Use ryg's much faster fp16 conversion code.

[movit] / fp16.h

diff --git a/fp16.h b/fp16.h
index 9f7eff926fca594d54198b14b3ec30d834124508..5f71fec593d8924deaee981a6aa3100d035171c1 100644 (file)
--- a/fp16.h
+++ b/fp16.h
@@ -1,26 +1,69 @@
 #ifndef _MOVIT_FP16_H
 #define _MOVIT_FP16_H 1
 
+#ifdef __F16C__
+#include <immintrin.h>
+#endif
+
 // Code for converting to and from fp16 (from fp64), without any particular
 // machine support, with proper IEEE round-to-even behavior (and correct
 // 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 {
 
-typedef unsigned int fp32_int_t;
-typedef unsigned short fp16_int_t;
+// structs instead of ints, so that they are not implicitly convertible.
+struct fp32_int_t {
+       unsigned int val;
+};
+struct fp16_int_t {
+       unsigned short val;
+};
+
+#ifdef __F16C__
+
+// Use the f16c instructions from Haswell if available (and we know that they
+// are at compile time).
+static inline float fp16_to_fp32(fp16_int_t x)
+{
+       return _cvtsh_ss(x.val);
+}
+
+static inline fp16_int_t fp32_to_fp16(float x)
+{
+       fp16_int_t ret;
+       ret.val = _cvtss_sh(x, 0);
+       return ret;
+}
+
+#else
+
+float fp16_to_fp32(fp16_int_t x);
+fp16_int_t fp32_to_fp16(float x);
+
+#endif
+
+// Overloads for use in templates.
+static inline float to_fp32(double x) { return x; }
+static inline float to_fp32(float x) { return x; }
+static inline float to_fp32(fp16_int_t x) { return fp16_to_fp32(x); }
+
+template<class T> inline T from_fp32(float x);
+template<> inline double from_fp32<double>(float x) { return x; }
+template<> inline float from_fp32<float>(float x) { return x; }
+template<> inline fp16_int_t from_fp32<fp16_int_t>(float x) { return fp32_to_fp16(x); }
 
-double fp16_to_fp64(fp16_int_t x);
-fp16_int_t fp64_to_fp16(double x);
+template<class From, class To>
+inline To convert_float(From x) { return from_fp32<To>(to_fp32(x)); }
 
-// These are not very useful by themselves, but are implemented using the same
-// code as the fp16 ones (just with different constants), so they are useful
-// for verifying against the FPU in unit tests.
-double fp32_to_fp64(fp32_int_t x);
-fp32_int_t fp64_to_fp32(double x);
+template<class Same>
+inline Same convert_float(Same x) { return x; }
 
 }  // namespace movit