1/*
2 * Copyright (C) 2021 Alyssa Rosenzweig <alyssa@rosenzweig.io>
3 *
4 * Permission is hereby granted, free of charge, to any person obtaining a
5 * copy of this software and associated documentation files (the "Software"),
6 * to deal in the Software without restriction, including without limitation
7 * the rights to use, copy, modify, merge, publish, distribute, sublicense,
8 * and/or sell copies of the Software, and to permit persons to whom the
9 * Software is furnished to do so, subject to the following conditions:
10 *
11 * The above copyright notice and this permission notice (including the next
12 * paragraph) shall be included in all copies or substantial portions of the
13 * Software.
14 *
15 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
16 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
17 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
18 * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
19 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
20 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
21 * SOFTWARE.
22 */
23
24#ifndef __AGX_MINIFLOAT_H_
25#define __AGX_MINIFLOAT_H_
26
27#include <math.h>
28#include "util/macros.h"
29
30/* AGX includes an 8-bit floating-point format for small dyadic immediates,
31 * consisting of 3 bits for the exponent, 4 bits for the mantissa, and 1-bit
32 * for sign, in the usual order. Zero exponent has special handling. */
33
34static inline float
35agx_minifloat_decode(uint8_t imm)
36{
37   float sign = (imm & 0x80) ? -1.0 : 1.0;
38   signed exp = (imm & 0x70) >> 4;
39   unsigned mantissa = (imm & 0xF);
40
41   if (exp)
42      return ldexpf(sign * (float) (mantissa | 0x10), exp - 7);
43   else
44      return ldexpf(sign * ((float) mantissa), -6);
45}
46
47/* Encodes a float. Results are only valid if the float can be represented
48 * exactly, if not the result of this function is UNDEFINED. signbit() is used
49 * to ensure -0.0 is handled correctly. */
50
51static inline uint8_t
52agx_minifloat_encode(float f)
53{
54   unsigned sign = signbit(f) ? 0x80 : 0;
55   f = fabsf(f);
56
57   /* frac is in [0.5, 1) and f = frac * 2^exp */
58   int exp = 0;
59   float frac = frexpf(f, &exp);
60
61   if (f >= 0.25) {
62      unsigned mantissa = (frac * 32.0);
63      exp -= 5; /* 2^5 = 32 */
64      exp = CLAMP(exp + 7, 0, 7);
65
66      assert(mantissa >= 0x10 && mantissa < 0x20);
67      assert(exp >= 1);
68
69      return sign | (exp << 4) | (mantissa & 0xF);
70   } else {
71      unsigned mantissa = (f * 64.0f);
72      assert(mantissa < 0x10);
73
74      return sign | mantissa;
75   }
76}
77
78static inline bool
79agx_minifloat_exact(float f)
80{
81   float f_ = agx_minifloat_decode(agx_minifloat_encode(f));
82   return memcmp(&f, &f_, sizeof(float)) == 0;
83}
84
85#ifndef NDEBUG
86static inline void
87agx_minifloat_tests(void)
88{
89   /* Decode some representative values */
90   assert(agx_minifloat_decode(0) == 0.0f);
91   assert(agx_minifloat_decode(25) == 0.390625f);
92   assert(agx_minifloat_decode(135) == -0.109375f);
93   assert(agx_minifloat_decode(255) == -31.0);
94
95   /* Verify exactness */
96   assert(agx_minifloat_exact(0.0f));
97   assert(agx_minifloat_exact(0.390625f));
98   assert(agx_minifloat_exact(-0.109375f));
99   assert(agx_minifloat_exact(-31.0));
100   assert(!agx_minifloat_exact(3.141f));
101   assert(!agx_minifloat_exact(2.718f));
102   assert(!agx_minifloat_exact(1.618f));
103
104   /* Check that all values round trip */
105   for (unsigned i = 0; i < 0x100; ++i) {
106      float f = agx_minifloat_decode(i);
107      assert(agx_minifloat_encode(f) == i);
108      assert(agx_minifloat_exact(f));
109   }
110}
111#endif
112
113#endif
114