Optimize indexing slices and strs with inclusive ranges #145024

Kmeakin · 2025-08-06T22:18:56Z

Instead of separately checking for end == usize::MAX and end + 1 > slice.len(), we can check for end >= slice.len(). Also consolidate all the slice indexing related panic functions into a single function which reports the correct error depending on the arguments, as the str indexing code already does.

The downside of all this is that the panic message is slightly less specific when trying to index with [..=usize::MAX]: instead of saying "attempted to index slice up to maximum usize" it just says "range end index {end} out of range for slice of length {len}". But this is a rare enough case that I think it is acceptable

rustbot · 2025-08-06T22:19:02Z

r? @ibraheemdev

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scottmcm

As you saw from the PR build failure, if you're touching things like slice_index_order_fail you'll need to look through every mention of those in the repo -- there are a bunch of codegen tests that look for their absence, and you need to make sure that you don't make those tests useless by renaming what they're looking for.

(You probably also need to re-bless some MIR tests that include indexing.)

library/core/src/slice/index.rs

rustbot · 2025-08-09T00:09:58Z

The Miri subtree was changed

cc @rust-lang/miri

oli-obk · 2025-08-09T20:10:22Z

@bors try @rust-timer queue

Optimize indexing slices and strs with inclusive ranges

rust-bors · 2025-08-09T20:10:26Z

⌛ Trying commit bd63d02 with merge 4249615…

To cancel the try build, run the command @bors try cancel.

Kmeakin · 2025-11-10T00:41:17Z

@scottmcm ping?

tests/codegen-llvm/slice-range-indexing.rs

library/core/src/str/mod.rs

it's my review.

scottmcm · 2026-01-27T02:45:27Z

@bors try @rust-timer queue

Optimize indexing slices and strs with inclusive ranges

rust-bors · 2026-01-27T05:01:52Z

☀️ Try build successful (CI)
Build commit: ddd9a12 (ddd9a12e711d4b329a39072cd3ed4cf138a43c4c, parent: b3cda168c8afd5c4240a9477f6a7f54e70e2589a)

rust-timer · 2026-01-27T05:42:59Z

Finished benchmarking commit (ddd9a12): comparison URL.

Overall result: ❌ regressions - 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

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.1%	[0.1%, 0.1%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

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

Cycles

Results (primary 9.4%, secondary 5.2%)

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

	mean	range	count
Regressions ❌ (primary)	9.4%	[2.4%, 16.9%]	12
Regressions ❌ (secondary)	5.2%	[4.3%, 6.1%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	9.4%	[2.4%, 16.9%]	12

Binary size

Results (secondary 0.0%)

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)	0.0%	[0.0%, 0.0%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Bootstrap: 472.506s -> 472.489s (-0.00%)
Artifact size: 383.60 MiB -> 383.59 MiB (-0.00%)

Add a `codegen-llvm` test to check the number of `icmp` instrucitons generated for each `SliceIndex` method on the various range types. This will be updated in the next commit when `SliceIndex::get` is optimized for `RangeInclusive`.

Mark-Simulacrum · 2026-02-01T19:16:47Z

tests/codegen-llvm/str-range-indexing.rs

+}
+
+// CHECK-LABEL: @get_range
+// CHECK-COUNT-9: %{{.+}} = icmp


Are you sure this is a useful test? The string code is comparatively complex and some of these are for the char boundaries and some of it is for slices, etc. Maybe at least worth annotating with why there's this number?

library/core/src/slice/index.rs

library/core/src/str/mod.rs

rustbot · 2026-02-01T19:34:40Z

Reminder, once the PR becomes ready for a review, use @rustbot ready.

rustbot · 2026-02-02T21:15:50Z

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.

The check for `self.end() == usize::MAX` can be combined with the `self.end() + 1 > slice.len()` check into `self.end() >= slice.len()`, since `self.end() < slice.len()` implies both `self.end() <= slice.len()` and `self.end() < usize::MAX`.

Same reasoning as previous two commits.

The panic message when slicing a string with a negative length range (eg `"abcdef"[4..3]`) is confusing: it gives the condition that failed to hold, whilst all the other panic messages give the condition that did hold. Before: begin <= end (4 <= 3) when slicing `abcdef` After: begin > end (4 > 3) when slicing `abcdef`

Rename `into_range` to `try_into_slice_range`: - Prepend `try_` to show that it returns `None` on error, like `try_range` - add `_slice` to make it consistent with `into_slice_range`

rustbot assigned ibraheemdev Aug 6, 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 Aug 6, 2025

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Kmeakin force-pushed the km/optimize-slice-index/v3 branch from eab010d to 61bcd82 Compare August 8, 2025 00:40

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Kmeakin force-pushed the km/optimize-slice-index/v3 branch from 61bcd82 to 2546221 Compare August 8, 2025 22:52

Kmeakin mentioned this pull request Aug 8, 2025

Consolidate panicking functions in slice/index.rs #145137

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Kmeakin force-pushed the km/optimize-slice-index/v3 branch from 2546221 to a61fde0 Compare August 9, 2025 00:09

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Kmeakin force-pushed the km/optimize-slice-index/v3 branch from 48b325f to 87a3889 Compare August 9, 2025 00:16

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Kmeakin force-pushed the km/optimize-slice-index/v3 branch from 87a3889 to bd63d02 Compare August 9, 2025 19:16

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rust-bors bot added a commit that referenced this pull request Aug 9, 2025

Auto merge of #145024 - Kmeakin:km/optimize-slice-index/v3, r=<try>

4249615

Optimize indexing slices and strs with inclusive ranges

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Aug 9, 2025

Kmeakin force-pushed the km/optimize-slice-index/v3 branch from bd63d02 to 194cfc3 Compare August 9, 2025 22:13

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rust-bors bot pushed a commit that referenced this pull request Jan 27, 2026

Auto merge of #145024 - Kmeakin:km/optimize-slice-index/v3, r=<try>

ddd9a12

Optimize indexing slices and strs with inclusive ranges

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Precommit tests for SliceIndex method codegen

d470fb0

Add a `codegen-llvm` test to check the number of `icmp` instrucitons generated for each `SliceIndex` method on the various range types. This will be updated in the next commit when `SliceIndex::get` is optimized for `RangeInclusive`.

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rustbot assigned Mark-Simulacrum and unassigned scottmcm Feb 1, 2026

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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 Feb 1, 2026

Kmeakin force-pushed the km/optimize-slice-index/v3 branch from ab33748 to 0f259ac Compare February 2, 2026 21:15

Kmeakin added 4 commits February 2, 2026 21:19

Optimize SliceIndex<str> for RangeInclusive

cc6a0e5

Same reasoning as previous two commits.

Give into_range more consistent name

0b26613

Rename `into_range` to `try_into_slice_range`: - Prepend `try_` to show that it returns `None` on error, like `try_range` - add `_slice` to make it consistent with `into_slice_range`

Kmeakin force-pushed the km/optimize-slice-index/v3 branch from 0f259ac to 0b26613 Compare February 2, 2026 21:19

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rust-timer commented Jan 27, 2026

Overall result: ❌ regressions - no action needed

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