Convert Color32 code to perfect blend.

Before we commit to blend_256_round_alt, let's make sure blend_perfect is
really slower in practice (i.e. regresses on perf.skia.org).

blend_perfect is really the most desirable algorithm if we can afford it.  Not
only is it correct, but it's easy to think about and break into correct pieces:
for instance, its div255() doesn't require any coordination with the multiply.

This looks like a 30% hit according to microbenches.  That said, microbenches
said my previous change would be a 20-25% perf improvement, but it didn't end
up showing a significant effect at a high level.

As for correctness, I see a bunch of off-by-1 compared to blend_256_round_alt
(exactly what we'd expect), and one off-by-3 in a GM that looks like it has a
bunch of overdraw.

BUG=skia:

Review URL: https://codereview.chromium.org/1098913002

1 files changed, 16 insertions, 27 deletions

diff --git a/src/opts/SkBlitRow_opts_SSE2.cpp b/src/opts/SkBlitRow_opts_SSE2.cpp
index 59375f1831..3fcb9e0e14 100644
--- a/src/opts/SkBlitRow_opts_SSE2.cpp
+++ b/src/opts/SkBlitRow_opts_SSE2.cpp
@@ -234,41 +234,30 @@ void S32A_Blend_BlitRow32_SSE2(SkPMColor* SK_RESTRICT dst,
 
 #define SK_SUPPORT_LEGACY_COLOR32_MATHx
 
-/* SSE2 version of Color32()
- * portable version is in core/SkBlitRow_D32.cpp
- */
-// Color32 and its SIMD specializations use the blend_256_round_alt algorithm
-// from tests/BlendTest.cpp.  It's not quite perfect, but it's never wrong in the
-// interesting edge cases, and it's quite a bit faster than blend_perfect.
-//
-// blend_256_round_alt is our currently blessed algorithm.  Please use it or an analogous one.
+/* SSE2 version of Color32(), portable version is in core/SkBlitRow_D32.cpp */
+// Color32 and its SIMD specializations use the blend_perfect algorithm from tests/BlendTest.cpp.
+// An acceptable alternative is blend_256_round_alt, which is faster but not quite perfect.
 void Color32_SSE2(SkPMColor dst[], const SkPMColor src[], int count, SkPMColor color) {
     switch (SkGetPackedA32(color)) {
         case   0: memmove(dst, src, count * sizeof(SkPMColor)); return;
         case 255: sk_memset32(dst, color, count);               return;
     }
 
-    __m128i colorHigh = _mm_unpacklo_epi8(_mm_setzero_si128(), _mm_set1_epi32(color));
-#ifdef SK_SUPPORT_LEGACY_COLOR32_MATH  // blend_256_plus1_trunc, busted
-    __m128i colorAndRound = colorHigh;
-#else                          // blend_256_round_alt, good
-    __m128i colorAndRound = _mm_add_epi16(colorHigh, _mm_set1_epi16(128));
-#endif
-
-    unsigned invA = 255 - SkGetPackedA32(color);
-#ifdef SK_SUPPORT_LEGACY_COLOR32_MATH  // blend_256_plus1_trunc, busted
-    __m128i invA16 = _mm_set1_epi16(invA);
-#else                          // blend_256_round_alt, good
-    SkASSERT(invA + (invA >> 7) < 256);  // We should still fit in the low byte here.
-    __m128i invA16 = _mm_set1_epi16(invA + (invA >> 7));
-#endif
+    __m128i color_2x_high = _mm_unpacklo_epi8(_mm_setzero_si128(), _mm_set1_epi32(color)),
+             invA_8x      = _mm_set1_epi16(255 - SkGetPackedA32(color));
 
     // Does the core work of blending color onto 4 pixels, returning the resulting 4 pixels.
-    auto kernel = [&](const __m128i& src4) -> __m128i {
-        __m128i lo = _mm_mullo_epi16(invA16, _mm_unpacklo_epi8(src4, _mm_setzero_si128())),
-                hi = _mm_mullo_epi16(invA16, _mm_unpackhi_epi8(src4, _mm_setzero_si128()));
-        return _mm_packus_epi16(_mm_srli_epi16(_mm_add_epi16(colorAndRound, lo), 8),
-                                _mm_srli_epi16(_mm_add_epi16(colorAndRound, hi), 8));
+    auto kernel = [&](const __m128i& src_4x) -> __m128i {
+        __m128i lo = _mm_mullo_epi16(invA_8x, _mm_unpacklo_epi8(src_4x, _mm_setzero_si128())),
+                hi = _mm_mullo_epi16(invA_8x, _mm_unpackhi_epi8(src_4x, _mm_setzero_si128()));
+    #ifndef SK_SUPPORT_LEGACY_COLOR32_MATH
+        lo = _mm_add_epi16(lo, _mm_set1_epi16(128));
+        hi = _mm_add_epi16(hi, _mm_set1_epi16(128));
+        lo = _mm_add_epi16(lo, _mm_srli_epi16(lo, 8));
+        hi = _mm_add_epi16(hi, _mm_srli_epi16(hi, 8));
+    #endif
+        return _mm_packus_epi16(_mm_srli_epi16(_mm_add_epi16(color_2x_high, lo), 8),
+                                _mm_srli_epi16(_mm_add_epi16(color_2x_high, hi), 8));
     };
 
     while (count >= 8) {