Support SIMD on Rust stable (#520)

* Remove dependency on `packed_simd`

* Support SIMD on stable Rust

* Move `packed_simd.rs` to `vector` module

* Add comment header to `packed_simd.rs`

* Initialize SIMD registers using intrinsics instead of `transmute`

* Use a splat inside of `unpack_pair`

* Update README: the AVX2 backend now works on stable Rust

* Add a CI job to also build the AVX2 SIMD backend on Rust stable

* Added SIMD MSRV test

.github/workflows/rust.yml

@@ -55,7 +55,7 @@ jobs:
       - run: cargo build --target thumbv7em-none-eabi --release
       - run: cargo build --target thumbv7em-none-eabi --release --features serde
 
-  build-simd:
+  build-simd-nightly:
     name: Build simd backend (nightly)
     runs-on: ubuntu-latest
     steps:
@@ -69,6 +69,16 @@ jobs:
         RUSTFLAGS: '--cfg curve25519_dalek_backend="simd" -C target_feature=+avx512ifma'
       run: cargo build --target x86_64-unknown-linux-gnu
 
+  test-simd-avx2:
+    name: Test simd backend (avx2)
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v3
+    - uses: dtolnay/rust-toolchain@stable
+    - env:
+        RUSTFLAGS: '--cfg curve25519_dalek_backend="simd" -C target_feature=+avx2'
+      run: cargo test --target x86_64-unknown-linux-gnu
+
   build-docs:
     name: Build docs
     runs-on: ubuntu-latest
@@ -151,6 +161,10 @@ jobs:
     # deps and the stated MSRV
     - uses: dtolnay/rust-toolchain@1.60.0
     - run: cargo build --no-default-features --features serde
+    # Also make sure the AVX2 build works
+    - env:
+        RUSTFLAGS: '--cfg curve25519_dalek_backend="simd" -C target_feature=+avx2'
+      run: cargo build --target x86_64-unknown-linux-gnu
 
   bench:
     name: Check that benchmarks compile

Cargo.toml

@@ -40,6 +40,7 @@ rand_core = { version = "0.6", default-features = false, features = ["getrandom"
 
 [build-dependencies]
 platforms = "3.0.2"
+rustc_version = "0.4.0"
 
 [[bench]]
 name = "dalek_benchmarks"
@@ -57,11 +58,6 @@ zeroize = { version = "1", default-features = false, optional = true }
 [target.'cfg(curve25519_dalek_backend = "fiat")'.dependencies]
 fiat-crypto = "0.1.19"
 
-# The original packed_simd package was orphaned, see
-# https://github.com/rust-lang/packed_simd/issues/303#issuecomment-701361161
-[target.'cfg(curve25519_dalek_backend = "simd")'.dependencies]
-packed_simd = { version = "0.3.8", package = "packed_simd_2", features = ["into_bits"] }
-
 [features]
 default = ["alloc", "precomputed-tables", "zeroize"]
 alloc = ["zeroize?/alloc"]

README.md

@@ -155,15 +155,15 @@ $ cargo build --target i686-unknown-linux-gnu
 Target backend selection within `simd` must be done manually by setting the
 `RUSTFLAGS` environment variable to one of the below options:
 
-| CPU feature | `RUSTFLAGS`                     |
-| :---        | :---                            |
-| avx2        | `-C target_feature=+avx2`       |
-| avx512ifma  | `-C target_feature=+avx512ifma` |
+| CPU feature | `RUSTFLAGS`                     | Requires nightly? |
+| :---        | :---                            | :---              |
+| avx2        | `-C target_feature=+avx2`       | no                |
+| avx512ifma  | `-C target_feature=+avx512ifma` | yes               |
 
 Or you can use `-C target_cpu=native` if you don't know what to set.
 
-The `simd` backend also requires using nightly, e.g. by running `cargo
-+nightly build`, to build.
+The AVX512 backend requires Rust nightly. If enabled and when compiled on a non-nightly
+compiler it will fall back to using the AVX2 backend.
 
 # Documentation
 

build.rs

@@ -19,6 +19,14 @@ fn main() {
         DalekBits::Dalek64 => println!("cargo:rustc-cfg=curve25519_dalek_bits=\"64\""),
         DalekBits::Dalek32 => println!("cargo:rustc-cfg=curve25519_dalek_bits=\"32\""),
     }
+
+    if rustc_version::version_meta()
+        .expect("failed to detect rustc version")
+        .channel
+        == rustc_version::Channel::Nightly
+    {
+        println!("cargo:rustc-cfg=nightly");
+    }
 }
 
 // Deterministic cfg(curve25519_dalek_bits) when this is not explicitly set.