chacha20: Add full NEON backend. (#310)

Observed performance changes on an Apple M1 Air:

```
 name                   chacha-soft ns/iter  chacha-neon ns/iter  diff ns/iter   diff %  speedup
 chacha12_bench1_16b    34 (470 MB/s)        28 (571 MB/s)                  -6  -17.65%   x 1.21
 chacha12_bench2_256b   477 (536 MB/s)       132 (1939 MB/s)              -345  -72.33%   x 3.61
 chacha12_bench3_1kib   1,897 (539 MB/s)     503 (2035 MB/s)            -1,394  -73.48%   x 3.77
 chacha12_bench4_16kib  30,811 (531 MB/s)    7,914 (2070 MB/s)         -22,897  -74.31%   x 3.89
 chacha20_bench1_16b    51 (313 MB/s)        47 (340 MB/s)                  -4   -7.84%   x 1.09
 chacha20_bench2_256b   777 (329 MB/s)       212 (1207 MB/s)              -565  -72.72%   x 3.67
 chacha20_bench3_1kib   3,088 (331 MB/s)     821 (1247 MB/s)            -2,267  -73.41%   x 3.76
 chacha20_bench4_16kib  50,251 (326 MB/s)    13,001 (1260 MB/s)        -37,250  -74.13%   x 3.87
 chacha8_bench1_16b     26 (615 MB/s)        19 (842 MB/s)                  -7  -26.92%   x 1.37
 chacha8_bench2_256b    335 (764 MB/s)       92 (2782 MB/s)               -243  -72.54%   x 3.64
 chacha8_bench3_1kib    1,328 (771 MB/s)     344 (2976 MB/s)              -984  -74.10%   x 3.86
 chacha8_bench4_16kib   21,184 (773 MB/s)    5,371 (3050 MB/s)         -15,813  -74.65%   x 3.94
 ```

* chacha20: Gate NEON backend behind `neon` feature.

* chacha20: Re-enable CI for NEON backend

* chacha20: Update README to include `neon`

Author: codahale

.github/workflows/chacha20.yml

@@ -197,9 +197,9 @@ jobs:
             rust: stable
 
           # ARM64 with NEON backend
-          #- target: aarch64-unknown-linux-gnu
-          #  rust: nightly
-          #  features: --features neon
+          - target: aarch64-unknown-linux-gnu
+            rust: stable
+            features: --features neon
 
           # PPC32
           - target: powerpc-unknown-linux-gnu

chacha20/Cargo.toml

@@ -32,6 +32,7 @@ hex-literal = "0.3.3"
 [features]
 std = ["cipher/std"]
 zeroize = ["cipher/zeroize"]
+neon = []
 
 [package.metadata.docs.rs]
 all-features = true

chacha20/README.md

@@ -34,6 +34,8 @@ work on stable Rust with the following `RUSTFLAGS`:
 - `x86` / `x86_64`
   - `avx2`: (~1.4cpb) `-Ctarget-cpu=haswell -Ctarget-feature=+avx2`
   - `sse2`: (~2.5cpb) `-Ctarget-feature=+sse2` (on by default on x86 CPUs)
+- `aarch64`
+  - `neon` (~2-3x faster than `soft`) requires Rust 1.61+ and the `neon` feature enabled
 - Portable
   - `soft`: (~5 cpb on x86/x86_64)
 
@@ -76,8 +78,8 @@ done with a minor version bump.
 
 Licensed under either of:
 
- * [Apache License, Version 2.0](http://www.apache.org/licenses/LICENSE-2.0)
- * [MIT license](http://opensource.org/licenses/MIT)
+- [Apache License, Version 2.0](http://www.apache.org/licenses/LICENSE-2.0)
+- [MIT license](http://opensource.org/licenses/MIT)
 
 at your option.
 

chacha20/src/backends.rs

@@ -15,6 +15,8 @@ cfg_if! {
                 pub(crate) mod sse2;
             }
         }
+    } else if #[cfg(all(feature = "neon", target_arch = "aarch64", target_feature = "neon"))] {
+        pub(crate) mod neon;
     } else {
         pub(crate) mod soft;
     }