2 files changed, 20 insertions, 4 deletions
diff --git a/internal/speed/speed.go b/internal/speed/speed.go
index 2f818d0..d5aa696 100644
--- a/internal/speed/speed.go
+++ b/internal/speed/speed.go
@@ -32,8 +32,8 @@ func Run() {
 		f         func(*testing.B)
 		preferred bool
 	}{
-		{name: cryptocore.BackendOpenSSL.Name, f: bStupidGCM, preferred: stupidgcm.PreferOpenSSL()},
-		{name: cryptocore.BackendGoGCM.Name, f: bGoGCM, preferred: !stupidgcm.PreferOpenSSL()},
+		{name: cryptocore.BackendOpenSSL.Name, f: bStupidGCM, preferred: stupidgcm.PreferOpenSSLAES256GCM()},
+		{name: cryptocore.BackendGoGCM.Name, f: bGoGCM, preferred: !stupidgcm.PreferOpenSSLAES256GCM()},
 		{name: cryptocore.BackendAESSIV.Name, f: bAESSIV, preferred: false},
 		{name: cryptocore.BackendXChaCha20Poly1305.Name, f: bXchacha20poly1305, preferred: false},
 		{name: cryptocore.BackendXChaCha20Poly1305OpenSSL.Name, f: bStupidXchacha, preferred: false},
diff --git a/internal/stupidgcm/prefer.go b/internal/stupidgcm/prefer.go
index bacd56a..ffbaf58 100644
--- a/internal/stupidgcm/prefer.go
+++ b/internal/stupidgcm/prefer.go
@@ -6,7 +6,8 @@ import (
 	"golang.org/x/sys/cpu"
 )
 
-// PreferOpenSSL tells us if OpenSSL is faster than Go GCM on this machine.
+// PreferOpenSSLAES256GCM tells us if OpenSSL AES-256-GCM is faster than Go stdlib
+// on this machine.
 //
 // Go GCM is only faster if the CPU either:
 //
@@ -16,7 +17,7 @@ import (
 //
 // See https://github.com/rfjakob/gocryptfs/wiki/CPU-Benchmarks
 // for benchmarks.
-func PreferOpenSSL() bool {
+func PreferOpenSSLAES256GCM() bool {
 	if BuiltWithoutOpenssl {
 		return false
 	}
@@ -33,3 +34,18 @@ func PreferOpenSSL() bool {
 	// OpenSSL is probably faster
 	return true
 }
+
+// PreferOpenSSLXchacha20poly1305 returns true if OpenSSL Xchacha20poly1305 is
+// faster than Go stdlib on this machine.
+func PreferOpenSSLXchacha20poly1305() bool {
+	if BuiltWithoutOpenssl {
+		return false
+	}
+	// Go x/crypto has optimized assembly for amd64:
+	// https://github.com/golang/crypto/blob/master/chacha20poly1305/chacha20poly1305_amd64.s
+	if runtime.GOARCH == "amd64" {
+		return false
+	}
+	// On arm64 and arm, OpenSSL is faster. Probably everwhere else too.
+	return true
+}