Stop emitting UbChecks on every Vec→Slice #150265

scottmcm · 2025-12-22T12:33:17Z

Spotted this in #148766's test changes. It doesn't seem like this ubcheck would catch anything useful; let's see if skipping it helps perf. (After all, this is inside every [] on a vec, among other things.)

scottmcm · 2025-12-22T12:41:20Z

@bors try @rust-timer queue

Stop emitting UbChecks on every Vec→Slice

rust-bors · 2025-12-22T14:58:52Z

☀️ Try build successful (CI)
Build commit: d3405d7 (d3405d79c1b6f5617f04715f0b349c565c6a743d, parent: 000ccd651d6dfeab13f7703d92a5fd7a9ff7510f)

rust-timer · 2025-12-22T15:40:26Z

Finished benchmarking commit (d3405d7): comparison URL.

Overall result: ❌✅ regressions and improvements - please read the text below

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please do so in sufficient writing along with @rustbot label: +perf-regression-triaged. If not, please fix the regressions and do another perf run. If its results are neutral or positive, the label will be automatically removed.

@bors rollup=never
@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)	1.6%	[0.6%, 2.4%]	4
Regressions ❌ (secondary)	0.4%	[0.0%, 2.3%]	7
Improvements ✅ (primary)	-1.4%	[-6.0%, -0.1%]	28
Improvements ✅ (secondary)	-0.7%	[-2.2%, -0.1%]	7
All ❌✅ (primary)	-1.0%	[-6.0%, 2.4%]	32

Max RSS (memory usage)

Results (primary 1.5%, secondary 1.9%)

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

	mean	range	count
Regressions ❌ (primary)	3.8%	[1.3%, 6.7%]	6
Regressions ❌ (secondary)	3.3%	[0.9%, 6.1%]	14
Improvements ✅ (primary)	-5.4%	[-7.2%, -3.5%]	2
Improvements ✅ (secondary)	-1.9%	[-2.6%, -1.5%]	5
All ❌✅ (primary)	1.5%	[-7.2%, 6.7%]	8

Cycles

Results (primary -1.0%, secondary -1.9%)

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

	mean	range	count
Regressions ❌ (primary)	2.6%	[2.3%, 2.8%]	2
Regressions ❌ (secondary)	2.3%	[2.1%, 2.8%]	3
Improvements ✅ (primary)	-2.4%	[-2.7%, -2.3%]	5
Improvements ✅ (secondary)	-4.0%	[-6.5%, -2.1%]	6
All ❌✅ (primary)	-1.0%	[-2.7%, 2.8%]	7

Binary size

Results (primary -0.2%, secondary -0.5%)

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

	mean	range	count
Regressions ❌ (primary)	0.2%	[0.0%, 1.2%]	11
Regressions ❌ (secondary)	0.6%	[0.0%, 1.2%]	2
Improvements ✅ (primary)	-0.3%	[-0.9%, -0.0%]	64
Improvements ✅ (secondary)	-0.6%	[-3.6%, -0.0%]	20
All ❌✅ (primary)	-0.2%	[-0.9%, 1.2%]	75

Bootstrap: 481.34s -> 483.129s (0.37%)
Artifact size: 390.37 MiB -> 390.63 MiB (0.07%)

rustbot · 2025-12-22T22:17:51Z

r? @ibraheemdev

rustbot has assigned @ibraheemdev.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

scottmcm · 2025-12-22T22:19:22Z

Reconfirming after rebasing, but should be basically the same
@bors try @rust-timer queue

Stop emitting UbChecks on every Vec→Slice

scottmcm · 2025-12-22T22:21:03Z

library/core/src/slice/cmp.rs

-    fn not_equal(&self, other: &[B]) -> bool {
-        !self.equal(other)
-    }


Annot: nothing actually overrode this anywhere, so removed it in favour of the usual PartialEq::ne.

scottmcm · 2025-12-22T22:33:21Z

tests/mir-opt/jump_threading.chained_conditions.JumpThreading.panic-abort.diff

+          StorageLive(_38);
+          _36 = copy _29 as &[u8] (Transmute);
+          _38 = copy _28 as &[u8] (Transmute);
+          _7 = <[u8] as PartialEq>::eq(move _36, move _38) -> [return: bb19, unwind unreachable];


annot: note that we're still inlining the whole &String → &str → &u8 part (since it'll essentially disappear in LLVM), just stopping at <[_]>::eq which sharing at the MIR level is probably best.

rust-bors · 2025-12-23T00:41:17Z

☀️ Try build successful (CI)
Build commit: fa07ba2 (fa07ba28fd0d71635c2fcc8ab0cb7944dd5fea85, parent: 04813e4de86a5e024e71756ef092637aa862c984)

rust-timer · 2025-12-23T01:22:03Z

Finished benchmarking commit (fa07ba2): comparison URL.

Overall result: ❌✅ regressions and improvements - please read the text below

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please do so in sufficient writing along with @rustbot label: +perf-regression-triaged. If not, please fix the regressions and do another perf run. If its results are neutral or positive, the label will be automatically removed.

@bors rollup=never
@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)	1.4%	[0.3%, 2.6%]	5
Regressions ❌ (secondary)	0.6%	[0.0%, 2.5%]	5
Improvements ✅ (primary)	-1.5%	[-6.0%, -0.2%]	26
Improvements ✅ (secondary)	-0.6%	[-2.0%, -0.1%]	10
All ❌✅ (primary)	-1.0%	[-6.0%, 2.6%]	31

Max RSS (memory usage)

Results (primary -0.1%, secondary 1.6%)

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

	mean	range	count
Regressions ❌ (primary)	4.3%	[2.1%, 5.4%]	3
Regressions ❌ (secondary)	3.5%	[0.8%, 5.8%]	9
Improvements ✅ (primary)	-4.6%	[-7.7%, -2.3%]	3
Improvements ✅ (secondary)	-1.7%	[-2.2%, -1.4%]	5
All ❌✅ (primary)	-0.1%	[-7.7%, 5.4%]	6

Cycles

Results (primary -1.0%, secondary -1.4%)

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

	mean	range	count
Regressions ❌ (primary)	2.2%	[2.2%, 2.2%]	1
Regressions ❌ (secondary)	2.5%	[2.2%, 2.9%]	3
Improvements ✅ (primary)	-2.6%	[-2.8%, -2.3%]	2
Improvements ✅ (secondary)	-3.3%	[-6.9%, -1.4%]	6
All ❌✅ (primary)	-1.0%	[-2.8%, 2.2%]	3

Binary size

Results (primary -0.2%, secondary -0.5%)

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

	mean	range	count
Regressions ❌ (primary)	0.2%	[0.0%, 1.2%]	8
Regressions ❌ (secondary)	0.6%	[0.0%, 1.2%]	2
Improvements ✅ (primary)	-0.3%	[-0.9%, -0.0%]	57
Improvements ✅ (secondary)	-0.6%	[-3.7%, -0.0%]	20
All ❌✅ (primary)	-0.2%	[-0.9%, 1.2%]	65

Bootstrap: 481.395s -> 479.879s (-0.31%)
Artifact size: 390.31 MiB -> 390.61 MiB (0.08%)

scottmcm · 2025-12-24T07:17:15Z

With the tweak to the attribute location, let's double-check it again
@bors try @rust-timer queue

Stop emitting UbChecks on every Vec→Slice

rust-bors · 2025-12-24T09:29:11Z

☀️ Try build successful (CI)
Build commit: 1c6122f (1c6122f63ca256ab772930aae21c88b8f1e8467a, parent: 5a7ad8ee060eb429a28d844080cfe6938d550b19)

rust-timer · 2025-12-24T10:10:03Z

Finished benchmarking commit (1c6122f): comparison URL.

Overall result: ❌✅ regressions and improvements - please read the text below

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please do so in sufficient writing along with @rustbot label: +perf-regression-triaged. If not, please fix the regressions and do another perf run. If its results are neutral or positive, the label will be automatically removed.

@bors rollup=never
@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)	1.6%	[1.1%, 2.2%]	2
Regressions ❌ (secondary)	1.1%	[0.1%, 2.1%]	2
Improvements ✅ (primary)	-1.4%	[-5.8%, -0.1%]	26
Improvements ✅ (secondary)	-0.5%	[-1.5%, -0.1%]	7
All ❌✅ (primary)	-1.2%	[-5.8%, 2.2%]	28

Max RSS (memory usage)

Results (primary 0.9%, secondary -3.3%)

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

	mean	range	count
Regressions ❌ (primary)	3.7%	[2.3%, 5.2%]	3
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-3.4%	[-4.4%, -2.4%]	2
Improvements ✅ (secondary)	-3.3%	[-3.3%, -3.3%]	1
All ❌✅ (primary)	0.9%	[-4.4%, 5.2%]	5

Cycles

Results (primary -3.1%, secondary 1.3%)

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)	4.2%	[4.2%, 4.2%]	1
Improvements ✅ (primary)	-3.1%	[-3.3%, -3.0%]	4
Improvements ✅ (secondary)	-1.6%	[-1.6%, -1.6%]	1
All ❌✅ (primary)	-3.1%	[-3.3%, -3.0%]	4

Binary size

Results (primary -0.2%, secondary -0.3%)

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

	mean	range	count
Regressions ❌ (primary)	0.2%	[0.0%, 1.1%]	9
Regressions ❌ (secondary)	0.6%	[0.0%, 1.1%]	2
Improvements ✅ (primary)	-0.3%	[-0.9%, -0.0%]	60
Improvements ✅ (secondary)	-0.4%	[-1.3%, -0.0%]	19
All ❌✅ (primary)	-0.2%	[-0.9%, 1.1%]	69

Bootstrap: 482.036s -> 484.285s (0.47%)
Artifact size: 392.48 MiB -> 392.35 MiB (-0.03%)

Spotted this in PR148766's test changes. It doesn't seem like this ubcheck would catch anything useful; let's see if skipping it helps perf.

rustbot · 2026-01-09T01:14:14Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

scottmcm · 2026-01-09T01:45:04Z

@rustbot reroll

jhpratt · 2026-01-09T06:10:27Z

@bors r+ rollup=never

rust-bors · 2026-01-09T06:10:31Z

📌 Commit c48df5d has been approved by jhpratt

It is now in the queue for this repository.

rust-bors · 2026-01-09T12:19:13Z

☀️ Test successful - CI
Approved by: jhpratt
Pushing 85d0cdf to main...

github-actions · 2026-01-09T12:22:15Z

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 3fda0e4 (parent) -> 85d0cdf (this PR)

Test differences

Show 268 test diffs

268 doctest diffs were found. These are ignored, as they are noisy.

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \
    test-dashboard 85d0cdfe3489ff1a4b86daeddba6fcf82b47bd65 --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

pr-check-1: 1529.9s -> 1916.3s (+25.3%)
dist-apple-various: 4285.9s -> 3506.0s (-18.2%)
x86_64-rust-for-linux: 2711.7s -> 3191.3s (+17.7%)
x86_64-gnu-aux: 7576.7s -> 6612.3s (-12.7%)
x86_64-gnu-tools: 3314.0s -> 3720.5s (+12.3%)
x86_64-gnu-gcc: 3408.2s -> 3809.6s (+11.8%)
x86_64-gnu-miri: 4477.3s -> 4972.6s (+11.1%)
x86_64-gnu-llvm-21-1: 4122.5s -> 4526.0s (+9.8%)
aarch64-gnu-debug: 3917.5s -> 4296.9s (+9.7%)
aarch64-apple: 9532.5s -> 10434.1s (+9.5%)

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 · 2026-01-09T13:01:33Z

Finished benchmarking commit (85d0cdf): comparison URL.

Overall result: ❌✅ regressions and improvements - please read the text below

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)	1.3%	[0.1%, 2.6%]	3
Regressions ❌ (secondary)	2.5%	[2.5%, 2.5%]	1
Improvements ✅ (primary)	-1.5%	[-5.8%, -0.1%]	25
Improvements ✅ (secondary)	-0.5%	[-1.4%, -0.1%]	6
All ❌✅ (primary)	-1.2%	[-5.8%, 2.6%]	28

Max RSS (memory usage)

Results (primary 1.5%, secondary 1.0%)

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

	mean	range	count
Regressions ❌ (primary)	5.9%	[3.1%, 10.5%]	4
Regressions ❌ (secondary)	3.6%	[3.6%, 3.6%]	1
Improvements ✅ (primary)	-4.4%	[-7.4%, -0.8%]	3
Improvements ✅ (secondary)	-1.6%	[-1.6%, -1.6%]	1
All ❌✅ (primary)	1.5%	[-7.4%, 10.5%]	7

Cycles

Results (primary -1.3%, secondary 1.0%)

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

	mean	range	count
Regressions ❌ (primary)	2.6%	[2.6%, 2.6%]	1
Regressions ❌ (secondary)	2.5%	[2.5%, 2.5%]	2
Improvements ✅ (primary)	-2.6%	[-2.9%, -2.4%]	3
Improvements ✅ (secondary)	-2.0%	[-2.0%, -2.0%]	1
All ❌✅ (primary)	-1.3%	[-2.9%, 2.6%]	4

Binary size

Results (primary -0.3%, secondary -0.3%)

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

	mean	range	count
Regressions ❌ (primary)	0.2%	[0.0%, 1.1%]	5
Regressions ❌ (secondary)	0.4%	[0.1%, 1.1%]	3
Improvements ✅ (primary)	-0.3%	[-1.6%, -0.0%]	42
Improvements ✅ (secondary)	-0.5%	[-1.3%, -0.0%]	14
All ❌✅ (primary)	-0.3%	[-1.6%, 1.1%]	47

Bootstrap: 474.629s -> 473.885s (-0.16%)
Artifact size: 390.98 MiB -> 390.84 MiB (-0.03%)

scottmcm · 2026-01-09T19:17:35Z

Wins clearly outweigh regressions here: check, debug, and doc are all green. opt is mixed -0.00% overall, with green for primary.

The one notable red is in syn-opt-full, which loses the gains it had in #148766, but I think overall this is still good. Might be interesting future work to see why it specifically got hit by this, though.

@rustbot label: +perf-regression-triaged

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rust-bors bot added a commit that referenced this pull request Dec 22, 2025

Auto merge of #150265 - scottmcm:vec-less-ubchecks, r=<try>

d3405d7

Stop emitting UbChecks on every Vec→Slice

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Dec 22, 2025

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scottmcm force-pushed the vec-less-ubchecks branch from 9ca160d to fd8744f Compare December 22, 2025 22:08

scottmcm marked this pull request as ready for review December 22, 2025 22:17

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Auto merge of #150265 - scottmcm:vec-less-ubchecks, r=<try>

fa07ba2

Stop emitting UbChecks on every Vec→Slice

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Dec 22, 2025

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rust-bors bot added a commit that referenced this pull request Dec 24, 2025

Auto merge of #150265 - scottmcm:vec-less-ubchecks, r=<try>

1c6122f

Stop emitting UbChecks on every Vec→Slice

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scottmcm added 2 commits January 8, 2026 17:14

Stop emitting UbChecks on every Vec→Slice

5932078

Spotted this in PR148766's test changes. It doesn't seem like this ubcheck would catch anything useful; let's see if skipping it helps perf.

Move the rustc_no_mir_inline down a level

c48df5d

scottmcm force-pushed the vec-less-ubchecks branch from f9695bf to c48df5d Compare January 9, 2026 01:14

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rust-bors bot added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jan 9, 2026

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rust-bors bot added the merged-by-bors This PR was explicitly merged by bors. label Jan 9, 2026

rust-bors bot merged commit 85d0cdf into rust-lang:main Jan 9, 2026
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rustbot added this to the 1.94.0 milestone Jan 9, 2026

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Ignore #[doc(hidden)] items when computing trimmed paths for printing #148623

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rustbot added the perf-regression-triaged The performance regression has been triaged. label Jan 9, 2026

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Stop emitting UbChecks on every Vec→Slice #150265

Stop emitting UbChecks on every Vec→Slice #150265

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Overall result: ❌✅ regressions and improvements - please read the text below

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Overall result: ❌✅ regressions and improvements - please read the text below

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Overall result: ❌✅ regressions and improvements - please read the text below

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Test differences

Job duration changes

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Overall result: ❌✅ regressions and improvements - please read the text below

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