Optimize `checked_ilog` and `pow` when `base` is a power of two by Kmeakin · Pull Request #147250 · rust-lang/rust

Kmeakin · 2025-10-02T00:33:17Z

Optimize checked_ilog and pow when the base is a power of two

rustbot · 2025-10-02T00:33:22Z

rustbot has assigned @scottmcm.
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workingjubilee · 2025-10-02T02:09:34Z

does this affect the codegen in practical circumstances? I would expect even a fairly weak optimizer to perform this optimization, making us hand-coding it irrelevant and potentially even harmful because we introduce more conditional logic to chew through.

Kmeakin · 2025-10-02T02:21:24Z

does this affect the codegen in practical circumstances?

It replaces a loop by some bit manipulations. Whether anyone actually calls either function with a compile-time known, power of 2 base is another question. But it can't hurt, since it is guarded by is_statically_known.

I would expect even a fairly weak optimizer to perform this optimization, making us hand-coding it irrelevant

No, in either function, LLVM is not able to see that the loop is performing a log/pow and apply the identities: https://godbolt.org/z/6vMsxc9Kh

and potentially even harmful because we introduce more conditional logic to chew through.

They're guarded by is_statically_known so will be ignored if the base is not known at compile-time

workingjubilee · 2025-10-02T02:40:04Z

No, in either function, LLVM is not able to see that the loop is performing a log/pow and apply the identities: https://godbolt.org/z/6vMsxc9Kh

...then I'm kinda surprised! Nice catch.

scottmcm · 2025-10-02T06:01:57Z

does this affect the codegen in practical circumstances?

Would be good to have codegen tests to demonstrate what this is doing -- especially since that way there's a way for people to try removing the special cases later if they think that LLVM no longer needs them.

workingjubilee · 2025-10-05T18:10:15Z

thanks for the codegen tests!

Kmeakin · 2025-10-05T21:25:43Z

@rustbot ready

Kmeakin · 2025-10-10T22:51:45Z

@scottmcm ping?

scottmcm

The code here is looking good, but can you make sure there's normal runtime behaviour tests for it too? Notably, after the base conversation (that's fixed in the code) it made me think that that's not currently covered by any tests, so we should have something -- maybe just add to the # Examples that it returns None for base zero or one? (They seem like perfectly reasonable and helpful examples, in addition to giving coverage for this stuff.)

And maybe add some should_panic tests to ensure that the overflow checking is still correct for pow when overflow checks are enabled?

View changes since this review

rust-bors · 2026-06-02T08:03:21Z

☀️ Try build successful (CI)
Build commit: 759376f (759376f9a4e9acc6447f3f4933a190e8a1bb0ba1, parent: 4f84d9fac456d973d592cf3fb48db958ecf22506)

rust-timer · 2026-06-02T08:44:08Z

Finished benchmarking commit (759376f): comparison URL.

Overall result: ❌ regressions - 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
@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)	0.6%	[0.4%, 0.8%]	3
Regressions ❌ (secondary)	0.2%	[0.2%, 0.2%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.6%	[0.4%, 0.8%]	3

Max RSS (memory usage)

Results (primary 3.0%, 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)	3.0%	[3.0%, 3.0%]	1
Regressions ❌ (secondary)	2.8%	[0.9%, 4.8%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.2%	[-3.0%, -0.7%]	9
All ❌✅ (primary)	3.0%	[3.0%, 3.0%]	1

Cycles

Results (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)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-1.6%	[-1.6%, -1.6%]	1
All ❌✅ (primary)	-	-	0

Binary size

Results (primary -0.0%, 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)	0.3%	[0.3%, 0.3%]	1
Regressions ❌ (secondary)	0.3%	[0.3%, 0.3%]	1
Improvements ✅ (primary)	-0.4%	[-0.4%, -0.4%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.0%	[-0.4%, 0.3%]	2

Bootstrap: 510.625s -> 510.701s (0.01%)
Artifact size: 400.75 MiB -> 400.68 MiB (-0.02%)

jhpratt · 2026-06-02T15:55:48Z

Same crates and for roughly the same amount. I'm clueless. Maybe try posting on Zulip, as I'm not sure who to ping.

jhpratt · 2026-06-07T03:36:26Z

Despite the unexpected perf result, it was recommended on Zulip to merge this anyway.

@bors r+

rust-bors · 2026-06-07T03:36:28Z

📌 Commit e845f15 has been approved by jhpratt

It is now in the queue for this repository.

JonathanBrouwer · 2026-06-07T08:08:37Z

@bors p=2
Putting this above the rollupable PRs to maximize throughput, since the queue is busy

…=jhpratt Optimize `checked_ilog` and `pow` when `base` is a power of two Optimize `checked_ilog` and `pow` when the base is a power of two

JonathanBrouwer · 2026-06-07T13:01:03Z

Stuck
@bors cancel

rust-bors · 2026-06-07T13:01:06Z

Auto build was cancelled. Cancelled workflows:

https://github.com/rust-lang/rust/actions/runs/27088352843

The next pull request likely to be tested is #157548.

Kmeakin · 2026-06-07T14:33:47Z

Despite the unexpected perf result, it was recommended on Zulip to merge this anyway.

@bors r+

Thanks. I was going to post to zulip but haven't had time this week. Can you link to the zulip discussion?

jhpratt · 2026-06-07T18:01:32Z

https://rust-lang.zulipchat.com/#narrow/channel/247081-t-compiler.2Fperformance/topic/sus.20perf.20results/near/600950235

rust-bors · 2026-06-07T21:44:00Z

☀️ Test successful - CI
Approved by: jhpratt
Duration: 3h 12m 18s
Pushing f20a92e to main...

github-actions · 2026-06-07T21:47:04Z

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 43a4909 (parent) -> f20a92e (this PR)

Test differences

Show 3966 test diffs

Stage 1

[codegen] tests/codegen-llvm/ilog_known_base.rs: [missing] -> pass (J1)
[codegen] tests/codegen-llvm/pow_known_base.rs: [missing] -> pass (J1)

Stage 2

[codegen] tests/codegen-llvm/ilog_known_base.rs: [missing] -> pass (J0)
[codegen] tests/codegen-llvm/pow_known_base.rs: [missing] -> pass (J0)

Additionally, 3962 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 f20a92ec01483dc5c58e90e246f266bdad822d86 --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

x86_64-gnu-gcc-core-tests: 14m 24s -> 7m 58s (-44.7%)
x86_64-gnu-llvm-21-1: 1h -> 40m 27s (-32.7%)
dist-i586-gnu-i586-i686-musl: 1h 16m -> 1h 36m (+27.2%)
x86_64-msvc-ext3: 1h 54m -> 1h 24m (-26.6%)
dist-i686-mingw: 1h 40m -> 2h 7m (+26.5%)
pr-check-1: 32m 46s -> 24m 55s (-24.0%)
dist-aarch64-linux: 2h 31m -> 1h 55m (-23.6%)
dist-x86_64-apple: 2h 10m -> 1h 41m (-22.4%)
dist-apple-various: 1h 41m -> 1h 19m (-22.1%)
dist-x86_64-linux-alt: 2h 31m -> 2h 1m (-19.7%)

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 · 2026-06-07T22:25:42Z

Finished benchmarking commit (f20a92e): comparison URL.

Overall result: ❌ regressions - 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)	0.6%	[0.3%, 0.7%]	4
Regressions ❌ (secondary)	0.4%	[0.4%, 0.4%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.6%	[0.3%, 0.7%]	4

Max RSS (memory usage)

Results (primary 5.0%, 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)	5.0%	[5.0%, 5.0%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-5.5%	[-5.5%, -5.5%]	1
All ❌✅ (primary)	5.0%	[5.0%, 5.0%]	1

Cycles

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

Binary size

Results (primary -0.0%, secondary 0.2%)

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

	mean	range	count
Regressions ❌ (primary)	0.1%	[0.1%, 0.3%]	4
Regressions ❌ (secondary)	0.3%	[0.3%, 0.3%]	1
Improvements ✅ (primary)	-0.1%	[-0.4%, -0.0%]	6
Improvements ✅ (secondary)	-0.0%	[-0.0%, -0.0%]	1
All ❌✅ (primary)	-0.0%	[-0.4%, 0.3%]	10

Bootstrap: 516.093s -> 516.04s (-0.01%)
Artifact size: 400.83 MiB -> 401.28 MiB (0.11%)

jhpratt · 2026-06-07T22:36:06Z

This was known before approving.

@rustbot label +perf-regression-triaged

rustbot assigned scottmcm Oct 2, 2025

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Oct 2, 2025

Kmeakin force-pushed the km/optimize-ilog-base-power-of-two branch from 8f19ce6 to e5ca8cd Compare October 2, 2025 01:31

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Kmeakin changed the title ~~Optimize checked_ilog when base is a power of two~~ Optimize checked_ilog and pow when base is a power of two Oct 2, 2025

Kmeakin force-pushed the km/optimize-ilog-base-power-of-two branch 2 times, most recently from 2faa397 to abb7f32 Compare October 2, 2025 02:38

scottmcm reviewed Oct 2, 2025

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Comment thread library/core/src/num/uint_macros.rs Outdated

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Comment thread library/core/src/num/uint_macros.rs Outdated

scottmcm added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Oct 2, 2025

Kmeakin force-pushed the km/optimize-ilog-base-power-of-two branch from abb7f32 to 9ab0f63 Compare October 2, 2025 23:13

nikic reviewed Oct 3, 2025

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Comment thread library/core/src/num/uint_macros.rs Outdated

Kmeakin force-pushed the km/optimize-ilog-base-power-of-two branch from 9ab0f63 to 1d0ac82 Compare October 3, 2025 21:16

Kmeakin force-pushed the km/optimize-ilog-base-power-of-two branch from 1d0ac82 to c84b99e Compare October 5, 2025 18:29

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Oct 5, 2025

scottmcm reviewed Oct 14, 2025

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Comment thread tests/codegen-llvm/ilog_known_base.rs

scottmcm requested changes Oct 14, 2025

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rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jun 2, 2026

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 Jun 7, 2026

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rust-bors Bot added merged-by-bors This PR was explicitly merged by bors. and removed S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels Jun 7, 2026

rust-bors Bot merged commit f20a92e into rust-lang:main Jun 7, 2026
13 checks passed

rustbot added this to the 1.98.0 milestone Jun 7, 2026

This was referenced Jun 7, 2026

Implement feature integer_casts #157402

Merged

Rollup of 21 pull requests #157582

Closed

rust-bors Bot mentioned this pull request Jun 7, 2026

[perf trials] how bad would it be if panic handlers returned unit #157424

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rustbot added the perf-regression-triaged The performance regression has been triaged. label Jun 7, 2026

Kmeakin deleted the km/optimize-ilog-base-power-of-two branch June 8, 2026 13:32

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Overall result: ❌ regressions - please read:

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Test differences

Stage 1

Stage 2

Job duration changes

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Overall result: ❌ regressions - please read:

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