Further optimize `SliceIndex<str>` impl for `Range<usize>` by Kmeakin · Pull Request #156119 · rust-lang/rust

Kmeakin · 2026-05-03T20:48:01Z

We can shave a further 2 icmps by inlining is_char_boundary and rearranging the checks.

Follow up to #145024

rustbot · 2026-05-03T20:48:06Z

rustbot has assigned @scottmcm.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

Why was this reviewer chosen?

The reviewer was selected based on:

Owners of files modified in this PR: @scottmcm, libs
@scottmcm, libs expanded to 8 candidates
Random selection from Mark-Simulacrum, jhpratt, nia-e, scottmcm

We can shave a further 2 `icmp`s by inlining `is_char_boundary` and rearranging the checks.

scottmcm · 2026-05-18T06:27:49Z

I'm leaving libs reviews.
@rustbot reroll

Mark-Simulacrum · 2026-06-06T18:44:57Z

@bors r+ rollup=never

rust-bors · 2026-06-06T18:45:00Z

📌 Commit ef9eff9 has been approved by Mark-Simulacrum

It is now in the queue for this repository.

jhpratt · 2026-06-07T01:05:13Z

Moving ahead of prioritized but rollup-able PRs

@bors p=2

jhpratt · 2026-06-07T07:01:34Z

rolling back priority

@bors p=0

JonathanBrouwer · 2026-06-07T07:23:14Z

@Mark-Simulacrum Is the reason this is not rollupable perf? Let's measure that
@bors try @rust-timer queue

Further optimize `SliceIndex<str>` impl for `Range<usize>`

rust-bors · 2026-06-07T09:33:19Z

☀️ Try build successful (CI)
Build commit: 4e4cf03 (4e4cf031367bb469ae04066e79966dc71acd2123, parent: 61d7280f3c4c63fa24c56bdaa9a446151b5a30dc)

rust-timer · 2026-06-07T10:35:13Z

Finished benchmarking commit (4e4cf03): comparison URL.

Overall result: ✅ improvements - no action needed

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.

@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
Improvements ✅ (primary)	-0.6%	[-0.6%, -0.6%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.6%	[-0.6%, -0.6%]	1

Max RSS (memory usage)

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

Cycles

Results (secondary -2.8%)

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

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
Improvements ✅ (primary)	-0.2%	[-0.4%, -0.1%]	8
Improvements ✅ (secondary)	-0.1%	[-0.1%, -0.0%]	35
All ❌✅ (primary)	-0.2%	[-0.4%, -0.1%]	8

Bootstrap: 514.705s -> 515.168s (0.09%)
Artifact size: 401.25 MiB -> 401.31 MiB (0.01%)

JonathanBrouwer · 2026-06-07T11:45:13Z

The perf improvement looks real to me, but is relatively small

The queue is a little large atm so let's do
@bors rollup=iffy

…ark-Simulacrum Further optimize `SliceIndex<str>` impl for `Range<usize>` We can shave a further 2 `icmp`s by inlining `is_char_boundary` and rearranging the checks. Follow up to rust-lang#145024

…uwer Rollup of 10 pull requests Successful merges: - #157447 (Move cross crate tests into the appropriate folder) - #145108 (Resolver: Batched Import Resolution) - #156119 (Further optimize `SliceIndex<str>` impl for `Range<usize>`) - #157289 (Add infallible primitive type lookups to template arg resolver) - #157540 (Cleanup and optimize `render_impls`) - #157543 (Reorganize `tests/ui/issues` [5/N]) - #156188 (riscv: promote d, e, and f target_features to CfgStableToggleUnstable) - #157323 (Document Repeat::last panic behavior) - #157545 (Suggest using comma to separate valid attribute list items) - #157559 (chore: Update annotate-snippets to 0.12.16)

…ark-Simulacrum Further optimize `SliceIndex<str>` impl for `Range<usize>` We can shave a further 2 `icmp`s by inlining `is_char_boundary` and rearranging the checks. Follow up to rust-lang#145024

Rollup of 25 pull requests Successful merges: - #157447 (Move cross crate tests into the appropriate folder) - #145108 (Resolver: Batched Import Resolution) - #156119 (Further optimize `SliceIndex<str>` impl for `Range<usize>`) - #157224 (Manually unroll loop in `str::floor_char_boundary`) - #157289 (Add infallible primitive type lookups to template arg resolver) - #157540 (Cleanup and optimize `render_impls`) - #157444 (Couple of work product cleanups) - #157543 (Reorganize `tests/ui/issues` [5/N]) - #153513 (Syntactically reject equality predicates) - #155797 (LineWriter: cap write_vectored newline scan to avoid quadratic write_all_vectored) - #156155 (macros: report unbound metavariables directly) - #156188 (riscv: promote d, e, and f target_features to CfgStableToggleUnstable) - #156666 (Clarify meaning of ranges in pointer offset docs) - #157078 (Document equivalence of `highest_one` and `ilog2` methods on integers) - #157129 (ci: update download-artifact action to v8) - #157169 (triagebot: Update messages to direct changes to appropriate repositories) - #157323 (Document Repeat::last panic behavior) - #157370 (Clarify MaybeUninit::zeroed padding docs) - #157399 (Silence llbc's output by default to prevent rustc's linker output warning) - #157500 (Improve documentation of `align_of` and `Alignment`.) - #157545 (Suggest using comma to separate valid attribute list items) - #157559 (chore: Update annotate-snippets to 0.12.16) - #157560 (In `copy_nonoverlapping`, use `mul nuw nsw` to compute the byte size) - #157580 (Importing suggestion reported twice when reporting privacy error) - #157581 (Test fixup)

Rollup of 25 pull requests Successful merges: - rust-lang/rust#157447 (Move cross crate tests into the appropriate folder) - rust-lang/rust#145108 (Resolver: Batched Import Resolution) - rust-lang/rust#156119 (Further optimize `SliceIndex<str>` impl for `Range<usize>`) - rust-lang/rust#157224 (Manually unroll loop in `str::floor_char_boundary`) - rust-lang/rust#157289 (Add infallible primitive type lookups to template arg resolver) - rust-lang/rust#157540 (Cleanup and optimize `render_impls`) - rust-lang/rust#157444 (Couple of work product cleanups) - rust-lang/rust#157543 (Reorganize `tests/ui/issues` [5/N]) - rust-lang/rust#153513 (Syntactically reject equality predicates) - rust-lang/rust#155797 (LineWriter: cap write_vectored newline scan to avoid quadratic write_all_vectored) - rust-lang/rust#156155 (macros: report unbound metavariables directly) - rust-lang/rust#156188 (riscv: promote d, e, and f target_features to CfgStableToggleUnstable) - rust-lang/rust#156666 (Clarify meaning of ranges in pointer offset docs) - rust-lang/rust#157078 (Document equivalence of `highest_one` and `ilog2` methods on integers) - rust-lang/rust#157129 (ci: update download-artifact action to v8) - rust-lang/rust#157169 (triagebot: Update messages to direct changes to appropriate repositories) - rust-lang/rust#157323 (Document Repeat::last panic behavior) - rust-lang/rust#157370 (Clarify MaybeUninit::zeroed padding docs) - rust-lang/rust#157399 (Silence llbc's output by default to prevent rustc's linker output warning) - rust-lang/rust#157500 (Improve documentation of `align_of` and `Alignment`.) - rust-lang/rust#157545 (Suggest using comma to separate valid attribute list items) - rust-lang/rust#157559 (chore: Update annotate-snippets to 0.12.16) - rust-lang/rust#157560 (In `copy_nonoverlapping`, use `mul nuw nsw` to compute the byte size) - rust-lang/rust#157580 (Importing suggestion reported twice when reporting privacy error) - rust-lang/rust#157581 (Test fixup)

avjewe · 2026-06-11T14:01:19Z

+        return false;
+    }
+
+    if start == slice.len() {


Suggested change

if start == slice.len() {

if start == end {

Any empty slice in range should be fine, not just at the end, and this will save an instruction or two for empty slices.

View changes since the review

I don't think this would be valid: slicing in the middle of a Unicode character is invalid, even if the slice is zero length

I can see how that could be argued.

rustbot assigned scottmcm May 3, 2026

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. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels May 3, 2026

Kmeakin force-pushed the km/optimize-str-index branch from 492232c to 3693b41 Compare May 3, 2026 21:00

This comment has been minimized.

Sign in to view

Kmeakin force-pushed the km/optimize-str-index branch from 3693b41 to 04d9b91 Compare May 3, 2026 21:11

This comment has been minimized.

Sign in to view

Further optimize SliceIndex<str> impl for Range<usize>

ef9eff9

We can shave a further 2 `icmp`s by inlining `is_char_boundary` and rearranging the checks.

Kmeakin force-pushed the km/optimize-str-index branch from 04d9b91 to ef9eff9 Compare May 3, 2026 23:12

rustbot assigned Mark-Simulacrum and unassigned scottmcm May 18, 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 6, 2026

This comment has been minimized.

Sign in to view

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jun 7, 2026

rust-bors Bot pushed a commit that referenced this pull request Jun 7, 2026

Auto merge of #156119 - Kmeakin:km/optimize-str-index, r=<try>

4e4cf03

Further optimize `SliceIndex<str>` impl for `Range<usize>`

This comment has been minimized.

Sign in to view

rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jun 7, 2026

JonathanBrouwer mentioned this pull request Jun 7, 2026

Rollup of 11 pull requests #157569

Closed

JonathanBrouwer mentioned this pull request Jun 7, 2026

Rollup of 10 pull requests #157570

Closed

JonathanBrouwer mentioned this pull request Jun 7, 2026

Rollup of 21 pull requests #157582

Closed

This was referenced Jun 7, 2026

Rollup of 21 pull requests #157583

Closed

Rollup of 25 pull requests #157586

Merged

rust-bors Bot merged commit 0efcd77 into rust-lang:main Jun 8, 2026
12 checks passed

rustbot added this to the 1.98.0 milestone Jun 8, 2026

Kmeakin deleted the km/optimize-str-index branch June 8, 2026 13:31

avjewe reviewed Jun 11, 2026

View reviewed changes

Uh oh!

Conversation

Kmeakin commented May 3, 2026

Uh oh!

rustbot commented May 3, 2026

Uh oh!

This comment has been minimized.

This comment has been minimized.

scottmcm commented May 18, 2026

Uh oh!

Mark-Simulacrum commented Jun 6, 2026

Uh oh!

rust-bors Bot commented Jun 6, 2026

Uh oh!

jhpratt commented Jun 7, 2026

Uh oh!

jhpratt commented Jun 7, 2026

Uh oh!

JonathanBrouwer commented Jun 7, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

This comment has been minimized.

This comment has been minimized.

rust-bors Bot commented Jun 7, 2026

Uh oh!

This comment has been minimized.

rust-timer commented Jun 7, 2026

Overall result: ✅ improvements - no action needed

Instruction count

Max RSS (memory usage)

Cycles

Binary size

Uh oh!

JonathanBrouwer commented Jun 7, 2026

Uh oh!

Uh oh!

avjewe Jun 11, 2026 • edited by rustbot Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Kmeakin Jun 11, 2026

Choose a reason for hiding this comment

Uh oh!

avjewe Jun 11, 2026

Choose a reason for hiding this comment

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

9 participants

JonathanBrouwer commented Jun 7, 2026 •

edited

Loading

avjewe Jun 11, 2026 •

edited by rustbot

Loading