Add ARGBExtractAlpha function

source/row_gcc.cc

 // VERSION 2
 /*
  *  Copyright 2011 The LibYuv Project Authors. All rights reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS. All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
@@ skipping 2918 matching lines @@
   : "+r"(src),   // %0
     "+r"(dst),   // %1
     "+r"(width)  // %2
   :
   : "memory", "cc"
     , "xmm0", "xmm1", "xmm2"
   );
 }
 #endif  // HAS_ARGBCOPYALPHAROW_AVX2
 
+#ifdef HAS_ARGBEXTRACTALPHAROW_SSE2
+// width in pixels
+void ARGBExtractAlphaRow_SSE2(const uint8* src_argb, uint8* dst_a, int width) {
+ asm volatile (
+    LABELALIGN
+  "1:                                          \n"
+    "movdqu    " MEMACCESS(0) ", %%xmm0        \n"
+    "movdqu    " MEMACCESS2(0x10, 0) ", %%xmm1 \n"
+    "lea       " MEMLEA(0x20, 0) ", %0         \n"
+    "psrld     $0x18, %%xmm0                   \n"
+    "psrld     $0x18, %%xmm1                   \n"
+    "packssdw  %%xmm1, %%xmm0                  \n"
+    "packuswb  %%xmm0, %%xmm0                  \n"
+    "movq      %%xmm0," MEMACCESS(1) "         \n"
+    "lea       " MEMLEA(0x8, 1) ", %1          \n"
+    "sub       $0x8, %2                        \n"
+    "jg        1b                              \n"
+  : "+r"(src_argb),  // %0
+    "+r"(dst_a),     // %1
+    "+rm"(width)     // %2
+  :
+  : "memory", "cc"
+    , "xmm0", "xmm1"
+  );
+}
+#endif  // HAS_ARGBEXTRACTALPHAROW_SSE2
+
 #ifdef HAS_ARGBCOPYYTOALPHAROW_SSE2
 // width in pixels
 void ARGBCopyYToAlphaRow_SSE2(const uint8* src, uint8* dst, int width) {
   asm volatile (
     "pcmpeqb   %%xmm0,%%xmm0                   \n"
     "pslld     $0x18,%%xmm0                    \n"
     "pcmpeqb   %%xmm1,%%xmm1                   \n"
     "psrld     $0x8,%%xmm1                     \n"
     LABELALIGN
   "1:                                          \n"
@@ skipping 2549 matching lines @@
   );
 }
 #endif  // HAS_ARGBLUMACOLORTABLEROW_SSSE3
 
 #endif  // defined(__x86_64__) || defined(__i386__)
 
 #ifdef __cplusplus
 }  // extern "C"
 }  // namespace libyuv
 #endif