Fix is_ascii performance regression on AVX-512 CPUs when compiling with -C target-cpu=native
## Summary
This PR fixes a severe performance regression in `slice::is_ascii` on AVX-512 CPUs when compiling with `-C target-cpu=native`.
On affected systems, the current implementation achieves only ~3 GB/s for large inputs, compared to ~60–70 GB/s previously (≈20–24× regression). This PR restores the original performance characteristics.
This change is intended as a **temporary workaround** for upstream LLVM poor codegen. Once the underlying LLVM issue is fixed and Rust is able to consume that fix, this workaround should be reverted.
## Problem
When `is_ascii` is compiled with AVX-512 enabled, LLVM's auto-vectorization generates ~31 `kshiftrd` instructions to extract mask bits one-by-one, instead of using the efficient `pmovmskb`
instruction. This causes a **~22x performance regression**.
Because `is_ascii` is marked `#[inline]`, it gets inlined and recompiled with the user's target settings, affecting anyone using `-C target-cpu=native` on AVX-512 CPUs.
## Root cause (upstream)
The underlying issue appears to be an LLVM vectorizer/backend bug affecting certain AVX-512 patterns.
An upstream issue has been filed by @folkertdev to track the root cause: llvm/llvm-project#176906
Until this is resolved in LLVM and picked up by rustc, this PR avoids triggering the problematic codegen pattern.
## Solution
Replace the counting loop with explicit SSE2 intrinsics (`_mm_movemask_epi8`) that force `pmovmskb` codegen regardless of CPU features.
## Godbolt Links (Rust 1.92)
| Pattern | Target | Link | Result |
|---------|--------|------|--------|
| Counting loop (old) | Default SSE2 | https://godbolt.org/z/sE86xz4fY | `pmovmskb` |
| Counting loop (old) | AVX-512 (znver4) | https://godbolt.org/z/b3jvMhGd3 | 31x `kshiftrd` (broken) |
| SSE2 intrinsics (fix) | Default SSE2 | https://godbolt.org/z/hMeGfeaPv | `pmovmskb` |
| SSE2 intrinsics (fix) | AVX-512 (znver4) | https://godbolt.org/z/Tdvdqjohn | `vpmovmskb` (fixed) |
## Benchmark Results
**CPU:** AMD Ryzen 5 7500F (Zen 4 with AVX-512)
### Default Target (SSE2) — Mixed
| Size | Before | After | Change |
|------|--------|-------|--------|
| 4 B | 1.8 GB/s | 2.0 GB/s | **+11%** |
| 8 B | 3.2 GB/s | 5.8 GB/s | **+81%** |
| 16 B | 5.3 GB/s | 8.5 GB/s | **+60%** |
| 32 B | 17.7 GB/s | 15.8 GB/s | -11% |
| 64 B | 28.6 GB/s | 25.1 GB/s | -12% |
| 256 B | 51.5 GB/s | 48.6 GB/s | ~same |
| 1 KB | 64.9 GB/s | 60.7 GB/s | ~same |
| 4 KB+ | ~68-70 GB/s | ~68-72 GB/s | ~same |
### Native Target (AVX-512) — Up to 24x Faster
| Size | Before | After | Speedup |
|------|--------|-------|---------|
| 4 B | 1.2 GB/s | 2.0 GB/s | **1.7x** |
| 8 B | 1.6 GB/s | 5.0 GB/s | **3.3x** |
| 16 B | ~7 GB/s | ~7 GB/s | ~same |
| 32 B | 2.9 GB/s | 14.2 GB/s | **4.9x** |
| 64 B | 2.9 GB/s | 23.2 GB/s | **8x** |
| 256 B | 2.9 GB/s | 47.2 GB/s | **16x** |
| 1 KB | 2.8 GB/s | 60.0 GB/s | **21x** |
| 4 KB+ | 2.9 GB/s | ~68-70 GB/s | **23-24x** |
### Summary
- **SSE2 (default):** Small inputs (4-16 B) 11-81% faster; 32-64 B ~11% slower; large inputs unchanged
- **AVX-512 (native):** 21-24x faster for inputs ≥1 KB, peak ~70 GB/s (was ~3 GB/s)
Note: this is the pure ascii path, but the story is similar for the others.
See linked bench project.
## Test Plan
- [x] Assembly test (`slice-is-ascii-avx512.rs`) verifies no `kshiftrd` with AVX-512
- [x] Existing codegen test updated to `loongarch64`-only (auto-vectorization still used there)
- [x] Fuzz testing confirms old/new implementations produce identical results (~53M iterations)
- [x] Benchmarks confirm performance improvement
- [x] Tidy checks pass
## Reproduction / Test Projects
Standalone validation tools: https://github.com/bonega/is-ascii-fix-validation
- `bench/` - Criterion benchmarks for SSE2 vs AVX-512 comparison
- `fuzz/` - Compares old/new implementations with libfuzzer
## Related Issues
- issue opened by @folkertdev llvm/llvm-project#176906
- Regression introduced in https://github.com/rust-lang/rust/pull/130733
One extension worth noting is the use of revisions as custom prefixes for
FileCheck. If your codegen test has different behavior based on the chosen
target or different compiler flags that you want to exercise, you can use a
revisions annotation, like so:
After specifying those variations, you can write different expected, or
explicitly unexpected output by using <prefix>-SAME: and <prefix>-NOT:,
like so:
Jonathan Brouwer
13f0399a57