Stop considering moved-out locals when computing auto traits for generators #112279

nbdd0121 · 2023-06-04T16:01:08Z

Addresses #94067 (but does not fix it since drop-tracking-mir is unstable)

This PR, unlike #110420 or #112050, does not attempt to reduce the number of live locals across suspension points. It however ignores moved-out locals for trait purposes. So this PR solves the non-send issue, but not the size issue.

Suggested by @RalfJung in rust-lang/unsafe-code-guidelines#188

cc @b-naber who's working on this from a different perspective.

r? @cjgillot

rustbot · 2023-06-04T16:01:15Z

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

cjgillot · 2023-06-11T13:07:36Z

compiler/rustc_mir_transform/src/generator.rs

+            if !init_locals.contains(local) {
+                // If only the storage is required to be live, but local is not initialized, then we can
+                // ignore such type for auto trait purposes.
+                true


Why do we require storage to be live if the locals is not initialized? Should the general algorithm be changed to avoid requiring storage? That'd kill 2 birds with one stone: the spurious auto trait limitation and the generator size.

As a general rule, I'd rather keep the "storage" and the "traits" algorithm as close to each other as possible: any deviation between the two is a potential source of unsoundness. This exact change is probably not, but the general rule should stand.

That's currently an open question rust-lang/unsafe-code-guidelines#188 and is much harder to solve.

cjgillot · 2023-07-08T15:43:35Z

There is a meeting on july 24th to discuss soundness of -Zdrop-tracking-mir. I'm waiting for this meeting before doing any review here.

bors · 2023-08-09T18:43:25Z

☔ The latest upstream changes (presumably #88936) made this pull request unmergeable. Please resolve the merge conflicts.

nbdd0121 · 2023-08-18T15:44:12Z

What's the outcome from the meeting?

cjgillot · 2023-08-24T16:30:52Z

The meeting happened and decided to stabilize -Zdrop-tracking-mir.

On the change itself: I'd still prefer to be prudent about any divergence between the "trait" algorithm and the "storage" algorithm.
I understand that this PR emulates drop elaboration for the "trait" algorithm: if a local is moved-from on all paths that lead to a drop, then it does not need dropping.
Moving drop elaboration earlier has long been in the borrowck wishlist, but is hard enough to stay there long.

My proposal:

document that this attempts to emulate drop elaboration, a kind of implicit RemoveUninitDrops pass;
wait after Enable -Zdrop-tracking-mir by default #107421 merges;
run perf then to check we don't have a regression (MaybeInitializedLocals can be quite slow, as it tracks places, not just locals).

bors · 2023-09-23T20:49:54Z

☔ The latest upstream changes (presumably #107421) made this pull request unmergeable. Please resolve the merge conflicts.

These tests are identical to the ones without drop-track prefix, so remove.

…rators

cjgillot · 2023-10-04T21:14:38Z

I said I would run perf, and forgot.
@bors try @rust-timer queue

bors · 2023-10-04T21:15:49Z

⌛ Trying commit 0259c6e with merge 258decf...

Stop considering moved-out locals when computing auto traits for generators Addresses rust-lang#94067 (but does not fix it since drop-tracking-mir is unstable) This PR, unlike rust-lang#110420 or rust-lang#112050, does not attempt to reduce the number of live locals across suspension points. It however ignores moved-out locals for trait purposes. So this PR solves the non-send issue, but not the size issue. Suggested by `@RalfJung` in [rust-lang/unsafe-code-guidelines#188](rust-lang/unsafe-code-guidelines#188 (comment)) cc `@b-naber` who's working on this from a different perspective. r? `@cjgillot`

cjgillot · 2023-10-04T21:18:32Z

One limitation of this PR is the interaction with drop-elaboration.
@RalfJung cited in #107421 (comment) an earlier adventure between drop elaboration and const checking.

I still have my concern about the auto-trait analysis and the layout analysis eventually drifting.

bors · 2023-10-04T22:29:52Z

☀️ Try build successful - checks-actions
Build commit: 258decf (258decfeb5bf65897ee095291294f5800ad13f51)

rust-timer · 2023-10-05T00:15:27Z

Finished benchmarking commit (258decf): comparison URL.

Overall result: ❌ regressions - ACTION NEEDED

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR may lead to changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please fix the regressions and do another perf run. If the next run shows neutral or positive results, the label will be automatically removed.

@bors rollup=never
@rustbot label: -S-waiting-on-perf +perf-regression

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	0.6%	[0.4%, 0.8%]	5
Regressions ❌ (secondary)	3.0%	[0.6%, 6.1%]	15
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.6%	[0.4%, 0.8%]	5

Max RSS (memory usage)

This benchmark run did not return any relevant results for this metric.

Cycles

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	3.7%	[2.8%, 6.4%]	7
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 623.925s -> 621.686s (-0.36%)
Artifact size: 272.03 MiB -> 272.00 MiB (-0.01%)

RalfJung · 2023-10-05T05:13:48Z

One limitation of this PR is the interaction with drop-elaboration.

So this passes uses post-drop-elaboration MIR? Yeah that would be concerning.
Can it use pre-drop-elaboration MIR instead?

cjgillot · 2023-10-05T10:51:54Z

One limitation of this PR is the interaction with drop-elaboration.

So this passes uses post-drop-elaboration MIR? Yeah that would be concerning. Can it use pre-drop-elaboration MIR instead?

The current pass uses pre-drop-elaboration MIR. This PR proposes to elaborate some drops as part of the analysis.

I'm not fully aware of the difficulties around drop elaboration, so we need a bit of background on the foreseeable difficulties.

apiraino · 2023-11-21T15:35:26Z

I think this might waiting on triaging the latest perf run

@rustbot author

Dylan-DPC · 2024-02-10T05:43:35Z

@nbdd0121 any updates on this?

Dylan-DPC · 2024-07-28T07:06:21Z

Closing this as inactive. Feel free to reöpen this pr or create a new pr if you get the time to work on this. Thanks

Stop considering moved-out locals when computing auto traits for generators (rebased) This PR revives rust-lang#112279. I wanted to reopen this to gauge `@cjgillot's` thoughts about this, since it's been a while since `-Zdrop-tracking-mir` has landed. If this PR looks OK, I can flesh it out and write up an FCP report -- I think this is a T-types FCP, since this has to do with the types that are considered live for auto traits, and doesn't otherwise affect the layout of coroutines. Open questions: * **"Why do we require storage to be live if the locals is not initialized?"** (rust-lang#112279 (comment)) * I agree that we should eventually fix the storage analysis for coroutines, but this seems like a problem that can be fixed after we fix witnesses for *the purposes of traits* here. * As far as I could tell, fixing the problem of moved locals for *storage* would require insertion of additional `StorageDead` statements, or would require further entangling the type system with the operational semantics in ways that T-opsem seemed uncomfortable with [when I asked](https://rust-lang.zulipchat.com/#narrow/stream/189540-t-compiler.2Fwg-mir-opt/topic/Inserting.20.60StorageDrop.60.20after.20unconditional.20moves). cc `@RalfJung` * Relevant: rust-lang/unsafe-code-guidelines#188 Fixes rust-lang#128095 Fixes rust-lang#94067 r? `@cjgillot`

rustbot assigned cjgillot Jun 4, 2023

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Jun 4, 2023

cjgillot reviewed Jun 11, 2023

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nbdd0121 added 2 commits September 24, 2023 04:58

Remove drop-track-field-assign[-nonsend] test

425e87e

These tests are identical to the ones without drop-track prefix, so remove.

Stop considering moved-out locals when computing auto traits for gene…

0259c6e

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nbdd0121 force-pushed the generator branch from 9652ea9 to 0259c6e Compare September 24, 2023 07:13

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compiler-errors mentioned this pull request Jul 24, 2024

rustc fails to prove Sendness for an async block where a !Send value is dropped before an .await point #128095

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Dylan-DPC closed this Jul 28, 2024

compiler-errors mentioned this pull request Aug 8, 2024

Stop considering moved-out locals when computing auto traits for generators (rebased) #128846

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Stop considering moved-out locals when computing auto traits for generators #112279

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Stop considering moved-out locals when computing auto traits for generators #112279

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rust-timer commented Oct 5, 2023

Overall result: ❌ regressions - ACTION NEEDED

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