fp8/amd/hip_float8_impl.h

#pragma once

#if defined(__HIPCC__) && \
    (defined(__gfx940__) || defined(__gfx941__) || defined(__gfx942__))
  #define __HIP__MI300__
#endif

#ifdef __HIPCC__
  #define HIP_FP8_HOST_DEVICE __host__ __device__
  #define HIP_FP8_HOST __host__
  #define HIP_FP8_DEVICE __device__
#else
  #define HIP_FP8_HOST_DEVICE
  #define HIP_FP8_HOST
  #define HIP_FP8_DEVICE
#endif

namespace hip_fp8_impl {

#ifdef __HIP__MI300__
HIP_FP8_DEVICE uint8_t to_fp8_from_fp32(float v) {
  uint8_t i8data;
  union {
    float fval;
    uint32_t i32val;
    uint8_t i8val[4];  // NOTE: not endian independent
  } val;

  uint32_t ival = 0;
  val.fval = v;

  if ((val.i32val & 0x7F800000) !=
      0x7F800000) {  /// propagate NAN/INF, no clipping
    val.fval = __builtin_amdgcn_fmed3f(val.fval, 240.0, -240.0);
  }

  ival = __builtin_amdgcn_cvt_pk_fp8_f32(val.fval, val.fval, ival,
                                         false);  // false -> WORD0
  val.i32val = ival;
  i8data = val.i8val[0];

  return i8data;
}
#endif  // __HIP__MI300__

HIP_FP8_HOST inline int clz(uint32_t x) { return __builtin_clz(x); }
#if defined(__HIPCC__) || defined(__CUDA_ARCH__)
HIP_FP8_DEVICE inline int clz(uint32_t x) { return __clz(x); }
#endif

template <int we, int wm, typename T, bool negative_zero_nan, bool clip>
HIP_FP8_HOST_DEVICE uint8_t to_float8(T _x, bool stoch = false,
                                      uint32_t rng = 0) {
#ifdef __HIPCC__
  constexpr bool is_half = std::is_same<T, _Float16>::value;
#else
  constexpr bool is_half = false;
#endif
  constexpr bool is_float = std::is_same<T, float>::value;
  static_assert(wm + we == 7, "wm+we==7");
  static_assert(is_half || is_float, "Only half and float can be cast to f8");

  const int mfmt = (sizeof(T) == 4) ? 23 : 10;
  uint32_t x;
  if (sizeof(T) == 4) {
    x = reinterpret_cast<uint32_t&>(_x);
  } else {
    x = reinterpret_cast<uint16_t&>(_x);
  }

  uint32_t head, mantissa;
  int exponent, bias;
  uint32_t sign;

  if (sizeof(T) == 4) {
    head = x & 0xFF800000;
    mantissa = x & 0x7FFFFF;
    exponent = (head >> 23) & 0xFF;
    sign = head >> 31;
    bias = 127;
  } else {
    head = x & 0xFC00;
    mantissa = x & 0x3FF;
    exponent = (head >> 10) & 0x1F;
    sign = head >> 15;
    bias = 15;
  }

  uint32_t signed_inf = (sign << 7) + (((1 << we) - 1) << wm);

  // Deal with inf and NaNs
  if (negative_zero_nan) {
    if (sizeof(T) == 4) {
      if ((x & 0x7F800000) == 0x7F800000) {
        return 0x80;
      }
    } else {
      // if(__hisinf(x) || __hisnan(x))
      if ((x & 0x7C00) == 0x7C00) {
        return 0x80;
      }
    }
  } else {
    if (sizeof(T) == 4) {
      if ((x & 0x7F800000) == 0x7F800000) {
        return signed_inf + (mantissa != 0 ? 1 : 0);
      }
    } else {
      if ((x & 0x7C00) == 0x7C00) {
        return signed_inf + (mantissa != 0 ? 1 : 0);
      }
    }
  }
  if (x == 0) {
    return 0;
  }

  // First need to check if it is normal or denorm as there is a difference of
  // implicit 1 Then need to adjust the exponent to align with the F8 exponent,
  // in the meanwhile, shift The mantissa. Then for stochastic rounding, add rng
  // to mantissa and truncate. And for RNE, no need to add rng. Then probably
  // need to check whether there is carry and adjust exponent and mantissa again

  // For IEEE bias mode, the bias is 2^(k-1) -1 where k is the width of exponent
  // bits
  const int f8_bias = (1 << (we - 1)) - 1 + (negative_zero_nan ? 1 : 0);
  const int f8_denormal_act_exponent =
      1 - f8_bias;  // actual exponent of f8 denormal
  // act_exponent is the actual exponent of fp32/fp16 (after subtracting bias)
  // f8_exponent is the converted f8 exponent with bias encoding
  // exponent_diff is the diff between fp32/fp16 exponent and f8 exponent,
  // the difference needs to be adjusted and mantissa shifted
  int act_exponent, f8_exponent, exponent_diff;

  if (exponent == 0) {  // fp32/fp16 is in denormal.
    /* fp32 denormal is below 2^-127 so it is usually not a concern here, we
mostly concern fp16 here. In this case, f8 is usually in denormal. But there
could be exceptions. fp16 denormal has exponent bias 15 while bf8 with NANOO has
exponent bias 16. It means that there are some numbers in fp16 denormal but they
are bf8 (NANOO) normals - smallest bf8 (NANOO) normal is 2^-15. fp16 numbers
where exponent==0 (actual exponent -14) and highest bit of mantissa is 1 are bf8
(NANOO) normal. In this case, the fp16 mantissa should be shift left by 1  */
    act_exponent = exponent - bias + 1;
    exponent_diff =
        f8_denormal_act_exponent -
        act_exponent;  // actual exponent is exponent-bias+1 as it is denormal
  } else {             // fp32/fp16 is normal with implicit 1
    act_exponent = exponent - bias;
    if (act_exponent <= f8_denormal_act_exponent) {
      /* This is the case where fp32/fp16 is normal but it is in f8 denormal
range. For example fp8 nanoo mode, denormal exponent is -7, but if the
fp32/fp16 actual exponent is -7, it is actually larger due to the implicit 1,
Therefore it needs to be adjust to -6 and mantissa shift right by 1.
So for fp32/fp16, exponent -8 is the cut point to convert to fp8 nanoo */
      exponent_diff = f8_denormal_act_exponent - act_exponent;
    } else {              // both fp32/fp16 and f8 are in normal range
      exponent_diff = 0;  // exponent_diff=0 does not mean there is no
                          // difference for this case, act_exponent could be
                          // larger. Just that it does not need shift mantissa
    }
    mantissa += (1 << mfmt);  // Add the implicit 1 into mantissa
  }

  bool midpoint = (mantissa & ((1 << (mfmt - wm + exponent_diff)) - 1)) ==
                  static_cast<uint32_t>(1 << (mfmt - wm + exponent_diff - 1));
  /* This part is a bit tricky. The judgment of whether it is a tie needs to be
 done before we shift right as shift right could rip off some residual part
 and make something not midpoint look like midpoint. For example, the fp16
 number 0x1002 (0 00100 0000000010), it is larger than midpoint, but after
 shift right by 4 bits, it would look like midpoint.
*/

  if (exponent_diff > 0) {
    mantissa >>= exponent_diff;
  } else if (exponent_diff == -1) {
    mantissa <<= -exponent_diff;
  }
  bool implicit_one = mantissa & (1 << mfmt);
  // if there is no implicit 1, it  means the f8 is denormal and need to adjust
  // to denorm exponent
  f8_exponent = (act_exponent + exponent_diff) /*actual f8 exponent*/ +
                f8_bias - (implicit_one ? 0 : 1);

  // Now we have the exponent and mantissa adjusted
  uint32_t drop_mask = (1 << (mfmt - wm)) - 1;
  bool odd = mantissa & (1 << (mfmt - wm));  // if the least significant bit
                                             // that is not truncated is 1
  mantissa +=
      (stoch ? rng : (midpoint ? (odd ? mantissa : mantissa - 1) : mantissa)) &
      drop_mask;

  // Now we deal with overflow
  if (f8_exponent == 0) {
    if ((1 << mfmt) & mantissa) {
      f8_exponent = 1;  // denormal overflow to become normal, promote exponent
    }
  } else {
    if ((1 << (mfmt + 1)) & mantissa) {
      mantissa >>= 1;
      f8_exponent++;
    }
  }

  mantissa >>= (mfmt - wm);

  // above range: quantize to maximum possible float of the same sign
  const int max_exp = (1 << we) - (negative_zero_nan ? 1 : 2);
  if (f8_exponent > max_exp) {
    if (clip) {
      mantissa = (1 << wm) - 1;
      f8_exponent = max_exp;
    } else {
      return signed_inf;
    }
  }

  if (f8_exponent == 0 && mantissa == 0) {
    return negative_zero_nan ? 0 : (sign << 7);
  }
  mantissa &= (1 << wm) - 1;
  return (sign << 7) | (f8_exponent << wm) | mantissa;
}

template <int we, int wm, typename T = float, bool negative_zero_nan = true>
inline HIP_FP8_HOST_DEVICE T from_float8(uint8_t x) {
#ifdef __HIPCC__
  constexpr bool is_half = std::is_same<T, _Float16>::value;
#else
  constexpr bool is_half = false;
#endif
  constexpr bool is_float = std::is_same<T, float>::value;
  static_assert(is_half || is_float, "only half and float are supported");

  constexpr int weo = is_half ? 5 : 8;
  constexpr int wmo = is_half ? 10 : (is_float ? 23 : 7);

  T fInf, fNegInf, fNaN, fNeg0;

#ifdef __HIPCC__
  if (is_half) {
    const uint16_t ihInf = 0x7C00;
    const uint16_t ihNegInf = 0xFC00;
    const uint16_t ihNaN = 0x7C01;
    const uint16_t ihNeg0 = 0x8000;
    fInf = reinterpret_cast<const _Float16&>(ihInf);
    fNegInf = reinterpret_cast<const _Float16&>(ihNegInf);
    fNaN = reinterpret_cast<const _Float16&>(ihNaN);
    fNeg0 = reinterpret_cast<const _Float16&>(ihNeg0);
  } else
#endif
      if (is_float) {
    const uint32_t ifInf = 0x7F800000;
    const uint32_t ifNegInf = 0xFF800000;
    const uint32_t ifNaN = 0x7F800001;
    const uint32_t ifNeg0 = 0x80000000;
    fInf = reinterpret_cast<const float&>(ifInf);
    fNegInf = reinterpret_cast<const float&>(ifNegInf);
    fNaN = reinterpret_cast<const float&>(ifNaN);
    fNeg0 = reinterpret_cast<const float&>(ifNeg0);
  }

  if (x == 0) {
    return 0;
  }

  uint32_t sign = x >> 7;
  uint32_t mantissa = x & ((1 << wm) - 1);
  int exponent = (x & 0x7F) >> wm;
  if (negative_zero_nan) {
    if (x == 0x80) {
      return fNaN;
    }
  } else {
    if (x == 0x80) {
      return fNeg0;
    }
    if (exponent == ((1 << we) - 1)) {
      return (mantissa == 0) ? (sign ? fNegInf : fInf) : fNaN;
    }
  }
  typename std::conditional<sizeof(T) == 2, uint16_t, uint32_t>::type retval;
  if (we == 5 && is_half && !negative_zero_nan) {
    retval = x << 8;
    return reinterpret_cast<const T&>(retval);
  }

  const int exp_low_cutoff =
      (1 << (weo - 1)) - (1 << (we - 1)) + 1 - (negative_zero_nan ? 1 : 0);

  // subnormal input
  if (exponent == 0) {
    // guaranteed mantissa!=0 since cases 0x0 and 0x80 are handled above
    int sh = 1 + clz(mantissa) - (32 - wm);
    mantissa <<= sh;
    exponent += 1 - sh;
    mantissa &= ((1 << wm) - 1);
  }
  exponent += exp_low_cutoff - 1;
  mantissa <<= wmo - wm;

  // subnormal output (occurs when T=half, we=5, negative_zero_nan=true)
  if (exponent <= 0) {
    mantissa |= 1 << wmo;
    mantissa >>= 1 - exponent;
    exponent = 0;
  }

  if (sizeof(T) == 2) {
    retval = (sign << 15) | (exponent << 10) | mantissa;
  } else {
    retval = (sign << 31) | (exponent << 23) | mantissa;
  }
  return reinterpret_cast<const T&>(retval);
}

}  // namespace hip_fp8_impl