Improve naive SkColorXform to half floats

src/core/SkHalf.h

 /*
  * Copyright 2014 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkHalf_DEFINED
 #define SkHalf_DEFINED
 
 #include "SkNx.h"
 #include "SkTypes.h"
 
 // 16-bit floating point value
 // format is 1 bit sign, 5 bits exponent, 10 bits mantissa
 // only used for storage
 typedef uint16_t SkHalf;
 
-#define SK_HalfMin      0x0400   // 2^-24  (minimum positive normal value)
-#define SK_HalfMax      0x7bff   // 65504
-#define SK_HalfEpsilon  0x1400   // 2^-10
+static constexpr uint16_t SK_HalfMin     = 0x0400; // 2^-24  (minimum positive normal value)
+static constexpr uint16_t SK_HalfMax     = 0x7bff; // 65504
+static constexpr uint16_t SK_HalfEpsilon = 0x1400; // 2^-10
+static constexpr uint16_t SK_Half1       = 0x3C00; // 1
 
 // convert between half and single precision floating point
 float SkHalfToFloat(SkHalf h);
 SkHalf SkFloatToHalf(float f);
 
 // Convert between half and single precision floating point,
 // assuming inputs and outputs are both finite.
-static inline     Sk4f SkHalfToFloat_finite(uint64_t);
-static inline uint64_t SkFloatToHalf_finite(const Sk4f&);
+static inline Sk4f SkHalfToFloat_finite(uint64_t);
+static inline Sk4h SkFloatToHalf_finite(const Sk4f&);
 
 // ~~~~~~~~~~~ impl ~~~~~~~~~~~~~~ //
 
 // Like the serial versions in SkHalf.cpp, these are based on
 // https://fgiesen.wordpress.com/2012/03/28/half-to-float-done-quic/
 
 // GCC 4.9 lacks the intrinsics to use ARMv8 f16<->f32 instructions, so we use inline assembly.
 
 static inline Sk4f SkHalfToFloat_finite(uint64_t hs) {
 #if !defined(SKNX_NO_SIMD) && defined(SK_CPU_ARM64)
@@ skipping 17 matching lines @@
     // denorm value V*2^-14 into a normalized float K + V*2^-14.  Then subtract off K.
     const Sk4i K = ((127-15) + (23-10) + 1) << 23;
     Sk4i mask_K = positive | K;
     Sk4f denorm = Sk4f::Load(&mask_K) - Sk4f::Load(&K);
 
     Sk4i merged = (sign << 16) | is_denorm.thenElse(Sk4i::Load(&denorm), norm);
     return Sk4f::Load(&merged);
 #endif
 }
 
-static inline uint64_t SkFloatToHalf_finite(const Sk4f& fs) {
-    uint64_t r;
+static inline Sk4h SkFloatToHalf_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));
 #else
     Sk4i bits           = Sk4i::Load(&fs),
          sign           = bits & 0x80000000,              // Save the sign bit for later...
          positive       = bits ^ sign,                    // ...but strip it off for now.
          will_be_denorm = positive < ((127-15+1) << 23);  // positve < smallest normal half?
 
     // For normal half floats, adjust the exponent from 127 bias to 15 bias,
     // then drop the bottom 13 mantissa bits.
     Sk4i norm = (positive - ((127 - 15) << 23)) >> 13;
 
     // This mechanically inverts the denorm half -> normal float conversion above.
     // Knowning that and reading its explanation will leave you feeling more confident
     // than reading my best attempt at explaining this directly.
     const Sk4i K = ((127-15) + (23-10) + 1) << 23;
     Sk4f plus_K = Sk4f::Load(&positive) + Sk4f::Load(&K);
     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
-    return r;
 }
 
 #endif