Performance regression: rustc failed to optimize _mm256_avg_epu8 after 1.75.0

[Nugine]

Code

I tried this code:

https://rust.godbolt.org/z/KG4cT6aPK

use std::arch::x86_64::*;

#[target_feature(enable = "avx2")]
pub unsafe fn decode(
    x: __m256i,
    ch: __m256i,
    ct: __m256i,
    dh: __m256i,
    dt: __m256i,
) -> Result<__m256i, ()> {
    let shr3 = _mm256_srli_epi32::<3>(x);

    let h1 = _mm256_avg_epu8(shr3, _mm256_shuffle_epi8(ch, x));
    let h2 = _mm256_avg_epu8(shr3, _mm256_shuffle_epi8(dh, x));

    let o1 = _mm256_shuffle_epi8(ct, h1);
    let o2 = _mm256_shuffle_epi8(dt, h2);

    let c1 = _mm256_adds_epi8(x, o1);
    let c2 = _mm256_add_epi8(x, o2);

    if _mm256_movemask_epi8(c1) != 0 {
        return Err(());
    }

    Ok(c2)
}

I expected to see this happen: This code should emit two vpavgb instructions.

Instead, this happened: One of the vpavgb instructions is missing.

Nugine/simd#43

Version it worked on

It most recently worked on: 1.74.1

Version with regression

1.75.0 ~ nightly

rustc 1.79.0-nightly (dbce3b43b 2024-04-20)
binary: rustc
commit-hash: dbce3b43b6cb34dd3ba12c3ec6f708fe68e9c3df
commit-date: 2024-04-20
host: x86_64-unknown-linux-gnu
release: 1.79.0-nightly
LLVM version: 18.1.4

@rustbot modify labels: +regression-from-stable-to-stable -regression-untriaged

[saethlin]

Blaming rust-lang/stdarch#1477

Did you confirm that this is the responsible change or are you guessing?

[workingjubilee]

@Nugine This is definitely more instructions and more bytes on each, so I'm marking it with I-heavy, but it appears this comes with a performance regression. Can you be precise about which of the ~19 benchmarks you appear to run have regressed, and on what architecture?

I would rather we not make the 2nd vpavgb instruction come back only for your algorithm to still be dog-slow because some of the other instructions are different.

Also, can you be more precise on what architectures and with what target features you're testing on? GitHub is allowed to change the CPU you run benchmarks on, and does, because their fleet is not perfectly uniform, so -Ctarget-cpu=native makes it more likely your benchmarks can be run-to-run and job-to-job inconsistent.

[Nugine]

Base64-decode in base64-simd has been slower than radix64 since Rust 1.75.0. By comparing the asm generated by 1.74.1 and 1.75.0, I found that one of vpavgb is missing. LLVM doesn't emit vpavgb for one of _mm256_avg_epu8, but a lot of equivalent instructions.

rust-lang/stdarch#1477 made the change. However, the root cause may be elsewhere, possibly LLVM.

To see the asm, you can use the following commands.

git clone https://github.com/Nugine/simd.git
cd simd
rustup override set 1.74.1 # or 1.75.0
RUSTFLAGS="--cfg vsimd_dump_symbols" cargo asm -p base64-simd --lib --simplify --target x86_64-unknown-linux-gnu  --context 1 -- base64_simd::multiversion::decode::avx2 > base64-decode-avx2.asm
cat base64-decode-avx2.asm

Target: x86_64-unknown-linux-gnu
Instruction: AVX2

I have extracted the decode function and reproduced the regression. https://rust.godbolt.org/z/KG4cT6aPK
I'm looking for:

• a stable workaround method to generate vpavgb
• why the optimization is missing

[workingjubilee]

@Nugine re: the workaround: On current Rust, stable, the decode_asm function here recovers exactly equivalent output to what you had before: https://rust.godbolt.org/z/fGEaYME1h

[jhorstmann]

Seems the early exit somehow makes llvm loose track of the equivalence to vpavgb instruction. Another workaround thus seems to be to force llvm to calculate both Ok and Err versions:

#[target_feature(enable = "avx2")]
pub unsafe fn decode(
    x: __m256i,
    ch: __m256i,
    ct: __m256i,
    dh: __m256i,
    dt: __m256i,
) -> Result<__m256i, __m256i> {
    let shr3 = _mm256_srli_epi32::<3>(x);

    let h1 = _mm256_avg_epu8(shr3, _mm256_shuffle_epi8(ch, x));
    let h2 = _mm256_avg_epu8(shr3, _mm256_shuffle_epi8(dh, x));

    let o1 = _mm256_shuffle_epi8(ct, h1);
    let o2 = _mm256_shuffle_epi8(dt, h2);

    let c1 = _mm256_adds_epi8(x, o1);
    let c2 = _mm256_add_epi8(x, o2);

    if _mm256_movemask_epi8(c1) != 0 {
        return Err(c2);
    }

    Ok(c2)
}

But I guess this will break down as soon as the function gets inlined if the error value is not otherwise used.

[Nugine]

@Nugine re: the workaround: On current Rust, stable, the decode_asm function here recovers exactly equivalent output to what you had before: https://rust.godbolt.org/z/fGEaYME1h

Cool! I'll try asm wrapper.

[workingjubilee]

based on jhorstmann's remark, it would be nicest to fix this in LLVM, since LLVM appears to have the information necessary to do this optimization, it just is missing it in the early-return case. I don't think partially reverting a diff is unwarranted, however.

[apiraino]

WG-prioritization assigning priority (Zulip discussion).

@rustbot label -I-prioritize +P-medium