Defer extra liveness calculation for Polonius Alpha by jackh726 · Pull Request #161938 · rust-lang/rust

jackh726 · 2026-08-28T15:29:27Z

Best reviewed by commit.

This moves liveness calculation of NLL-boring/Polonius-relevant locals to be lazy. This allows us to skip unnecessary work.

r? lqd

jackh726 · 2026-08-28T15:29:52Z

@bors try @rust-time queue

Defer extra liveness calculation for Polonius Alpha

jackh726 · 2026-08-28T15:31:57Z

@rust-timer queue

jackh726 · 2026-08-28T15:34:44Z

Disclosure: An LLM was used for experiments here, but all code here is written fresh.

rust-bors · 2026-08-28T15:39:48Z

💔 Test for 254d008 failed: CI. Failed job:

try - dist-x86_64-linux-quick (web logs, enhanced plaintext logs)

jackh726 · 2026-08-28T16:33:28Z

@bors try

Defer extra liveness calculation for Polonius Alpha

rust-bors · 2026-08-28T17:34:15Z

☀️ Try build successful (CI)
Build commit: f904a04 (f904a046d815a8f17021ba39511de661f74c1612)
Base parent: 344f790 (344f7902949345394fa40a5d7dda31f012ccbc0d)

rust-timer · 2026-08-28T18:17:13Z

Finished benchmarking commit (f904a04): comparison URL.

Overall result: ❌✅ regressions and improvements - please read:

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

Next, please: If you can, justify the regressions found in this try perf run in writing along with @rustbot label: +perf-regression-triaged. If not, fix the regressions and do another perf run. Neutral or positive results will clear the label automatically.

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

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	0.4%	[0.3%, 0.5%]	11
Regressions ❌ (secondary)	1.2%	[0.1%, 3.5%]	20
Improvements ✅ (primary)	-3.3%	[-11.9%, -0.2%]	18
Improvements ✅ (secondary)	-4.5%	[-11.6%, -0.1%]	19
All ❌✅ (primary)	-1.9%	[-11.9%, 0.5%]	29

Max RSS (memory usage)

Results (primary -2.0%, secondary 1.2%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	2.9%	[0.8%, 9.2%]	16
Improvements ✅ (primary)	-2.0%	[-2.0%, -2.0%]	1
Improvements ✅ (secondary)	-4.4%	[-5.3%, -3.1%]	5
All ❌✅ (primary)	-2.0%	[-2.0%, -2.0%]	1

Cycles

Results (primary -3.6%, secondary -5.5%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	4.6%	[4.2%, 5.0%]	2
Improvements ✅ (primary)	-3.6%	[-4.4%, -3.0%]	6
Improvements ✅ (secondary)	-6.9%	[-19.1%, -2.3%]	14
All ❌✅ (primary)	-3.6%	[-4.4%, -3.0%]	6

Binary size

Results (secondary -0.0%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.0%	[-0.0%, -0.0%]	4
All ❌✅ (primary)	-	-	0

Bootstrap: 475.673s -> 475.822s (0.03%)
Artifact size: 403.60 MiB -> 402.91 MiB (-0.17%)

jackh726 · 2026-09-04T15:03:55Z

@craterbot check

As discussed today, this adds more region constraints for points that have drop locations but are not drop live - which could have some implications on things that use sccs.

craterbot · 2026-09-04T15:04:39Z

👌 Experiment pr-161938 created and queued.
🤖 Automatically detected try build f904a04
⚠️ Try build based on commit 29b9558, but latest commit is fead75b. Did you forget to make a new try build?
🔍 You can check out the queue and this experiment's details.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

…rsal type hold liveness and live_region_variances

jackh726 · 2026-09-08T02:17:36Z

@craterbot abort

We no longer register region constraints without checking if there are drop-live points.

craterbot · 2026-09-08T02:17:51Z

🗑️ Experiment pr-161938 deleted!

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

lqd · 2026-09-08T05:48:02Z

We no longer register region constraints without checking if there are drop-live points.

I’ve already read the PR but the force pushes make it hard to see this change. What code changed from our last discussion about this?

jackh726 · 2026-09-08T12:10:58Z

+        // them, but *only if the local is drop-live*.
+        // It doesn't really make sense to only check drop-liveness but defer use-liveness,
+        // so we just treat this as eager.
+        if drop_data.region_constraint_data.is_some() {


If there are region constraints to push, we compute everything eagerly (which gates pushing them on drop-liveness).

View changes since the review

rust-bors · 2026-09-09T06:32:05Z

☔ The latest upstream changes (presumably #162488) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.

rustbot added the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label Aug 28, 2026

rustbot assigned lqd Aug 28, 2026

rustbot added the T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. label Aug 28, 2026

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rust-bors Bot pushed a commit that referenced this pull request Aug 28, 2026

Auto merge of #161938 - jackh726:polonius-lazy-liveness-final, r=<try>

254d008

Defer extra liveness calculation for Polonius Alpha

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rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Aug 28, 2026

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rust-cloud-vms Bot force-pushed the polonius-lazy-liveness-final branch 3 times, most recently from ef46fa3 to 29b9558 Compare August 28, 2026 15:52

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rust-bors Bot pushed a commit that referenced this pull request Aug 28, 2026

Auto merge of #161938 - jackh726:polonius-lazy-liveness-final, r=<try>

f904a04

Defer extra liveness calculation for Polonius Alpha

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rustbot added perf-regression Performance regression. and removed S-waiting-on-perf Status: Waiting on a perf run to be completed. labels Aug 28, 2026

panstromek mentioned this pull request Aug 29, 2026

Store locals in borrowck in BitSets #161930

Draft

jackh726 mentioned this pull request Aug 31, 2026

Store LiveLoans more densely packed #161850

Merged

lqd mentioned this pull request Aug 31, 2026

A series of Polonius Alpha refactors #161937

Merged

rust-cloud-vms Bot force-pushed the polonius-lazy-liveness-final branch from 29b9558 to fead75b Compare September 1, 2026 17:12

jackh726 marked this pull request as ready for review September 1, 2026 17:13

rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Sep 1, 2026

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craterbot added S-waiting-on-crater Status: Waiting on a crater run to be completed. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Sep 4, 2026

rust-cloud-vms Bot force-pushed the polonius-lazy-liveness-final branch 4 times, most recently from 808923f to e24813e Compare September 7, 2026 13:27

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jackh726 added 4 commits September 8, 2026 01:03

Make polonius constraint graph traversal two-phase and make the trave…

f425d5d

…rsal type hold liveness and live_region_variances

Refactor LivenessResults into LivenessCalculation, without typeck

9e2030a

Basics needed for deferred liveness

97b7b58

Defer nll-boring/polonius-relevant locals

5166f9c

rust-cloud-vms Bot force-pushed the polonius-lazy-liveness-final branch from e24813e to 5166f9c Compare September 8, 2026 01:19

craterbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-crater Status: Waiting on a crater run to be completed. labels Sep 8, 2026

jackh726 commented Sep 8, 2026

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rust-timer commented Aug 28, 2026

Overall result: ❌✅ regressions and improvements - please read:

Instruction count

Max RSS (memory usage)

Cycles

Binary size

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jackh726 commented Sep 4, 2026

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