Flush denorm half floats to zero.

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;
 
 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 Sk4h SkFloatToHalf_finite(const Sk4f&);
+// assuming inputs and outputs are both finite, and
+// flushing values which would be denormal half floats to zero.
+static inline Sk4f SkHalfToFloat_finite_ftz(uint64_t);
+static inline Sk4h SkFloatToHalf_finite_ftz(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(const Sk4h& hs) {
+static inline Sk4f SkHalfToFloat_finite_ftz(const Sk4h& hs) {
 #if !defined(SKNX_NO_SIMD) && defined(SK_CPU_ARM64)
     float32x4_t fs;
     asm ("fcvtl %[fs].4s, %[hs].4h   \n"   // vcvt_f32_f16(...)
         : [fs] "=w" (fs)                   // =w: write-only NEON register
         : [hs] "w" (hs.fVec));             //  w: read-only NEON register
     return fs;
 #else
-    Sk4i bits      = SkNx_cast<int>(hs),   // Expand to 32 bit.
-         sign      = bits & 0x00008000,    // Save the sign bit for later...
-         positive  = bits ^ sign,          // ...but strip it off for now.
-         is_denorm = positive < (1<<10);   // Exponent == 0?
+    Sk4i bits     = SkNx_cast<int>(hs),  // Expand to 32 bit.
+         sign     = bits & 0x00008000,   // Save the sign bit for later...
+         positive = bits ^ sign,         // ...but strip it off for now.
+         is_norm  = 0x03ff < positive;   // Exponent > 0?
 
     // For normal half floats, extend the mantissa by 13 zero bits,
     // then adjust the exponent from 15 bias to 127 bias.
     Sk4i norm = (positive << 13) + ((127 - 15) << 23);
 
-    // For denorm half floats, mask in the exponent-only float K that turns our
-    // 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);
+    Sk4i merged = (sign << 16) | (norm & is_norm);
     return Sk4f::Load(&merged);
 #endif
 }
 
-static inline Sk4f SkHalfToFloat_finite(uint64_t hs) {
-    return SkHalfToFloat_finite(Sk4h::Load(&hs));
+static inline Sk4f SkHalfToFloat_finite_ftz(uint64_t hs) {
+    return SkHalfToFloat_finite_ftz(Sk4h::Load(&hs));
 }
 
-static inline Sk4h SkFloatToHalf_finite(const Sk4f& fs) {
+static inline Sk4h SkFloatToHalf_finite_ftz(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)
         : [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?
+    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_norm = 0x387fdfff < positive;  // greater than largest denorm 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);
+    Sk4i merged = (sign >> 16) | (will_be_norm & norm);
     return SkNx_cast<uint16_t>(merged);
 #endif
 }
 
 #endif