crypto/aes: use asm for BenchmarkExpand on amd64

This reverses the change to this benchmark made in 9b6bf20a.

Change-Id: I79ab88286c3028d3be561957140375bbc413e7ab
Reviewed-on: https://go-review.googlesource.com/22340
Run-TryBot: Michael Munday <munday@ca.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Minux Ma <minux@golang.org>

src/crypto/aes/aes_test.go

@@ -380,6 +380,6 @@ func BenchmarkExpand(b *testing.B) {
 	c := &aesCipher{make([]uint32, n), make([]uint32, n)}
 	b.ResetTimer()
 	for i := 0; i < b.N; i++ {
-		expandKeyGo(tt.key, c.enc, c.dec)
+		expandKey(tt.key, c.enc, c.dec)
 	}
 }

src/crypto/aes/cipher_amd64.go

@@ -64,3 +64,20 @@ func (c *aesCipherAsm) Decrypt(dst, src []byte) {
 	}
 	decryptBlockAsm(len(c.dec)/4-1, &c.dec[0], &dst[0], &src[0])
 }
+
+// expandKey is used by BenchmarkExpand to ensure that the asm implementation
+// of key expansion is used for the benchmark when it is available.
+func expandKey(key []byte, enc, dec []uint32) {
+	if useAsm {
+		rounds := 10 // rounds needed for AES128
+		switch len(key) {
+		case 192 / 8:
+			rounds = 12
+		case 256 / 8:
+			rounds = 14
+		}
+		expandKeyAsm(rounds, &key[0], &enc[0], &dec[0])
+	} else {
+		expandKeyGo(key, enc, dec)
+	}
+}

src/crypto/aes/cipher_generic.go

@@ -18,3 +18,9 @@ import (
 func newCipher(key []byte) (cipher.Block, error) {
 	return newCipherGeneric(key)
 }
+
+// expandKey is used by BenchmarkExpand and should
+// call an assembly implementation if one is available.
+func expandKey(key []byte, enc, dec []uint32) {
+	expandKeyGo(key, enc, dec)
+}

src/crypto/aes/cipher_s390x.go

@@ -82,3 +82,9 @@ func (c *aesCipherAsm) Decrypt(dst, src []byte) {
 	// The decrypt function code is equal to the function code + 128.
 	cryptBlocks(c.function+128, &c.key[0], &dst[0], &src[0], BlockSize)
 }
+
+// expandKey is used by BenchmarkExpand. cipher message (KM) does not need key
+// expansion so there is no assembly equivalent.
+func expandKey(key []byte, enc, dec []uint32) {
+	expandKeyGo(key, enc, dec)
+}