Revisit MIR opt levels

Meeting proposal info

• Title: Revisit MIR opt levels
• Type: technical

Summary

The current system of MIR opt levels 0-4 is suboptimal, in part because there is confusion what the individual levels mean. As we continue to add new MIR optimizations and revise existing ones, we should take the chance to improve the opt level system.

One potential option is to remove the levels entirely and derive what passes and (and to what strength or complexity) from other options the user has provided such as -Copt-level and -Cdebuginfo. If there are additional axis that are important to capture, other flags could be introduced for those.

Another option is to redefine the opt levels. Having four levels is confusing as it doesn't intuitively map to the 0-2 levels provided by -Copt-level (although that flag also has s and z opt levels which are not present in MIR opt levels at all). We could decide to redefine MIR opt levels to be 0-2 based. One challenge here is defining what the levels themselves mean (when does an optimization fall into opt level 1 or opt level 2?).

Prior discussions:

• https://rust-lang.zulipchat.com/#narrow/stream/189540-t-compiler.2Fwg-mir-opt/topic/Revisiting.20MIR.20opt.20levels
• https://rust-lang.zulipchat.com/#narrow/stream/238009-t-compiler.2Fmeetings/topic/.5Bweekly.5D.202022-12-08/near/314703565
• https://rust-lang.zulipchat.com/#narrow/stream/238009-t-compiler.2Fmeetings/topic/.5Bweekly.5D.202022-12-08/near/314706273
• https://github.com/rust-lang/rust/pull/103657#issuecomment-1294199487

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).

Start with the MIR opt-level proposal and review the linked Zulip discussions and rust-lang/rust#103657 comment for prior context. The issue is a steering-meeting topic rather than an implementation task; work is complete when the compiler team reaches and records a decision about the opt-level design.