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, 13 insertions, 0 deletions

diff --git a/internal/prefer_openssl/prefer_go1.6.go b/internal/prefer_openssl/prefer_go1.6.go
new file mode 100644
index 0000000..5d0f426
--- /dev/null
+++ b/internal/prefer_openssl/prefer_go1.6.go
@@ -0,0 +1,13 @@
+// +build go1.6
+// = go 1.6 or higher
+
+package prefer_openssl
+
+// PreferOpenSSL 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.
+func PreferOpenSSL() bool {
+	return filePreferOpenSSL("/proc/cpuinfo")
+}