prefer_openssl: autodetect whether to use OpenSSL or Go GCM

Go GCM is faster than OpenSSL if the CPU has AES instructions
and you are running Go 1.6+.

The "-openssl" option now defaults to "auto".

"gocryptfs -debug -version" displays the result of the autodetection.

See https://github.com/rfjakob/gocryptfs/issues/23 for details and
benchmarks.

1 files changed, 9 insertions, 0 deletions

diff --git a/internal/prefer_openssl/prefer_go1.5.go b/internal/prefer_openssl/prefer_go1.5.go
new file mode 100644
index 0000000..4b681e7
--- /dev/null
+++ b/internal/prefer_openssl/prefer_go1.5.go
@@ -0,0 +1,9 @@
+// +build !go1.6
+// = go 1.5 or lower
+
+package prefer_openssl
+
+func PreferOpenSSL() bool {
+	// OpenSSL is always faster than Go GCM on old Go versions.
+	return true
+}