Skip to content
New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Jump to bottom

Stacked Borrows: update for shallow retagging #187

Merged
merged 1 commit into from
Aug 6, 2019
Merged

Stacked Borrows: update for shallow retagging #187

Changes from all commits
Commits
Show all changes
1 commit
Select commit
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
12 changes: 9 additions & 3 deletions wip/stacked-borrows.md
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -13,6 +13,10 @@ For more background, see the following blog-posts:
* [Stacked Borrows 2.0](https://www.ralfj.de/blog/2019/04/30/stacked-borrows-2.html) is a re-design of Stacked Borrows 1 that maintains the original core ideas, but changes the mechanism to support more precise tracking of shared references.
* [Stacked Borrows 2.1](https://www.ralfj.de/blog/2019/05/21/stacked-borrows-2.1.html) slightly tweaks the rules for read and write accesses and describes a high-level way of thinking about the new shape of the "stack" in Stacked Borrows 2.

Changes compared to the latest post (2.1):

* Retags are "shallow" instead of recursively looking for references inside compound types.

[Miri]: https://github.com/solson/miri/
[all-hands]: https://paper.dropbox.com/doc/Topic-Stacked-borrows--AXAkoFfUGViWL_PaSryqKK~hAg-2q57v4UM7cIkxCq9PQc22

Expand Down Expand Up @@ -109,7 +113,7 @@ pub enum RetagKind {

`Retag` is inserted into the MIR for the following situations:

* A retag happens after every assignment MIR statement where the assigned type may contain a reference type (checking recursively below compound types but not below references).
* A retag happens after every assignment MIR statement where the assigned type may by of reference or box type.
This is usually a `Default` retag. However, if the RHS of this assignment is a `Ref` which allows two-phase borrows, then this is a `TwoPhase` retag.

Currently, if the LHS of the assignment involves a `Deref`, no `Retag` is inserted.
Expand Down Expand Up @@ -311,14 +315,16 @@ location.stack.grant(

### Retagging

When executing `Retag(kind, place)`, we recursively visit all fields of this place, descending into compound types (`struct`, `enum`, arrays and so on) but not below any pointers.
For each reference (`&[mut] _`) and box (`Box<_>`) we encounter, and if `kind == Raw` also for each raw pointer (`*[const,mut] _`), we perform the following steps:
When executing `Retag(kind, place)`, we check if `place` holds a reference (`&[mut] _`) or box (`Box<_>`), and if `kind == Raw` we also check each raws pointer (`*[const,mut] _`).
For those we perform the following steps:

1. We compute a fresh tag: `Untagged` for raw pointers, `Tag(Tracking::new_ptr_id())` for everything else.
2. We determine if we will want to protect the items we are going to generate:
This is the case only if `kind == FnEntry` and the type of this pointer is a reference (not a box).
3. We perform reborrowing of the memory this pointer points to with the new tag and indicating whether we want protection, treating boxes as `RefKind::Unique { two_phase: false }`.

We do not recurse into fields of structs or other compound types, only "bare" references/... get retagged.

### Deallocating memory

Memory deallocation first acts like a write access through the pointer used for deallocation.
Expand Down