Sk4px: alphas() and Load[24]Alphas()

src/opts/Sk4px_SSE2.h

Index: src/opts/Sk4px_SSE2.h
diff --git a/src/opts/Sk4px_SSE2.h b/src/opts/Sk4px_SSE2.h
index d036328c148904a8fd776a9bda30d4797f2d1687..55e1a69135d561a42d51f5dd2e456d7c1d05564c 100644
--- a/src/opts/Sk4px_SSE2.h
+++ b/src/opts/Sk4px_SSE2.h
@@ -37,3 +37,42 @@ inline Sk4px Sk4px::Wide::addNarrowHi(const Sk16h& other) const {
     Sk4px::Wide r = (*this + other) >> 8;
     return Sk4px(_mm_packus_epi16(r.fLo.fVec, r.fHi.fVec));
 }
+
+// Load4Alphas and Load2Alphas use possibly-unaligned loads (SkAlpha[] -> uint16_t or uint32_t).
+// These are safe on x86, often with no speed penalty.
+
+#if SK_CPU_SSE_LEVEL >= SK_CPU_SSE_LEVEL_SSSE3
+    inline Sk4px Sk4px::alphas() const {
+        static_assert(SK_A32_SHIFT == 24, "Intel's always little-endian.");
+        __m128i splat = _mm_set_epi8(15,15,15,15, 11,11,11,11, 7,7,7,7, 3,3,3,3);
+        return Sk16b(_mm_shuffle_epi8(this->fVec, splat));
+    }
+
+    inline Sk4px Sk4px::Load4Alphas(const SkAlpha a[4]) {
+        uint32_t as = *(const uint32_t*)a;
+        __m128i splat = _mm_set_epi8(3,3,3,3, 2,2,2,2, 1,1,1,1, 0,0,0,0);
+        return Sk16b(_mm_shuffle_epi8(_mm_cvtsi32_si128(as), splat));
+    }
+#else
+    inline Sk4px Sk4px::alphas() const {
+        static_assert(SK_A32_SHIFT == 24, "Intel's always little-endian.");
+        __m128i as = _mm_srli_epi32(this->fVec, 24);   // ___3 ___2 ___1 ___0
+        as = _mm_or_si128(as, _mm_slli_si128(as, 1));  // __33 __22 __11 __00
+        as = _mm_or_si128(as, _mm_slli_si128(as, 2));  // 3333 2222 1111 0000
+        return Sk16b(as);
+    }
+
+    inline Sk4px Sk4px::Load4Alphas(const SkAlpha a[4]) {
+        __m128i as = _mm_cvtsi32_si128(*(const uint32_t*)a);  // ____ ____ ____ 3210
+        as = _mm_unpacklo_epi8 (as, _mm_setzero_si128());     // ____ ____ _3_2 _1_0
+        as = _mm_unpacklo_epi16(as, _mm_setzero_si128());     // ___3 ___2 ___1 ___0
+        as = _mm_or_si128(as, _mm_slli_si128(as, 1));         // __33 __22 __11 __00
+        as = _mm_or_si128(as, _mm_slli_si128(as, 2));         // 3333 2222 1111 0000
+        return Sk16b(as);
+    }
+#endif
+
+inline Sk4px Sk4px::Load2Alphas(const SkAlpha a[2]) {
+    uint32_t as = *(const uint16_t*)a;   // Aa -> Aa00
+    return Load4Alphas((const SkAlpha*)&as);
+}