For those who would like to use portable code for float-to-half conversions, rather than the platform-specific means that I pointed out earlier, you may want to consider the code below. I only implemented the round-to-nearest-or-even variant. I tested this exhaustively against the device function __float2half_rn() on an sm_50 GPU, which maps to the F2F.F16.F32 instruction.
/*
Copyright (c) 2015, Norbert Juffa
All rights reserved.
Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions
are met:
1. Redistributions of source code must retain the above copyright
notice, this list of conditions and the following disclaimer.
2. Redistributions in binary form must reproduce the above copyright
notice, this list of conditions and the following disclaimer in the
documentation and/or other materials provided with the distribution.
THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/
__fp16 uint16_as_fp16 (uint16_t a)
{
__fp16 res;
#if defined (__cplusplus)
memcpy (&res, &a, sizeof (res));
#else /* __cplusplus */
volatile union {
__fp16 f;
uint16_t i;
} cvt;
cvt.i = a;
res = cvt.f;
#endif /* __cplusplus */
return res;
}
uint32_t fp32_as_uint32 (float a)
{
uint32_t res;
#if defined (__cplusplus)
memcpy (&res, &a, sizeof (res));
#else /* __cplusplus */
volatile union {
float f;
uint32_t i;
} cvt;
cvt.f = a;
res = cvt.i;
#endif /* __cplusplus */
return res;
}
/* host version of device function __float2half_rn() */
__fp16 float2half_rn (float a)
{
uint32_t ia = fp32_as_uint32 (a);
uint16_t ir;
ir = (ia >> 16) & 0x8000;
if ((ia & 0x7f800000) == 0x7f800000) {
if ((ia & 0x7fffffff) == 0x7f800000) {
ir |= 0x7c00; /* infinity */
} else {
ir = 0x7fff; /* canonical NaN */
}
} else if ((ia & 0x7f800000) >= 0x33000000) {
int shift = (int)((ia >> 23) & 0xff) - 127;
if (shift > 15) {
ir |= 0x7c00; /* infinity */
} else {
ia = (ia & 0x007fffff) | 0x00800000; /* extract mantissa */
if (shift < -14) { /* denormal */
ir |= ia >> (-1 - shift);
ia = ia << (32 - (-1 - shift));
} else { /* normal */
ir |= ia >> (24 - 11);
ia = ia << (32 - (24 - 11));
ir = ir + ((14 + shift) << 10);
}
/* IEEE-754 round to nearest of even */
if ((ia > 0x80000000) || ((ia == 0x80000000) && (ir & 1))) {
ir++;
}
}
}
return uint16_as_fp16 (ir);
}