A unified framework for dataflow analysis

[ecstatic-morse]

Meeting proposal info

After rust-lang/rust#64566, which was a prerequisite for dataflow-based const validation, there are two different frameworks for dataflow analysis in the compiler. I've written a prototype along with a design document for unifying the two frameworks.

The dataflow framework is an internal API (with the exception of one use in cargo-clippy), so this is a good time to improve it if desired. Accordingly, I've made some changes in the prototype, like passing Statements and Terminators directly into the effect methods, and making the signature of call_return_effect more similar to the others. If anyone has had difficulties implementing or inspecting the results of a dataflow analysis, it would be awesome to get your feedback here.

• Title: A unified framework for dataflow analysis
• Type: technical
• Proposed by: @ecstatic-morse
• Link: https://hackmd.io/@39Qr_z9cQhasi25sGjmFnA/Skvd9rztS
• Scheduled dates for discussion:
  • Not yet scheduled, we'll add content here when it gets scheduled

About this issue

This issue corresponds to a meeting proposal for the compiler team
steering meeting. It corresponds to a possible topic of
discussion. You can read more about the steering meeting procedure
here.

Comment policy

These issues are meant to be used as an "announcements channel"
regarding the proposal, and not as a place to discuss the technical
details. Feel free to subscribe to updates. We'll post comments when
reviewing the proposal in meetings or making a scheduling decision.
In the meantime, if you have questions or ideas, ping the proposers
on Zulip (or elsewhere).

[nikomatsakis]

We discussed this in our planning meeting today and decided to schedule it for discussion on Nov 1, 2019.

Calendar event

[mark-i-m]

Match in const fn here we come!

[nikomatsakis]

Update: Since @oli-obk will be unavailable, we've moved the date to Nov 8

[ecstatic-morse]

To clarify, this isn't blocking if and match in const fn. The necessary dataflow analysis was merged in rust-lang/rust#64566, and it has been used to check const bodies since rust-lang/rust#64470. It's just that this framework is completely separate from the existing BitDenotation: they don't share a ResultsCursor implementation, a graphviz visualizer, etc.

This design meeting is about merging the two frameworks into a single, shared one. Since this will require modifications wherever either framework is used, it is a good time to make changes to the existing API to make it easier to comprehend. It's also a good time to consider possible optimizations and extensions to this framework (extended basic blocks, customizable dataflow lattices, etc.) and make sure that the unified framework does not make any of these infeasible.

[nikomatsakis]

We had the meeting and added minutes here, closing.