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Use a safe BucketIndex abstraction in VecCache#153434

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Zalathar:bucket-index
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Use a safe BucketIndex abstraction in VecCache#153434
Zalathar wants to merge 1 commit intorust-lang:mainfrom
Zalathar:bucket-index

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@Zalathar Zalathar commented Mar 5, 2026

The current code for indexing into bucket arrays is quite tricky and unsafe, partly because it has to keep manually assuring the compiler that a bucket index is always less than 21.

By encapsulating that knowledge in a 21-value enum, we can make the code clearer and safer, without giving up performance.

Having a dedicated BucketIndex type could also help with further cleanups of VecCache indexing.

@rustbot rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Mar 5, 2026
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rustbot commented Mar 5, 2026

r? @wesleywiser

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Zalathar commented Mar 5, 2026

@bors try @rust-timer queue

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rust-bors bot pushed a commit that referenced this pull request Mar 5, 2026
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Auto merge of #153434 - Zalathar:bucket-index, r=<try>
a141722 
Use a safe `BucketIndex` abstraction in `VecCache`
@rustbot rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Mar 5, 2026
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rust-bors bot commented Mar 5, 2026

☀️ Try build successful (CI)
Build commit: a141722 (a141722f73cfed3d09e312058bc234370a64db49, parent: f8704be04fe1150527fc2cf21dd44327f0fe87fb)

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rust-timer commented Mar 5, 2026

Finished benchmarking commit (a141722): 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% [0.1%, 0.2%] 5
Regressions ❌
(secondary)		 0.1% [0.0%, 0.3%] 7
Improvements ✅
(primary)		 -0.3% [-0.3%, -0.2%] 4
Improvements ✅
(secondary)		 - - 0
All ❌✅ (primary) -0.0% [-0.3%, 0.2%] 9

Max RSS (memory usage)

Results (primary -2.3%, secondary 0.8%)

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)		 3.5% [3.5%, 3.5%] 1
Improvements ✅
(primary)		 -2.3% [-2.4%, -2.2%] 2
Improvements ✅
(secondary)		 -1.9% [-1.9%, -1.9%] 1
All ❌✅ (primary) -2.3% [-2.4%, -2.2%] 2

Cycles

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

Binary size

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

Bootstrap: 480.998s -> 485.163s (0.87%)
Artifact size: 394.98 MiB -> 394.96 MiB (-0.00%)

@rustbot rustbot added perf-regression Performance regression. and removed S-waiting-on-perf Status: Waiting on a perf run to be completed. labels Mar 5, 2026
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Zalathar commented Mar 5, 2026

Hmm, did my harmless tweaks somehow trigger another regression?

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rustbot commented Mar 5, 2026

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.

@Zalathar
Use a safe BucketIndex abstraction in VecCache
431f2a9 
The current code for indexing into bucket arrays is quite tricky and unsafe,
partly because it has to keep manually assuring the compiler that a bucket
index is always less than 21.

By encapsulating that knowledge in a 21-value enum, we can make the code
clearer and safer, without giving up performance.

Having a dedicated `BucketIndex` type could also help with further cleanups of
`VecCache` indexing.
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Zalathar commented Mar 5, 2026

@bors try @rust-timer queue

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@rustbot rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Mar 5, 2026
rust-bors bot pushed a commit that referenced this pull request Mar 5, 2026
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Auto merge of #153434 - Zalathar:bucket-index, r=<try>
a6e26e0 
Use a safe `BucketIndex` abstraction in `VecCache`
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rust-bors bot commented Mar 5, 2026

☀️ Try build successful (CI)
Build commit: a6e26e0 (a6e26e04fe48bab5cea884f2973be488f752b03e, parent: 70d86e3abeecf3a655264d9a716c5d08160176b7)

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rust-timer commented Mar 5, 2026

Finished benchmarking commit (a6e26e0): comparison URL.

Overall result: no relevant changes - no action needed

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.

@bors rollup=never
@rustbot label: -S-waiting-on-perf -perf-regression

Instruction count

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

Max RSS (memory usage)

Results (primary 2.3%)

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)		 - - 0
Improvements ✅
(primary)		 - - 0
Improvements ✅
(secondary)		 - - 0
All ❌✅ (primary) 2.3% [2.3%, 2.3%] 1

Cycles

Results (primary -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)		 - - 0
Improvements ✅
(primary)		 -1.3% [-1.3%, -1.3%] 1
Improvements ✅
(secondary)		 - - 0
All ❌✅ (primary) -1.3% [-1.3%, -1.3%] 1

Binary size

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

Bootstrap: 480.787s -> 481.672s (0.18%)
Artifact size: 395.02 MiB -> 394.99 MiB (-0.01%)

@rustbot rustbot removed S-waiting-on-perf Status: Waiting on a perf run to be completed. perf-regression Performance regression. labels Mar 5, 2026
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