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, 29 insertions, 0 deletions

diff --git a/internal/prefer_openssl/prefer.go b/internal/prefer_openssl/prefer.go
new file mode 100644
index 0000000..16788e0
--- /dev/null
+++ b/internal/prefer_openssl/prefer.go
@@ -0,0 +1,29 @@
+package prefer_openssl
+
+import (
+	"fmt"
+	"io/ioutil"
+	"regexp"
+)
+
+// filePreferOpenSSL tells us if OpenSSL is faster than Go GCM on this machine.
+// Go GCM is fastern when the CPU has AES instructions and Go is v1.6 or higher.
+//
+// See https://github.com/rfjakob/gocryptfs/issues/23#issuecomment-218286502
+// for benchmarks.
+//
+// filePreferOpenSSL takes an explicit filename so it can be tested with saved
+// cpuinfo files instead of /proc/cpuinfo.
+func filePreferOpenSSL(file string) bool {
+	ci, err := ioutil.ReadFile(file)
+	if err != nil {
+		fmt.Println(err)
+		return true
+	}
+	haveAes, err := regexp.Match(`(?m)^flags.*\baes\b`, ci)
+	if err != nil {
+		fmt.Println(err)
+		return true
+	}
+	return !haveAes
+}