`rustc_scalable_vector(N)` #143924

davidtwco · 2025-07-14T11:13:12Z

Supercedes #118917.

Initial experimental implementation of rust-lang/rfcs#3838. Introduces a rustc_scalable_vector(N) attribute that can be applied to types with a single [$ty] field (for u{16,32,64}, i{16,32,64}, f{32,64}, bool). rustc_scalable_vector types are lowered to scalable vectors in the codegen backend.

As with any unstable feature, there will necessarily be follow-ups as we experiment and find cases that we've not considered or still need some logic to handle, but this aims to be a decent baseline to start from.

See #145052 for request for a lang experiment.

rustbot · 2025-07-14T11:13:17Z

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rustbot · 2025-07-14T11:13:21Z

Some changes occurred in compiler/rustc_attr_parsing

cc @jdonszelmann

Some changes occurred in compiler/rustc_attr_data_structures

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Some changes occurred in compiler/rustc_passes/src/check_attr.rs

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Some changes occurred to the intrinsics. Make sure the CTFE / Miri interpreter
gets adapted for the changes, if necessary.

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

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri

Some changes occurred to the platform-builtins intrinsics. Make sure the
LLVM backend as well as portable-simd gets adapted for the changes.

cc @antoyo, @GuillaumeGomez, @bjorn3, @calebzulawski, @programmerjake

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

Some changes occurred in compiler/rustc_codegen_ssa

cc @WaffleLapkin

Some changes occurred in compiler/rustc_codegen_gcc

cc @antoyo, @GuillaumeGomez

changes to the core type system

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Some changes occurred to the CTFE machinery

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davidtwco · 2025-07-14T11:21:46Z

I've changed this back to a draft and marked it as S-waiting-on-author while I rebase it and get the approval for an experimental implementation to go ahead.

workingjubilee

I do not intend to have repr(simd) survive this year, so I do not think this should be added.

davidtwco · 2025-07-14T16:09:12Z

I do not intend to have repr(simd) survive this year, so I do not think this should be added.

Could you elaborate?

workingjubilee · 2025-07-14T16:10:18Z

I intend to replace it with an approach based on lang items for a variety of reasons, one of them being that to start with, the repr(simd) "attribute" does not compose and should not.

r? @workingjubilee

davidtwco · 2025-08-12T12:42:35Z

Updated this reflecting changes in rust-lang/rfcs#3838. It doesn't implement everything in it yet, and is definitely incomplete, but it's a starting point.

Extend parsing of `ReprOptions` with `rustc_scalable_vector(N)` which optionally accepts a single literal integral value - the base multiple of lanes that are in a scalable vector. Can only be applied to structs. Co-authored-by: Jamie Cunliffe <Jamie.Cunliffe@arm.com>

Extend well-formedness checking and HIR analysis to prohibit the use of scalable vectors in structs, enums, unions, tuples and arrays. LLVM does not support scalable vectors being members of other types, so these restrictions are necessary. Co-authored-by: Jamie Cunliffe <Jamie.Cunliffe@arm.com>

`simd_reinterpret` is a replacement for `transmute`, specifically for use with scalable SIMD types. It is used in the tests for scalable vectors and in stdarch. Co-authored-by: Jamie Cunliffe <Jamie.Cunliffe@arm.com>

davidtwco · 2025-11-25T15:39:04Z

@workingjubilee are you open to reviewing this so that we can land it as an incomplete feature?

There's clearly still missing bits and ways that one could use the vectors defined using this incorrectly, but it's a lot easier to experiment with solutions and iterate when there's something landed and we can do those experiments in individual PRs. None of the details are set in stone, we're happy to change the attribute name, etc - can discuss some of that on the RFC.

I've rebased and this should pass CI.

cc @Amanieu

Introduces `BackendRepr::ScalableVector` corresponding to scalable vector types annotated with `repr(scalable)` which lowers to a scalable vector type in LLVM. Co-authored-by: Jamie Cunliffe <Jamie.Cunliffe@arm.com>

LLVM doesn't handle stores on `<vscale x N x i1>` for `N != 16`, a type used internally in SVE intrinsics. Spilling to the stack to create debuginfo will cause errors during instruction selection. These types that are an internal implementation detail to the intrinsic, so users should never see them types and won't need any debuginfo. Co-authored-by: Jamie Cunliffe <Jamie.Cunliffe@arm.com>

Scalable vectors cannot be members of ADTs and thus cannot be kept over await points in async functions.

Scalable vector types only work with the relevant target features enabled, so require this for any function with the types in its signature.

davidtwco · 2025-11-25T16:53:08Z

That test has subtly different output for me locally (it was passing), but none of the differences seem important to so I've added a wildcard for the parts that seem to change.

workingjubilee · 2025-11-25T20:44:15Z

@davidtwco Yes, I hadn't prioritized it because it became unclear how many changes it would receive from its then-current draft. As you have addressed my main request that I have seen, I'm happy for this to land in incomplete form. I'll try to take a look soon.

rustbot · 2025-11-26T14:28:17Z

Changes to the size of AST and/or HIR nodes.

cc @nnethercote

Some changes occurred in compiler/rustc_hir/src/attrs

cc @jdonszelmann

This PR changes rustc_public

cc @oli-obk, @celinval, @ouz-a

nnethercote · 2025-11-26T21:15:12Z

compiler/rustc_hir/src/attrs/data_structures.rs

+    RustcScalableVector {
+        /// The base multiple of lanes that are in a scalable vector, if provided. `element_count`
+        /// is not provided for representing tuple types.
+        element_count: Option<u128>,


The size of Attribute jumps from 200 to 240. Probably worth boxing this to avoid that.

rustbot assigned compiler-errors Jul 14, 2025

davidtwco mentioned this pull request Jul 14, 2025

Support for a scalable simd representation #118917

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davidtwco marked this pull request as draft July 14, 2025 11:21

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 Jul 14, 2025

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davidtwco changed the title ~~repr(scalable)~~ rustc_scalable_vector(N) Aug 12, 2025

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This was referenced Nov 25, 2025

Integrate Arm SVE types and intrinsics into stdarch #149297

Closed

Integrate Arm SVE types and intrinsics into stdarch davidtwco/rust#2

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davidtwco mentioned this pull request Nov 25, 2025

SVE types and intrinsics rust-lang/stdarch#1509

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davidtwco and others added 3 commits November 25, 2025 13:23

core: add simd_reinterpret

7f6f8c8

`simd_reinterpret` is a replacement for `transmute`, specifically for use with scalable SIMD types. It is used in the tests for scalable vectors and in stdarch. Co-authored-by: Jamie Cunliffe <Jamie.Cunliffe@arm.com>

davidtwco force-pushed the sve-infrastructure branch from 26fb3ae to 8a0189a Compare November 25, 2025 15:34

davidtwco 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 Nov 25, 2025

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davidtwco and others added 4 commits November 25, 2025 16:52

codegen: implement repr(scalable)

a05dd71

Introduces `BackendRepr::ScalableVector` corresponding to scalable vector types annotated with `repr(scalable)` which lowers to a scalable vector type in LLVM. Co-authored-by: Jamie Cunliffe <Jamie.Cunliffe@arm.com>

mir_transform: prohibit scalable vectors in async

fec9a6c

Scalable vectors cannot be members of ADTs and thus cannot be kept over await points in async functions.

mono: require target feature for scalable vectors

6cdc8f7

Scalable vector types only work with the relevant target features enabled, so require this for any function with the types in its signature.

davidtwco force-pushed the sve-infrastructure branch from 8a0189a to 6cdc8f7 Compare November 25, 2025 16:52

davidtwco marked this pull request as ready for review November 26, 2025 14:28

nnethercote reviewed Nov 26, 2025

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rustc_scalable_vector(N) #143924

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`rustc_scalable_vector(N)` #143924

`rustc_scalable_vector(N)` #143924

davidtwco commented Jul 14, 2025 •

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