Revisit MIR opt levels

[wesleywiser]

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
• Fix variable debuginfo being optimized away at mir-opt-level=2  rust#103657 (comment)

About this issue

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In the meantime, if you have questions or ideas, ping the proposers
on Zulip (or elsewhere).

[scottmcm]

Linking another conversation:

• [mir-opt] SimplifyLocals should also clean up debuginfo rust#110702 (comment)