Convert SkPMFloat to [0,1] range and prune its API.

src/opts/SkPMFloat_neon.h

Index: src/opts/SkPMFloat_neon.h
diff --git a/src/opts/SkPMFloat_neon.h b/src/opts/SkPMFloat_neon.h
index cabb29a2a80ee8b399878a21e26aa723f0c90e31..57f613751d1e657264325be1ad620f9d5b911c34 100644
--- a/src/opts/SkPMFloat_neon.h
+++ b/src/opts/SkPMFloat_neon.h
@@ -7,70 +7,24 @@
 
 namespace { // See SkPMFloat.h
 
-// For SkPMFloat(SkPMFColor), we widen our 8 bit components (fix8) to 8-bit components in 16 bits
-// (fix8_16), then widen those to 8-bit-in-32-bits (fix8_32), and finally convert those to floats.
-
-// round() and roundClamp() do the opposite, working from floats to 8-bit-in-32-bit,
-// to 8-bit-in-16-bit, back down to 8-bit components.
-// roundClamp() uses vqmovn to clamp while narrowing instead of just narrowing with vmovn.
-
 inline SkPMFloat::SkPMFloat(SkPMColor c) {
     SkPMColorAssert(c);
     uint8x8_t   fix8    = (uint8x8_t)vdup_n_u32(c);
     uint16x8_t  fix8_16 = vmovl_u8(fix8);
     uint32x4_t  fix8_32 = vmovl_u16(vget_low_u16(fix8_16));
-    fVec = vcvtq_f32_u32(fix8_32);
+    fVec = vcvtq_n_f32_u32(fix8_32, 8);
     SkASSERT(this->isValid());
 }
 
-inline SkPMColor SkPMFloat::trunc() const {
-    uint32x4_t  fix8_32  = vcvtq_u32_f32(fVec);  // vcvtq_u32_f32 truncates
-    uint16x4_t  fix8_16  = vmovn_u32(fix8_32);
-    uint8x8_t   fix8     = vmovn_u16(vcombine_u16(fix8_16, vdup_n_u16(0)));
-    SkPMColor c = vget_lane_u32((uint32x2_t)fix8, 0);
-    SkPMColorAssert(c);
-    return c;
-}
-
 inline SkPMColor SkPMFloat::round() const {
-    return SkPMFloat(Sk4f(0.5f) + *this).trunc();
-}
-
-inline SkPMColor SkPMFloat::roundClamp() const {
-    float32x4_t add_half = vaddq_f32(fVec, vdupq_n_f32(0.5f));
-    uint32x4_t  fix8_32  = vcvtq_u32_f32(add_half);  // vcvtq_u32_f32 truncates, so round manually
-    uint16x4_t  fix8_16  = vqmovn_u32(fix8_32);
-    uint8x8_t   fix8     = vqmovn_u16(vcombine_u16(fix8_16, vdup_n_u16(0)));
+    // vcvtq_n_u32_f32 truncates, so we round manually by adding a half before converting.
+    float32x4_t rounded = vaddq_f32(fVec, vdupq_n_f32(0.5f/255));
+    uint32x4_t  fix8_32 = vcvtq_n_u32_f32(rounded, 8);
+    uint16x4_t  fix8_16 = vqmovn_u32(fix8_32);
+    uint8x8_t   fix8    = vqmovn_u16(vcombine_u16(fix8_16, vdup_n_u16(0)));
     SkPMColor c = vget_lane_u32((uint32x2_t)fix8, 0);
     SkPMColorAssert(c);
     return c;
 }
 
-// TODO: we should be able to beat these loops on all three methods.
-inline void SkPMFloat::From4PMColors(const SkPMColor colors[4],
-                                     SkPMFloat* a, SkPMFloat* b, SkPMFloat* c, SkPMFloat* d) {
-    *a = FromPMColor(colors[0]);
-    *b = FromPMColor(colors[1]);
-    *c = FromPMColor(colors[2]);
-    *d = FromPMColor(colors[3]);
-}
-
-inline void SkPMFloat::RoundTo4PMColors(
-        const SkPMFloat& a, const SkPMFloat& b, const SkPMFloat&c, const SkPMFloat& d,
-        SkPMColor colors[4]) {
-    colors[0] = a.round();
-    colors[1] = b.round();
-    colors[2] = c.round();
-    colors[3] = d.round();
-}
-
-inline void SkPMFloat::RoundClampTo4PMColors(
-        const SkPMFloat& a, const SkPMFloat& b, const SkPMFloat&c, const SkPMFloat& d,
-        SkPMColor colors[4]) {
-    colors[0] = a.roundClamp();
-    colors[1] = b.roundClamp();
-    colors[2] = c.roundClamp();
-    colors[3] = d.roundClamp();
-}
-
 }  // namespace