Optimize `Extend for String` by paolobarbolini · Pull Request #149705 · rust-lang/rust

paolobarbolini · 2025-12-06T09:00:15Z

Implements the idea in #148604 (comment).
Enhancement to #149694.

Could we get a perf run?

rustbot · 2025-12-06T09:00:20Z

rustbot has assigned @joboet.
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Urgau · 2025-12-06T10:37:37Z

@bors try @rust-timer queue

Optimize `Extend for String`

rust-bors · 2025-12-06T12:49:59Z

☀️ Try build successful (CI)
Build commit: 3f3acdf (3f3acdfbe44242af2a9a5419227d7c20c118425e, parent: fbab541a7ad1c22fc51783d03c7d75fa577f5633)

rust-timer · 2025-12-06T15:06:35Z

Finished benchmarking commit (3f3acdf): 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
Regressions ❌ (secondary)	0.1%	[0.1%, 0.2%]	3
Improvements ✅ (primary)	-1.1%	[-1.1%, -1.1%]	1
Improvements ✅ (secondary)	-0.4%	[-1.3%, -0.1%]	4
All ❌✅ (primary)	-1.1%	[-1.1%, -1.1%]	1

Max RSS (memory usage)

Results (secondary -2.4%)

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)	1.1%	[0.8%, 1.7%]	4
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.7%	[-6.4%, -0.9%]	11
All ❌✅ (primary)	-	-	0

Cycles

Results (primary -3.2%, secondary 1.4%)

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)	3.7%	[2.1%, 6.6%]	7
Improvements ✅ (primary)	-3.2%	[-3.2%, -3.2%]	1
Improvements ✅ (secondary)	-6.5%	[-6.8%, -6.2%]	2
All ❌✅ (primary)	-3.2%	[-3.2%, -3.2%]	1

Binary size

Results (primary -0.1%, 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)	-0.1%	[-1.1%, -0.0%]	30
Improvements ✅ (secondary)	-0.1%	[-0.1%, -0.0%]	23
All ❌✅ (primary)	-0.1%	[-1.1%, -0.0%]	30

Bootstrap: 471.701s -> 471.964s (0.06%)
Artifact size: 388.90 MiB -> 388.53 MiB (-0.09%)

paolobarbolini · 2025-12-07T10:24:50Z

Interesting perf results. Let's see what happens when we specialize on array and Vec iterators...

@Urgau: Could we do another perf run please?

I hope this is ok for you @lolbinarycat since it takes from #148604, #148604 (comment) and #149694

Urgau · 2025-12-07T11:07:25Z

@bors try @rust-timer queue

Optimize `Extend for String`

rust-bors · 2025-12-07T17:11:23Z

💥 Test timed out after 21600s

lolbinarycat · 2025-12-07T18:10:24Z

I would appreciate a Co-Authored-by field on the first commit if you could (git rebase -i then edit on that commit)

library/alloc/src/string.rs

Co-authored-by: binarycat <binarycat@envs.net>

paolobarbolini · 2025-12-10T06:51:14Z

Could we run perf again? The previous attempt failed after the try build timed out.

lolbinarycat · 2025-12-10T19:40:30Z

@bors try @rust-timer queue

Optimize `Extend for String`

rust-bors · 2025-12-10T21:52:59Z

☀️ Try build successful (CI)
Build commit: ed1b0fb (ed1b0fb66582f4a21549e973103af3f8652d8b66, parent: 198328ad7960b1bece0dc48496bff6c62dd5d339)

rust-timer · 2025-12-10T22:34:23Z

Finished benchmarking commit (ed1b0fb): 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
Regressions ❌ (secondary)	0.5%	[0.3%, 0.8%]	3
Improvements ✅ (primary)	-0.4%	[-0.4%, -0.4%]	1
Improvements ✅ (secondary)	-0.1%	[-0.1%, -0.0%]	6
All ❌✅ (primary)	-0.4%	[-0.4%, -0.4%]	1

Max RSS (memory usage)

Results (primary 0.0%, secondary -3.5%)

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

	mean	range	count
Regressions ❌ (primary)	3.5%	[3.5%, 3.5%]	1
Regressions ❌ (secondary)	0.9%	[0.8%, 1.0%]	2
Improvements ✅ (primary)	-1.1%	[-1.5%, -0.9%]	3
Improvements ✅ (secondary)	-4.4%	[-6.9%, -1.6%]	10
All ❌✅ (primary)	0.0%	[-1.5%, 3.5%]	4

Cycles

Results (secondary 3.7%)

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)	4.4%	[1.8%, 7.5%]	9
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.0%	[-2.0%, -2.0%]	1
All ❌✅ (primary)	-	-	0

Binary size

Results (primary -0.4%)

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)	-0.4%	[-1.1%, -0.1%]	4
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.4%	[-1.1%, -0.1%]	4

Bootstrap: 471.708s -> 474.648s (0.62%)
Artifact size: 389.07 MiB -> 388.75 MiB (-0.08%)

joboet · 2025-12-16T06:32:54Z

library/alloc/src/string.rs

+            // SAFETY:
+            // - `val` is a valid &str, so `val.as_ptr()` is valid
+            //   for `val.len()` bytes and properly initialized.
+            // - `spare` points to valid spare capacity in the Vec


This is unsound – it's possible to pass the same string multiple times (or overlapping strings), in which case this sum will saturate. But in that case, the capacity will not suffice here.

reserve will abort the program with an OOM error in that case, no?

Yes, esp since no single allocation can be isize::MAX bytes large, reserve is guaranteed to fail if the sum saturates (in this case it will panic when it tries to calculate the new layout of the allocation, not OOM).

joboet · 2025-12-16T06:39:50Z

library/alloc/src/string.rs

+            let chunk = match iter.next_chunk::<8>() {
+                Ok(chunk) => chunk.into_iter(),
+                Err(partial_chunk) => {
+                    repeat = false;
+                    partial_chunk
+                }
+            };


Is this necessary for performance? Otherwise, I'd rather you use a loop and return instead of testing repeat.

rustbot · 2025-12-16T06:40:15Z

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

lolbinarycat · 2025-12-29T18:10:39Z

note: #149694 has been accepted.

bors · 2025-12-30T02:43:26Z

☔ The latest upstream changes (presumably #149694) made this pull request unmergeable. Please resolve the merge conflicts.

rustbot assigned joboet Dec 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 Dec 6, 2025

paolobarbolini force-pushed the string-extend-chunked branch from 28e8c60 to 207e484 Compare December 6, 2025 09:06

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Auto merge of #149705 - paolobarbolini:string-extend-chunked, r=<try>

3f3acdf

Optimize `Extend for String`

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rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Dec 6, 2025

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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 Dec 6, 2025

paolobarbolini force-pushed the string-extend-chunked branch from 207e484 to b6110e5 Compare December 7, 2025 10:11

rustbot added the T-rustdoc-frontend Relevant to the rustdoc-frontend team, which will review and decide on the web UI/UX output. label Dec 7, 2025

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Auto merge of #149705 - paolobarbolini:string-extend-chunked, r=<try>

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Optimize `Extend for String`

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rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Dec 7, 2025

bjorn3 reviewed Dec 8, 2025

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library/alloc/src/string.rs Outdated Show resolved Hide resolved

paolobarbolini and others added 2 commits December 10, 2025 04:18

Optimize Extend for String

9b1de06

Co-authored-by: binarycat <binarycat@envs.net>

Specialize for array and Vec iterators

64f63c3

Convert consecutive String::push_str to String::extend

9961b10

paolobarbolini force-pushed the string-extend-chunked branch from b6110e5 to 9961b10 Compare December 10, 2025 03:20

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Auto merge of #149705 - paolobarbolini:string-extend-chunked, r=<try>

ed1b0fb

Optimize `Extend for String`

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rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Dec 10, 2025

joboet requested changes Dec 16, 2025

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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 Dec 16, 2025

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

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

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