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Use more readable assembly for the struct example

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Also do the thing I said I did
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Bradlee Speice 2019-02-02 23:27:03 -05:00
parent 6b2cb968a4
commit 7a949bc9bf
1 changed files with 14 additions and 9 deletions
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_drafts/stacking-up.md
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@ -29,14 +29,14 @@ to design allocators:
and [Reference counting](https://en.wikipedia.org/wiki/Reference_counting) and [Reference counting](https://en.wikipedia.org/wiki/Reference_counting)
(used in [Python](https://docs.python.org/3/extending/extending.html#reference-counts)) (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) - 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 - Arena structures used in [jemalloc](http://jemalloc.net/), which
[until recently](https://blog.rust-lang.org/2019/01/17/Rust-1.32.0.html#jemalloc-is-removed-by-default)
was the primary allocator for Rust programs! was the primary allocator for Rust programs!
But no matter how fast your allocator is, the principle remains: the But no matter how fast your allocator is, the principle remains: the
fastest allocator is the one you never use. As such, we're not going to go fastest allocator is the one you never use. As such, we're not going to discuss how exactly the
in detail on how exactly the
[`push` and `pop` instructions work](http://www.cs.virginia.edu/~evans/cs216/guides/x86.html), [`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 but we'll focus instead on the conditions that enable the Rust compiler to use
the faster stack-based allocation for variables. the faster stack-based allocation for variables.
With that in mind, let's get into the details. How do we know when Rust will or will not use With that in mind, let's get into the details. How do we know when Rust will or will not use
@ -106,8 +106,8 @@ to hold their contents:
```rust ```rust
struct Point { struct Point {
x: f64, x: u64,
y: f64, y: u64,
} }
struct Line { struct Line {
@ -116,13 +116,18 @@ struct Line {
} }
pub fn make_line() { pub fn make_line() {
let origin = Point { x: 0., y: 0. }; // `origin` is stored in the first 16 bytes of memory
let point = Point { x: 1., y: 2. }; // starting at location `rsp`
let origin = Point { x: 0, y: 0 };
// `point` makes up the next 16 bytes of memory
let point = Point { x: 1, y: 2 };
// When creating `ray`, we just move the content out of
// `origin` and `point` into the next 32 bytes of memory
let ray = Line { a: origin, b: point }; let ray = Line { a: origin, b: point };
} }
``` ```
-- [Compiler Explorer](https://godbolt.org/z/WZRa1D) -- [Compiler Explorer](https://godbolt.org/z/vri9BE)
Note that while some extra-fancy instructions are used for memory manipulation in the assembly, Note that while some extra-fancy instructions are used for memory manipulation in the assembly,
the `sub rsp, 64` instruction indicates we're still working with the stack. the `sub rsp, 64` instruction indicates we're still working with the stack.