Improve naive SkColorXform to half floats

src/core/SkHalf.h

Index: src/core/SkHalf.h
diff --git a/src/core/SkHalf.h b/src/core/SkHalf.h
index 2f2ed66c6a64016ee0ae512f92c2876298f2988e..5a95ab9a1ce253569081c7dad0d3f691720c3cc8 100644
--- a/src/core/SkHalf.h
+++ b/src/core/SkHalf.h
@@ -65,14 +65,12 @@ static inline Sk4f SkHalfToFloat_finite(uint64_t hs) {
 #endif
 }
 
-static inline uint64_t SkFloatToHalf_finite(const Sk4f& fs) {
-    uint64_t r;
+static inline Sk4h SkFloatToVectorHalf_finite(const Sk4f& fs) {
 #if !defined(SKNX_NO_SIMD) && defined(SK_CPU_ARM64)
     float32x4_t vec = fs.fVec;
     asm ("fcvtn %[vec].4h, %[vec].4s  \n"   // vcvt_f16_f32(vec)
-         "fmov  %[r], %d[vec]         \n"   // vst1_f16(&r, ...)
-        : [r] "=r" (r)                      // =r: write-only 64-bit general register
-        , [vec] "+w" (vec));                // +w: read-write NEON register
+        : [vec] "+w" (vec));                // +w: read-write NEON register
+    return vreinterpret_u16_f32(vget_low_f32(vec));

[mtklein, 2016/07/19 12:45:46]

Duh, nice.

 #else
     Sk4i bits           = Sk4i::Load(&fs),
          sign           = bits & 0x80000000,              // Save the sign bit for later...
@@ -91,8 +89,14 @@ static inline uint64_t SkFloatToHalf_finite(const Sk4f& fs) {
     Sk4i denorm = Sk4i::Load(&plus_K) ^ K;
 
     Sk4i merged = (sign >> 16) | will_be_denorm.thenElse(denorm, norm);
-    SkNx_cast<uint16_t>(merged).store(&r);
+    return SkNx_cast<uint16_t>(merged);
 #endif
+}
+
+static inline uint64_t SkFloatToHalf_finite(const Sk4f& fs) {

[mtklein, 2016/07/19 12:45:45]

Let's port over the other uses and call your new signature SkFloatToHalf_finite.
 They'll also likely benefit.

[msarett, 2016/07/19 15:24:49]

Done.

+    uint64_t r;
+    Sk4h v = SkFloatToVectorHalf_finite(fs);
+    v.store(&r);
     return r;
 }