RustCrypto / RustCrypto/block-ciphers
aes: SIMD-acceleration for WASM
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- Dominant language
- Rust
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Description
WASM targets currently use the software fixslice implementation which processes up to 4 blocks at a time (fixslice64). WebAssembly has had the simd128 proposal stabilized in the spec for a while now, and the corresponding intrinsics ([core::arch::wasm32::*]) have been stable in Rust since 1.33. Every major WebAssembly runtime ships simd128 support today.
We use AES extensively in our projects which target running in browsers, the throughput of AES in WASM is a major performance lever for us. Translating the existing fixslice64 implementation (AI assisted, not sure what your policy is on that) to use 128 bit lanes yields the expected >2x throughput increase in V8 (chromium), and >3x in SpiderMonkey (firefox).
(AMD Ryzen 7 5800X)
| Unit: MB/s | wasmtime soft | wasmtime simd128 | wasmtime relative | v8 soft | v8 simd128 | v8 relative | sm soft | sm simd128 | sm relative |
|---|---|---|---|---|---|---|---|---|---|
| AES-128 enc | 186 | 184 | 0.99× | 258 | 604 | 2.34× | 213 | 657 | 3.08× |
| AES-128 dec | 185 | 122 | 0.66× | 235 | 508 | 2.16× | 182 | 598 | 3.29× |
| AES-192 enc | 159 | 150 | 0.94× | 224 | 522 | 2.33× | 185 | 588 | 3.18× |
| AES-192 dec | 159 | 100 | 0.63× | 202 | 486 | 2.41× | 153 | 577 | 3.77× |
| AES-256 enc | 138 | 140 | 1.01× | 192 | 455 | 2.37× | 159 | 491 | 3.09× |
| AES-256 dec | 135 | 89 | 0.66× | 171 | 424 | 2.48× | 134 | 453 | 3.38× |
As you can see, this causes a regression in Wasmtime decrypt performance due to bad JIT by Cranelift. I haven't been able to mitigate that without incurring significant throughput loss in the browser runtimes. I'm not sure whether the right approach is to eat the regression by default, or to put the SIMD behind a flag.
Can reproduce the benchmarks using wasm-harness
Would love to upstream this if possible. If so, I will open a PR.
Contributor guide
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First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Start by reviewing the linked implementation commit and the existing WASM AES fixslice64 path, then reproduce the reported measurements with wasm-harness. Done means an upstreamable SIMD implementation with benchmark coverage and a documented decision on whether SIMD is enabled by default or behind a flag, including the Wasmtime decryption regression.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- rust, wasm
- Domain
- cryptography, performance
- Issue type
- Feature
- Difficulty
- 5/5
- Estimated time
- Over a week
- Activity status
- Quiet
- Clarity
- Mostly clear
- Newbie friendliness
- 45/100