reduce perf impact of scalar size checks by RalfJung · Pull Request #161456 · rust-lang/rust

RalfJung · 2026-08-21T12:04:12Z

Attempting to undo the perf regression in #161238.

RalfJung · 2026-08-21T12:05:44Z

try to reduce impact of scalar size checks

rust-bors · 2026-08-21T13:04:51Z

☀️ Try build successful (CI)
Build commit: 5e1a5a6 (5e1a5a6052b5d6d684b94bb5c2842e53fa58a153)
Base parent: 16a623a (16a623ad672a92409b5c04beb303583c6cf72a7e)

rust-timer · 2026-08-21T15:30:42Z

Finished benchmarking commit (5e1a5a6): comparison URL.

Overall result: ✅ improvements - no action needed

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

@bors rollup=never rustc-perf
@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
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.3%	[-0.4%, -0.2%]	6
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.3%	[-0.4%, -0.2%]	6

Max RSS (memory usage)

Results (primary 2.1%, secondary 2.1%)

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

	mean	range	count
Regressions ❌ (primary)	2.1%	[0.7%, 3.5%]	2
Regressions ❌ (secondary)	6.4%	[6.4%, 6.4%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.2%	[-2.2%, -2.2%]	1
All ❌✅ (primary)	2.1%	[0.7%, 3.5%]	2

Cycles

Results (primary 2.4%, secondary 6.9%)

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

	mean	range	count
Regressions ❌ (primary)	2.4%	[2.2%, 2.6%]	2
Regressions ❌ (secondary)	6.9%	[6.9%, 6.9%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.4%	[2.2%, 2.6%]	2

Binary size

Results (primary 0.1%, secondary 0.1%)

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

	mean	range	count
Regressions ❌ (primary)	0.1%	[0.0%, 0.2%]	116
Regressions ❌ (secondary)	0.1%	[0.0%, 0.2%]	121
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.1%	[0.0%, 0.2%]	116

Bootstrap: 457.604s -> 457.186s (-0.09%)
Artifact size: 398.89 MiB -> 398.99 MiB (0.03%)

RalfJung · 2026-08-21T15:44:38Z

Looks like the right direction, but not quite enough. So I guess it's code size here, where previously we just constructed a little struct and now we invoke the panic machinery.

RalfJung · 2026-08-21T15:54:50Z

@bors try
@rust-timer queue

try to reduce impact of scalar size checks

rust-bors · 2026-08-21T16:54:42Z

☀️ Try build successful (CI)
Build commit: d8dc741 (d8dc741a46ffa8d3eb34711760f4c5154fecbe5c)
Base parent: b662191 (b66219142c937562f2ff8e083be2a2b25b20199e)

rust-timer · 2026-08-21T17:45:06Z

Finished benchmarking commit (d8dc741): comparison URL.

Overall result: ✅ improvements - no action needed

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

@bors rollup=never rustc-perf
@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
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.4%	[-0.5%, -0.4%]	6
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.4%	[-0.5%, -0.4%]	6

Max RSS (memory usage)

Results (primary 2.9%, secondary -1.3%)

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

	mean	range	count
Regressions ❌ (primary)	2.9%	[2.5%, 3.3%]	2
Regressions ❌ (secondary)	2.9%	[2.9%, 2.9%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-5.5%	[-5.5%, -5.5%]	1
All ❌✅ (primary)	2.9%	[2.5%, 3.3%]	2

Cycles

Results (primary -2.6%, secondary -1.9%)

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)	1.6%	[1.6%, 1.6%]	1
Improvements ✅ (primary)	-2.6%	[-3.0%, -2.1%]	2
Improvements ✅ (secondary)	-3.1%	[-3.6%, -2.7%]	3
All ❌✅ (primary)	-2.6%	[-3.0%, -2.1%]	2

Binary size

Results (primary -0.1%, secondary -0.1%)

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)	-0.1%	[-0.2%, -0.0%]	93
Improvements ✅ (secondary)	-0.1%	[-0.2%, -0.0%]	66
All ❌✅ (primary)	-0.1%	[-0.2%, -0.0%]	93

Bootstrap: 470.067s -> 467.991s (-0.44%)
Artifact size: 400.02 MiB -> 400.02 MiB (0.00%)

RalfJung · 2026-08-21T18:17:15Z

That's at least reasonably close to the red numbers in the original PR I think.
r? @petrochenkov
since you reviewed the previous PR

rustbot · 2026-08-21T18:17:20Z

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri, @oli-obk, @lcnr

Rollup of 7 pull requests Successful merges: - #150075 (Implement clamp_to) - #157145 (Codegen Overloaded LLVM intrinsics based on their name) - #161866 (delegation: add tests fixating behavior of delegating to default trait implementations) - #161456 (reduce perf impact of scalar size checks) - #161730 (Improve type mismatch annotation for lets with block-wrapped initializers) - #161828 (Never type after-stabilization cleanup) - #161860 (atomicptr.rs test: remove unused import)

matthiaskrgr · 2026-08-27T15:44:02Z

@bors r-
looks like this failed here:
#161871 (comment)

rust-bors · 2026-08-27T15:44:12Z

This pull request was unapproved.

This PR was contained in a rollup (#161871), which was unapproved.

View changes since this unapproval

rustbot · 2026-08-27T15:47:31Z

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.

RalfJung · 2026-08-27T15:48:27Z

I doubt that this PR could do anything to affect the scalable vector tests.

@bors try jobs=aarch64-gnu

reduce perf impact of scalar size checks try-job: aarch64-gnu

RalfJung · 2026-08-27T16:50:13Z

Yeah it was the other one.
@bors r=petrochenkov

rust-bors · 2026-08-27T16:50:16Z

📌 Commit d802d17 has been approved by petrochenkov

It is now in the queue for this repository.

rust-bors · 2026-08-27T18:10:43Z

☀️ Try build successful (CI)
Build commit: e4be0ad (e4be0adb48accec0cd74df0b9fd13f216e808948)
Base parent: e457a7b (e457a7b0d326d67b4322ef0d11bd715cfaeda48f)

…ochenkov reduce perf impact of scalar size checks Attempting to undo the perf regression in rust-lang#161238.

Rollup of 14 pull requests Successful merges: - #150075 (Implement clamp_to) - #159103 (fix(reborrow): recursive implementation) - #160848 (std: avoid aliasing violations when wrapping opaque C types) - #161421 (Include startup crt objects on WASI for more outputs) - #161805 (Prefer ambiguous candidates when deduplicating traits in scope, so `ambiguous_glob_imported_traits` doesn't depend on import order) - #161862 (Put data segment in specified section with link_section on wasm) - #161866 (delegation: add tests fixating behavior of delegating to default trait implementations) - #161456 (reduce perf impact of scalar size checks) - #161666 (Print vendor instructions in `x vendor`) - #161730 (Improve type mismatch annotation for lets with block-wrapped initializers) - #161828 (Never type after-stabilization cleanup) - #161860 (atomicptr.rs test: remove unused import) - #161870 (bind to [::1] instead of 127.0.0.1 in documentation examples for v6 UDP methods) - #161876 (rustdoc: Correctly handle when a macro generates multiple items in `--generate-macro-expansion`)

…ochenkov reduce perf impact of scalar size checks Attempting to undo the perf regression in rust-lang#161238.

…uwer Rollup of 21 pull requests Successful merges: - #158609 (Update sccache to 0.16.0) - #150075 (Implement clamp_to) - #159103 (fix(reborrow): recursive implementation) - #160562 (add target feature ABI checks for SPARC) - #160848 (std: avoid aliasing violations when wrapping opaque C types) - #161421 (Include startup crt objects on WASI for more outputs) - #161805 (Prefer ambiguous candidates when deduplicating traits in scope, so `ambiguous_glob_imported_traits` doesn't depend on import order) - #161862 (Put data segment in specified section with link_section on wasm) - #161866 (delegation: add tests fixating behavior of delegating to default trait implementations) - #161456 (reduce perf impact of scalar size checks) - #161528 (Add regression test to ensure optimal compilation) - #161666 (Print vendor instructions in `x vendor`) - #161730 (Improve type mismatch annotation for lets with block-wrapped initializers) - #161828 (Never type after-stabilization cleanup) - #161859 (Do not optimize MIR for comptime ConstFns) - #161860 (atomicptr.rs test: remove unused import) - #161870 (bind to [::1] instead of 127.0.0.1 in documentation examples for v6 UDP methods) - #161876 (rustdoc: Correctly handle when a macro generates multiple items in `--generate-macro-expansion`) - #161889 (Add link to ownership section in ptr::read docs) - #161890 (rustdoc: some clarifying comments) - #161891 (Mark `extern_item_impls` feature as incomplete) Failed merges: - #161702 (Use `drop_guard` in some places in {core,alloc,std})

…uwer Rollup of 21 pull requests Successful merges: - #150075 (Implement clamp_to) - #159103 (fix(reborrow): recursive implementation) - #160562 (add target feature ABI checks for SPARC) - #160848 (std: avoid aliasing violations when wrapping opaque C types) - #161421 (Include startup crt objects on WASI for more outputs) - #161805 (Prefer ambiguous candidates when deduplicating traits in scope, so `ambiguous_glob_imported_traits` doesn't depend on import order) - #161862 (Put data segment in specified section with link_section on wasm) - #161866 (delegation: add tests fixating behavior of delegating to default trait implementations) - #157218 (Track items behind `cfg_select` in the same way we do for `cfg`) - #161456 (reduce perf impact of scalar size checks) - #161528 (Add regression test to ensure optimal compilation) - #161666 (Print vendor instructions in `x vendor`) - #161730 (Improve type mismatch annotation for lets with block-wrapped initializers) - #161828 (Never type after-stabilization cleanup) - #161859 (Do not optimize MIR for comptime ConstFns) - #161860 (atomicptr.rs test: remove unused import) - #161870 (bind to [::1] instead of 127.0.0.1 in documentation examples for v6 UDP methods) - #161876 (rustdoc: Correctly handle when a macro generates multiple items in `--generate-macro-expansion`) - #161889 (Add link to ownership section in ptr::read docs) - #161890 (rustdoc: some clarifying comments) - #161891 (Mark `extern_item_impls` feature as incomplete) Failed merges: - #161702 (Use `drop_guard` in some places in {core,alloc,std})

Rollup merge of #161456 - RalfJung:scalar-size-check, r=petrochenkov reduce perf impact of scalar size checks Attempting to undo the perf regression in #161238.

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Aug 21, 2026

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rust-bors Bot pushed a commit that referenced this pull request Aug 21, 2026

Auto merge of #161456 - RalfJung:scalar-size-check, r=<try>

5e1a5a6

try to reduce impact of scalar size checks

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RalfJung force-pushed the scalar-size-check branch from 74c6e8a to 8fbb81d Compare August 21, 2026 15:54

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Auto merge of #161456 - RalfJung:scalar-size-check, r=<try>

d8dc741

try to reduce impact of scalar size checks

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rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Aug 21, 2026

RalfJung force-pushed the scalar-size-check branch from 8fbb81d to ae77f4b Compare August 21, 2026 18:16

RalfJung marked this pull request as ready for review August 21, 2026 18:17

rustbot assigned petrochenkov Aug 21, 2026

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 Aug 21, 2026

rustbot assigned jackh726 Aug 21, 2026

matthiaskrgr mentioned this pull request Aug 27, 2026

Rollup of 7 pull requests #161871

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try to reduce impact of scalar size checks

d802d17

RalfJung force-pushed the scalar-size-check branch from ae77f4b to d802d17 Compare August 27, 2026 15:47

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Auto merge of #161456 - RalfJung:scalar-size-check, r=<try>

e4be0ad

reduce perf impact of scalar size checks try-job: aarch64-gnu

rust-bors Bot added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Aug 27, 2026

GuillaumeGomez added a commit to GuillaumeGomez/rust that referenced this pull request Aug 27, 2026

Rollup merge of rust-lang#161456 - RalfJung:scalar-size-check, r=petr…

ee9c7b7

…ochenkov reduce perf impact of scalar size checks Attempting to undo the perf regression in rust-lang#161238.

GuillaumeGomez mentioned this pull request Aug 27, 2026

Rollup of 14 pull requests #161884

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JonathanBrouwer mentioned this pull request Aug 28, 2026

Rollup of 18 pull requests #161901

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JonathanBrouwer mentioned this pull request Aug 28, 2026

Rollup of 21 pull requests #161905

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JonathanBrouwer mentioned this pull request Aug 28, 2026

Rollup of 21 pull requests #161906

Merged

rust-bors Bot merged commit 0d704a3 into rust-lang:main Aug 28, 2026
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rustbot added this to the 1.100.0 milestone Aug 28, 2026

rust-bors Bot pushed a commit that referenced this pull request Aug 28, 2026

Unrolled build for #161456 in rollup 161906

cc2e3a1

Rollup merge of #161456 - RalfJung:scalar-size-check, r=petrochenkov reduce perf impact of scalar size checks Attempting to undo the perf regression in #161238.

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rust-timer commented Aug 21, 2026

Overall result: ✅ improvements - no action needed

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Overall result: ✅ improvements - no action needed

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