revise row blits to keep intermediate precision so that color is preserved when blended against its…

src/opts/SkBlitRow_opts_arm_neon.cpp

Index: src/opts/SkBlitRow_opts_arm_neon.cpp
diff --git a/src/opts/SkBlitRow_opts_arm_neon.cpp b/src/opts/SkBlitRow_opts_arm_neon.cpp
index 3cb5a92f86b1d9310a47a62af9582099c6bc770b..7998a8951771423c34722c2284dc58394a75e983 100644
--- a/src/opts/SkBlitRow_opts_arm_neon.cpp
+++ b/src/opts/SkBlitRow_opts_arm_neon.cpp
@@ -909,7 +909,12 @@ void S32_Blend_BlitRow32_neon(SkPMColor* SK_RESTRICT dst,
         vdst_wide = vmull_u8(vdst, vdup_n_u8(dst_scale));
 
         // Combine
+#ifdef SK_SUPPORT_LEGACY_BROKEN_LERP
         vres = vshrn_n_u16(vdst_wide, 8) + vshrn_n_u16(vsrc_wide, 8);
+#else
+        vdst_wide += vsrc_wide;
+        vres = vshrn_n_u16(vdst_wide, 8);
+#endif
 
         // Store
         vst1_u32(dst, vreinterpret_u32_u8(vres));
@@ -931,7 +936,12 @@ void S32_Blend_BlitRow32_neon(SkPMColor* SK_RESTRICT dst,
         vsrc_wide = vmovl_u8(vsrc);
         vsrc_wide = vmulq_u16(vsrc_wide, vdupq_n_u16(src_scale));
         vdst_wide = vmull_u8(vdst, vdup_n_u8(dst_scale));
+#ifdef SK_SUPPORT_LEGACY_BROKEN_LERP
         vres = vshrn_n_u16(vdst_wide, 8) + vshrn_n_u16(vsrc_wide, 8);
+#else
+        vdst_wide += vsrc_wide;
+        vres = vshrn_n_u16(vdst_wide, 8);
+#endif
 
         // Store
         vst1_lane_u32(dst, vreinterpret_u32_u8(vres), 0);
@@ -943,7 +953,7 @@ void S32A_Blend_BlitRow32_neon(SkPMColor* SK_RESTRICT dst,
                          const SkPMColor* SK_RESTRICT src,
                          int count, U8CPU alpha) {
 
-    SkASSERT(255 >= alpha);
+    SkASSERT(255 > alpha);
 
     if (count <= 0) {
         return;
@@ -963,9 +973,7 @@ void S32A_Blend_BlitRow32_neon(SkPMColor* SK_RESTRICT dst,
 
         // Calc dst_scale
         dst_scale = vget_lane_u8(vsrc, 3);
-        dst_scale *= alpha256;
-        dst_scale >>= 8;
-        dst_scale = 256 - dst_scale;
+        dst_scale = SkAlphaMulInv256(dst_scale, alpha256);
 
         // Process src
         vsrc_wide = vmovl_u8(vsrc);
@@ -976,7 +984,12 @@ void S32A_Blend_BlitRow32_neon(SkPMColor* SK_RESTRICT dst,
         vdst_wide = vmulq_n_u16(vdst_wide, dst_scale);
 
         // Combine
+#ifdef SK_SUPPORT_LEGACY_BROKEN_LERP
         vres = vshrn_n_u16(vdst_wide, 8) + vshrn_n_u16(vsrc_wide, 8);
+#else
+        vdst_wide += vsrc_wide;
+        vres = vshrn_n_u16(vdst_wide, 8);
+#endif
 
         vst1_lane_u32(dst, vreinterpret_u32_u8(vres), 0);
         dst++;
@@ -1007,9 +1020,20 @@ void S32A_Blend_BlitRow32_neon(SkPMColor* SK_RESTRICT dst,
             // Calc dst_scale
             vsrc_alphas = vtbl1_u8(vsrc, alpha_mask);
             vdst_scale = vmovl_u8(vsrc_alphas);
+#ifdef SK_SUPPORT_LEGACY_BROKEN_LERP
             vdst_scale *= vsrc_scale;
             vdst_scale = vshrq_n_u16(vdst_scale, 8);
             vdst_scale = vsubq_u16(vdupq_n_u16(256), vdst_scale);
+#else
+            // Calculate SkAlphaMulInv256(vdst_scale, vsrc_scale).
+            // A 16-bit lane would overflow if we used 0xFFFF here,
+            // so use an approximation with 0xFF00 that is off by 1,
+            // and add back 1 after to get the correct value.
+            // This is valid if alpha256 <= 255.
+            vdst_scale = vmlsq_u16(vdupq_n_u16(0xFF00), vdst_scale, vsrc_scale);
+            vdst_scale = vsraq_n_u16(vdst_scale, vdst_scale, 8);
+            vdst_scale = vsraq_n_u16(vdupq_n_u16(1), vdst_scale, 8);
+#endif
 
             // Process src
             vsrc_wide = vmovl_u8(vsrc);
@@ -1020,7 +1044,12 @@ void S32A_Blend_BlitRow32_neon(SkPMColor* SK_RESTRICT dst,
             vdst_wide *= vdst_scale;
 
             // Combine
+#ifdef SK_SUPPORT_LEGACY_BROKEN_LERP
             vres = vshrn_n_u16(vdst_wide, 8) + vshrn_n_u16(vsrc_wide, 8);
+#else
+            vdst_wide += vsrc_wide;
+            vres = vshrn_n_u16(vdst_wide, 8);
+#endif
 
             vst1_u32(dst, vreinterpret_u32_u8(vres));