Improve naive SkColorXform to half floats

src/core/SkXfermodeF16.cpp

 /*
  * Copyright 2016 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkHalf.h"
 #include "SkPM4fPriv.h"
 #include "SkUtils.h"
 #include "SkXfermode.h"
 
 static Sk4f lerp_by_coverage(const Sk4f& src, const Sk4f& dst, uint8_t srcCoverage) {
     return dst + (src - dst) * Sk4f(srcCoverage * (1/255.0f));
 }
 
 ///////////////////////////////////////////////////////////////////////////////////////////////////
 
 static void xfer_1(const SkXfermode* xfer, uint64_t dst[], const SkPM4f* src, int count,
                        const SkAlpha aa[]) {
     SkXfermodeProc4f proc = xfer->getProc4f();
     SkPM4f d;
     if (aa) {
         for (int i = 0; i < count; ++i) {
             Sk4f d4 = SkHalfToFloat_finite(dst[i]);
             d4.store(d.fVec);
             Sk4f r4 = Sk4f::Load(proc(*src, d).fVec);
-            dst[i] = SkFloatToHalf_finite(lerp_by_coverage(r4, d4, aa[i]));
+            SkFloatToHalf_finite(lerp_by_coverage(r4, d4, aa[i])).store(&dst[i]);
         }
     } else {
         for (int i = 0; i < count; ++i) {
             SkHalfToFloat_finite(dst[i]).store(d.fVec);
             Sk4f r4 = Sk4f::Load(proc(*src, d).fVec);
-            dst[i] = SkFloatToHalf_finite(r4);
+            SkFloatToHalf_finite(r4).store(&dst[i]);
         }
     }
 }
 
 static void xfer_n(const SkXfermode* xfer, uint64_t dst[], const SkPM4f src[], int count,
                        const SkAlpha aa[]) {
     SkXfermodeProc4f proc = xfer->getProc4f();
     SkPM4f d;
     if (aa) {
         for (int i = 0; i < count; ++i) {
             Sk4f d4 = SkHalfToFloat_finite(dst[i]);
             d4.store(d.fVec);
             Sk4f r4 = Sk4f::Load(proc(src[i], d).fVec);
-            dst[i] = SkFloatToHalf_finite(lerp_by_coverage(r4, d4, aa[i]));
+            SkFloatToHalf_finite(lerp_by_coverage(r4, d4, aa[i])).store(&dst[i]);
         }
     } else {
         for (int i = 0; i < count; ++i) {
             SkHalfToFloat_finite(dst[i]).store(d.fVec);
             Sk4f r4 = Sk4f::Load(proc(src[i], d).fVec);
-            dst[i] = SkFloatToHalf_finite(r4);
+            SkFloatToHalf_finite(r4).store(&dst[i]);
         }
     }
 }
 
 const SkXfermode::F16Proc gProcs_General[] = { xfer_n, xfer_n, xfer_1, xfer_1 };
 
 ///////////////////////////////////////////////////////////////////////////////////////////////////
 
 static void clear(const SkXfermode*, uint64_t dst[], const SkPM4f*, int count, const SkAlpha aa[]) {
     if (aa) {
         for (int i = 0; i < count; ++i) {
             if (aa[i]) {
                 const Sk4f d4 = SkHalfToFloat_finite(dst[i]);
-                dst[i] = SkFloatToHalf_finite(d4 * Sk4f((255 - aa[i]) * 1.0f/255));
+                SkFloatToHalf_finite(d4 * Sk4f((255 - aa[i]) * 1.0f/255)).store(&dst[i]);
             }
         }
     } else {
         sk_memset64(dst, 0, count);
     }
 }
 
 const SkXfermode::F16Proc gProcs_Clear[] = { clear, clear, clear, clear };
 
 ///////////////////////////////////////////////////////////////////////////////////////////////////
 
 static void src_1(const SkXfermode*, uint64_t dst[], const SkPM4f* src, int count,
                   const SkAlpha aa[]) {
     const Sk4f s4 = Sk4f::Load(src->fVec);
     if (aa) {
         for (int i = 0; i < count; ++i) {
             const Sk4f d4 = SkHalfToFloat_finite(dst[i]);
-            dst[i] = SkFloatToHalf_finite(lerp_by_coverage(s4, d4, aa[i]));
+            SkFloatToHalf_finite(lerp_by_coverage(s4, d4, aa[i])).store(&dst[i]);
         }
     } else {
-        sk_memset64(dst, SkFloatToHalf_finite(s4), count);
+        uint64_t s4h;
+        SkFloatToHalf_finite(s4).store(&s4h);
+        sk_memset64(dst, s4h, count);
     }
 }
 
 static void src_n(const SkXfermode*, uint64_t dst[], const SkPM4f src[], int count,
                   const SkAlpha aa[]) {
     if (aa) {
         for (int i = 0; i < count; ++i) {
             const Sk4f s4 = Sk4f::Load(src[i].fVec);
             const Sk4f d4 = SkHalfToFloat_finite(dst[i]);
-            dst[i] = SkFloatToHalf_finite(lerp_by_coverage(s4, d4, aa[i]));
+            SkFloatToHalf_finite(lerp_by_coverage(s4, d4, aa[i])).store(&dst[i]);
         }
     } else {
         for (int i = 0; i < count; ++i) {
             const Sk4f s4 = Sk4f::Load(src[i].fVec);
-            dst[i] = SkFloatToHalf_finite(s4);
+            SkFloatToHalf_finite(s4).store(&dst[i]);
         }
     }
 }
 
 const SkXfermode::F16Proc gProcs_Src[] = { src_n, src_n, src_1,  src_1 };
 
 ///////////////////////////////////////////////////////////////////////////////////////////////////
 
 static void dst(const SkXfermode*, uint64_t*, const SkPM4f*, int count, const SkAlpha[]) {}
 
 const SkXfermode::F16Proc gProcs_Dst[] = { dst, dst, dst, dst };
 
 ///////////////////////////////////////////////////////////////////////////////////////////////////
 
 static void srcover_1(const SkXfermode*, uint64_t dst[], const SkPM4f* src, int count,
                       const SkAlpha aa[]) {
     const Sk4f s4 = Sk4f::Load(src->fVec);
     const Sk4f dst_scale = Sk4f(1 - get_alpha(s4));
     for (int i = 0; i < count; ++i) {
         const Sk4f d4 = SkHalfToFloat_finite(dst[i]);
         const Sk4f r4 = s4 + d4 * dst_scale;
         if (aa) {
-            dst[i] = SkFloatToHalf_finite(lerp_by_coverage(r4, d4, aa[i]));
+            SkFloatToHalf_finite(lerp_by_coverage(r4, d4, aa[i])).store(&dst[i]);
         } else {
-            dst[i] = SkFloatToHalf_finite(r4);
+            SkFloatToHalf_finite(r4).store(&dst[i]);
         }
     }
 }
 
 static void srcover_n(const SkXfermode*, uint64_t dst[], const SkPM4f src[], int count,
                       const SkAlpha aa[]) {
     for (int i = 0; i < count; ++i) {
         Sk4f s = Sk4f::Load(src+i),
              d = SkHalfToFloat_finite(dst[i]),
              r = s + d*(1.0f - SkNx_shuffle<3,3,3,3>(s));
         if (aa) {
             r = lerp_by_coverage(r, d, aa[i]);
         }
-        dst[i] = SkFloatToHalf_finite(r);
+        SkFloatToHalf_finite(r).store(&dst[i]);
     }
 }
 
 const SkXfermode::F16Proc gProcs_SrcOver[] = { srcover_n, src_n, srcover_1, src_1 };
 
 ///////////////////////////////////////////////////////////////////////////////////////////////////
 
 static SkXfermode::F16Proc find_proc(SkXfermode::Mode mode, uint32_t flags) {
     SkASSERT(0 == (flags & ~3));
     flags &= 3;
@@ skipping 13 matching lines @@
     SkASSERT(0 == (flags & ~3));
     flags &= 3;
 
     Mode mode;
     return this->asMode(&mode) ? find_proc(mode, flags) : gProcs_General[flags];
 }
 
 SkXfermode::F16Proc SkXfermode::GetF16Proc(SkXfermode* xfer, uint32_t flags) {
     return xfer ? xfer->onGetF16Proc(flags) : find_proc(SkXfermode::kSrcOver_Mode, flags);
 }