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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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@EFanZh EFanZh commented Nov 3, 2024 •
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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.
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rustbot commented Nov 3, 2024

r? @jhpratt

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@rustbot rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. 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 Nov 3, 2024
@EFanZh EFanZh force-pushed the zero-cost-rc-arc-deref branch from b283c44 to ae36f44 Compare November 3, 2024 09:14
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marmeladema commented Nov 3, 2024

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?
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EFanZh commented Nov 3, 2024

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.
@EFanZh EFanZh force-pushed the zero-cost-rc-arc-deref branch from ae36f44 to 0d6165f Compare November 3, 2024 11:21
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@EFanZh EFanZh force-pushed the zero-cost-rc-arc-deref branch from 0d6165f to 98edd5b Compare November 3, 2024 13:06
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jhpratt commented Nov 3, 2024

r? libs
@rustbot rustbot assigned joboet and unassigned jhpratt Nov 3, 2024
@EFanZh EFanZh force-pushed the zero-cost-rc-arc-deref branch from 98edd5b to 8beb51d Compare November 4, 2024 16:29
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@EFanZh EFanZh force-pushed the zero-cost-rc-arc-deref branch from 8beb51d to d7879fa Compare November 4, 2024 17:26
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@EFanZh EFanZh force-pushed the zero-cost-rc-arc-deref branch from d7879fa to 317aa0e Compare November 4, 2024 18:40
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joboet commented Nov 7, 2024

@EFanZh Is this ready for review? If so, please un-draft the PR.
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EFanZh commented Nov 7, 2024 •
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@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.
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joboet commented Nov 8, 2024

No worries! I just didn't want to keep you waiting in case you had forgotten to change the state.
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@rustbot 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
@EFanZh EFanZh force-pushed the zero-cost-rc-arc-deref branch 3 times, most recently from f243654 to 1308bf6 Compare November 11, 2024 18:35
@EFanZh EFanZh force-pushed the zero-cost-rc-arc-deref branch from 4055bda to 3dacbcd Compare November 22, 2025 03:55
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@EFanZh EFanZh force-pushed the zero-cost-rc-arc-deref branch from 3dacbcd to d824f59 Compare November 22, 2025 06:46
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bors commented Nov 26, 2025

☔ The latest upstream changes (presumably #147799) made this pull request unmergeable. Please resolve the merge conflicts.
@EFanZh EFanZh force-pushed the zero-cost-rc-arc-deref branch from d824f59 to a84cde1 Compare November 30, 2025 07:08
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EFanZh added 20 commits November 30, 2025 21:06
@EFanZh
Add RefCounts and RcLayout types
f940c9f
@EFanZh
Add allocation functions
91a1484
@EFanZh
Add RefCounter trait
87b82ad
@EFanZh
Add RawWeak type
f746dcc
@EFanZh
Add RawWeak methods for sized values
a5931bc
@EFanZh
Add RawWeak methods for slice values
81e12ce
@EFanZh
Implement necessary traits for RawWeak
8c8aee5
@EFanZh
Add RawRc type
87177cf
@EFanZh
Add RawRc methods for sized values
64f14a1
@EFanZh
Add RawRc methods for MaybeUninit<T> values
c0329f1
@EFanZh
Add RawRc methods for slice values
2332c17
@EFanZh
Add RawRc methods for dyn Any type
532882d
@EFanZh
Implement necessary traits for RawRc
9f6c2db
@EFanZh
Add RawUniqueRc type
308edb7
@EFanZh
Add RawUniqueRc methods for sized values
effa938
@EFanZh
Implement necessary traits for RawUniqueRc
32f11f0
@EFanZh
Implement alloc::rc::{Rc,Weak,UniqueRc} with alloc::raw_rc types
9c74df7
@EFanZh
Add LLVM codegen tests for new Rc implementation
7552777
@EFanZh
Implement alloc::sync::{Arc,Weak,UniqueArc} with alloc::raw_rc types
c7da147
@EFanZh
Add LLVM codegen tests for new Arc implementation
0bc78ef
@EFanZh EFanZh force-pushed the zero-cost-rc-arc-deref branch from a84cde1 to 0bc78ef Compare November 30, 2025 13:06
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rustbot commented Nov 30, 2025

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.
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perf-regression Performance regression. S-blocked Status: Blocked on something else such as an RFC or other implementation work. T-libs Relevant to the library team, which will review and decide on the PR/issue.