Skip computing extra liveness info if borrow_set is empty

[jackh726]

With polonius alpha, we currently compute liveness for a larger set of locals than NLL. However, this extra liveness only matters if there are borrows; so we can skip the extra work if there aren't any.

r? lqd

[jackh726]

@bors try @rust-timer queue

[jackh726]

Disclosure: An LLM was used to dig into the icu4x (and other) regressions, and identified that that and other cases have no loans - and so that this is a candidate fix.

[rust-bors]

☀️ Try build successful (CI)
Build commit: 4b205ca (4b205caca087d3a42150f9866c94ad2cad745472)
Base parent: 6d656b1 (6d656b1efca82491a110b476ac9bbe712628ccb6)

[rust-timer]

Finished benchmarking commit (4b205ca): 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
Regressions ❌ (secondary)	0.1%	[0.0%, 0.2%]	5
Improvements ✅ (primary)	-0.6%	[-1.5%, -0.1%]	15
Improvements ✅ (secondary)	-0.3%	[-0.8%, -0.1%]	16
All ❌✅ (primary)	-0.6%	[-1.5%, -0.1%]	15

Max RSS (memory usage)

Results (primary 1.3%, 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)	1.3%	[0.5%, 2.5%]	5
Regressions ❌ (secondary)	3.2%	[3.2%, 3.2%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.8%	[-0.8%, -0.8%]	1
All ❌✅ (primary)	1.3%	[0.5%, 2.5%]	5

Cycles

This perf run didn't have relevant results for this metric.

Binary size

This perf run didn't have relevant results for this metric.

Bootstrap: 468.493s -> 459.261s (-1.97%)
Artifact size: 398.93 MiB -> 398.95 MiB (0.01%)

[lqd]

The small regressions in the perf run look like noise to me.

r=me with:

• the comment + FIXME suggestion applied
• filling the PR description with a quick summary

@rustbot author

[jackh726]

@bors r=lqd

[rust-bors]

📌 Commit ea0a481 has been approved by lqd

It is now in the queue for this repository.

[rust-bors]

☀️ Test successful - CI
Approved by: lqd
Duration: 3h 4m 18s
Pushing c656540 to main...

[github-actions]

What is this?

This is an experimental post-merge analysis report that shows differences in test outcomes between the merged PR and its parent PR.

Comparing a872286 (parent) -> c656540 (this PR)

Test differences

No test diffs found

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \
    test-dashboard c656540d6467dee1381f0cbd882412d6bd1cd5ae --output-dir test-dashboard

And then open test-dashboard/index.html in your browser to see an overview of all executed tests.

Job duration changes

1. x86_64-msvc-ext3: 1h 56m -> 1h 6m (-43.2%)
2. x86_64-gnu-miri: 1h 31m -> 52m 53s (-42.3%)
3. i686-gnu-2: 1h 47m -> 1h 4m (-39.7%)
4. dist-x86_64-solaris: 1h 13m -> 1h 42m (+39.0%)
5. x86_64-gnu-stdlib-semver-check: 18m 49s -> 11m 43s (-37.7%)
6. dist-s390x-linux: 1h 6m -> 1h 29m (+35.1%)
7. x86_64-gnu-llvm-21-2: 1h 13m -> 1h 36m (+31.3%)
8. optional-x86_64-gnu-autodiff: 36m 56s -> 48m 29s (+31.3%)
9. x86_64-gnu-gcc-core-tests: 17m 41s -> 12m 14s (-30.8%)
10. aarch64-apple-1: 1h 35m -> 2h 3m (+30.1%)

How to interpret the job duration changes?

Job durations can vary a lot, based on the actual runner instance
that executed the job, system noise, invalidated caches, etc. The table above is provided
mostly for t-infra members, for simpler debugging of potential CI slow-downs.

[rust-timer]

Finished benchmarking commit (c656540): comparison URL.

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

Our benchmarks found a performance regression caused by this PR.
This might be an actual regression, but it can also be just noise.

Next Steps:

• If the regression was expected or you think it can be justified,
  please write a comment with sufficient written justification, and add
  @rustbot label: +perf-regression-triaged to it, to mark the regression as triaged.
• If you think that you know of a way to resolve the regression, try to create
  a new PR with a fix for the regression.
• If you do not understand the regression or you think that it is just noise,
  you can ask the @rust-lang/wg-compiler-performance working group for help (members of this group
  were already notified of this PR).

@rustbot label: +perf-regression
cc @rust-lang/wg-compiler-performance

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)	0.1%	[0.1%, 0.1%]	1
Regressions ❌ (secondary)	0.3%	[0.2%, 0.5%]	27
Improvements ✅ (primary)	-0.7%	[-1.5%, -0.1%]	11
Improvements ✅ (secondary)	-0.3%	[-0.9%, -0.0%]	19
All ❌✅ (primary)	-0.6%	[-1.5%, 0.1%]	12

Max RSS (memory usage)

Results (primary 3.4%)

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

	mean	range	count
Regressions ❌ (primary)	3.4%	[1.5%, 5.2%]	2
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	3.4%	[1.5%, 5.2%]	2

Cycles

Results (secondary -0.6%)

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)	1.7%	[1.7%, 1.7%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.9%	[-2.9%, -2.9%]	1
All ❌✅ (primary)	-	-	0

Binary size

Results (primary 0.0%)

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

	mean	range	count
Regressions ❌ (primary)	0.0%	[0.0%, 0.0%]	3
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.0%	[0.0%, 0.0%]	3

Bootstrap: 468.499s -> 470.079s (0.34%)
Artifact size: 400.07 MiB -> 400.19 MiB (0.03%)

[Mark-Simulacrum]

@rustbot label: +perf-regression-triaged

As mentioned above (#161252 (comment)), regressions are maybe noise and definitely outweighed by the improvements.