Add LLVM range attributes to slice length parameters #148350

scottmcm · 2025-10-31T23:13:03Z

It'll take a bunch more work to do this in layout -- because of cycles in struct Foo<'a>(&'a Foo<'a>); -- so until we figure out how to do that well, just look for slices specifically and add the proper range for the length.

rustbot · 2025-10-31T23:13:07Z

r? @JonathanBrouwer

rustbot has assigned @JonathanBrouwer.
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scottmcm · 2025-10-31T23:15:42Z

tests/codegen-llvm/slice-len-math.rs

+pub fn len_plus_ten_b(s: &[u32]) -> usize {
+    // CHECK: start:
+    // CHECK-NOT: add
+    // CHECK: %[[R:.+]] = add nuw nsw i{{.+}} %s.1, 10


This file is the simplest demonstration of LLVM taking advantage of it. See https://rust.godbolt.org/z/7jxqsaaTx showing that it didn't know nuw/nsw for these things previously.

scottmcm · 2025-10-31T23:17:15Z

@bors try @rust-timer queue

Add LLVM range attributes to slice length parameters

rust-bors · 2025-11-01T01:28:56Z

☀️ Try build successful (CI)
Build commit: 398e28a (398e28a631e9d9248dbf3cef0409d1b7a684b917, parent: 82ae0ee6487e93bd6c05167ccb2ef3485fdbc890)

rust-timer · 2025-11-01T02:39:44Z

Finished benchmarking commit (398e28a): 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)	0.2%	[0.2%, 0.2%]	1
Regressions ❌ (secondary)	1.0%	[0.1%, 2.0%]	20
Improvements ✅ (primary)	-0.4%	[-0.4%, -0.4%]	1
Improvements ✅ (secondary)	-0.4%	[-0.7%, -0.1%]	15
All ❌✅ (primary)	-0.1%	[-0.4%, 0.2%]	2

Max RSS (memory usage)

Results (secondary -1.1%)

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)	2.0%	[2.0%, 2.0%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-4.1%	[-4.1%, -4.1%]	1
All ❌✅ (primary)	-	-	0

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)	2.7%	[2.1%, 3.8%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.5%	[-2.9%, -2.1%]	2
All ❌✅ (primary)	-	-	0

Binary size

Results (primary -0.1%, secondary -0.1%)

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)	0.1%	[0.1%, 0.1%]	1
Improvements ✅ (primary)	-0.1%	[-0.2%, -0.0%]	14
Improvements ✅ (secondary)	-0.1%	[-0.1%, -0.1%]	37
All ❌✅ (primary)	-0.1%	[-0.2%, -0.0%]	14

Bootstrap: 475.716s -> 474.193s (-0.32%)
Artifact size: 390.89 MiB -> 390.71 MiB (-0.04%)

scottmcm · 2025-11-01T06:26:40Z

The size numbers are really interesting here. 112KB off librustc_driver.so, which has to be coming from a whole bunch of stuff optimizing out from knowing more about the lengths, since there's no other changes here.

I think instruction-wide it's close enough to neutral to be tolerable. Especially since the bootstrap time is green.

Kobzol · 2025-11-01T10:08:41Z

Note that the compiler toolchain size sometimes changes e.g. by tens of KiB even on no-op/README changes :( Probably due to BOLT/PGO.

JonathanBrouwer · 2025-11-01T11:23:01Z

I've reviewed the code and it is correct, the tests also look good.

I'm however struggling to interpret the performance results. From my experience the bootstrap time & toolchain size are so noisy that I don't even look at them. The benchmarks are a bit more red than I like (and expect from this change) tho, I don't really understand why. Especially check builds should not be affected by this change, since this code does not even run for them. Out of interest, lets rerun perf and see how reproducible the results are: @rust-timer queue

(To other reviewers, feel free to r=me if you can explain the perf)

JonathanBrouwer · 2025-11-01T11:25:41Z

Do we need to make a new build?
@bors try

Add LLVM range attributes to slice length parameters

rust-bors · 2025-11-01T13:36:18Z

☀️ Try build successful (CI)
Build commit: 2d5ecaf (2d5ecaf90d3d6ce5f705cf9a7cd2b5821e180a9d, parent: d85276b256a8ab18e03b6394b4f7a7b246176db7)

rust-timer · 2025-11-01T19:01:00Z

Finished benchmarking commit (2d5ecaf): 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)	0.2%	[0.2%, 0.2%]	1
Regressions ❌ (secondary)	1.0%	[0.1%, 1.9%]	20
Improvements ✅ (primary)	-1.2%	[-3.0%, -0.3%]	3
Improvements ✅ (secondary)	-0.3%	[-0.6%, -0.2%]	17
All ❌✅ (primary)	-0.9%	[-3.0%, 0.2%]	4

Max RSS (memory usage)

Results (primary 7.6%, secondary -3.1%)

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

	mean	range	count
Regressions ❌ (primary)	7.6%	[7.6%, 7.6%]	1
Regressions ❌ (secondary)	3.1%	[3.1%, 3.1%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.6%	[-5.7%, -0.9%]	12
All ❌✅ (primary)	7.6%	[7.6%, 7.6%]	1

Cycles

Results (primary -2.8%, 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
Regressions ❌ (secondary)	4.1%	[0.8%, 8.6%]	6
Improvements ✅ (primary)	-2.8%	[-2.8%, -2.8%]	1
Improvements ✅ (secondary)	-4.2%	[-7.0%, -1.5%]	5
All ❌✅ (primary)	-2.8%	[-2.8%, -2.8%]	1

Binary size

Results (primary -0.2%, secondary -0.1%)

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)	0.1%	[0.1%, 0.1%]	1
Improvements ✅ (primary)	-0.2%	[-1.1%, -0.0%]	16
Improvements ✅ (secondary)	-0.1%	[-0.1%, -0.1%]	37
All ❌✅ (primary)	-0.2%	[-1.1%, -0.0%]	16

Bootstrap: 476.797s -> 476.639s (-0.03%)
Artifact size: 390.82 MiB -> 388.62 MiB (-0.56%)

JonathanBrouwer · 2025-11-01T20:39:50Z

Ok so it is reproducible, good to know :)

Still very confusing perf tho. I think that both improvements in the primary benchmarks may actually be noise, looking at the history graph.

I think the perf is probably good enough to merge regardless? I'm not sure though, re-assigning to someone else to make the final decision r? compiler

r=me,you if you agree performance is acceptable, I already reviewed the code and that looks good

davidtwco

I think the perf results are okay, the primary benchmarks basically just see small improvements and its only really secondary benchmarks that have small regressions. More information is typically good and could be used more by LLVM in future, so I'd prefer to output this.

View changes since this review

davidtwco · 2025-11-03T16:45:43Z

@bors r+

bors · 2025-11-03T16:45:45Z

📌 Commit 0ba7bcf has been approved by davidtwco

It is now in the queue for this repository.

JonathanBrouwer · 2025-11-03T17:20:30Z

@bors r=JonathanBrouwer,davidtwco rollup=never

bors · 2025-11-03T17:20:32Z

💡 This pull request was already approved, no need to approve it again.

There's another pull request that is currently being tested, blocking this pull request: Rollup of 8 pull requests #148446

bors · 2025-11-03T17:20:33Z

📌 Commit 0ba7bcf has been approved by JonathanBrouwer,davidtwco

It is now in the queue for this repository.

bors · 2025-11-03T19:36:47Z

⌛ Testing commit 0ba7bcf with merge 20383c9...

bors · 2025-11-03T22:45:32Z

☀️ Test successful - checks-actions
Approved by: JonathanBrouwer,davidtwco
Pushing 20383c9 to master...

github-actions · 2025-11-03T22:48:24Z

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 1f880d9 (parent) -> 20383c9 (this PR)

Test differences

Show 4 test diffs

Stage 1

[codegen] tests/codegen-llvm/slice-len-math.rs: [missing] -> pass (J0)

Stage 2

[codegen] tests/codegen-llvm/slice-len-math.rs: [missing] -> pass (J1)

Additionally, 2 doctest diffs were found. These are ignored, as they are noisy.

Job group index

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \
    test-dashboard 20383c9f1d84eb9b9c6668a1668ef68a81eae274 --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

dist-aarch64-apple: 8534.6s -> 6996.5s (-18.0%)
dist-apple-various: 4254.6s -> 3609.7s (-15.2%)
x86_64-gnu-llvm-20-3: 6505.1s -> 5581.8s (-14.2%)
pr-check-1: 1710.1s -> 1482.8s (-13.3%)
aarch64-apple: 10260.3s -> 8956.0s (-12.7%)
dist-loongarch64-musl: 5787.4s -> 5141.4s (-11.2%)
armhf-gnu: 5139.6s -> 4589.6s (-10.7%)
dist-x86_64-windows-gnullvm: 5314.8s -> 4780.0s (-10.1%)
x86_64-gnu-llvm-21-3: 6246.9s -> 5679.4s (-9.1%)
dist-powerpc64-linux: 5682.5s -> 5168.6s (-9.0%)

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 · 2025-11-04T16:55:52Z

Finished benchmarking commit (20383c9): 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)	0.2%	[0.2%, 0.2%]	1
Regressions ❌ (secondary)	0.7%	[0.1%, 2.0%]	29
Improvements ✅ (primary)	-0.3%	[-0.5%, -0.1%]	6
Improvements ✅ (secondary)	-0.3%	[-0.7%, -0.1%]	27
All ❌✅ (primary)	-0.3%	[-0.5%, 0.2%]	7

Max RSS (memory usage)

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

Cycles

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

Binary size

Results (primary -0.1%, secondary -0.1%)

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)	0.1%	[0.1%, 0.1%]	1
Improvements ✅ (primary)	-0.1%	[-0.2%, -0.1%]	14
Improvements ✅ (secondary)	-0.1%	[-0.1%, -0.1%]	37
All ❌✅ (primary)	-0.1%	[-0.2%, -0.1%]	14

Bootstrap: 474.478s -> 472.257s (-0.47%)
Artifact size: 390.89 MiB -> 390.71 MiB (-0.05%)

Add LLVM range attributes to slice length parameters

0ba7bcf

rustbot assigned JonathanBrouwer Oct 31, 2025

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Auto merge of #148350 - scottmcm:slice-range-attributes, r=<try>

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Add LLVM range attributes to slice length parameters

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Auto merge of #148350 - scottmcm:slice-range-attributes, r=<try>

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Add LLVM range attributes to slice length parameters

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rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Nov 1, 2025

rustbot assigned davidtwco and unassigned JonathanBrouwer Nov 1, 2025

davidtwco approved these changes Nov 3, 2025

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bors 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 Nov 3, 2025

bors added the merged-by-bors This PR was explicitly merged by bors. label Nov 3, 2025

bors merged commit 20383c9 into rust-lang:master Nov 3, 2025
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rustbot added this to the 1.93.0 milestone Nov 3, 2025

Add LLVM range attributes to slice length parameters #148350

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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

Stage 1

Stage 2

Job duration changes

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

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