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

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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

jhpratt · 2026-04-21T06:07:42Z

With the one nit resolved (assuming there's no reason to leave as-is), LGTM.

@bors try @rust-timer queue

if base == 2 ** k, then log(base, n) == log(2, n) / k

rustbot · 2026-04-21T14:05:09Z

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.

Increase test coverage to check all interesting edge cases and all variants.

`strict_pow` can be implemented in terms of `checked_pow`, `wrapping_pow` can be implemented in terms of `overflowing_pow`, and `pow` can be implemented in terms of `strict_pow` or `wrapping_pow`.

Copy the optimization that unrolls the loop from `pow` to `checked_pow` and `overflowing_pow`.

if base == 2 ** k, then (2 ** k) ** n == 2 ** (k * n) == 1 << (k * n)

Kmeakin · 2026-05-17T00:14:16Z

ping @jhpratt?

jhpratt · 2026-05-17T04:44:50Z

The last email I had received was the CI failure. I'll take a look when I can, but I'm busy in Europe at the moment.

jhpratt · 2026-05-30T17:17:33Z

@bors try @rust-timer queue

…=<try> Optimize `checked_ilog` and `pow` when `base` is a power of two

rust-bors · 2026-05-30T19:28:02Z

☀️ Try build successful (CI)
Build commit: c0638d9 (c0638d923bca5c150f08ba8dd83d5ac9020f5095, parent: c58275e0369d09fc3959b8ba87dcbcbe73797465)

rust-timer · 2026-05-30T20:09:24Z

Finished benchmarking commit (c0638d9): 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
@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%, 0.9%]	3
Regressions ❌ (secondary)	0.4%	[0.2%, 0.6%]	2
Improvements ✅ (primary)	-0.1%	[-0.1%, -0.1%]	1
Improvements ✅ (secondary)	-0.1%	[-0.1%, -0.1%]	1
All ❌✅ (primary)	0.4%	[-0.1%, 0.9%]	4

Max RSS (memory usage)

Results (primary 2.8%, 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)	2.8%	[2.8%, 2.8%]	1
Regressions ❌ (secondary)	1.6%	[1.6%, 1.6%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.8%	[2.8%, 2.8%]	1

Cycles

Results (primary 2.3%, secondary 2.4%)

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

	mean	range	count
Regressions ❌ (primary)	2.3%	[2.3%, 2.3%]	1
Regressions ❌ (secondary)	2.4%	[2.4%, 2.4%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.3%	[2.3%, 2.3%]	1

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: 515.367s -> 512.478s (-0.56%)
Artifact size: 400.82 MiB -> 400.82 MiB (0.00%)

jhpratt · 2026-05-30T23:24:33Z

Hmm...those regressions are far beyond what would be considered significant (both statistically and by looking at the graphs). Any thoughts?

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

scottmcm reviewed Oct 2, 2025

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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 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 14, 2025

rustbot unassigned scottmcm Apr 20, 2026

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jhpratt reviewed Apr 21, 2026

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

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rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Apr 21, 2026

optimize: checked_ilog when base is a power of two

616f71d

if base == 2 ** k, then log(base, n) == log(2, n) / k

Kmeakin force-pushed the km/optimize-ilog-base-power-of-two branch from f23f1e5 to f20dfa8 Compare April 21, 2026 14:05

quaternic reviewed Apr 21, 2026

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

Kmeakin added 3 commits April 30, 2026 18:23

refactor: Increase test coverage for pow

3cd4e9e

Increase test coverage to check all interesting edge cases and all variants.

refactor: Deduplicate pow implementations

e378588

`strict_pow` can be implemented in terms of `checked_pow`, `wrapping_pow` can be implemented in terms of `overflowing_pow`, and `pow` can be implemented in terms of `strict_pow` or `wrapping_pow`.

optimize: pow if exp is statically known

72fdfb1

Copy the optimization that unrolls the loop from `pow` to `checked_pow` and `overflowing_pow`.

Kmeakin force-pushed the km/optimize-ilog-base-power-of-two branch from f20dfa8 to 6986ad2 Compare May 1, 2026 00:29

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optimize: pow when base is a power of two

e845f15

if base == 2 ** k, then (2 ** k) ** n == 2 ** (k * n) == 1 << (k * n)

Kmeakin force-pushed the km/optimize-ilog-base-power-of-two branch from 6986ad2 to e845f15 Compare May 3, 2026 14:30

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rust-bors Bot pushed a commit that referenced this pull request May 30, 2026

Auto merge of #147250 - Kmeakin:km/optimize-ilog-base-power-of-two, r…

c0638d9

…=<try> Optimize `checked_ilog` and `pow` when `base` is a power of two

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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 May 30, 2026

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rust-timer commented May 30, 2026

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

Instruction count

Max RSS (memory usage)

Cycles

Binary size

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