Sample code to use __fp16 in HCC. Adopted __gnu_f2h_ieee() and __gnu_h2f_ieee() from compiler-rt in LLVM

fp_extend.cpp

#include <limits.h>
#include <stdint.h>

typedef uint16_t src_t;
typedef uint16_t src_rep_t;
#define SRC_REP_C UINT16_C
static const int srcSigBits = 10;
#define src_rep_t_clz __builtin_clz

typedef float dst_t;
typedef uint32_t dst_rep_t;
#define DST_REP_C UINT32_C
static const int dstSigBits = 23;

// End of specialization parameters.  Two helper routines for conversion to and
// from the representation of floating-point data as integer values follow.

static __inline src_rep_t srcToRep(src_t x) {
    const union { src_t f; src_rep_t i; } rep = {.f = x};
    return rep.i;
}

static __inline dst_t dstFromRep(dst_rep_t x) {
    const union { dst_t f; dst_rep_t i; } rep = {.i = x};
    return rep.f;
}
// End helper routines.  Conversion implementation follows.

static __inline dst_t __extendXfYf2__(src_t a) {
    // Various constants whose values follow from the type parameters.
    // Any reasonable optimizer will fold and propagate all of these.
    const int srcBits = sizeof(src_t)*CHAR_BIT;
    const int srcExpBits = srcBits - srcSigBits - 1;
    const int srcInfExp = (1 << srcExpBits) - 1;
    const int srcExpBias = srcInfExp >> 1;

    const src_rep_t srcMinNormal = SRC_REP_C(1) << srcSigBits;
    const src_rep_t srcInfinity = (src_rep_t)srcInfExp << srcSigBits;
    const src_rep_t srcSignMask = SRC_REP_C(1) << (srcSigBits + srcExpBits);
    const src_rep_t srcAbsMask = srcSignMask - 1;
    const src_rep_t srcQNaN = SRC_REP_C(1) << (srcSigBits - 1);
    const src_rep_t srcNaNCode = srcQNaN - 1;

    const int dstBits = sizeof(dst_t)*CHAR_BIT;
    const int dstExpBits = dstBits - dstSigBits - 1;
    const int dstInfExp = (1 << dstExpBits) - 1;
    const int dstExpBias = dstInfExp >> 1;

    const dst_rep_t dstMinNormal = DST_REP_C(1) << dstSigBits;

    // Break a into a sign and representation of the absolute value
    const src_rep_t aRep = srcToRep(a);
    const src_rep_t aAbs = aRep & srcAbsMask;
    const src_rep_t sign = aRep & srcSignMask;
    dst_rep_t absResult;

    // If sizeof(src_rep_t) < sizeof(int), the subtraction result is promoted
    // to (signed) int.  To avoid that, explicitly cast to src_rep_t.
    if ((src_rep_t)(aAbs - srcMinNormal) < srcInfinity - srcMinNormal) {
        // a is a normal number.
        // Extend to the destination type by shifting the significand and
        // exponent into the proper position and rebiasing the exponent.
        absResult = (dst_rep_t)aAbs << (dstSigBits - srcSigBits);
        absResult += (dst_rep_t)(dstExpBias - srcExpBias) << dstSigBits;
    }

    else if (aAbs >= srcInfinity) {
        // a is NaN or infinity.
        // Conjure the result by beginning with infinity, then setting the qNaN
        // bit (if needed) and right-aligning the rest of the trailing NaN
        // payload field.
        absResult = (dst_rep_t)dstInfExp << dstSigBits;
        absResult |= (dst_rep_t)(aAbs & srcQNaN) << (dstSigBits - srcSigBits);
        absResult |= (dst_rep_t)(aAbs & srcNaNCode) << (dstSigBits - srcSigBits);
    }

    else if (aAbs) {
        // a is denormal.
        // renormalize the significand and clear the leading bit, then insert
        // the correct adjusted exponent in the destination type.
        const int scale = src_rep_t_clz(aAbs) - src_rep_t_clz(srcMinNormal);
        absResult = (dst_rep_t)aAbs << (dstSigBits - srcSigBits + scale);
        absResult ^= dstMinNormal;
        const int resultExponent = dstExpBias - srcExpBias - scale + 1;
        absResult |= (dst_rep_t)resultExponent << dstSigBits;
    }

    else {
        // a is zero.
        absResult = 0;
    }

    // Apply the signbit to (dst_t)abs(a).
    const dst_rep_t result = absResult | (dst_rep_t)sign << (dstBits - srcBits);
    return dstFromRep(result);
}
// Use a forwarding definition and noinline to implement a poor man's alias,
// as there isn't a good cross-platform way of defining one.
__attribute__((noinline)) float __extendhfsf2(uint16_t a) {
    return __extendXfYf2__(a);
}

extern "C" float __gnu_h2f_ieee(uint16_t a) {
    return __extendhfsf2(a);
}

fp_trunc.cpp

#include <limits.h>
#include <stdint.h>

typedef float src_t;
typedef uint32_t src_rep_t;
#define SRC_REP_C UINT32_C
static const int srcSigBits = 23;

typedef uint16_t dst_t;
typedef uint16_t dst_rep_t;
#define DST_REP_C UINT16_C
static const int dstSigBits = 10;

// End of specialization parameters.  Two helper routines for conversion to and
// from the representation of floating-point data as integer values follow.

static __inline src_rep_t srcToRep(src_t x) {
    const union { src_t f; src_rep_t i; } rep = {.f = x};
    return rep.i;
}

static __inline dst_t dstFromRep(dst_rep_t x) {
    const union { dst_t f; dst_rep_t i; } rep = {.i = x};
    return rep.f;
}

static __inline dst_t __truncXfYf2__(src_t a) {
    // Various constants whose values follow from the type parameters.
    // Any reasonable optimizer will fold and propagate all of these.
    const int srcBits = sizeof(src_t)*CHAR_BIT;
    const int srcExpBits = srcBits - srcSigBits - 1;
    const int srcInfExp = (1 << srcExpBits) - 1;
    const int srcExpBias = srcInfExp >> 1;

    const src_rep_t srcMinNormal = SRC_REP_C(1) << srcSigBits;
    const src_rep_t srcSignificandMask = srcMinNormal - 1;
    const src_rep_t srcInfinity = (src_rep_t)srcInfExp << srcSigBits;
    const src_rep_t srcSignMask = SRC_REP_C(1) << (srcSigBits + srcExpBits);
    const src_rep_t srcAbsMask = srcSignMask - 1;
    const src_rep_t roundMask = (SRC_REP_C(1) << (srcSigBits - dstSigBits)) - 1;
    const src_rep_t halfway = SRC_REP_C(1) << (srcSigBits - dstSigBits - 1);
    const src_rep_t srcQNaN = SRC_REP_C(1) << (srcSigBits - 1);
    const src_rep_t srcNaNCode = srcQNaN - 1;

    const int dstBits = sizeof(dst_t)*CHAR_BIT;
    const int dstExpBits = dstBits - dstSigBits - 1;
    const int dstInfExp = (1 << dstExpBits) - 1;
    const int dstExpBias = dstInfExp >> 1;
    const int underflowExponent = srcExpBias + 1 - dstExpBias;
    const int overflowExponent = srcExpBias + dstInfExp - dstExpBias;
    const src_rep_t underflow = (src_rep_t)underflowExponent << srcSigBits;
    const src_rep_t overflow = (src_rep_t)overflowExponent << srcSigBits;

    const dst_rep_t dstQNaN = DST_REP_C(1) << (dstSigBits - 1);
    const dst_rep_t dstNaNCode = dstQNaN - 1;

    // Break a into a sign and representation of the absolute value
    const src_rep_t aRep = srcToRep(a);
    const src_rep_t aAbs = aRep & srcAbsMask;
    const src_rep_t sign = aRep & srcSignMask;
    dst_rep_t absResult;

    if (aAbs - underflow < aAbs - overflow) {
        // The exponent of a is within the range of normal numbers in the
        // destination format.  We can convert by simply right-shifting with
        // rounding and adjusting the exponent.
        absResult = aAbs >> (srcSigBits - dstSigBits);
        absResult -= (dst_rep_t)(srcExpBias - dstExpBias) << dstSigBits;

        const src_rep_t roundBits = aAbs & roundMask;
        // Round to nearest
        if (roundBits > halfway)
            absResult++;
        // Ties to even
        else if (roundBits == halfway)
            absResult += absResult & 1;
    }
    else if (aAbs > srcInfinity) {
        // a is NaN.
        // Conjure the result by beginning with infinity, setting the qNaN
        // bit and inserting the (truncated) trailing NaN field.
        absResult = (dst_rep_t)dstInfExp << dstSigBits;
        absResult |= dstQNaN;
        absResult |= ((aAbs & srcNaNCode) >> (srcSigBits - dstSigBits)) & dstNaNCode;
    }
    else if (aAbs >= overflow) {
        // a overflows to infinity.
        absResult = (dst_rep_t)dstInfExp << dstSigBits;
    }
    else {
        // a underflows on conversion to the destination type or is an exact
        // zero.  The result may be a denormal or zero.  Extract the exponent
        // to get the shift amount for the denormalization.
        const int aExp = aAbs >> srcSigBits;
        const int shift = srcExpBias - dstExpBias - aExp + 1;

        const src_rep_t significand = (aRep & srcSignificandMask) | srcMinNormal;

        // Right shift by the denormalization amount with sticky.
        if (shift > srcSigBits) {
            absResult = 0;
        } else {
            const bool sticky = significand << (srcBits - shift);
            src_rep_t denormalizedSignificand = significand >> shift | sticky;
            absResult = denormalizedSignificand >> (srcSigBits - dstSigBits);
            const src_rep_t roundBits = denormalizedSignificand & roundMask;
            // Round to nearest
            if (roundBits > halfway)
                absResult++;
            // Ties to even
            else if (roundBits == halfway)
                absResult += absResult & 1;
        }
    }

    // Apply the signbit to (dst_t)abs(a).
    const dst_rep_t result = absResult | sign >> (srcBits - dstBits);
    return dstFromRep(result);
}

// Use a forwarding definition and noinline to implement a poor man's alias,
// as there isn't a good cross-platform way of defining one.
__attribute__((noinline)) uint16_t __truncsfhf2(float a) {
    return __truncXfYf2__(a);
}

extern "C" uint16_t __gnu_f2h_ieee(float a) {
    return __truncsfhf2(a);
}

hcc-clang-upgrade.diff

diff --git a/lib/Frontend/CompilerInvocation.cpp b/lib/Frontend/CompilerInvocation.cpp
index c234cae..4ad9362 100644
--- a/lib/Frontend/CompilerInvocation.cpp
+++ b/lib/Frontend/CompilerInvocation.cpp
@@ -1470,6 +1470,12 @@ void CompilerInvocation::setLangDefaults(LangOptions &Opts, InputKind IK,
   Opts.ImplicitInt = Std.hasImplicitInt();
   Opts.CPlusPlusAMP = Std.isCPlusPlusAMP();
 
+  // Set C++AMP-specific defaults
+  if (Opts.CPlusPlusAMP) {
+    Opts.NativeHalfType = 1;
+    Opts.NativeHalfArgsAndReturns = 1;
+  }
+
   // Set OpenCL Version.
   Opts.OpenCL = LangStd == LangStandard::lang_opencl || IK == IK_OpenCL;
   if (LangStd == LangStandard::lang_opencl)

test_fp16.cpp

__fp16 foo(__fp16 a, __fp16 b) [[hc]][[cpu]] {
  return a + b;
}

int main() {
  __fp16 ret = foo(1.0, 2.0);

  return 0;
}

Build it with:

hcc `hcc-config --cxxflags --ldflags` test_fp16.cpp fp_trunc.cpp fp_extend.cpp -o foo

Need to use ToT-based HCC frontend (git@github.com:RadeonOpenCompute/hcc-clang-upgrade.git , develop branch), with patch (hcc-clang-upgrade.diff) in this gist applied.