Add ARGBExtractAlpha function

source/row_win.cc

 /*
  *  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 3514 matching lines @@
     lea        edx, [edx + 64]
     sub        ecx, 16
     jg         convertloop
 
     vzeroupper
     ret
   }
 }
 #endif  // HAS_ARGBCOPYALPHAROW_AVX2
 
+#ifdef HAS_ARGBEXTRACTALPHAROW_SSE2
+// width in pixels
+__declspec(naked)
+void ARGBExtractAlphaRow_SSE2(const uint8* src_argb, uint8* dst_a, int width) {
+  __asm {
+    mov        eax, [esp + 4]   // src_argb
+    mov        edx, [esp + 8]   // dst_a
+    mov        ecx, [esp + 12]  // width
+
+  extractloop:
+    movdqu     xmm0, [eax]
+    movdqu     xmm1, [eax + 16]
+    lea        eax, [eax + 32]
+    psrld      xmm0, 24
+    psrld      xmm1, 24
+    packssdw   xmm0, xmm1
+    packuswb   xmm0, xmm0
+    movq       qword ptr [edx], xmm0
+    lea        edx, [edx + 8]
+    sub        ecx, 8
+    jg         extractloop
+
+    ret
+  }
+}
+#endif  // HAS_ARGBEXTRACTALPHAROW_SSE2
+
 #ifdef HAS_ARGBCOPYYTOALPHAROW_SSE2
 // width in pixels
 __declspec(naked)
 void ARGBCopyYToAlphaRow_SSE2(const uint8* src, uint8* dst, int width) {
   __asm {
     mov        eax, [esp + 4]   // src
     mov        edx, [esp + 8]   // dst
     mov        ecx, [esp + 12]  // count
     pcmpeqb    xmm0, xmm0       // generate mask 0xff000000
     pslld      xmm0, 24
@@ skipping 2687 matching lines @@
 }
 #endif  // HAS_ARGBLUMACOLORTABLEROW_SSSE3
 
 #endif  // defined(_M_X64)
 #endif  // !defined(LIBYUV_DISABLE_X86) && (defined(_M_IX86) || defined(_M_X64))
 
 #ifdef __cplusplus
 }  // extern "C"
 }  // namespace libyuv
 #endif