Update memory.md
Minor updates to formatting. documentation updates Update memory.md minor edits
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# Memory Management in Rustc
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Rustc tries to be pretty careful how it manages memory. The compiler allocates
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_a lot_ of data structures throughout compilation, and if we are not careful,
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it will take a lot of time and space to do so.
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Generally rustc tries to be pretty careful how it manages memory.
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The compiler allocates _a lot_ of data structures throughout compilation,
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and if we are not careful, it will take a lot of time and space to do so.
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One of the main way the compiler manages this is using [arena]s and [interning].
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@ -11,16 +11,18 @@ One of the main way the compiler manages this is using [arena]s and [interning].
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## Arenas and Interning
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We create a LOT of data structures during compilation. For performance reasons,
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we allocate them from a global memory pool; they are each allocated once from a
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long-lived *arena*. This is called _arena allocation_. This system reduces
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allocations/deallocations of memory. It also allows for easy comparison of
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types for equality: for each interned type `X`, we implemented [`PartialEq for
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X`][peqimpl], so we can just compare pointers. The [`CtxtInterners`] type
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contains a bunch of maps of interned types and the arena itself.
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Since A LOT of data structures are created during compilation, for performance
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reasons, we allocate them from a global memory pool.
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Each are allocated once from a long-lived *arena*.
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This is called _arena allocation_.
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This system reduces allocations/deallocations of memory.
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It also allows for easy comparison of types (more on types [here](./ty.md)) for equality:
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for each interned type `X`, we implemented [`PartialEq` for X][peqimpl],
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so we can just compare pointers.
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The [`CtxtInterners`] type contains a bunch of maps of interned types and the arena itself.
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[peqimpl]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.Ty.html#implementations
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[`CtxtInterners`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.CtxtInterners.html#structfield.arena
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[peqimpl]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.Ty.html#implementations
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### Example: `ty::TyKind`
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@ -30,7 +32,7 @@ compiler doesn’t naively allocate from the buffer. Instead, we check if that
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type was already constructed. If it was, we just get the same pointer we had
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before, otherwise we make a fresh pointer. With this schema if we want to know
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if two types are the same, all we need to do is compare the pointers which is
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efficient. `TyKind` should never be constructed on the stack, and it would be unusable
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efficient. [`TyKind`] should never be constructed on the stack, and it would be unusable
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if done so.
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You always allocate them from this arena and you always intern them so they are
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unique.
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@ -43,26 +45,33 @@ to that buffer is freed and our `'tcx` references would be invalid.
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In addition to types, there are a number of other arena-allocated data structures that you can
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allocate, and which are found in this module. Here are a few examples:
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- [`GenericArgs`], allocated with `mk_args` – this will intern a slice of types, often used
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- [`GenericArgs`], allocated with [`mk_args`] – this will intern a slice of types, often used
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to specify the values to be substituted for generics args (e.g. `HashMap<i32, u32>` would be
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represented as a slice `&'tcx [tcx.types.i32, tcx.types.u32]`).
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- [`TraitRef`], typically passed by value – a **trait reference** consists of a reference to a trait
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along with its various type parameters (including `Self`), like `i32: Display` (here, the def-id
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would reference the `Display` trait, and the args would contain `i32`). Note that `def-id` is
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defined and discussed in depth in the [`AdtDef and DefId`][adtdefid] section.
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- [`Predicate`] defines something the trait system has to prove (see `traits` module).
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- [`Predicate`] defines something the trait system has to prove (see [traits] module).
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[`GenericArgs`]: ./ty_module/generic_arguments.md#the-genericargs-type
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[adtdefid]: ./ty_module/generic_arguments.md#adtdef-and-defid
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[`TraitRef`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/type.TraitRef.html
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[`AdtDef` and `DefId`]: ./ty.md#adts-representation
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[`def-id`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/def_id/struct.DefId.html
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[`GenericArgs`]: ./generic_arguments.html#GenericArgs
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[`mk_args`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/context/struct.TyCtxt.html#method.mk_args
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[adtdefid]: ./ty_module/generic_arguments.md#adtdef-and-defid
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[`Predicate`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.Predicate.html
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[`TraitRef`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.TraitRef.html
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[`ty::TyKind`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/sty/type.TyKind.html
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[`TyKind`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/ty_kind/enum.TyKind.html
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[traits]: ./traits/resolution.md
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## The tcx and how it uses lifetimes
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## The `tcx` and how it uses lifetimes
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The `tcx` ("typing context") is the central data structure in the compiler. It is the context that
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you use to perform all manner of queries. The struct `TyCtxt` defines a reference to this shared
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The typing context (`tcx`) is the central data structure in the compiler. It is the context that
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you use to perform all manner of queries. The `struct` [`TyCtxt`] defines a reference to this shared
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context:
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```rust,ignore
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@ -76,10 +85,13 @@ As you can see, the `TyCtxt` type takes a lifetime parameter. When you see a ref
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lifetime like `'tcx`, you know that it refers to arena-allocated data (or data that lives as long as
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the arenas, anyhow).
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[`TyCtxt`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.TyCtxt.html
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### A Note On Lifetimes
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The Rust compiler is a fairly large program containing lots of big data
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structures (e.g. the AST, HIR, and the type system) and as such, arenas and
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structures (e.g. the [Abstract Syntax Tree (AST)][ast], [High-Level Intermediate
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Representation (`HIR`)][hir], and the type system) and as such, arenas and
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references are heavily relied upon to minimize unnecessary memory use. This
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manifests itself in the way people can plug into the compiler (i.e. the
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[driver](./rustc-driver.md)), preferring a "push"-style API (callbacks) instead
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@ -90,4 +102,6 @@ duplication while also preventing a lot of the ergonomic issues due to many
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pervasive lifetimes. The [`rustc_middle::ty::tls`][tls] module is used to access these
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thread-locals, although you should rarely need to touch it.
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[ast]: ./ast-validation.md
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[hir]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir/index.html
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[tls]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/tls/index.html
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