Use a `ArrayVec` in `CastTarget` by alexcrichton · Pull Request #157130 · rust-lang/rust

alexcrichton · 2026-05-29T22:48:17Z

This commit switches a fixed-size list of [Option<Reg>; 8] to instead
holding ArrayVec<Reg, 8> in the CastTarget type used when
calculating ABIs. This is inspired by discussion on Zulip where
I'm hoping to in the near future extend the usage of this to possibly
beyond 8 elements for a new WebAssembly ABI taking advantage of
multi-value. For now though this mostly just switches to
array/slice-like idioms of accessors rather than dealing with
Option<Reg> as the unit.

rustbot · 2026-05-29T22:48:20Z

These commits modify the Cargo.lock file. Unintentional changes to Cargo.lock can be introduced when switching branches and rebasing PRs.

If this was unintentional then you should revert the changes before this PR is merged.
Otherwise, you can ignore this comment.

rustbot · 2026-05-29T22:48:22Z

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rustbot · 2026-05-29T23:04:54Z

The Cranelift subtree was changed

cc @bjorn3

nnethercote

The GCC backend still needs fixing. Probably worth squashing the commits, too; there's no reason for them to be separate.

View changes since this review

nnethercote · 2026-05-30T02:13:36Z

 /// (and all data in the padding between the registers is dropped).
 #[derive(Clone, PartialEq, Eq, Hash, Debug, StableHash)]
 pub struct CastTarget {
-    pub prefix: [Option<Reg>; 8],


[Option<Reg>; 8], lol, what an awful representation.

If you wanted a nicer representation while still retaining the "at most 8 elements" constraint you could use ArrayVec<[Reg; 8]> instead. That would be very similar to the code you've written, and get the nicer iterations/pushes/etc. Then the change to allow more than 8 elements (later) would just be a conversion to SmallVec.

They hypothetical wasm use case I have will need the more-than-8-is-possible behavior of this, but that's still in the works as well. Would you prefer to have the interim period use ArrayVec and only switch to SmallVec when necessary?

So... the current code has a hard limit of 8. It's unclear why that was chosen and if it's meaningful. ArrayVec<8> would preserve that limit while making the code nicer.

Allowing arbitrary lengths seems reasonable, but SmallVec<8> says to me "this vec frequently has up to 8 elements and not heap allocating them is important for performance". Which probably isn't true in this case? I bet the most common case is a single element, in which case SmallVec<1> would probably make more sense. But if this code isn't hot then Vec would also be fine and simplest.

So I see two sensible options:

Use ArrayVec<8> and add a comment about the 8 probably being arbitrary.

Use Vec.

I don't have a strong preference. I guess it comes down to how confident you are that you'll end up needing the arbitrary length.

Ok I don't want to rock the boat too much yet, so I've switched to using ArrayVec everywhere. I'm not sure if this is a performance hot spot or not to know whether Vec is worth it over SmallVec

rustbot · 2026-05-31T16:17:17Z

The GCC codegen subtree was changed

cc @antoyo, @GuillaumeGomez

This commit switches a fixed-size list of `[Option<Reg>; 8]` to instead holding `ArrayVec<Reg, 8>` in the `CastTarget` type used when calculating ABIs. This is inspired by [discussion on Zulip][link] where I'm hoping to in the near future extend the usage of this to possibly beyond 8 elements for a new WebAssembly ABI taking advantage of multi-value. For now though this mostly just switches to array/slice-like idioms of accessors rather than dealing with `Option<Reg>` as the unit. [link]: https://rust-lang.zulipchat.com/#narrow/channel/131828-t-compiler/topic/Using.20.60ArgAbi.3A.3Amake_direct_deprecated.60/with/598607139

nnethercote · 2026-06-01T19:00:56Z

Thanks, the ArrayVec code looks good. I'll be surprised if this is hot, but let's play it safe.

@bors try @rust-timer queue

…r=<try> Use a `ArrayVec` in `CastTarget`

rust-bors · 2026-06-01T21:15:39Z

☀️ Try build successful (CI)
Build commit: 32cd855 (32cd855918a7ef6654ed0d726a6c1ee9c86eec14, parent: c0bb140a37c81cf59a0b40c21c9413059644e294)

rust-timer · 2026-06-01T21:56:37Z

Finished benchmarking commit (32cd855): comparison URL.

Overall result: no relevant changes - no action needed

Benchmarking means the PR may be perf-sensitive. Consider adding rollup=never if this change is not fit for rolling up.

@rustbot label: -S-waiting-on-perf -perf-regression

Instruction count

This perf run didn't have relevant results for this metric.

Max RSS (memory usage)

Results (secondary 0.0%)

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)	6.2%	[6.2%, 6.2%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-6.1%	[-6.1%, -6.1%]	1
All ❌✅ (primary)	-	-	0

Cycles

Results (secondary 1.8%)

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)	1.8%	[1.8%, 1.8%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

This perf run didn't have relevant results for this metric.

Bootstrap: 511.375s -> 509.277s (-0.41%)
Artifact size: 400.63 MiB -> 400.66 MiB (0.01%)

nnethercote · 2026-06-01T22:15:18Z

@bors r+ rollup

rust-bors · 2026-06-01T22:15:21Z

📌 Commit c668b83 has been approved by nnethercote

It is now in the queue for this repository.

…internals, r=nnethercote Use a `ArrayVec` in `CastTarget` This commit switches a fixed-size list of `[Option<Reg>; 8]` to instead holding `ArrayVec<Reg, 8>` in the `CastTarget` type used when calculating ABIs. This is inspired by [discussion on Zulip][link] where I'm hoping to in the near future extend the usage of this to possibly beyond 8 elements for a new WebAssembly ABI taking advantage of multi-value. For now though this mostly just switches to array/slice-like idioms of accessors rather than dealing with `Option<Reg>` as the unit. [link]: https://rust-lang.zulipchat.com/#narrow/channel/131828-t-compiler/topic/Using.20.60ArgAbi.3A.3Amake_direct_deprecated.60/with/598607139

Rollup of 8 pull requests Successful merges: - #157240 (Enable Enzyme for aarch64-apple-darwin) - #157276 (miri subtree update) - #154742 (Add APIs for case folding to the standard library) - #157130 (Use a `ArrayVec` in `CastTarget`) - #157195 (Move feature gating to the new attr parsing infrastructure) - #157256 (tests: adapt for LLVM codegen change) - #157265 (Update books) - #157277 (triagebot.toml: add LawnGnome to libs reviewers)

…internals, r=nnethercote Use a `ArrayVec` in `CastTarget` This commit switches a fixed-size list of `[Option<Reg>; 8]` to instead holding `ArrayVec<Reg, 8>` in the `CastTarget` type used when calculating ABIs. This is inspired by [discussion on Zulip][link] where I'm hoping to in the near future extend the usage of this to possibly beyond 8 elements for a new WebAssembly ABI taking advantage of multi-value. For now though this mostly just switches to array/slice-like idioms of accessors rather than dealing with `Option<Reg>` as the unit. [link]: https://rust-lang.zulipchat.com/#narrow/channel/131828-t-compiler/topic/Using.20.60ArgAbi.3A.3Amake_direct_deprecated.60/with/598607139

Rollup of 8 pull requests Successful merges: - #157240 (Enable Enzyme for aarch64-apple-darwin) - #157276 (miri subtree update) - #157130 (Use a `ArrayVec` in `CastTarget`) - #157131 (Add regression test for issue #144888) - #157195 (Move feature gating to the new attr parsing infrastructure) - #157256 (tests: adapt for LLVM codegen change) - #157265 (Update books) - #157277 (triagebot.toml: add LawnGnome to libs reviewers)

…internals, r=nnethercote Use a `ArrayVec` in `CastTarget` This commit switches a fixed-size list of `[Option<Reg>; 8]` to instead holding `ArrayVec<Reg, 8>` in the `CastTarget` type used when calculating ABIs. This is inspired by [discussion on Zulip][link] where I'm hoping to in the near future extend the usage of this to possibly beyond 8 elements for a new WebAssembly ABI taking advantage of multi-value. For now though this mostly just switches to array/slice-like idioms of accessors rather than dealing with `Option<Reg>` as the unit. [link]: https://rust-lang.zulipchat.com/#narrow/channel/131828-t-compiler/topic/Using.20.60ArgAbi.3A.3Amake_direct_deprecated.60/with/598607139

…uwer Rollup of 14 pull requests Successful merges: - #156467 (mark the 'import linkage' statics as unnamed_addr) - #156923 (Couple of diagnostics improvements for EII) - #157240 (Enable Enzyme for aarch64-apple-darwin) - #157244 (Privacy: tweak macros + more tests) - #157276 (miri subtree update) - #157130 (Use a `ArrayVec` in `CastTarget`) - #157131 (Add regression test for issue #144888) - #157195 (Move feature gating to the new attr parsing infrastructure) - #157233 (rustdoc: Fix trait impl ordering) - #157256 (tests: adapt for LLVM codegen change) - #157265 (Update books) - #157277 (triagebot.toml: add LawnGnome to libs reviewers) - #157291 (Clean up attribute target checking diagnostics) - #157301 (Remove unused import from a test)

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Comment thread compiler/rustc_target/src/callconv/mod.rs Outdated

alexcrichton force-pushed the refactor-cast-target-internals branch from 7bba23d to 6fee567 Compare May 31, 2026 16:17

alexcrichton force-pushed the refactor-cast-target-internals branch from 6fee567 to c668b83 Compare June 1, 2026 14:34

alexcrichton changed the title ~~Use a SmallVec in CastTarget~~ Use a ArrayVec in CastTarget Jun 1, 2026

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rust-bors Bot pushed a commit that referenced this pull request Jun 1, 2026

Auto merge of #157130 - alexcrichton:refactor-cast-target-internals, …

32cd855

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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-review Status: Awaiting review from the assignee but also interested parties. labels Jun 1, 2026

jhpratt mentioned this pull request Jun 2, 2026

Rollup of 8 pull requests #157279

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jhpratt mentioned this pull request Jun 2, 2026

Rollup of 8 pull requests #157284

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JonathanBrouwer mentioned this pull request Jun 2, 2026

Rollup of 14 pull requests #157303

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Overall result: no relevant changes - no action needed

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