Use `iter::repeat_n` to implement `Vec::extend_with` by paolobarbolini · Pull Request #133662 · rust-lang/rust

paolobarbolini · 2024-11-30T06:22:29Z

This replaces the Vec::extend_with manual implementation with iter::repeat_n, simplifying the code and fixing the issue identified in #120050.

rustbot · 2024-11-30T06:22:37Z

r? @Noratrieb

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joboet · 2024-11-30T12:47:40Z

Let's see what effect this has in the compiler!
@bors try @rust-timer queue

bors · 2024-11-30T12:48:51Z

⌛ Trying commit 4bf7072 with merge 243c5cc...

…epeatn, r=<try> Use `iter::repeat_n` to implement `Vec::extend_with` This replaces the `Vec::extend_with` manual implementation, which is used by `Vec::resize` and `Vec` `SpecFromElem`, with `iter::repeat_n`. I've compared the codegen output between: 1. the current `Vec::resize` impl 2. this branch 3. this branch + rust-lang#130887 3 gives the closest codegen output to 1, with some output improvements. 2 doesn't look good: https://rust.godbolt.org/z/Yrc83EhjY. May also help rust-lang#120050?

Noratrieb · 2024-11-30T13:48:12Z

given that this branch doesn't look good without #130887, let's block it on that.
@rustbot blocked

bors · 2024-11-30T14:37:20Z

☀️ Try build successful - checks-actions
Build commit: 243c5cc (243c5cc6905efbdf778394d96f4b83701962964f)

rust-timer · 2024-11-30T17:09:58Z

Finished benchmarking commit (243c5cc): comparison URL.

Overall result: ❌✅ regressions and improvements - please read the text below

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR may lead to changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please fix the regressions and do another perf run. If the next run shows neutral or positive results, the label will be automatically removed.

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

Instruction count

This is the most reliable metric that we have; it was used to determine the overall result at the top of this comment. However, even this metric can sometimes exhibit noise.

	mean	range	count
Regressions ❌ (primary)	0.3%	[0.1%, 0.7%]	16
Regressions ❌ (secondary)	0.4%	[0.1%, 0.6%]	9
Improvements ✅ (primary)	-0.9%	[-0.9%, -0.9%]	1
Improvements ✅ (secondary)	-1.3%	[-1.8%, -0.5%]	8
All ❌✅ (primary)	0.3%	[-0.9%, 0.7%]	17

Max RSS (memory usage)

Results (primary 0.1%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	5.6%	[0.9%, 11.4%]	3
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-8.1%	[-11.1%, -5.0%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.1%	[-11.1%, 11.4%]	5

Cycles

Results (secondary -2.1%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.1%	[-2.1%, -2.1%]	1
All ❌✅ (primary)	-	-	0

Binary size

Results (primary 0.4%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	0.5%	[0.0%, 1.3%]	43
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.1%	[-0.2%, -0.0%]	7
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.4%	[-0.2%, 1.3%]	50

Bootstrap: 773.805s -> 774.205s (0.05%)
Artifact size: 332.32 MiB -> 331.64 MiB (-0.20%)

paolobarbolini · 2024-12-01T09:37:08Z

Could we do another perf run? I've cherry-picked the commit from #130887 since, at least via Compiler Explorer, the codegen looked better

joboet · 2024-12-01T11:04:19Z

Sure!
@bors try @rust-timer queue

bors · 2024-12-01T11:05:32Z

⌛ Trying commit b080e18 with merge 78d5ed4...

…epeatn, r=<try> Use `iter::repeat_n` to implement `Vec::extend_with` This replaces the `Vec::extend_with` manual implementation, which is used by `Vec::resize` and `Vec` `SpecFromElem`, with `iter::repeat_n`. I've compared the codegen output between: 1. the current `Vec::resize` impl 2. this branch 3. this branch + rust-lang#130887 3 gives the closest codegen output to 1, with some output improvements. 2 doesn't look good: https://rust.godbolt.org/z/Yrc83EhjY. May also help rust-lang#120050? --- WARNING: DO NOT MERGE - in order to run the perf run in rust-lang#133662 (comment) this PR currently also contains commits from rust-lang#130887

bors · 2024-12-01T12:50:21Z

☀️ Try build successful - checks-actions
Build commit: 78d5ed4 (78d5ed40d7c459a7e5ea4cb6bea9918a493a800f)

rust-timer · 2024-12-01T14:18:32Z

Finished benchmarking commit (78d5ed4): comparison URL.

Overall result: ❌ regressions - please read the text below

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR may lead to changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please fix the regressions and do another perf run. If the next run shows neutral or positive results, the label will be automatically removed.

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

Instruction count

This is the most reliable metric that we have; it was used to determine the overall result at the top of this comment. However, even this metric can sometimes exhibit noise.

	mean	range	count
Regressions ❌ (primary)	0.4%	[0.1%, 0.7%]	17
Regressions ❌ (secondary)	1.3%	[0.1%, 3.2%]	16
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.4%	[0.1%, 0.7%]	17

Max RSS (memory usage)

Results (primary -5.6%, secondary 0.3%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	5.3%	[5.3%, 5.3%]	1
Regressions ❌ (secondary)	1.5%	[1.0%, 1.8%]	3
Improvements ✅ (primary)	-11.1%	[-15.5%, -6.8%]	2
Improvements ✅ (secondary)	-3.2%	[-3.2%, -3.2%]	1
All ❌✅ (primary)	-5.6%	[-15.5%, 5.3%]	3

Cycles

Results (primary -1.5%, secondary 35.1%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	0.9%	[0.9%, 0.9%]	1
Regressions ❌ (secondary)	35.1%	[33.0%, 39.1%]	6
Improvements ✅ (primary)	-4.0%	[-4.0%, -4.0%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-1.5%	[-4.0%, 0.9%]	2

Binary size

Results (primary 0.6%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	0.7%	[0.1%, 1.7%]	33
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.1%	[-0.2%, -0.0%]	9
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.6%	[-0.2%, 1.7%]	42

Bootstrap: 767.632s -> 769.266s (0.21%)
Artifact size: 332.19 MiB -> 332.16 MiB (-0.01%)

paolobarbolini · 2024-12-02T09:12:17Z

Looking at performance data borrow checking time in the Cycles benchmark for wg-grammar is the main culprit. I'm confused why that is.

The fact that this also fixes #120050 makes these changes still worth it to me.

scottmcm

I had a couple of shotgun debugging ideas -- no idea if they'll actually help, but since the perf results were still pretty red it might be good to try some stuff.

Aside, if I'm reading https://doc.rust-lang.org/nightly/std/clone/trait.TrivialClone.html right it doesn't actually say that Drop needs to be trivial? So can we just not specialize away the drop guards any more, maybe?

View changes since this review

paolobarbolini · 2025-12-08T07:35:02Z

I've made both changes. Can we rerun perf?

yotamofek · 2025-12-12T07:39:17Z

@bors try @rust-timer queue

…=<try> Use `iter::repeat_n` to implement `Vec::extend_with`

rust-bors · 2025-12-12T09:55:20Z

☀️ Try build successful (CI)
Build commit: 74054d7 (74054d7c18f8567db8284711aa3dea05a0a78d6d, parent: 5b150d238fbd4fe7bc2cd3140d8e6fb4406099fa)

rust-timer · 2025-12-12T11:06:46Z

Finished benchmarking commit (74054d7): comparison URL.

Overall result: ❌ regressions - 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)	0.4%	[0.2%, 0.9%]	4
Regressions ❌ (secondary)	4.3%	[0.2%, 5.4%]	7
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.4%	[0.2%, 0.9%]	4

Max RSS (memory usage)

Results (primary 4.7%)

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

	mean	range	count
Regressions ❌ (primary)	4.7%	[3.2%, 6.7%]	3
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	4.7%	[3.2%, 6.7%]	3

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)	2.8%	[2.7%, 2.8%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

Results (primary 0.4%, 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)	0.4%	[0.2%, 0.9%]	23
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.0%	[-0.0%, -0.0%]	4
Improvements ✅ (secondary)	-0.1%	[-0.1%, -0.1%]	4
All ❌✅ (primary)	0.4%	[-0.0%, 0.9%]	27

Bootstrap: 473.727s -> 474.786s (0.22%)
Artifact size: 389.29 MiB -> 389.44 MiB (0.04%)

clarfonthey · 2026-09-22T02:05:36Z

@rustbot reroll

Based upon your comments in #155928, are you interested in continuing this work, or would you rather close it? Just going through old issues and my understanding is that this hasn't really yielded any solid perf benefits.

clarfonthey · 2026-09-22T02:06:03Z

also @paolobarbolini since "you" is not a valid way to ping people

This replaces the manual `Vec::extend_with` implementation with `iter::repeat_n` and `Vec::extend_trusted`. This simplifies the code and gets LLVM to remove the special case for the last element when `T` is trivial to clone.

After running perf on the previous commit, it was found that the compile-time regression was too significant to ignore. This introduces a specialized, simplified implementation for `T: TrivialClone` that will reduce the perf regression.

rustbot · 2026-10-31T17:13:28Z

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.

paolobarbolini · 2026-10-31T17:18:45Z

I've rebased on top of main and I've run the cachegrind benchmarks locally both from the December 2025 and the current compiler, and things seem to have improved.

Could we get a new perf run?

rust-bors · 2026-11-01T23:58:57Z

☔ The latest upstream changes (presumably #165286) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.

clarfonthey · 2026-11-02T05:34:17Z

Can rerun perf for you after rebase since presumably rebasing might affect things.

paolobarbolini · 2026-11-02T15:54:04Z

I guess someone else got to implementing this same thing, which got merged in #163757. Closing.

clarfonthey · 2026-11-02T18:40:38Z

Yeah, unfortunately these old issues only getting triaged now has lead to a few mishaps like that. Hopefully shouldn't happen as often in the future now that we've been going through the libs backlog.

rustbot assigned Noratrieb Nov 30, 2024

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 Nov 30, 2024

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rustbot added S-blocked Status: Blocked on something else such as an RFC or other implementation work. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Nov 30, 2024

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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 Nov 30, 2024

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rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Dec 1, 2024

paolobarbolini mentioned this pull request Dec 1, 2024

Safer implementation of RepeatN #130887

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rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Dec 1, 2024

paolobarbolini mentioned this pull request Dec 2, 2024

Vec::resize for bytes should be a single memset #120050

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scottmcm reviewed Dec 7, 2025

View reviewed changes

Comment thread library/alloc/src/vec/mod.rs Resolved

Comment thread library/alloc/src/vec/spec_extend_with.rs Resolved

scottmcm mentioned this pull request Dec 8, 2025

Another attempt at Vec::extend_with via iter::repeat_n #149751

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paolobarbolini force-pushed the vec-extend-with-via-repeatn branch from 164c6cf to 28e44dc Compare December 8, 2025 07:34

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rust-bors Bot added a commit that referenced this pull request Dec 12, 2025

Auto merge of #133662 - paolobarbolini:vec-extend-with-via-repeatn, r…

74054d7

…=<try> Use `iter::repeat_n` to implement `Vec::extend_with`

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Dec 12, 2025

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rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Dec 12, 2025

paolobarbolini mentioned this pull request May 2, 2026

Use repeat_n in Vec::extend_with #155928

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rustbot assigned JohnTitor and unassigned Noratrieb Sep 22, 2026

paolobarbolini added 2 commits October 31, 2026 17:13

Use iter::repeat_n to implement Vec::extend_with

2fcd070

This replaces the manual `Vec::extend_with` implementation with `iter::repeat_n` and `Vec::extend_trusted`. This simplifies the code and gets LLVM to remove the special case for the last element when `T` is trivial to clone.

Specialize Vec::extend_with for T: TrivialClone

fd362c2

After running perf on the previous commit, it was found that the compile-time regression was too significant to ignore. This introduces a specialized, simplified implementation for `T: TrivialClone` that will reduce the perf regression.

paolobarbolini force-pushed the vec-extend-with-via-repeatn branch from 28e44dc to fd362c2 Compare October 31, 2026 17:13

paolobarbolini closed this Nov 2, 2026

rustbot removed the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label Nov 2, 2026

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Overall result: ❌✅ regressions and improvements - please read the text below

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Overall result: ❌ regressions - please read the text below

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Overall result: ❌ regressions - please read the text below

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