Alloc `String::retain` optimization - #150067

fereidani · 2025-12-16T18:33:20Z

Hi again!
This uses the exact same algorithm as my other PR: #149784

Technically it should improve performance for String::retain too, But let's see what bors thinks.

rustbot · 2025-12-16T18:33:24Z

rustbot has assigned @Mark-Simulacrum.
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

ZuseZ4 · 2025-12-16T20:11:44Z

@bors try @rust-timer queue

Alloc `String::retain` optimization

rust-bors · 2025-12-16T22:28:36Z

☀️ Try build successful (CI)
Build commit: 2621d01 (2621d0177c40d6c9165ad462e5826558b57ea69a, parent: 31010ca61c3ff019e1480dda0a7ef16bd2bd51c0)

rust-timer · 2025-12-16T23:09:44Z

Finished benchmarking commit (2621d01): comparison URL.

Overall result: no relevant changes - 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

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

Max RSS (memory usage)

Results (primary -2.1%, secondary 0.5%)

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%	[3.7%, 3.7%]	1
Improvements ✅ (primary)	-2.1%	[-2.1%, -2.1%]	1
Improvements ✅ (secondary)	-2.8%	[-2.8%, -2.8%]	1
All ❌✅ (primary)	-2.1%	[-2.1%, -2.1%]	1

Cycles

Results (primary 68.5%, 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)	68.5%	[68.5%, 68.5%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.4%	[-2.4%, -2.4%]	1
All ❌✅ (primary)	68.5%	[68.5%, 68.5%]	1

Binary size

Results (primary 0.0%)

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

	mean	range	count
Regressions ❌ (primary)	0.0%	[0.0%, 0.0%]	4
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.0%	[0.0%, 0.0%]	4

Bootstrap: 479.038s -> 478.434s (-0.13%)
Artifact size: 390.32 MiB -> 390.32 MiB (-0.00%)

Mark-Simulacrum · 2025-12-28T15:11:01Z

r? joboet

Probably makes sense to have the same reviewer for both.

rustbot · 2025-12-28T15:11:04Z

joboet is not on the review rotation at the moment.
They may take a while to respond.

rustbot · 2026-01-04T02:17:54Z

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.

joboet

I think this could be even faster by avoiding some UTF-8 reencoding...

View changes since this review

rustbot · 2026-01-05T10:47:27Z

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

Kobzol · 2026-01-07T13:04:00Z

@bors try @rust-timer queue

rust-timer · 2026-01-07T13:04:03Z

Awaiting bors try build completion.

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

rust-bors · 2026-01-07T13:04:05Z

⌛ Trying commit 590ba93 with merge 886f85e…

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

Workflow: https://github.com/rust-lang/rust/actions/runs/20782421026

Alloc `String::retain` optimization

joboet · 2026-01-07T14:04:27Z

@bors try cancel
Since this will need a rerun once fixed anyway.

rust-bors · 2026-01-07T14:04:31Z

Try build cancelled. Cancelled workflows:

https://github.com/rust-lang/rust/actions/runs/20782421026

SpriteOvO · 2026-05-03T17:05:04Z

@fereidani Hi, ping from triage team. This PR has been inactive for a while. There are still some review comments that need to be addressed. Would you like to proceed with this PR? Thanks.

fereidani · 2026-05-03T18:46:57Z

@SpriteOvO @joboet Please excuse the delay. My country is currently at war, and our government has cut off internet access for our people for over two months now. Now that I have a somewhat workable connection, I’ll try to complete the PR within the next few days. Thank you for your understanding.

SpriteOvO · 2026-05-04T02:09:52Z

@fereidani Oh, so sorry to hear that. No need to hurry at all here. Feel free to come back to this when you have time, staying safe first.

rustbot · 2026-08-01T21:32:58Z

⚠️ Warning ⚠️

This PR is based on an upstream commit that is older than 28 days.

It's recommended to update your branch according to the rustc-dev-guide.

fereidani · 2026-08-01T21:36:03Z

Please excuse my delay. Let me know when to clean up the tree and make it a single commit.

joboet

I'm very glad to hear from you again! This looks nice and sound, I only have one minor suggestion.

View changes since this review

joboet · 2026-08-02T08:48:34Z

+            let ch = unsafe { g.s.get_unchecked(read..len).chars().next().unwrap_unchecked() };
            let ch_len = ch.len_utf8();


Calculating the length using char_indices tends to optimise a bit better. Try doing

Suggested change

let ch = unsafe { g.s.get_unchecked(read..len).chars().next().unwrap_unchecked() };

let ch_len = ch.len_utf8();

let mut iter = unsafe { g.s.get_unchecked(read..len).char_indices() };

let ch = unsafe { iter.next().unwrap_unchecked() };

let ch_len = iter.offset();

fereidani · 2026-08-02T19:57:29Z

Thank you my friend, I'm happy to be back, It's a privilege for me to have you as my reviewer.
I checked the assembly for both, for retain(|c| c != ASCII) current version is generating smaller x64 assembly and optimizes better as ch.len_utf8() provides more information about the length for LLVM which it can optimize.
but while reviewing the assembly/IR I found out that ptr::copy turns into a slow call qword ptr [rip + memmove@GOTPCREL] call which is not optimal.
I would suggest that we switch it to:

                // SAFETY: `read..read + ch_len` is in bounds and `g.write < read`.
                //
                // The match makes every copy length constant so LLVM expands
                // it inline; a dynamic-length `ptr::copy` lowers to a
                // `memmove` call per retained character.
                unsafe {
                    let src = ptr.add(read);
                    let dst = ptr.add(g.write);
                    match ch_len {
                        1 => ptr::copy(src, dst, 1),
                        2 => ptr::copy(src, dst, 2),
                        3 => ptr::copy(src, dst, 3),
                        _ => ptr::copy(src, dst, 4),
                    }
                }

It optimizes better and it is about 3.5x faster when match is optimizable.
I also checked core::char::encode_utf8_raw_unchecked and self.encode_utf8; they both generate sub-optimal assembly and recalculate character length, self.encode_utf8 also generates unnecessary panic unwind handler and can't prove code could not panic. there might be a optimization chance for other functions using same ptr::copy match optimization too but I haven't benchmarked it yet for the others.
I also tried to do a cleaner optimization using assert_unchecked(ch_len>=1 && ch_len<=4) and then single ptr::copy, but LLVM can't optimize that like the match.
But whatever you think suits the project better is fine with me. Please let me know what I should do.

rustbot assigned Mark-Simulacrum Dec 16, 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 16, 2025

This comment has been minimized.

Sign in to view

rust-bors Bot added a commit that referenced this pull request Dec 16, 2025

Auto merge of #150067 - fereidani:string_retain, r=<try>

2621d01

Alloc `String::retain` optimization

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 Dec 16, 2025

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 Dec 16, 2025

rustbot assigned joboet and unassigned Mark-Simulacrum Dec 28, 2025

This comment has been minimized.

Sign in to view

rustbot added has-merge-commits PR has merge commits, merge with caution. S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Jan 3, 2026

fereidani force-pushed the string_retain branch from bc23951 to 875c63d Compare January 4, 2026 02:17

rustbot removed has-merge-commits PR has merge commits, merge with caution. S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Jan 4, 2026

fereidani mentioned this pull request Jan 5, 2026

Improve alloc Vec::retain_mut performance #149784

Merged

fereidani added 2 commits January 5, 2026 13:25

Add feature flag likely/unlikely for alloc

c68f67c

Improve and optimize string::retain implementation

2af59e0

fereidani force-pushed the string_retain branch from 66f0f66 to 2af59e0 Compare January 5, 2026 09:56

joboet requested changes Jan 5, 2026

View reviewed changes

Comment thread library/alloc/src/string.rs Outdated

Comment thread library/alloc/src/string.rs Outdated

Comment thread library/alloc/src/string.rs Outdated

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 Jan 5, 2026

Improve string::retain implementation

590ba93

fereidani requested a review from joboet January 7, 2026 11:46

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Jan 7, 2026

rust-bors Bot added a commit that referenced this pull request Jan 7, 2026

Auto merge of #150067 - fereidani:string_retain, r=<try>

886f85e

Alloc `String::retain` optimization

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

joboet requested changes Jan 7, 2026

View reviewed changes

Comment thread library/alloc/src/string.rs Outdated

Comment thread library/alloc/src/string.rs Outdated

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 Jan 7, 2026

Fix 'string::retain` implementation

fb8db60

fereidani requested a review from joboet August 1, 2026 21:34

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Aug 1, 2026

This comment has been minimized.

Sign in to view

Fix string::retain debug_assert

7373b61

joboet reviewed Aug 2, 2026

View reviewed changes

		let ch = unsafe { g.s.get_unchecked(read..len).chars().next().unwrap_unchecked() };
		let ch_len = ch.len_utf8();

-            let ch = unsafe { g.s.get_unchecked(read..len).chars().next().unwrap_unchecked() };
-            let ch_len = ch.len_utf8();
+            let mut iter = unsafe { g.s.get_unchecked(read..len).char_indices() };
+            let ch = unsafe { iter.next().unwrap_unchecked() };
+            let ch_len = iter.offset();

Uh oh!

Uh oh!

Conversation

fereidani commented Dec 16, 2025 • edited by rustbot Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

rustbot commented Dec 16, 2025

Uh oh!

ZuseZ4 commented Dec 16, 2025

Uh oh!

This comment has been minimized.

This comment has been minimized.

rust-bors Bot commented Dec 16, 2025

Uh oh!

This comment has been minimized.

rust-timer commented Dec 16, 2025

Overall result: no relevant changes - no action needed

Instruction count

Max RSS (memory usage)

Cycles

Binary size

Uh oh!

Mark-Simulacrum commented Dec 28, 2025

Uh oh!

rustbot commented Dec 28, 2025

Uh oh!

This comment has been minimized.

This comment has been minimized.

rustbot commented Jan 4, 2026

Uh oh!

joboet left a comment • 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!

Uh oh!

Uh oh!

Uh oh!

rustbot commented Jan 5, 2026

Uh oh!

Kobzol commented Jan 7, 2026

Uh oh!

rust-timer commented Jan 7, 2026

Uh oh!

rust-bors Bot commented Jan 7, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

Uh oh!

Uh oh!

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

Uh oh!

Uh oh!

rust-bors Bot commented Jan 7, 2026

Uh oh!

SpriteOvO commented May 3, 2026

Uh oh!

fereidani commented May 3, 2026

Uh oh!

SpriteOvO commented May 4, 2026

Uh oh!

rustbot commented Aug 1, 2026

Uh oh!

fereidani commented Aug 1, 2026

Uh oh!

This comment has been minimized.

joboet left a comment • 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!

joboet Aug 2, 2026

Choose a reason for hiding this comment

Uh oh!

fereidani commented Aug 2, 2026

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

fereidani commented Dec 16, 2025 •

edited by rustbot

Loading

joboet left a comment •

edited by rustbot

Loading

rust-bors Bot commented Jan 7, 2026 •

edited

Loading

joboet commented Jan 7, 2026 •

edited

Loading

joboet left a comment •

edited by rustbot

Loading