Regression on nightly in AVX2 byte shift intrinsics

[marcgalois]

Code

I tried this code:

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

fn main() {
    let input: [u8; 32] = [
        1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25,
        26, 27, 28, 29, 30, 31, 32,
    ];
    let output: [u8; 32] = unsafe {
        let input: __m256i = std::mem::transmute(input);
        let output = _mm256_bslli_epi128(input, 8);
        std::mem::transmute(output)
    };
    dbg!(output);
}

I expected to see this:

[src/main.rs:13] output = [ 0, 0, 0, 0, 0, 0, 0, 0, 1, 2, 3, 4, 5, 6, 7, 8, 0, 0, 0, 0, 0, 0, 0, 0, 17, 18, 19, 20, 21, 22, 23, 24]

(which matches clang on the equivalent C++ code)

Instead, this happened:

[src/main.rs:13] output = [ 0, 0, 0, 0, 0, 0, 0, 0, 1, 2, 3, 4, 5, 6, 7, 8, 0, 0, 0, 0, 0, 0, 0, 0, 1, 2, 3, 4, 5, 6, 7, 8]

Version it worked on

It most recently worked on: Stable rust 1.52.1

Version with regression

rustc --version --verbose:

rustc 1.54.0-nightly (fe72845f7 2021-05-16)
binary: rustc
commit-hash: fe72845f7bb6a77b9e671e6a4f32fe714962cec4
commit-date: 2021-05-16
host: x86_64-apple-darwin
release: 1.54.0-nightly
LLVM version: 12.0.1

I suspect that this issue was caused by rust-lang/stdarch#1067

[LeSeulArtichaut]

Let's bisect this: @rustbot ping cleanup

[rustbot]

Hey Cleanup Crew ICE-breakers! This bug has been identified as a good
"Cleanup ICE-breaking candidate". In case it's useful, here are some
instructions for tackling these sorts of bugs. Maybe take a look?
Thanks! <3

cc @AminArria @camelid @chrissimpkins @contrun @DutchGhost @elshize @h-michael @HallerPatrick @hdhoang @hellow554 @henryboisdequin @imtsuki @JamesPatrickGill @kanru @KarlK90 @LeSeulArtichaut @MAdrianMattocks @matheus-consoli @mental32 @nmccarty @Noah-Kennedy @pard68 @PeytonT @pierreN @Redblueflame @RobbieClarken @RobertoSnap @robjtede @SarthakSingh31 @shekohex @sinato @smmalis37 @steffahn @Stupremee @tamuhey @turboladen @woshilapin @yerke

[LeSeulArtichaut]

(I ended up doing the bisection myself, sorry for the ping everyone!)

cargo-bisect-rustc finds a regression in #83278 (which does contain rust-lang/stdarch#1067), cc @Amanieu @Mark-Simulacrum

[Amanieu]

cc @minybot

[apiraino]

Assigning priority as discussed in the Zulip thread of the Prioritization Working Group.

@rustbot label -I-prioritize +P-critical +T-compiler