// Copyright 2025 Google LLC
//
// This source code is licensed under the BSD-style license found in the
// LICENSE file in the root directory of this source tree.

#ifndef XNNPACK_YNNPACK_BASE_FP16_H_
#define XNNPACK_YNNPACK_BASE_FP16_H_

#include <stdint.h>

// This file is an excerpt from
// https://github.com/Maratyszcza/FP16/blob/master/include/fp16/fp16.h,
// including only the minimal functionality we need in XNNPACK. This works
// around some issues that we haven't been able to fix upstream
// (https://github.com/Maratyszcza/FP16/pull/32). See also:
// - https://github.com/microsoft/onnxruntime/pull/22294/files
// - https://github.com/google/XNNPACK/issues/6989
// We also don't need a lot of the functionality in the upstream library.

static inline float fp32_from_bits(uint32_t w) {
  union {
    uint32_t as_bits;
    float as_value;
  } fp32 = {w};
  return fp32.as_value;
}

static inline uint32_t fp32_to_bits(float f) {
  union {
    float as_value;
    uint32_t as_bits;
  } fp32 = {f};
  return fp32.as_bits;
}

/*
 * Convert a 16-bit floating-point number in IEEE half-precision format, in bit
 * representation, to a 32-bit floating-point number in IEEE single-precision
 * format.
 *
 * @note The implementation relies on IEEE-like (no assumption about rounding
 * mode and no operations on denormals) floating-point operations and bitcasts
 * between integer and floating-point variables.
 */
static inline float fp16_ieee_to_fp32_value(uint16_t h) {
  /*
   * Extend the half-precision floating-point number to 32 bits and shift to the
   * upper part of the 32-bit word:
   *      +---+-----+------------+-------------------+
   *      | S |EEEEE|MM MMMM MMMM|0000 0000 0000 0000|
   *      +---+-----+------------+-------------------+
   * Bits  31  26-30    16-25            0-15
   *
   * S - sign bit, E - bits of the biased exponent, M - bits of the mantissa, 0
   * - zero bits.
   */
  const uint32_t w = (uint32_t)h << 16;
  /*
   * Extract the sign of the input number into the high bit of the 32-bit word:
   *
   *      +---+----------------------------------+
   *      | S |0000000 00000000 00000000 00000000|
   *      +---+----------------------------------+
   * Bits  31                 0-31
   */
  const uint32_t sign = w & UINT32_C(0x80000000);
  /*
   * Extract mantissa and biased exponent of the input number into the high bits
   * of the 32-bit word:
   *
   *      +-----+------------+---------------------+
   *      |EEEEE|MM MMMM MMMM|0 0000 0000 0000 0000|
   *      +-----+------------+---------------------+
   * Bits  27-31    17-26            0-16
   */
  const uint32_t two_w = w + w;

  /*
   * Shift mantissa and exponent into bits 23-28 and bits 13-22 so they become
   * mantissa and exponent of a single-precision floating-point number:
   *
   *       S|Exponent |          Mantissa
   *      +-+---+-----+------------+----------------+
   *      |0|000|EEEEE|MM MMMM MMMM|0 0000 0000 0000|
   *      +-+---+-----+------------+----------------+
   * Bits   | 23-31   |           0-22
   *
   * Next, there are some adjustments to the exponent:
   * - The exponent needs to be corrected by the difference in exponent bias
   * between single-precision and half-precision formats (0x7F - 0xF = 0x70)
   * - Inf and NaN values in the inputs should become Inf and NaN values after
   * conversion to the single-precision number. Therefore, if the biased
   * exponent of the half-precision input was 0x1F (max possible value), the
   * biased exponent of the single-precision output must be 0xFF (max possible
   * value). We do this correction in two steps:
   *   - First, we adjust the exponent by (0xFF - 0x1F) = 0xE0 (see exp_offset
   * below) rather than by 0x70 suggested by the difference in the exponent bias
   * (see above).
   *   - Then we multiply the single-precision result of exponent adjustment by
   * 2**(-112) to reverse the effect of exponent adjustment by 0xE0 less the
   * necessary exponent adjustment by 0x70 due to difference in exponent bias.
   *     The floating-point multiplication hardware would ensure than Inf and
   * NaN would retain their value on at least partially IEEE754-compliant
   * implementations.
   *
   * Note that the above operations do not handle denormal inputs (where biased
   * exponent == 0). However, they also do not operate on denormal inputs, and
   * do not produce denormal results.
   */
  const uint32_t exp_offset = UINT32_C(0xE0) << 23;
#if defined(__STDC_VERSION__) && (__STDC_VERSION__ >= 199901L) || \
    defined(__GNUC__) && !defined(__STRICT_ANSI__)
  const float exp_scale = 0x1.0p-112f;
#else
  const float exp_scale = fp32_from_bits(UINT32_C(0x7800000));
#endif
  const float normalized_value =
      fp32_from_bits((two_w >> 4) + exp_offset) * exp_scale;

  /*
   * Convert denormalized half-precision inputs into single-precision results
   * (always normalized). Zero inputs are also handled here.
   *
   * In a denormalized number the biased exponent is zero, and mantissa has
   * on-zero bits. First, we shift mantissa into bits 0-9 of the 32-bit word.
   *
   *                  zeros           |  mantissa
   *      +---------------------------+------------+
   *      |0000 0000 0000 0000 0000 00|MM MMMM MMMM|
   *      +---------------------------+------------+
   * Bits             10-31                0-9
   *
   * Now, remember that denormalized half-precision numbers are represented as:
   *    FP16 = mantissa * 2**(-24).
   * The trick is to construct a normalized single-precision number with the
   * same mantissa and thehalf-precision input and with an exponent which would
   * scale the corresponding mantissa bits to 2**(-24). A normalized
   * single-precision floating-point number is represented as: FP32 = (1 +
   * mantissa * 2**(-23)) * 2**(exponent - 127) Therefore, when the biased
   * exponent is 126, a unit change in the mantissa of the input denormalized
   * half-precision number causes a change of the constructud single-precision
   * number by 2**(-24), i.e. the same ammount.
   *
   * The last step is to adjust the bias of the constructed single-precision
   * number. When the input half-precision number is zero, the constructed
   * single-precision number has the value of FP32 = 1 * 2**(126 - 127) =
   * 2**(-1) = 0.5 Therefore, we need to subtract 0.5 from the constructed
   * single-precision number to get the numerical equivalent of the input
   * half-precision number.
   */
  const uint32_t magic_mask = UINT32_C(126) << 23;
  const float magic_bias = 0.5f;
  const float denormalized_value =
      fp32_from_bits((two_w >> 17) | magic_mask) - magic_bias;

  /*
   * - Choose either results of conversion of input as a normalized number, or
   * as a denormalized number, depending on the input exponent. The variable
   * two_w contains input exponent in bits 27-31, therefore if its smaller than
   * 2**27, the input is either a denormal number, or zero.
   * - Combine the result of conversion of exponent and mantissa with the sign
   * of the input number.
   */
  const uint32_t denormalized_cutoff = UINT32_C(1) << 27;
  const uint32_t result =
      sign | (two_w < denormalized_cutoff ? fp32_to_bits(denormalized_value)
                                          : fp32_to_bits(normalized_value));
  return fp32_from_bits(result);
}

/*
 * Convert a 32-bit floating-point number in IEEE single-precision format to a
 * 16-bit floating-point number in IEEE half-precision format, in bit
 * representation.
 *
 * @note The implementation relies on IEEE-like (no assumption about rounding
 * mode and no operations on denormals) floating-point operations and bitcasts
 * between integer and floating-point variables.
 */
static inline uint16_t fp16_ieee_from_fp32_value(float f) {
#if defined(__STDC_VERSION__) && (__STDC_VERSION__ >= 199901L) || \
    defined(__GNUC__) && !defined(__STRICT_ANSI__)
  const float scale_to_inf = 0x1.0p+112f;
  const float scale_to_zero = 0x1.0p-110f;
#else
  const float scale_to_inf = fp32_from_bits(UINT32_C(0x77800000));
  const float scale_to_zero = fp32_from_bits(UINT32_C(0x08800000));
#endif
  const uint32_t w = fp32_to_bits(f);
  const float abs_f = fp32_from_bits(w & UINT32_C(0x7FFFFFFF));
  float base = (abs_f * scale_to_inf) * scale_to_zero;

  const uint32_t shl1_w = w + w;
  const uint32_t sign = w & UINT32_C(0x80000000);
  uint32_t bias = shl1_w & UINT32_C(0xFF000000);
  if (bias < UINT32_C(0x71000000)) {
    bias = UINT32_C(0x71000000);
  }

  base = fp32_from_bits((bias >> 1) + UINT32_C(0x07800000)) + base;
  const uint32_t bits = fp32_to_bits(base);
  const uint32_t exp_bits = (bits >> 13) & UINT32_C(0x00007C00);
  const uint32_t mantissa_bits = bits & UINT32_C(0x00000FFF);
  const uint32_t nonsign = exp_bits + mantissa_bits;
  return (sign >> 16) |
         (shl1_w > UINT32_C(0xFF000000) ? UINT16_C(0x7E00) : nonsign);
}

#endif  // XNNPACK_YNNPACK_BASE_FP16_H_