Prefer Go stdlib aes-gcm on arm64 with aes instructions

We used to prefer openssl in this situation, which
used to make sense, but now Go gained an optimized
assembly implementation for aes-gcm on arm64 with
aes instructions:

  root@q1:~/go/src/github.com/rfjakob/gocryptfs# ./gocryptfs -speed
  gocryptfs v1.7.1-46-g73436d9; go-fuse v1.0.1-0.20190319092520-161a16484456; 2020-04-13 go1.14.2 linux/arm64
  AES-GCM-256-OpenSSL      212.30 MB/s    (selected in auto mode)
  AES-GCM-256-Go           452.30 MB/s
  AES-SIV-512-Go           100.25 MB/s
  XChaCha20-Poly1305-Go    137.35 MB/s

https://github.com/rfjakob/gocryptfs/issues/452

1 files changed, 7 insertions, 5 deletions

diff --git a/internal/stupidgcm/prefer.go b/internal/stupidgcm/prefer.go
index a08d154..c067d09 100644
--- a/internal/stupidgcm/prefer.go
+++ b/internal/stupidgcm/prefer.go
@@ -6,11 +6,11 @@ import (
 
 // PreferOpenSSL tells us if OpenSSL is faster than Go GCM on this machine.
 //
-// Go GCM is only faster if the CPU:
+// Go GCM is only faster if the CPU either:
 //
-// 1) Is X86
-// 2) Has AES instructions
-// 3) Go is v1.6 or higher
+// 1) Is X86_64 && has AES instructions && Go is v1.6  or higher
+// 2) Is ARM64  && has AES instructions && Go is v1.11 or higher
+//    (commit https://github.com/golang/go/commit/4f1f503373cda7160392be94e3849b0c9b9ebbda)
 //
 // See https://github.com/rfjakob/gocryptfs/wiki/CPU-Benchmarks
 // for benchmarks.
@@ -19,8 +19,10 @@ func PreferOpenSSL() bool {
 		return false
 	}
 	// Safe to call on other architectures - will just read false.
-	if cpu.X86.HasAES {
+	if cpu.X86.HasAES || cpu.ARM64.HasAES {
+		// Go stdlib is probably faster
 		return false
 	}
+	// Openssl is probably faster
 	return true
 }