage_collection_(computer_science))
strategies like [Tracing](https://en.wikipedia.org/wiki/Tracing_garbage_collection)
(used in [Java](https://www.oracle.com/technetwork/java/javase/tech/g1-intro-jsp-135488.html))
and [Reference counting](https://en.wikipedia.org/wiki/Reference_counting)
(used in [Python](https://docs.python.org/3/extending/extending.html#reference-counts))
- Thread-local structures to prevent locking the allocator in [tcmalloc](https://jamesgolick.com/2013/5/19/how-tcmalloc-works.html)
- Arena structures used in [jemalloc](http://jemalloc.net/), which until recently
was the primary allocator for Rust programs!
But no matter how sophisticated your allocator is, the principle remains: the
fastest allocator is the one you never use. As such, we're not going to go
in detail on how exactly the
[`push` and `pop` instructions work](http://www.cs.virginia.edu/~evans/cs216/guides/x86.html),
and we'll focus instead on the conditions that enable the Rust compiler to use
stack-based allocation for variables.
Now, one question I hope you're asking is "how do we distinguish stack- and
heap-based allocations in Rust code?" There are three strategies I'm going
to use for this:
1. Any time the `push` or `pop` instructions are used, or the `rsp` register is modified,
this is a stack allocation:
```rust
pub fn stack_alloc(x: u32) -> u32 {
// Space for `y` is allocated by subtracting from `rsp`,
// and then populated
let y = [1u8, 2, 3, 4];
// Space for `y` is deallocated by adding back to `rsp`
x
}
```
-- [Compiler Explorer](https://godbolt.org/z/gKFOgB)
2. Any time `call core::ptr::drop_in_place` occurs, a heap allocation has occurred
sometime in the past and it is now time for us to de-allocate the memory:
```rust
pub fn heap_alloc(x: usize) -> usize {
// Space for elements in a vector has to be allocated
// on the heap, and is then de-allocated once the
// vector goes out of scope
let y: Vec<u8> = Vec::with_capacity(x);
x
}
```
-- [Compiler Explorer](https://godbolt.org/z/T2xoh8) (`drop_in_place` happens on line 1321)
3. Using a special [`GlobalAlloc`](https://doc.rust-lang.org/std/alloc/trait.GlobalAlloc.html)
implementation to track when heap allocations occur. For this post, I'll be using
[qadapt](https://crates.io/crates/qadapt) to trigger a panic if heap allocations
occur; code that doesn't panic doesn't use heap allocations, and by necessity
uses stack allocation instead.
With all that in mind, let's talk about how to use the stack in Rust.
Example: Why doesn't `Vec::new()` go to the allocator?
Questions:
@ -304,6 +504,8 @@ Questions:
7. Legal to pass an array as an argument?
8. Can you force a heap allocation with arrays that are larger than stack size?
- Check `ulimit -s`
9. Can you force heap allocation by returning something that escapes the stack?
- Will `#[inline(always)]` move this back to a stack allocation?
# Piling On - Rust and the Heap
@ -322,7 +524,9 @@ Questions:
# Compiler Optimizations Make Everything Complicated
Example: Compiler stripping out allocations of Box<>, Vec::push()
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
# Appendix and Further Reading