simplify `slice::Iter[Mut]::next_chunk` implementation by WaffleLapkin · Pull Request #158879 · rust-lang/rust

WaffleLapkin · 2026-07-06T19:10:26Z

I verified that this doesn't pessimize codegen using the example from the PR that inroduced the optimization of next_chunk (#149131):

#![feature(iter_next_chunk)]

#[no_mangle]
pub fn simd_sum_slow(arr: &[u32]) -> u32 {
    const STEP_SIZE: usize = 16;

    let mut result = [0; STEP_SIZE];

    let mut iter = arr.iter();

    while let Ok(c) = iter.next_chunk::<STEP_SIZE>() {
        for (&n, r) in c.iter().zip(result.iter_mut()) {
            *r += n;
        }
    }

    result.iter().sum()
}

I compiled this example with

./build/host/stage1/bin/rustc t.rs --emit=asm -O --crate-type=lib

Before and after this change; the only difference is the choice of the jump instruction, which I think shouldn't make any difference:

28,29c28,29
< 	cmpq	$64, %rsi
< 	jae	.LBB0_2
---
> 	cmpq	$60, %rsi
> 	ja	.LBB0_2

r? libs
cc @bend-n

bend-n · 2026-07-07T06:59:26Z

i dont think theres any optimization problem here in anything but compile time, but i'm not sure as to the purpose of the pr?

WaffleLapkin · 2026-07-07T16:16:15Z

The purpose of the PR is to make the code simpler and easier to read/maintain.

bend-n · 2026-07-07T16:18:22Z

i feel as though its not exactly simpler, merely smaller.

I verified that this doesn't pessimize codegen using the example from the PR that inroduced the optimization of `next_chunk` (# 149131): ```rust #![feature(iter_next_chunk)] #[no_mangle] pub fn simd_sum_slow(arr: &[u32]) -> u32 { const STEP_SIZE: usize = 16; let mut result = [0; STEP_SIZE]; let mut iter = arr.iter(); while let Ok(c) = iter.next_chunk::<STEP_SIZE>() { for (&n, r) in c.iter().zip(result.iter_mut()) { *r += n; } } result.iter().sum() } ``` I compiled this example with ```shell ./build/host/stage1/bin/rustc t.rs --emit=asm -O --crate-type=lib ``` Before and after this change; the only difference is the choice of the jump instruction, which I think shouldn't make any difference: ```diff 28,29c28,29 < cmpq $64, %rsi < jae .LBB0_2 --- > cmpq $60, %rsi > ja .LBB0_2 ```

WaffleLapkin · 2026-07-07T16:33:05Z

I feel like it is simpler? It avoids the whole thing with copying things into a MaybeUninit array and creating array::IntoIter, etc...

JohnTitor · 2026-07-27T22:02:39Z

Let's perf-run just in case:
@bors try @rust-timer queue

simplify `slice::Iter[Mut]::next_chunk` implementation

JohnTitor

r=me if the result is fine

View changes since this review

rust-bors · 2026-07-28T00:07:56Z

☀️ Try build successful (CI)
Build commit: 2277e36 (2277e3696d57fbbfdcbb2f2b84752dcafdd5afb3)
Base parent: 09ee43b (09ee43b2d6055539771bee8ac30a6e56eb4db773)

rust-timer · 2026-07-28T01:10:35Z

Finished benchmarking commit (2277e36): comparison URL.

Overall result: no relevant changes - no action needed

Benchmarking means the PR may be perf-sensitive. Consider adding rollup=never if this change is not fit for rolling up.

@rustbot label: -S-waiting-on-perf -perf-regression

Instruction count

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

Max RSS (memory usage)

Results (secondary -2.2%)

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)	2.2%	[2.2%, 2.2%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-6.5%	[-6.5%, -6.5%]	1
All ❌✅ (primary)	-	-	0

Cycles

Results (secondary -2.2%)

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)	-2.2%	[-2.4%, -2.1%]	2
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 487.408s -> 488.179s (0.16%)
Artifact size: 387.99 MiB -> 387.99 MiB (0.00%)

JohnTitor · 2026-07-28T20:49:18Z

@bors r+

rust-bors · 2026-07-28T20:49:22Z

📌 Commit aec1f2a has been approved by JohnTitor

It is now in the queue for this repository.

Rollup of 11 pull requests Successful merges: - #158168 (Added implementation on `set_permissions_nofollow` for all primary platforms) - #160055 (Simplify `MaybeRequiresStorage`) - #157226 (Partially stabilize `box_vec_non_null`) - #158879 (simplify `slice::Iter[Mut]::next_chunk` implementation) - #159413 (Enable `#[diagnostic::on_unknown]` during late res) - #160091 (Fix rustdoc toolbar height when title is taller than one line) - #158615 (fix: don't fire `explicit_outlives_requirements` on `?Sized` type params) - #159666 (fix(ld64.lld): route version mismatch warnings to linker_info on macOS) - #160032 (rustdoc-json: Make `Stability` compatible with non-self-describing serde formats) - #160039 (Add regression test for enum unconstrained parameter ) - #160049 (Use assert_eq! in splat codegen tests)

@bend-n

Rollup merge of #158879 - WaffleLapkin:slice_iter_next_chunk_simp, r=JohnTitor simplify `slice::Iter[Mut]::next_chunk` implementation I verified that this doesn't pessimize codegen using the example from the PR that inroduced the optimization of `next_chunk` (#149131): ```rust #![feature(iter_next_chunk)] #[no_mangle] pub fn simd_sum_slow(arr: &[u32]) -> u32 { const STEP_SIZE: usize = 16; let mut result = [0; STEP_SIZE]; let mut iter = arr.iter(); while let Ok(c) = iter.next_chunk::<STEP_SIZE>() { for (&n, r) in c.iter().zip(result.iter_mut()) { *r += n; } } result.iter().sum() } ``` I compiled this example with ```shell ./build/host/stage1/bin/rustc t.rs --emit=asm -O --crate-type=lib ``` Before and after this change; the only difference is the choice of the jump instruction, which I think shouldn't make any difference: ```diff 28,29c28,29 < cmpq $64, %rsi < jae .LBB0_2 --- > cmpq $60, %rsi > ja .LBB0_2 ``` r? libs cc @bend-n

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 Jul 6, 2026

rustbot assigned JohnTitor Jul 6, 2026

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WaffleLapkin force-pushed the slice_iter_next_chunk_simp branch from ac14155 to aec1f2a Compare July 7, 2026 16:35

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rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jul 27, 2026

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rust-bors Bot pushed a commit that referenced this pull request Jul 27, 2026

Auto merge of #158879 - WaffleLapkin:slice_iter_next_chunk_simp, r=<try>

2277e36

simplify `slice::Iter[Mut]::next_chunk` implementation

JohnTitor approved these changes Jul 27, 2026

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rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jul 28, 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 Jul 28, 2026

jhpratt mentioned this pull request Jul 29, 2026

Rollup of 11 pull requests #160112

Merged

rust-bors Bot merged commit 8c6ef9f into rust-lang:main Jul 29, 2026
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rustbot added this to the 1.99.0 milestone Jul 29, 2026

WaffleLapkin deleted the slice_iter_next_chunk_simp branch July 29, 2026 18:24

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simplify `slice::Iter[Mut]::next_chunk` implementation - #158879

simplify `slice::Iter[Mut]::next_chunk` implementation#158879
rust-bors[bot] merged 1 commit into
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WaffleLapkin:slice_iter_next_chunk_simp

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Overall result: no relevant changes - no action needed

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