Don't do a pointless << 0.

src/opts/SkColor_opts_SSE2.h

 /*
  * Copyright 2014 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkColor_opts_SSE2_DEFINED
 #define SkColor_opts_SSE2_DEFINED
 
@@ skipping 63 matching lines @@
     rb = _mm_mulhi_epu16(rb, s);
 
     __m128i ag = _mm_andnot_si128(mask, c);
     ag = _mm_mulhi_epu16(ag, s);     // Alpha and green values are in the higher byte of each word.
     ag = _mm_andnot_si128(mask, ag);
 
     return _mm_or_si128(rb, ag);
 }
 
 static inline __m128i SkGetPackedA32_SSE2(const __m128i& src) {
+#if SK_A32_SHIFT == 24                // It's very common (universal?) that alpha is the top byte.
+    return _mm_srli_epi32(src, 24);   // You'd hope the compiler would remove the left shift then,
+#else                                 // but I've seen Clang just do a dumb left shift of zero. :(
     __m128i a = _mm_slli_epi32(src, (24 - SK_A32_SHIFT));
     return _mm_srli_epi32(a, 24);
+#endif
 }
 
 static inline __m128i SkGetPackedR32_SSE2(const __m128i& src) {
     __m128i r = _mm_slli_epi32(src, (24 - SK_R32_SHIFT));
     return _mm_srli_epi32(r, 24);
 }
 
 static inline __m128i SkGetPackedG32_SSE2(const __m128i& src) {
     __m128i g = _mm_slli_epi32(src, (24 - SK_G32_SHIFT));
     return _mm_srli_epi32(g, 24);
@@ skipping 130 matching lines @@
     dst_scale = _mm_mullo_epi16(dst_scale, src_scale);
     dst_scale = _mm_srli_epi16(dst_scale, 8);
     dst_scale = _mm_sub_epi32(_mm_set1_epi32(256), dst_scale);
 
     __m128i result = SkAlphaMulQ_SSE2(src, alpha);
     return _mm_add_epi8(result, SkAlphaMulQ_SSE2(dst, dst_scale));
 }
 
 #undef ASSERT_EQ
 #endif // SkColor_opts_SSE2_DEFINED