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

EFanZh · 2024-11-03T07:48:34Z

Currently, Rc<T> and Arc<T> store pointers to RcInner<T> and ArcInner<T>. This PR changes the pointers so that they point to T directly instead.

This is based on the assumption that we access the T value more frequently than accessing reference counts. With this change, accessing the data can be done without offsetting pointers from RcInner<T> and ArcInner<T> to their contained data. This change might also enables some possibly useful future optimizations, such as:

Convert &[Rc<T>] into &[&T] within O(1) time.
Convert &[Rc<T>] into Vec<&T> utilizing memcpy.
Convert &Option<Rc<T>> into Option<&T> without branching.
Make Rc<T> and Arc<T> FFI compatible types where T: Sized.

rustbot · 2024-11-03T07:48:42Z

r? @jhpratt

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marmeladema · 2024-11-03T09:22:09Z

Would it potentially enable those types to have an ffi compatible ABI? So that they could be returned and passed directly from /to ffi function, like Box?

EFanZh · 2024-11-03T10:36:08Z

Would it potentially enable those types to have an ffi compatible ABI? So that they could be returned and passed directly from /to ffi function, like Box?

I think in theory it is possible, at least for sized types, but I am not familiar with how to formally make it so.

jhpratt · 2024-11-03T23:07:24Z

r? libs

joboet · 2024-11-07T16:17:52Z

@EFanZh Is this ready for review? If so, please un-draft the PR.

EFanZh · 2024-11-07T17:12:22Z

@joboet: The source code part is mostly done, but I haven’t finished updating LLDB and CDB pretty printers. The CI doesn’t seem to run those tests.

joboet · 2024-11-08T12:56:42Z

No worries! I just didn't want to keep you waiting in case you had forgotten to change the state.
@rustbot author

EFanZh · 2025-05-24T14:31:32Z

@joboet: I have split this PR into two commits:

The first commit contains alloc::raw_rc and alloc::rc changes, which has also be submitted separately in Make Rc<T>::deref zero-cost #141348.
The second commit contains alloc::sync changes.

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 · 2025-06-27T04:13:49Z

☔ The latest upstream changes (presumably #143074) made this pull request unmergeable. Please resolve the merge conflicts.

Kobzol · 2025-08-07T11:30:55Z

For running perf. here, the PR needs to be rebased first.

EFanZh · 2025-08-07T13:26:33Z

I’ll fix the conflicts in a few days.

Kobzol · 2025-08-09T17:34:43Z

@bors try @rust-timer queue

rust-bors · 2025-08-09T17:34:47Z

⌛ Trying commit 817b116 with merge ec7bf66…

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

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

rust-bors · 2025-08-09T19:50:45Z

☀️ Try build successful (CI)
Build commit: ec7bf66 (ec7bf66bfb634dca39e8b1a65c3a872c3cd13fd4, parent: ca77504943887037504c7fc0b9bf06dab3910373)

rust-timer · 2025-08-09T21:00:48Z

Finished benchmarking commit (ec7bf66): 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)	1.8%	[0.3%, 14.7%]	14
Regressions ❌ (secondary)	0.7%	[0.1%, 2.6%]	22
Improvements ✅ (primary)	-0.5%	[-1.5%, -0.1%]	21
Improvements ✅ (secondary)	-1.2%	[-3.0%, -0.0%]	19
All ❌✅ (primary)	0.4%	[-1.5%, 14.7%]	35

Max RSS (memory usage)

Results (primary 1.0%, 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)	3.2%	[1.3%, 6.1%]	4
Regressions ❌ (secondary)	3.6%	[2.1%, 4.8%]	9
Improvements ✅ (primary)	-3.3%	[-4.3%, -2.3%]	2
Improvements ✅ (secondary)	-3.5%	[-6.4%, -2.0%]	4
All ❌✅ (primary)	1.0%	[-4.3%, 6.1%]	6

Cycles

Results (primary 4.8%, secondary -0.6%)

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

	mean	range	count
Regressions ❌ (primary)	4.8%	[2.0%, 12.7%]	5
Regressions ❌ (secondary)	2.8%	[2.1%, 3.1%]	7
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-5.3%	[-7.1%, -1.6%]	5
All ❌✅ (primary)	4.8%	[2.0%, 12.7%]	5

Binary size

Results (primary 0.5%, secondary 0.6%)

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

	mean	range	count
Regressions ❌ (primary)	0.5%	[0.0%, 2.2%]	80
Regressions ❌ (secondary)	0.6%	[0.0%, 5.5%]	96
Improvements ✅ (primary)	-0.3%	[-0.4%, -0.2%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.5%	[-0.4%, 2.2%]	82

Bootstrap: 463.988s -> 464.881s (0.19%)
Artifact size: 377.65 MiB -> 377.46 MiB (-0.05%)

Kobzol · 2025-08-10T06:13:25Z

@bors try @rust-timer queue

rust-bors · 2025-08-10T06:13:29Z

⌛ Trying commit d54c877 with merge 38a5d4f…

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

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

rust-bors · 2025-08-10T08:26:31Z

☀️ Try build successful (CI)
Build commit: 38a5d4f (38a5d4f4c9ded25243c2946db27e12396bfed6a4, parent: 915a766b2f9fd53a8cd7b1fad003d3f8e488ff4b)

rust-timer · 2025-08-10T09:36:55Z

Finished benchmarking commit (38a5d4f): 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)	2.2%	[0.3%, 14.7%]	11
Regressions ❌ (secondary)	0.6%	[0.1%, 2.6%]	22
Improvements ✅ (primary)	-0.5%	[-1.5%, -0.2%]	28
Improvements ✅ (secondary)	-1.1%	[-3.0%, -0.1%]	26
All ❌✅ (primary)	0.3%	[-1.5%, 14.7%]	39

Max RSS (memory usage)

Results (primary 1.0%, secondary -0.8%)

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

	mean	range	count
Regressions ❌ (primary)	3.2%	[1.6%, 5.6%]	5
Regressions ❌ (secondary)	2.9%	[2.6%, 3.1%]	4
Improvements ✅ (primary)	-2.6%	[-3.3%, -2.0%]	3
Improvements ✅ (secondary)	-3.3%	[-6.1%, -2.1%]	6
All ❌✅ (primary)	1.0%	[-3.3%, 5.6%]	8

Cycles

Results (primary 5.1%, secondary -0.7%)

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

	mean	range	count
Regressions ❌ (primary)	5.1%	[2.1%, 13.3%]	4
Regressions ❌ (secondary)	2.4%	[2.1%, 2.8%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.8%	[-4.2%, -3.3%]	2
All ❌✅ (primary)	5.1%	[2.1%, 13.3%]	4

Binary size

Results (primary 0.5%, secondary 0.6%)

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

	mean	range	count
Regressions ❌ (primary)	0.5%	[0.0%, 2.2%]	77
Regressions ❌ (secondary)	0.6%	[0.0%, 5.6%]	96
Improvements ✅ (primary)	-0.3%	[-0.4%, -0.2%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.5%	[-0.4%, 2.2%]	79

Bootstrap: 463.478s -> 463.036s (-0.10%)
Artifact size: 377.38 MiB -> 377.46 MiB (0.02%)

rustbot assigned jhpratt Nov 3, 2024

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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 Nov 8, 2024

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rust-bors bot added a commit that referenced this pull request Aug 9, 2025

Auto merge of #132553 - EFanZh:zero-cost-rc-arc-deref, r=<try>

ec7bf66

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

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Aug 9, 2025

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tgross35 self-assigned this Aug 9, 2025

rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Aug 9, 2025

EFanZh added 2 commits August 10, 2025 09:35

Make Rc<T>::deref zero-cost

e345c2f

Make Arc<T>::deref zero-cost

d54c877

EFanZh force-pushed the zero-cost-rc-arc-deref branch from 817b116 to d54c877 Compare August 10, 2025 01:36

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rust-bors bot added a commit that referenced this pull request Aug 10, 2025

Auto merge of #132553 - EFanZh:zero-cost-rc-arc-deref, r=<try>

38a5d4f

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

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

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

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

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

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

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