nix: try to make armv8.2-a / cortex-a55 optimised builds if target is aarch64

Signed-off-by: strawberry <strawberry@puppygock.gay>

nix/pkgs/main/cross-compilation-env.nix

@@ -16,6 +16,9 @@ lib.optionalAttrs stdenv.hostPlatform.isStatic {
         ++ lib.optionals
           stdenv.targetPlatform.isx86_64
           [ "-C" "target-cpu=x86-64-v2" ]
+        ++ lib.optionals
+          stdenv.targetPlatform.isAarch64
+          [ "-C" "target-cpu=cortex-a55" ] # cortex-a55 == ARMv8.2-a
         # This disables PIE for static builds, which isn't great in terms
         # of security. Unfortunately, my hand is forced because nixpkgs'
         # `libstdc++.a` is built without `-fPIE`, which precludes us from

nix/pkgs/main/default.nix

@@ -87,6 +87,12 @@ buildDepsOnlyEnv =
           "-DHAVE_SSE=1"
           "-DHAVE_SSE42=1"
         ]
+        else if stdenv.targetPlatform.isAarch64
+        then lib.subtractLists [ "-DPORTABLE=1" ] old.cmakeFlags
+        ++ lib.optionals stdenv.targetPlatform.isAarch64 [
+          # cortex-a55 == ARMv8.2-a
+          "-DPORTABLE=armv8.2-a"
+        ]
         else old.cmakeFlags;
     });
   in
@@ -116,7 +122,9 @@ buildPackageEnv = {
     + lib.optionalString (enableLiburing && stdenv.hostPlatform.isStatic)
       " -L${lib.getLib liburing}/lib -luring"
     + lib.optionalString stdenv.targetPlatform.isx86_64
-      " -Ctarget-cpu=x86-64-v2";
+      " -Ctarget-cpu=x86-64-v2"
+    + lib.optionalString stdenv.targetPlatform.isAarch64
+      " -Ctarget-cpu=cortex-a55"; # cortex-a55 == ARMv8.2-a
 };