Bad codegen for widen followed by ARM vdupq_n_*

Code

I tried this code:

use std::arch::aarch64::*;

#[no_mangle]
pub unsafe fn half_dup(x: u16) -> uint16x8_t {
    vaddq_u16(vreinterpretq_u16_u32(vdupq_n_u32(x as u32)), vdupq_n_u16(1))
}

I expected to see this happen: The compiled output would use the dup instruction

Instead, this happened: The compiled output uses a bunch of movs instead

Godbolt

Version it worked on

It most recently worked on: Rust 1.81.0

Version with regression

rustc --version --verbose:

rustc 1.82.0 (f6e511eec 2024-10-15)
binary: rustc
commit-hash: f6e511eec7342f59a25f7c0534f1dbea00d01b14
commit-date: 2024-10-15
host: x86_64-unknown-linux-gnu
release: 1.82.0
LLVM version: 19.1.1

and

rustc 1.87.0-nightly (f280acf4c 2025-02-19)
binary: rustc
commit-hash: f280acf4c743806abbbbcfe65050ac52ec4bdec0
commit-date: 2025-02-19
host: x86_64-unknown-linux-gnu
release: 1.87.0-nightly
LLVM version: 20.1.0

@jieyouxu

@jieyouxu Rust 1.82 would be the LLVM 19 upgrade.

@jieyouxu

@jieyouxu Rust 1.82 would be the LLVM 19 upgrade.

Yeah sorry somehow I didn't notice the 1.82

@rustbot

@rustbot label +A-SIMD +O-aarch64

Rollup merge of rust-lang#151346 - folkertdev:simd-splat, r=workingju…

Unrolled build for #151346

Rollup merge of #151346 - folkertdev:simd-splat, r=workingjubilee

This issue was resolved by

and now produces the expected:

half_dup:
	movi v0.8h, #1
	and w9, w0, #0xffff
	dup v1.4s, w9
	add v0.8h, v1.8h, v0.8h
	str q0, [x8]
	ret

Rollup merge of #151346 - folkertdev:simd-splat, r=workingjubilee

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Bad codegen for widen followed by ARM `vdupq_n_*` #137407

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Bad codegen for widen followed by ARM vdupq_n_* #137407

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jieyouxu commented on Feb 22, 2025

nikic commented on Feb 22, 2025

jieyouxu commented on Feb 22, 2025

hkratz commented on Feb 22, 2025

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folkertdev commented on Jan 27, 2026

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Bad codegen for widen followed by ARM `vdupq_n_*` #137407