Alloc `String::retain` optimization by fereidani · Pull Request #150067 · rust-lang/rust

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.
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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

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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

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rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Dec 16, 2025

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rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Dec 16, 2025

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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

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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

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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

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Fix string::retain debug_assert

7373b61

joboet reviewed Aug 2, 2026

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		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();

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Overall result: no relevant changes - no action needed

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