diff --git a/_drafts/stacking-up.md b/_drafts/stacking-up.md index d61caf2..d7b6b16 100644 --- a/_drafts/stacking-up.md +++ b/_drafts/stacking-up.md @@ -29,14 +29,14 @@ to design allocators: 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 +- 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! 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 -in detail on how exactly the +fastest allocator is the one you never use. As such, we're not going to discuss 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 +but we'll focus instead on the conditions that enable the Rust compiler to use 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 @@ -106,8 +106,8 @@ to hold their contents: ```rust struct Point { - x: f64, - y: f64, + x: u64, + y: u64, } struct Line { @@ -116,13 +116,18 @@ struct Line { } pub fn make_line() { - let origin = Point { x: 0., y: 0. }; - let point = Point { x: 1., y: 2. }; + // `origin` is stored in the first 16 bytes of memory + // 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 }; } ``` --- [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, the `sub rsp, 64` instruction indicates we're still working with the stack.