AArch64:add SumSquareError_NEON armv8 assembly version

BUG=none
TESTED=libyuv_unittest
R=fbarchard@google.com

Review URL: https://webrtc-codereview.appspot.com/16259004/

the benckmarking result is as follows:
toolchain: gcc 4.9
hardware: A53

| count | C Times/NEON times |
| 16    | 3.35               |
| 128   | 6.63               |
| 512   | 7.47               |
| 1024  | 7.72               |

Change-Id: Ic10bf22d77d069a1a2074b68bd5a310c579ec490

Review URL: https://webrtc-codereview.appspot.com/21159004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@1043 16f28f9a-4ce2-e073-06de-1de4eb20be90

source/compare.cc

@@ -80,7 +80,7 @@ uint32 HashDjb2(const uint8* src, uint64 count, uint32 seed) {
 
 uint32 SumSquareError_C(const uint8* src_a, const uint8* src_b, int count);
 #if !defined(LIBYUV_DISABLE_NEON) && \
-    (defined(__ARM_NEON__) || defined(LIBYUV_NEON))
+    (defined(__ARM_NEON__) || defined(LIBYUV_NEON) || defined(__aarch64__))
 #define HAS_SUMSQUAREERROR_NEON
 uint32 SumSquareError_NEON(const uint8* src_a, const uint8* src_b, int count);
 #endif

source/compare_neon.cc

@@ -56,6 +56,45 @@ uint32 SumSquareError_NEON(const uint8* src_a, const uint8* src_b, int count) {
   return sse;
 }
 
+#elif !defined(LIBYUV_DISABLE_NEON) && defined(__aarch64__)
+
+uint32 SumSquareError_NEON(const uint8* src_a, const uint8* src_b, int count) {
+  volatile uint32 sse;
+  asm volatile (
+    "eor        v16.16b, v16.16b, v16.16b      \n"
+    "eor        v18.16b, v18.16b, v18.16b      \n"
+    "eor        v17.16b, v17.16b, v17.16b      \n"
+    "eor        v19.16b, v19.16b, v19.16b      \n"
+
+    ".p2align  2                               \n"
+  "1:                                          \n"
+    MEMACCESS(0)
+    "ld1        {v0.16b}, [%0], #16            \n"
+    MEMACCESS(1)
+    "ld1        {v1.16b}, [%1], #16            \n"
+    "subs       %2, %2, #16                    \n"
+    "usubl      v2.8h, v0.8b, v1.8b            \n"
+    "usubl2     v3.8h, v0.16b, v1.16b          \n"
+    "smlal      v16.4s, v2.4h, v2.4h           \n"
+    "smlal      v17.4s, v3.4h, v3.4h           \n"
+    "smlal2     v18.4s, v2.8h, v2.8h           \n"
+    "smlal2     v19.4s, v3.8h, v3.8h           \n"
+    "bgt        1b                             \n"
+
+    "add        v16.4s, v16.4s, v17.4s         \n"
+    "add        v18.4s, v18.4s, v19.4s         \n"
+    "add        v19.4s, v16.4s, v18.4s         \n"
+    "addv       s0, v19.4s                     \n"
+    "fmov       %w3, s0                        \n"
+    : "+r"(src_a),
+      "+r"(src_b),
+      "+r"(count),
+      "=r"(sse)
+    :
+    : "cc", "v0", "v1", "v2", "v3", "v16", "v17", "v18", "v19");
+  return sse;
+}
+
 #endif  // __ARM_NEON__
 
 #ifdef __cplusplus