Add aarch64 optimise_barrier

graviola/src/low/aarch64/mod.rs

@@ -51,6 +51,7 @@ pub(crate) mod cpu;
 pub(crate) mod curve25519_x25519;
 pub(crate) mod curve25519_x25519base;
 pub(crate) mod ghash;
+pub(crate) mod optimise_barrier;
 pub(crate) mod p256_montjadd;
 pub(crate) mod p256_montjdouble;
 pub(crate) mod p256_montjmixadd;

graviola/src/low/aarch64/optimise_barrier.rs

@@ -0,0 +1,14 @@
+// SPDX-License-Identifier: Apache-2.0 OR ISC OR MIT-0
+
+pub fn optimise_barrier_u8(v: u8) -> u8 {
+    let ret: u8;
+    // SAFETY: inline assembly, which does nothing but block the optimiser
+    // from seeing the data dependency between `v` and `ret`.
+    unsafe {
+        core::arch::asm!(
+            "/* {v:w} */",
+            v = inout(reg) v => ret,
+        )
+    };
+    ret
+}

graviola/src/low/mod.rs

@@ -16,8 +16,6 @@ mod generic {
     pub(super) mod ct_equal;
     #[cfg(test)]
     pub(crate) mod ghash;
-    #[cfg(target_arch = "aarch64")]
-    pub(super) mod optimise_barrier;
     pub(crate) mod poly1305;
     pub(super) mod sha256;
     pub(super) mod sha512;
@@ -152,11 +150,11 @@ cfg_if::cfg_if! {
         pub(crate) use aarch64::p384_montjadd::p384_montjadd;
         pub(crate) use aarch64::p384_montjdouble::p384_montjdouble;
         pub(crate) use aarch64::sha256::sha256_compress_blocks;
+        pub(crate) use aarch64::optimise_barrier::optimise_barrier_u8;
 
         pub(crate) use generic::chacha20;
         pub(crate) use generic::aes_gcm;
         pub(crate) use generic::sha512::sha512_compress_blocks;
-        pub(crate) use generic::optimise_barrier::optimise_barrier_u8;
     } else {
         compile_error!("This crate only supports x86_64 or aarch64");
     }