crypto/rc4: naïve ARM assembly implementation

On 800MHz Cortex-A8:
benchmark           old ns/op    new ns/op    delta
BenchmarkRC4_128         9395         2838  -69.79%
BenchmarkRC4_1K         74497        22120  -70.31%
BenchmarkRC4_8K        587243       171435  -70.81%

benchmark            old MB/s     new MB/s  speedup
BenchmarkRC4_128        13.62        45.09    3.31x
BenchmarkRC4_1K         13.75        46.29    3.37x
BenchmarkRC4_8K         13.79        47.22    3.42x

Result for "OpenSSL 1.0.1c 10 May 2012" from Debian/armhf sid:
type             16 bytes     64 bytes    256 bytes   1024 bytes   8192 bytes
rc4              39553.81k    46522.39k    49336.11k    50085.63k    50258.06k

R=golang-dev, agl, dave
CC=golang-dev
https://golang.org/cl/7310051

src/pkg/crypto/rc4/rc4_arm.s

+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Registers
+dst = 0
+src = 1
+n = 2
+state = 3
+pi = 4
+pj = 5
+i = 6
+j = 7
+k = 8
+t = 11
+t2 = 12
+
+// func xorKeyStream(dst, src *byte, n int, state *[256]byte, i, j *uint8)
+TEXT ·xorKeyStream(SB),7,$0
+	MOVW 0(FP), R(dst)
+	MOVW 4(FP), R(src)
+	MOVW 8(FP), R(n)
+	MOVW 12(FP), R(state)
+	MOVW 16(FP), R(pi)
+	MOVW 20(FP), R(pj)
+	MOVBU (R(pi)), R(i)
+	MOVBU (R(pj)), R(j)
+	MOVW $0, R(k)
+
+loop:
+	// i += 1; j += state[i]
+	ADD $1, R(i)
+	AND $0xff, R(i)
+	MOVBU R(i)<<0(R(state)), R(t)
+	ADD R(t), R(j)
+	AND $0xff, R(j)
+
+	// swap state[i] <-> state[j]
+	MOVBU R(j)<<0(R(state)), R(t2)
+	MOVB R(t2), R(i)<<0(R(state))
+	MOVB R(t), R(j)<<0(R(state))
+
+	// dst[k] = src[k] ^ state[state[i] + state[j]]
+	ADD R(t2), R(t)
+	AND $0xff, R(t)
+	MOVBU R(t)<<0(R(state)), R(t)
+	MOVBU R(k)<<0(R(src)), R(t2)
+	EOR R(t), R(t2)
+	MOVB R(t2), R(k)<<0(R(dst))
+
+	ADD $1, R(k)
+	CMP R(k), R(n)
+	BNE loop
+
+done:
+	MOVB R(i), (R(pi))
+	MOVB R(j), (R(pj))
+	RET

src/pkg/crypto/rc4/rc4_asm.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build amd64
+// +build amd64 arm
 
 package rc4
 

src/pkg/crypto/rc4/rc4_ref.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build !amd64
+// +build !amd64,!arm
 
 package rc4