SSE2 implementation of RGBA box blurs. This yields ~2X perf improvement on

src/opts/SkBlurImage_opts_SSE2.cpp

Index: src/opts/SkBlurImage_opts_SSE2.cpp
diff --git a/src/opts/SkBlurImage_opts_SSE2.cpp b/src/opts/SkBlurImage_opts_SSE2.cpp
new file mode 100644
index 0000000000000000000000000000000000000000..97cd2fdb59eb6ead3e1888d31024e8cce3b84ed8
--- /dev/null
+++ b/src/opts/SkBlurImage_opts_SSE2.cpp
@@ -0,0 +1,115 @@
+/*
+ * Copyright 2013 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.
+ */
+
+
+#include "SkBitmap.h"
+#include "SkColorPriv.h"
+#include "SkBlurImage_opts_SSE2.h"
+#include "SkRect.h"
+
+#include <emmintrin.h>
+
+namespace {
+
+enum BlurDirection {
+    kX, kY
+};
+
+/**
+ * Helper function to spread the components of a 32-bit integer into the
+ * lower 8 bits of each 32-bit element of an SSE register.
+ */
+
+inline __m128i expand(int a) {
+      const __m128i zero = _mm_setzero_si128();
+
+      // 0 0 0 0   0 0 0 0   0 0 0 0   A R G B
+      __m128i result = _mm_cvtsi32_si128(a);
+
+      // 0 0 0 0   0 0 0 0   0 A 0 R   0 G 0 B
+      result = _mm_unpacklo_epi8(result, zero);
+
+      // 0 0 0 A   0 0 0 R   0 0 0 G   0 0 0 B
+      return _mm_unpacklo_epi16(result, zero);
+}
+
+template<BlurDirection srcDirection, BlurDirection dstDirection>
+void SkBoxBlur_SSE2(const SkPMColor* src, int srcStride, SkPMColor* dst, int kernelSize,
+                    int leftOffset, int rightOffset, int width, int height)
+{
+    const int rightBorder = SkMin32(rightOffset + 1, width);
+    const int srcStrideX = srcDirection == kX ? 1 : srcStride;
+    const int dstStrideX = dstDirection == kX ? 1 : height;
+    const int srcStrideY = srcDirection == kX ? srcStride : 1;
+    const int dstStrideY = dstDirection == kX ? width : 1;
+    const __m128i scale = _mm_set1_epi32((1 << 24) / kernelSize);
+    const __m128i half = _mm_set1_epi32(1 << 23);
+    const __m128i zero = _mm_setzero_si128();
+    for (int y = 0; y < height; ++y) {
+        __m128i sum = zero;
+        const SkPMColor* p = src;
+        for (int i = 0; i < rightBorder; ++i) {
+            sum = _mm_add_epi32(sum, expand(*p));
+            p += srcStrideX;
+        }
+
+        const SkPMColor* sptr = src;
+        SkColor* dptr = dst;
+        for (int x = 0; x < width; ++x) {
+#if 0
+            // In SSE4.1, this would be
+            __m128i result = _mm_mullo_epi32(sum, scale);
+#else
+            // But SSE2 has no PMULLUD, so we must do AG and RB separately.
+            __m128i tmp1 = _mm_mul_epu32(sum, scale);
+            __m128i tmp2 = _mm_mul_epu32(_mm_srli_si128(sum, 4),
+                                         _mm_srli_si128(scale, 4));
+            __m128i result = _mm_unpacklo_epi32(_mm_shuffle_epi32(tmp1, _MM_SHUFFLE(0,0,2,0)),
+                                                _mm_shuffle_epi32(tmp2, _MM_SHUFFLE(0,0,2,0)));
+#endif
+            // sumA*scale+.5 sumB*scale+.5 sumG*scale+.5 sumB*scale+.5
+            result = _mm_add_epi32(result, half);
+
+            // 0 0 0 A   0 0 0 R   0 0 0 G   0 0 0 B
+            result = _mm_srli_epi32(result, 24);
+
+            // 0 0 0 0   0 0 0 0   0 A 0 R   0 G 0 B
+            result = _mm_packs_epi32(result, zero);
+
+            // 0 0 0 0   0 0 0 0   0 0 0 0   A R G B
+            result = _mm_packus_epi16(result, zero);
+            *dptr = _mm_cvtsi128_si32(result);
+            if (x >= leftOffset) {
+                SkColor l = *(sptr - leftOffset * srcStrideX);
+                sum = _mm_sub_epi32(sum, expand(l));
+            }
+            if (x + rightOffset + 1 < width) {
+                SkColor r = *(sptr + (rightOffset + 1) * srcStrideX);
+                sum = _mm_add_epi32(sum, expand(r));
+            }
+            sptr += srcStrideX;
+            if (srcDirection == kY) {
+                _mm_prefetch(reinterpret_cast<const char*>(sptr + (rightOffset + 1) * srcStrideX),
+                             _MM_HINT_T0);
+            }
+            dptr += dstStrideX;
+        }
+        src += srcStrideY;
+        dst += dstStrideY;
+    }
+}
+
+} // namespace
+
+bool SkBoxBlurGetPlatformProcs_SSE2(SkBoxBlurProc* boxBlurX,
+                                    SkBoxBlurProc* boxBlurY,
+                                    SkBoxBlurProc* boxBlurXY) {
+    *boxBlurX = SkBoxBlur_SSE2<kX, kX>;
+    *boxBlurY = SkBoxBlur_SSE2<kY, kY>;
+    *boxBlurXY = SkBoxBlur_SSE2<kX, kY>;
+    return true;
+}