Convert a loop to range-based for.

[movit] / fp16.h

diff --git a/fp16.h b/fp16.h
index 49828bb62af922b6069c3884f765c2ad64c96094..535eccf36b04c181f08cc432b3fca5785ca54da4 100644 (file)
--- a/fp16.h
+++ b/fp16.h
@@ -10,7 +10,11 @@
 // handling of NaNs and infinities). This is needed because some OpenGL
 // drivers don't properly round off when asked to convert data themselves.
 //
-// These routines are not particularly fast.
+// These routines are originally written by Fabian Giesen, and released by
+// him into the public domain;
+// see https://fgiesen.wordpress.com/2012/03/28/half-to-float-done-quic/.
+// They are quite fast, and can be vectorized if need be; of course, using
+// the f16c instructions (see below) will be faster still.
 
 namespace movit {
 
@@ -40,8 +44,79 @@ static inline fp16_int_t fp32_to_fp16(float x)
 
 #else
 
-float fp16_to_fp32(fp16_int_t x);
-fp16_int_t fp32_to_fp16(float x);
+union fp32 {
+       float f;
+       unsigned int u;
+};
+
+static inline float fp16_to_fp32(fp16_int_t h)
+{
+       fp32 magic; magic.u = 113 << 23;
+       unsigned int shifted_exp = 0x7c00 << 13;  // exponent mask after shift
+       fp32 o;
+
+       // mantissa+exponent
+       unsigned int shifted = (h.val & 0x7fff) << 13;
+       unsigned int exponent = shifted & shifted_exp;
+
+       // exponent cases
+       o.u = shifted;
+       if (exponent == 0) {  // Zero / Denormal
+               o.u += magic.u;
+               o.f -= magic.f;
+       } else if (exponent == shifted_exp) {  // Inf/NaN
+               o.u += (255 - 31) << 23;
+       } else {
+               o.u += (127 - 15) << 23;
+       }
+
+       o.u |= (h.val & 0x8000) << 16;  // copy sign bit
+       return o.f;
+}
+
+static inline fp16_int_t fp32_to_fp16(float x)
+{
+       fp32 f; f.f = x;
+       unsigned int f32infty = 255 << 23;
+       unsigned int f16max = (127 + 16) << 23;
+       fp32 denorm_magic; denorm_magic.u = ((127 - 15) + (23 - 10) + 1) << 23;
+       unsigned int sign_mask = 0x80000000u;
+       fp16_int_t o = { 0 };
+
+       unsigned int sign = f.u & sign_mask;
+       f.u ^= sign;
+
+       // NOTE all the integer compares in this function can be safely
+       // compiled into signed compares since all operands are below
+       // 0x80000000. Important if you want fast straight SSE2 code
+       // (since there's no unsigned PCMPGTD).
+
+       if (f.u >= f16max) {  // result is Inf or NaN (all exponent bits set)
+               o.val = (f.u > f32infty) ? 0x7e00 : 0x7c00; // NaN->qNaN and Inf->Inf
+       } else {  // (De)normalized number or zero
+               if (f.u < (113 << 23)) {  // resulting FP16 is subnormal or zero
+                       // use a magic value to align our 10 mantissa bits at the bottom of
+                       // the float. as long as FP addition is round-to-nearest-even this
+                       // just works.
+                       f.f += denorm_magic.f;
+
+                       // and one integer subtract of the bias later, we have our final float!
+                       o.val = f.u - denorm_magic.u;
+               } else {
+                       unsigned int mant_odd = (f.u >> 13) & 1; // resulting mantissa is odd
+
+                       // update exponent, rounding bias part 1
+                       f.u += (unsigned(15 - 127) << 23) + 0xfff;
+                       // rounding bias part 2
+                       f.u += mant_odd;
+                       // take the bits!
+                       o.val = f.u >> 13;
+               }
+       }
+
+       o.val |= sign >> 16;
+       return o;
+}
 
 #endif