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.

https://github.com/Nugine/simd/issues/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 https://github.com/rust-lang/stdarch/pull/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.

https://github.com/rust-lang/stdarch/pull/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

[Deniskore]

I've encountered the same issue. I reverted the minor change concerning pavgb on the stable branch, rebuilt the compiler, and can confirm that this pull request was the primary cause. I understand that the changes in this PR were made with good intentions, so I'm going to use asm! for now.

diff --git a/crates/core_arch/src/x86/avx2.rs b/crates/core_arch/src/x86/avx2.rs
index 75a393d..b4dba69 100644
--- a/crates/core_arch/src/x86/avx2.rs
+++ b/crates/core_arch/src/x86/avx2.rs
@@ -365,10 +365,7 @@ pub unsafe fn _mm256_avg_epu16(a: __m256i, b: __m256i) -> __m256i {
 #[cfg_attr(test, assert_instr(vpavgb))]
 #[stable(feature = "simd_x86", since = "1.27.0")]
 pub unsafe fn _mm256_avg_epu8(a: __m256i, b: __m256i) -> __m256i {
-    let a = simd_cast::<_, u16x32>(a.as_u8x32());
-    let b = simd_cast::<_, u16x32>(b.as_u8x32());
-    let r = simd_shr(simd_add(simd_add(a, b), u16x32::splat(1)), u16x32::splat(1));
-    transmute(simd_cast::<_, u8x32>(r))
+    transmute(pavgb(a.as_u8x32(), b.as_u8x32()))
 }

 /// Blends packed 32-bit integers from `a` and `b` using control mask `IMM4`.
@@ -3645,6 +3642,8 @@ pub unsafe fn _mm256_extract_epi16<const INDEX: i32>(a: __m256i) -> i32 {

 #[allow(improper_ctypes)]
 extern "C" {
+    #[link_name = "llvm.x86.avx2.pavg.b"]
+    fn pavgb(a: u8x32, b: u8x32) -> u8x32;
     #[link_name = "llvm.x86.avx2.phadd.w"]
     fn phaddw(a: i16x16, b: i16x16) -> i16x16;
     #[link_name = "llvm.x86.avx2.phadd.d"]

I've used the code given by @Nugine since the example is much smaller. You can compare the differences between the stable branch and my changes. The issue remains regardless of whether the function is inlined or not, tested with #[inline(always)] and #[inline(never)]

diff --git a/./issue_stable.s b/./issue_my.s
index fc2df6963c0..a11b08d6c92 100644
--- a/./issue_stable.s
+++ b/./issue_my.s
@@ -165,17 +165,11 @@ _ZN5issue6decode17hb5cdd31d54f96558E:
 .LBB6_1:
        mov     rdx, qword ptr [rsp + 40]
        mov     rax, qword ptr [rsp + 48]
-       vpmovzxbw       zmm1, ymm1
-       vpternlogd      zmm2, zmm2, zmm2, 255
-       vpsubw  zmm1, zmm1, zmm2
        vmovdqa ymm3, ymmword ptr [rdx]
        vmovdqa ymm2, ymmword ptr [rax]
        xor     eax, eax
        vpshufb ymm3, ymm3, ymm0
-       vpmovzxbw       zmm3, ymm3
-       vpaddw  zmm1, zmm1, zmm3
-       vpsrlw  zmm1, zmm1, 1
-       vpmovwb ymm1, zmm1
+       vpavgb  ymm1, ymm1, ymm3
        vpshufb ymm1, ymm2, ymm1
        vpaddb  ymm0, ymm1, ymm0
        vmovdqa ymmword ptr [rcx + 32], ymm0

[workingjubilee]

The -Ctarget-cpu=native in this Godbolt is somewhat confounding, because the host CPU can change, enabling various optimizations, or disabling them. It is also unnecessary to get the correct codegen.

However, what is more interesting is that this:

pub unsafe extern "C" fn genericized_mm256_avg_epu8(a: __m256i, b: __m256i) -> __m256i {
    let a = simd_cast::<u8x32, u16x32>(a.into());
    let b = simd_cast::<u8x32, u16x32>(b.into());
    let r = simd_shr(simd_add(simd_add(a, b), u16x32::splat(1)), u16x32::splat(1));
    transmute(simd_cast::<_, u8x32>(r))
}

compiles to this:

example::genericized_mm256_avg_epu8::hcdc2eb986be9a161:
        vpavgb  ymm0, ymm0, ymm1
        ret

https://rust.godbolt.org/z/sxGY1ffWq

[Deniskore]

@workingjubilee Because LLVM InstCombineVectorOps or other InstCombiners can recognize and optimize this code into the vpavgb instruction, I've proven it using an IR function.

llc -O3 -mcpu=haswell -mattr=+avx2

define <32 x i8> @genericized_mm256_avg_epu8(<32 x i8> %a, <32 x i8> %b) {
entry:
  ; Cast <32 x i8> to <32 x i16>
  %a_wide = zext <32 x i8> %a to <32 x i16>
  %b_wide = zext <32 x i8> %b to <32 x i16>

  ; Add the two vectors: a + b
  %sum_ab = add <32 x i16> %a_wide, %b_wide

  ; Add 1 to each element in the result
  %sum_ab_plus_one = add <32 x i16> %sum_ab, <i16 1, i16 1, i16 1, i16 1, i16 1, i16 1, i16 1, i16 1, 
                                                    i16 1, i16 1, i16 1, i16 1, i16 1, i16 1, i16 1, i16 1, 
                                                    i16 1, i16 1, i16 1, i16 1, i16 1, i16 1, i16 1, i16 1, 
                                                    i16 1, i16 1, i16 1, i16 1, i16 1, i16 1, i16 1, i16 1>

  ; Right shift each element by 1 -> dividing by 2
  %average = lshr <32 x i16> %sum_ab_plus_one, <i16 1, i16 1, i16 1, i16 1, i16 1, i16 1, i16 1, i16 1, 
                                             i16 1, i16 1, i16 1, i16 1, i16 1, i16 1, i16 1, i16 1, 
                                             i16 1, i16 1, i16 1, i16 1, i16 1, i16 1, i16 1, i16 1, 
                                             i16 1, i16 1, i16 1, i16 1, i16 1, i16 1, i16 1, i16 1>

  ; Truncate <32 x i16> back to <32 x i8>
  %result = trunc <32 x i16> %average to <32 x i8>

  ret <32 x i8> %result
}

Output:

genericized_mm256_avg_epu8:             # @genericized_mm256_avg_epu8
        vpavgb  ymm0, ymm0, ymm1
        ret