Make `Rc<T>::deref` zero-cost #141348

EFanZh · 2025-05-21T14:14:40Z

This PR makes Rc::deref zero-cost by changing the internal pointer to point directly to the value instead of to the allocation.

This PR is split from #132553, which will also make Arc::deref zero-cost.

Review status:

rustbot · 2025-05-24T10:22:23Z

The Miri subtree was changed

cc @rust-lang/miri

library/alloc/src/raw_rc/mod.rs

oli-obk · 2025-05-26T15:03:21Z

@bors try @rust-timer queue

bors · 2025-05-26T15:04:31Z

⌛ Trying commit f5245ba with merge 8ef4a25...

Make `Rc<T>::deref` zero-cost This PR makes `Rc::deref` zero-cost by changing the internal pointer so that it points to the value directly instead of the allocation. This is split out from #132553, which will also make `Arc::deref` zero-cost.

bors · 2025-05-26T17:13:37Z

☀️ Try build successful - checks-actions
Build commit: 8ef4a25 (8ef4a25b05973cfbd577205c507a891d07f0ae5f)

rust-timer · 2025-05-26T19:26:48Z

Finished benchmarking commit (8ef4a25): 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)	0.5%	[0.2%, 0.7%]	4
Regressions ❌ (secondary)	1.1%	[0.2%, 2.5%]	9
Improvements ✅ (primary)	-1.0%	[-2.0%, -0.5%]	7
Improvements ✅ (secondary)	-1.3%	[-2.0%, -0.1%]	4
All ❌✅ (primary)	-0.5%	[-2.0%, 0.7%]	11

Max RSS (memory usage)

Results (primary -0.3%, secondary -0.0%)

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)	8.4%	[8.4%, 8.4%]	1
Regressions ❌ (secondary)	3.1%	[2.0%, 4.8%]	6
Improvements ✅ (primary)	-4.6%	[-7.1%, -2.1%]	2
Improvements ✅ (secondary)	-4.7%	[-6.4%, -2.9%]	4
All ❌✅ (primary)	-0.3%	[-7.1%, 8.4%]	3

Cycles

Results (primary -0.5%, secondary -1.9%)

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)	2.1%	[2.1%, 2.1%]	2
Improvements ✅ (primary)	-1.9%	[-1.9%, -1.9%]	1
Improvements ✅ (secondary)	-3.9%	[-9.3%, -1.5%]	4
All ❌✅ (primary)	-0.5%	[-1.9%, 0.9%]	2

Binary size

Results (primary 0.2%, secondary 1.6%)

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)	0.3%	[0.0%, 1.8%]	29
Regressions ❌ (secondary)	1.8%	[0.0%, 6.4%]	9
Improvements ✅ (primary)	-0.3%	[-0.6%, -0.1%]	5
Improvements ✅ (secondary)	-0.4%	[-0.4%, -0.4%]	1
All ❌✅ (primary)	0.2%	[-0.6%, 1.8%]	34

Bootstrap: 775.728s -> 776.531s (0.10%)
Artifact size: 366.25 MiB -> 366.33 MiB (0.02%)

apiraino · 2025-07-31T13:34:22Z

Would anyone comment the perf. run? cc @EFanZh thanks!

@rustbot author

rustbot · 2025-07-31T13:34:26Z

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

library/alloc/src/raw_rc/mod.rs

bors · 2025-09-30T23:22:36Z

☔ The latest upstream changes (presumably #147197) made this pull request unmergeable. Please resolve the merge conflicts.

bors · 2025-10-04T21:57:07Z

☔ The latest upstream changes (presumably #147340) made this pull request unmergeable. Please resolve the merge conflicts.

rustbot · 2025-10-05T07:16:02Z

This PR was rebased onto a different master 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.

the8472 · 2025-10-05T09:21:36Z

library/alloc/src/raw_rc/mod.rs

+//! | Padding     | `size_of::<RefCounts>().next_multiple_of(align_of::<T>()) - size_of::<RefCounts>()` |
+//! | `RefCounts` | `size_of::<RefCounts>()`                                                            |
+//! | `T`         | `size_of::<T>()`                                                                    |


So this puts the data after the refcount. Would it later still be possible to change it to data, padding and then the refcounts? That'd make it possible to convert a Rc<[T]> to Vec<T> without memcpy.

@the8472: I think there are several problems:

The current design employs the strategy from [DRAFT] Rc: allow deduping both deref and clone across types #133061, where using a fixed offset of the reference counter could potentially allowing sharing the same clone implementation across different Rc types.

Not using a fixed offset makes the debug pretty printer more difficult to write.

The buffer allocated with Rc has alignment of align_of::<RefCounts>().max(align_of::<T>()), which might be different with align_of::<T>(). Since Vec<T> will deallocate using a Layout with align_of::<T>(), and deallocating requires the alignment being exactly the same as the one that was used to do allocation according to https://doc.rust-lang.org/stable/std/alloc/trait.Allocator.html#memory-fitting, Vec<T> might not be able to deallocate the memory allocated by Rc<[T]>, at least not with the current Vec implementation.

rustbot assigned joboet May 21, 2025

rustbot added T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels May 21, 2025

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EFanZh force-pushed the zero-cost-rc-deref branch from df34f84 to d3a7429 Compare May 24, 2025 05:01

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EFanZh force-pushed the zero-cost-rc-deref branch 2 times, most recently from bc84ec6 to 19fb34b Compare May 24, 2025 09:00

EFanZh marked this pull request as ready for review May 24, 2025 10:22

rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label May 24, 2025

EFanZh mentioned this pull request May 24, 2025

Make Rc<T>::deref and Arc<T>::deref zero-cost #132553

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teor2345 reviewed May 26, 2025

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EFanZh force-pushed the zero-cost-rc-deref branch from 19fb34b to f5245ba Compare May 26, 2025 15:02

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rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label May 26, 2025

bors mentioned this pull request May 26, 2025

Remove an unnecessary use of Box::into_inner. #141599

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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 May 26, 2025

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 Jul 31, 2025

teor2345 reviewed Jul 31, 2025

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EFanZh force-pushed the zero-cost-rc-deref branch from 62c8b9c to eccd4c2 Compare September 9, 2025 16:42

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EFanZh force-pushed the zero-cost-rc-deref branch from eccd4c2 to 1266321 Compare September 10, 2025 16:27

Dylan-DPC 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 Sep 20, 2025

EFanZh force-pushed the zero-cost-rc-deref branch from 1266321 to 616991d Compare October 1, 2025 14:22

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EFanZh added 18 commits October 5, 2025 15:03

Add RefCounts and RcLayout types

44e6414

Add allocation functions

699ac9c

Add RefCounter trait

c483f03

Add RawWeak type

e273cb2

Add RawWeak methods for sized values

be73265

Add RawWeak methods for slice values

e8fb46c

Implement necessary traits for RawWeak

84a7178

Add RawRc type

61d8455

Add RawRc methods for sized values

2e2591a

Add RawRc methods for MaybeUninit<T> values

1b449cc

Add RawRc methods for slice values

0fac2bb

Add RawRc methods for dyn Any type

9414e2a

Implement necessary traits for RawRc

977f83c

Add RawUniqueRc type

74f3386

Add RawUniqueRc methods for sized values

17fab92

Implement necessary traits for RawUniqueRc

c68950e

Implement alloc::rc::{Rc,Weak,UniqueRc} with alloc::raw_rc types

1dd28dc

Add LLVM codegen tests for new Rc implementation

31d5740

EFanZh force-pushed the zero-cost-rc-deref branch from 616991d to 31d5740 Compare October 5, 2025 07:15

the8472 reviewed Oct 5, 2025

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Make Rc<T>::deref zero-cost #141348

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rust-timer commented May 26, 2025

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

Instruction count

Max RSS (memory usage)

Cycles

Binary size

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Make `Rc<T>::deref` zero-cost #141348

Make `Rc<T>::deref` zero-cost #141348

EFanZh commented May 21, 2025 •

edited by tgross35

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