mesa/src/asahi/compiler/agx_minifloat.h

/*
 * Copyright 2021 Alyssa Rosenzweig
 * SPDX-License-Identifier: MIT
 */

#ifndef __AGX_MINIFLOAT_H_
#define __AGX_MINIFLOAT_H_

#include <math.h>
#include "util/macros.h"

/* AGX includes an 8-bit floating-point format for small dyadic immediates,
 * consisting of 3 bits for the exponent, 4 bits for the mantissa, and 1-bit
 * for sign, in the usual order. Zero exponent has special handling. */

static inline float
agx_minifloat_decode(uint8_t imm)
{
   float sign = (imm & 0x80) ? -1.0 : 1.0;
   signed exp = (imm & 0x70) >> 4;
   unsigned mantissa = (imm & 0xF);

   if (exp)
      return ldexpf(sign * (float)(mantissa | 0x10), exp - 7);
   else
      return ldexpf(sign * ((float)mantissa), -6);
}

/* Encodes a float. Results are only valid if the float can be represented
 * exactly, if not the result of this function is UNDEFINED. However, it is
 * guaranteed that this function will not crash on out-of-spec inputs, so it is
 * safe to call on any input. signbit() is used to ensure -0.0 is handled
 * correctly.
 */
static inline uint8_t
agx_minifloat_encode(float f)
{
   unsigned sign = signbit(f) ? 0x80 : 0;
   f = fabsf(f);

   /* frac is in [0.5, 1) and f = frac * 2^exp */
   int exp = 0;
   float frac = frexpf(f, &exp);

   if (f >= 0.25) {
      unsigned mantissa = (frac * 32.0);
      exp -= 5; /* 2^5 = 32 */
      exp = CLAMP(exp + 7, 0, 7);

      return sign | (exp << 4) | (mantissa & 0xF);
   } else {
      unsigned mantissa = (f * 64.0f);

      return sign | mantissa;
   }
}

static inline bool
agx_minifloat_exact(float f)
{
   float f_ = agx_minifloat_decode(agx_minifloat_encode(f));
   return memcmp(&f, &f_, sizeof(float)) == 0;
}

#endif