Fix simd default on backend

build.rs

@@ -50,20 +50,28 @@ fn main() {
         Ok("simd") => {
             // simd override is not guaranteed as:
             // simd can only be enabled on x86_64 & 64bit target_pointer_width
-            if target_arch == "x86_64" && curve25519_dalek_bits == DalekBits::Dalek64 {
+            if is_capable_simd(&target_arch, curve25519_dalek_bits) {
                 "simd"
             // fallback to serial with a warning
             } else {
                 println!("cargo:warning=Could not override curve25519_dalek_backend to simd - defaulting to serial");
                 "serial"
             }
         }
-        // default serial
-        _ => "serial",
+        // default between serial / simd (if potentially capable)
+        _ => match is_capable_simd(&target_arch, curve25519_dalek_bits) {
+            true => "simd",
+            false => "serial",
+        },
     };
     println!("cargo:rustc-cfg=curve25519_dalek_backend=\"{curve25519_dalek_backend}\"");
 }
 
+// Is the target arch & curve25519_dalek_bits potentially simd capable ?
+fn is_capable_simd(arch: &str, bits: DalekBits) -> bool {
+    arch == "x86_64" && bits == DalekBits::Dalek64
+}
+
 // Deterministic cfg(curve25519_dalek_bits) when this is not explicitly set.
 mod deterministic {
 

tests/build_tests.sh

@@ -19,7 +19,7 @@ function match_and_report() {
 cargo clean
 OUT=build_1.txt
 env RUSTFLAGS="--cfg curve25519_dalek_diagnostics=\"build\"" cargo build > "$OUT" 2>&1
-match_and_report "curve25519_dalek_backend is 'serial'" "$OUT"
+match_and_report "curve25519_dalek_backend is 'simd'" "$OUT"
 match_and_report "curve25519_dalek_bits is '64'" "$OUT"
 
 # Override to 32 bits assuming naively 64 bit build host