Simplify internals of `{Rc,Arc}::default` by alexcrichton · Pull Request #152591 · rust-lang/rust

alexcrichton · 2026-02-13T22:59:00Z

This commit simplifies the internal implementation of Default for these two pointer types to have the same performance characteristics as before (a side effect of changes in #131460) while avoid use of internal private APIs of Rc/Arc. To preserve the same codegen as before some non-generic functions needed to be tagged as #[inline] as well, but otherwise the same IR is produced before/after this change.

The motivation of this commit is I was studying up on the state of initialization of Arc and Rc and figured it'd be nicer to reduce the use of internal APIs and instead use public stable APIs where possible, even in the implementation itself.

rustbot · 2026-02-13T22:59:05Z

r? @Mark-Simulacrum

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alexcrichton · 2026-02-14T00:38:57Z

tests/codegen-llvm/issues/issue-111603.rs

@@ -30,8 +31,7 @@ pub fn new_uninit(x: u64) -> Arc<[u64; 1000]> {
 // CHECK-LABEL: @new_uninit_slice
 #[no_mangle]
 pub fn new_uninit_slice(x: u64) -> Arc<[u64]> {
-    // CHECK: call alloc::sync::arcinner_layout_for_value_layout
-    // CHECK-NOT: call alloc::sync::arcinner_layout_for_value_layout
+    // CHECK-NOT: %[[B:.+]] = alloca


The breadcrumbs for this codegen test points back at #111634 which had to do with internal refactorings about how methods were done. I don't believe that these codegen tests are really all that applicable any more so I did my best to at least keep them somewhat, but the extra #[inline] in this PR effectively means that the internals this test relies on also changed which means that the test needed an update one way or another.

library/alloc/src/rc.rs

This commit simplifies the internal implementation of `Default` for these two pointer types to have the same performance characteristics as before (a side effect of changes in 131460) while avoid use of internal private APIs of Rc/Arc. To preserve the same codegen as before some non-generic functions needed to be tagged as `#[inline]` as well, but otherwise the same IR is produced before/after this change. The motivation of this commit is I was studying up on the state of initialization of `Arc` and `Rc` and figured it'd be nicer to reduce the use of internal APIs and instead use public stable APIs where possible, even in the implementation itself.

rustbot · 2026-02-14T18:56:54Z

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 · 2026-02-21T20:04:54Z

Good idea!
@bors r+
r? joboet

rust-bors · 2026-02-21T20:04:57Z

📌 Commit ce4c17f has been approved by joboet

It is now in the queue for this repository.

…lt, r=joboet Simplify internals of `{Rc,Arc}::default` This commit simplifies the internal implementation of `Default` for these two pointer types to have the same performance characteristics as before (a side effect of changes in rust-lang#131460) while avoid use of internal private APIs of Rc/Arc. To preserve the same codegen as before some non-generic functions needed to be tagged as `#[inline]` as well, but otherwise the same IR is produced before/after this change. The motivation of this commit is I was studying up on the state of initialization of `Arc` and `Rc` and figured it'd be nicer to reduce the use of internal APIs and instead use public stable APIs where possible, even in the implementation itself.

Rollup of 14 pull requests Successful merges: - #150468 (rustc_target: callconv: powerpc64: Use the ABI set in target options instead of guessing) - #151628 (Fix ICE in const eval of packed SIMD types with non-power-of-two element counts) - #151740 (Fix short backtraces from stripped executables) - #151871 (don't use env with infer vars) - #152591 (Simplify internals of `{Rc,Arc}::default`) - #152865 (Fixed ByteStr not padding within its Display trait when no specific alignment is mentioned) - #152908 (Enable rust.remap-debuginfo in the dist profile) - #152705 (Test(lib/win/proc): Skip `raw_attributes` doctest under Win7) - #152767 (fix typo in `carryless_mul` macro invocation) - #152837 (fix(codegen): Use `body_codegen_attrs` For Caller In `adjust_target_feature_sig`) - #152871 (Fix warnings in rs{begin,end}.rs files) - #152921 (Add build.rustdoc option to bootstrap config) - #152933 (Start migration for `LintDiagnostic` items by adding API and migrating `LinkerOutput` lint) - #152937 (remove unneeded reboxing)

…lt, r=joboet Simplify internals of `{Rc,Arc}::default` This commit simplifies the internal implementation of `Default` for these two pointer types to have the same performance characteristics as before (a side effect of changes in rust-lang#131460) while avoid use of internal private APIs of Rc/Arc. To preserve the same codegen as before some non-generic functions needed to be tagged as `#[inline]` as well, but otherwise the same IR is produced before/after this change. The motivation of this commit is I was studying up on the state of initialization of `Arc` and `Rc` and figured it'd be nicer to reduce the use of internal APIs and instead use public stable APIs where possible, even in the implementation itself.

Rollup of 15 pull requests Successful merges: - #149366 (GVN: consider constants of primitive types as deterministic) - #150468 (rustc_target: callconv: powerpc64: Use the ABI set in target options instead of guessing) - #151628 (Fix ICE in const eval of packed SIMD types with non-power-of-two element counts) - #151871 (don't use env with infer vars) - #152591 (Simplify internals of `{Rc,Arc}::default`) - #152657 (std: move `exit` out of PAL) - #152865 (Fixed ByteStr not padding within its Display trait when no specific alignment is mentioned) - #152908 (Enable rust.remap-debuginfo in the dist profile) - #152705 (Test(lib/win/proc): Skip `raw_attributes` doctest under Win7) - #152767 (fix typo in `carryless_mul` macro invocation) - #152837 (fix(codegen): Use `body_codegen_attrs` For Caller In `adjust_target_feature_sig`) - #152871 (Fix warnings in rs{begin,end}.rs files) - #152921 (Add build.rustdoc option to bootstrap config) - #152933 (Start migration for `LintDiagnostic` items by adding API and migrating `LinkerOutput` lint) - #152937 (remove unneeded reboxing)

…lt, r=joboet Simplify internals of `{Rc,Arc}::default` This commit simplifies the internal implementation of `Default` for these two pointer types to have the same performance characteristics as before (a side effect of changes in rust-lang#131460) while avoid use of internal private APIs of Rc/Arc. To preserve the same codegen as before some non-generic functions needed to be tagged as `#[inline]` as well, but otherwise the same IR is produced before/after this change. The motivation of this commit is I was studying up on the state of initialization of `Arc` and `Rc` and figured it'd be nicer to reduce the use of internal APIs and instead use public stable APIs where possible, even in the implementation itself.

Rollup of 14 pull requests Successful merges: - #150468 (rustc_target: callconv: powerpc64: Use the ABI set in target options instead of guessing) - #151628 (Fix ICE in const eval of packed SIMD types with non-power-of-two element counts) - #151871 (don't use env with infer vars) - #152591 (Simplify internals of `{Rc,Arc}::default`) - #152657 (std: move `exit` out of PAL) - #152865 (Fixed ByteStr not padding within its Display trait when no specific alignment is mentioned) - #152908 (Enable rust.remap-debuginfo in the dist profile) - #152705 (Test(lib/win/proc): Skip `raw_attributes` doctest under Win7) - #152767 (fix typo in `carryless_mul` macro invocation) - #152837 (fix(codegen): Use `body_codegen_attrs` For Caller In `adjust_target_feature_sig`) - #152871 (Fix warnings in rs{begin,end}.rs files) - #152921 (Add build.rustdoc option to bootstrap config) - #152933 (Start migration for `LintDiagnostic` items by adding API and migrating `LinkerOutput` lint) - #152937 (remove unneeded reboxing)

…lt, r=joboet Simplify internals of `{Rc,Arc}::default` This commit simplifies the internal implementation of `Default` for these two pointer types to have the same performance characteristics as before (a side effect of changes in rust-lang#131460) while avoid use of internal private APIs of Rc/Arc. To preserve the same codegen as before some non-generic functions needed to be tagged as `#[inline]` as well, but otherwise the same IR is produced before/after this change. The motivation of this commit is I was studying up on the state of initialization of `Arc` and `Rc` and figured it'd be nicer to reduce the use of internal APIs and instead use public stable APIs where possible, even in the implementation itself.

Rollup of 13 pull requests Successful merges: - #150468 (rustc_target: callconv: powerpc64: Use the ABI set in target options instead of guessing) - #151628 (Fix ICE in const eval of packed SIMD types with non-power-of-two element counts) - #151871 (don't use env with infer vars) - #152591 (Simplify internals of `{Rc,Arc}::default`) - #152865 (Fixed ByteStr not padding within its Display trait when no specific alignment is mentioned) - #152908 (Enable rust.remap-debuginfo in the dist profile) - #152705 (Test(lib/win/proc): Skip `raw_attributes` doctest under Win7) - #152767 (fix typo in `carryless_mul` macro invocation) - #152837 (fix(codegen): Use `body_codegen_attrs` For Caller In `adjust_target_feature_sig`) - #152871 (Fix warnings in rs{begin,end}.rs files) - #152921 (Add build.rustdoc option to bootstrap config) - #152933 (Start migration for `LintDiagnostic` items by adding API and migrating `LinkerOutput` lint) - #152937 (remove unneeded reboxing)

…lt, r=joboet Simplify internals of `{Rc,Arc}::default` This commit simplifies the internal implementation of `Default` for these two pointer types to have the same performance characteristics as before (a side effect of changes in rust-lang#131460) while avoid use of internal private APIs of Rc/Arc. To preserve the same codegen as before some non-generic functions needed to be tagged as `#[inline]` as well, but otherwise the same IR is produced before/after this change. The motivation of this commit is I was studying up on the state of initialization of `Arc` and `Rc` and figured it'd be nicer to reduce the use of internal APIs and instead use public stable APIs where possible, even in the implementation itself.

Rollup of 15 pull requests Successful merges: - #150468 (rustc_target: callconv: powerpc64: Use the ABI set in target options instead of guessing) - #151628 (Fix ICE in const eval of packed SIMD types with non-power-of-two element counts) - #151871 (don't use env with infer vars) - #152591 (Simplify internals of `{Rc,Arc}::default`) - #152865 (Fixed ByteStr not padding within its Display trait when no specific alignment is mentioned) - #152908 (Enable rust.remap-debuginfo in the dist profile) - #147859 (reduce the amount of panics in `{TokenStream, Literal}::from_str` calls) - #152705 (Test(lib/win/proc): Skip `raw_attributes` doctest under Win7) - #152767 (fix typo in `carryless_mul` macro invocation) - #152837 (fix(codegen): Use `body_codegen_attrs` For Caller In `adjust_target_feature_sig`) - #152871 (Fix warnings in rs{begin,end}.rs files) - #152879 (Remove `impl IntoQueryParam<P> for &'a P`.) - #152933 (Start migration for `LintDiagnostic` items by adding API and migrating `LinkerOutput` lint) - #152937 (remove unneeded reboxing) - #152953 (Fix typo in armv7a-vex-v5.md)

…lt, r=joboet Simplify internals of `{Rc,Arc}::default` This commit simplifies the internal implementation of `Default` for these two pointer types to have the same performance characteristics as before (a side effect of changes in rust-lang#131460) while avoid use of internal private APIs of Rc/Arc. To preserve the same codegen as before some non-generic functions needed to be tagged as `#[inline]` as well, but otherwise the same IR is produced before/after this change. The motivation of this commit is I was studying up on the state of initialization of `Arc` and `Rc` and figured it'd be nicer to reduce the use of internal APIs and instead use public stable APIs where possible, even in the implementation itself.

…uwer Rollup of 14 pull requests Successful merges: - #150468 (rustc_target: callconv: powerpc64: Use the ABI set in target options instead of guessing) - #151628 (Fix ICE in const eval of packed SIMD types with non-power-of-two element counts) - #151871 (don't use env with infer vars) - #152591 (Simplify internals of `{Rc,Arc}::default`) - #152865 (Fixed ByteStr not padding within its Display trait when no specific alignment is mentioned) - #147859 (reduce the amount of panics in `{TokenStream, Literal}::from_str` calls) - #152705 (Test(lib/win/proc): Skip `raw_attributes` doctest under Win7) - #152767 (fix typo in `carryless_mul` macro invocation) - #152837 (fix(codegen): Use `body_codegen_attrs` For Caller In `adjust_target_feature_sig`) - #152871 (Fix warnings in rs{begin,end}.rs files) - #152879 (Remove `impl IntoQueryParam<P> for &'a P`.) - #152933 (Start migration for `LintDiagnostic` items by adding API and migrating `LinkerOutput` lint) - #152937 (remove unneeded reboxing) - #152953 (Fix typo in armv7a-vex-v5.md)

Rollup merge of #152591 - alexcrichton:simplify-rc-arc-default, r=joboet Simplify internals of `{Rc,Arc}::default` This commit simplifies the internal implementation of `Default` for these two pointer types to have the same performance characteristics as before (a side effect of changes in #131460) while avoid use of internal private APIs of Rc/Arc. To preserve the same codegen as before some non-generic functions needed to be tagged as `#[inline]` as well, but otherwise the same IR is produced before/after this change. The motivation of this commit is I was studying up on the state of initialization of `Arc` and `Rc` and figured it'd be nicer to reduce the use of internal APIs and instead use public stable APIs where possible, even in the implementation itself.

…uwer Rollup of 14 pull requests Successful merges: - rust-lang/rust#150468 (rustc_target: callconv: powerpc64: Use the ABI set in target options instead of guessing) - rust-lang/rust#151628 (Fix ICE in const eval of packed SIMD types with non-power-of-two element counts) - rust-lang/rust#151871 (don't use env with infer vars) - rust-lang/rust#152591 (Simplify internals of `{Rc,Arc}::default`) - rust-lang/rust#152865 (Fixed ByteStr not padding within its Display trait when no specific alignment is mentioned) - rust-lang/rust#147859 (reduce the amount of panics in `{TokenStream, Literal}::from_str` calls) - rust-lang/rust#152705 (Test(lib/win/proc): Skip `raw_attributes` doctest under Win7) - rust-lang/rust#152767 (fix typo in `carryless_mul` macro invocation) - rust-lang/rust#152837 (fix(codegen): Use `body_codegen_attrs` For Caller In `adjust_target_feature_sig`) - rust-lang/rust#152871 (Fix warnings in rs{begin,end}.rs files) - rust-lang/rust#152879 (Remove `impl IntoQueryParam<P> for &'a P`.) - rust-lang/rust#152933 (Start migration for `LintDiagnostic` items by adding API and migrating `LinkerOutput` lint) - rust-lang/rust#152937 (remove unneeded reboxing) - rust-lang/rust#152953 (Fix typo in armv7a-vex-v5.md)

JonathanBrouwer · 2026-02-25T12:12:57Z

@rust-timer build 367f121

rust-timer · 2026-02-25T15:27:20Z

Finished benchmarking commit (367f121): comparison URL.

Overall result: ❌ regressions - 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)	0.4%	[0.3%, 0.7%]	7
Regressions ❌ (secondary)	0.5%	[0.5%, 0.5%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.4%	[0.3%, 0.7%]	7

Max RSS (memory usage)

Results (primary -1.2%)

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

	mean	range	count
Regressions ❌ (primary)	4.1%	[4.1%, 4.1%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-6.5%	[-6.5%, -6.5%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-1.2%	[-6.5%, 4.1%]	2

Cycles

Results (secondary -3.4%)

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)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.4%	[-6.1%, -2.0%]	3
All ❌✅ (primary)	-	-	0

Binary size

Results (primary 1.0%)

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

	mean	range	count
Regressions ❌ (primary)	1.0%	[1.0%, 1.0%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.0%	[1.0%, 1.0%]	1

Bootstrap: 481.835s -> 480.386s (-0.30%)
Artifact size: 397.84 MiB -> 397.96 MiB (0.03%)

JonathanBrouwer · 2026-02-25T15:36:35Z

This caused the perf regression in #152965

alexcrichton · 2026-02-25T15:43:40Z

Is this at the threshold that I should proactively revert this pending further investigation? Or should I be the one digging in to see what the regression stems from? Given how minor this is I'd probably just revert and abandon the change in that case, but I wanted to be sure to clarify if the responsibility for addressing this lies with me.

JonathanBrouwer · 2026-02-25T16:09:03Z

The regression is not very large but large enough that if there is no benefit to this PR other than simpler code I'd propose to revert it

Revert "Simplify internals of `{Rc,Arc}::default`" This reverts rust-lang#152591 following a [perf run being done](rust-lang#152591 (comment)). I'm not really positioned at this time to dive in further and understand the performance regressions, so I'll back away from `Rc`/`Arc` and leave them as-is.

Revert "Simplify internals of `{Rc,Arc}::default`" This reverts #152591 following a [perf run being done](#152591 (comment)). I'm not really positioned at this time to dive in further and understand the performance regressions, so I'll back away from `Rc`/`Arc` and leave them as-is.

rustbot assigned Mark-Simulacrum Feb 13, 2026

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 Feb 13, 2026

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alexcrichton force-pushed the simplify-rc-arc-default branch from da1ddcf to ba5ff3d Compare February 14, 2026 00:36

alexcrichton commented Feb 14, 2026

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alexcrichton force-pushed the simplify-rc-arc-default branch from ba5ff3d to ce4c17f Compare February 14, 2026 18:56

rustbot assigned joboet Feb 21, 2026

rust-bors bot added the S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. label Feb 21, 2026

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rust-bors bot removed the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Feb 21, 2026

jhpratt mentioned this pull request Feb 21, 2026

Rollup of 14 pull requests #152945

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Rollup of 15 pull requests #152950

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Rollup of 14 pull requests #152951

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Rollup of 13 pull requests #152957

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Rollup of 15 pull requests #152960

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Rollup of 13 pull requests #152964

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Rollup of 14 pull requests #152965

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rust-bors bot merged commit 00d849a into rust-lang:main Feb 22, 2026
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rustbot added the perf-regression Performance regression. label Feb 25, 2026

alexcrichton deleted the simplify-rc-arc-default branch February 25, 2026 15:42

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Revert "Simplify internals of {Rc,Arc}::default" #153108

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Simplify internals of `{Rc,Arc}::default`#152591

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Overall result: ❌ regressions - please read the text below

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