rust-lang / rust-lang/portable-simd

Mask operations are not optimized well

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C-bug
Dominant language
Rust
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Merged PRs (30d)
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Description

I tried this code (godbolt):

#![feature(portable_simd)]

use std::simd::prelude::*;

type Simd = u8x16;

#[no_mangle]
pub fn check_hex(chunk: Simd) -> bool {
    let valid_digit = chunk.simd_ge(Simd::splat(b'0')) & chunk.simd_le(Simd::splat(b'9'));
    let valid_upper = chunk.simd_ge(Simd::splat(b'A')) & chunk.simd_le(Simd::splat(b'F'));
    let valid_lower = chunk.simd_ge(Simd::splat(b'a')) & chunk.simd_le(Simd::splat(b'f'));
    let valid = valid_digit | valid_upper | valid_lower;
    valid.all()
}

I expected to see this happen: optimizes as well as the hand-rolled x86 version

Instead, this happened: doesn't, see godbolt

Meta

rustc --version --verbose:

rustc 1.91.0-nightly (1ebbd87a6 2025-08-11)
binary: rustc
commit-hash: 1ebbd87a62ce96a72b22da61b7c2c43893534842
commit-date: 2025-08-11
host: x86_64-unknown-linux-gnu
release: 1.91.0-nightly
LLVM version: 21.1.0

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First steps

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Research direction

Start with the Rust reproducer and its Godbolt link, then compare the generated code for the portable-SIMD mask operations with the hand-rolled x86 version. Identify why the mask expression is not optimized equivalently; done means the reproducer generates comparably efficient code.

Written by the indexing model from the issue text.

Assessment

Tech stack
rust
Domain
performance
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Needs clarification
Newbie friendliness
30/100

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