1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
// origin: FreeBSD /usr/src/lib/msun/src/s_exp2.c */
//-
// Copyright (c) 2005 David Schultz <das@FreeBSD.ORG>
// All rights reserved.
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions
// are met:
// 1. Redistributions of source code must retain the above copyright
//    notice, this list of conditions and the following disclaimer.
// 2. Redistributions in binary form must reproduce the above copyright
//    notice, this list of conditions and the following disclaimer in the
//    documentation and/or other materials provided with the distribution.
//
// THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
// ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
// IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
// ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
// FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
// DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
// OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
// HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
// LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
// OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
// SUCH DAMAGE.

use super::scalbn;

const TBLSIZE: usize = 256;

#[cfg_attr(rustfmt, rustfmt_skip)]
static TBL: [u64; TBLSIZE * 2] = [
    //  exp2(z + eps)          eps
    0x3fe6a09e667f3d5d, 0x3d39880000000000,
    0x3fe6b052fa751744, 0x3cd8000000000000,
    0x3fe6c012750bd9fe, 0xbd28780000000000,
    0x3fe6cfdcddd476bf, 0x3d1ec00000000000,
    0x3fe6dfb23c651a29, 0xbcd8000000000000,
    0x3fe6ef9298593ae3, 0xbcbc000000000000,
    0x3fe6ff7df9519386, 0xbd2fd80000000000,
    0x3fe70f7466f42da3, 0xbd2c880000000000,
    0x3fe71f75e8ec5fc3, 0x3d13c00000000000,
    0x3fe72f8286eacf05, 0xbd38300000000000,
    0x3fe73f9a48a58152, 0xbd00c00000000000,
    0x3fe74fbd35d7ccfc, 0x3d2f880000000000,
    0x3fe75feb564267f1, 0x3d03e00000000000,
    0x3fe77024b1ab6d48, 0xbd27d00000000000,
    0x3fe780694fde5d38, 0xbcdd000000000000,
    0x3fe790b938ac1d00, 0x3ce3000000000000,
    0x3fe7a11473eb0178, 0xbced000000000000,
    0x3fe7b17b0976d060, 0x3d20400000000000,
    0x3fe7c1ed0130c133, 0x3ca0000000000000,
    0x3fe7d26a62ff8636, 0xbd26900000000000,
    0x3fe7e2f336cf4e3b, 0xbd02e00000000000,
    0x3fe7f3878491c3e8, 0xbd24580000000000,
    0x3fe80427543e1b4e, 0x3d33000000000000,
    0x3fe814d2add1071a, 0x3d0f000000000000,
    0x3fe82589994ccd7e, 0xbd21c00000000000,
    0x3fe8364c1eb942d0, 0x3d29d00000000000,
    0x3fe8471a4623cab5, 0x3d47100000000000,
    0x3fe857f4179f5bbc, 0x3d22600000000000,
    0x3fe868d99b4491af, 0xbd32c40000000000,
    0x3fe879cad931a395, 0xbd23000000000000,
    0x3fe88ac7d98a65b8, 0xbd2a800000000000,
    0x3fe89bd0a4785800, 0xbced000000000000,
    0x3fe8ace5422aa223, 0x3d33280000000000,
    0x3fe8be05bad619fa, 0x3d42b40000000000,
    0x3fe8cf3216b54383, 0xbd2ed00000000000,
    0x3fe8e06a5e08664c, 0xbd20500000000000,
    0x3fe8f1ae99157807, 0x3d28280000000000,
    0x3fe902fed0282c0e, 0xbd1cb00000000000,
    0x3fe9145b0b91ff96, 0xbd05e00000000000,
    0x3fe925c353aa2ff9, 0x3cf5400000000000,
    0x3fe93737b0cdc64a, 0x3d17200000000000,
    0x3fe948b82b5f98ae, 0xbd09000000000000,
    0x3fe95a44cbc852cb, 0x3d25680000000000,
    0x3fe96bdd9a766f21, 0xbd36d00000000000,
    0x3fe97d829fde4e2a, 0xbd01000000000000,
    0x3fe98f33e47a23a3, 0x3d2d000000000000,
    0x3fe9a0f170ca0604, 0xbd38a40000000000,
    0x3fe9b2bb4d53ff89, 0x3d355c0000000000,
    0x3fe9c49182a3f15b, 0x3d26b80000000000,
    0x3fe9d674194bb8c5, 0xbcec000000000000,
    0x3fe9e86319e3238e, 0x3d17d00000000000,
    0x3fe9fa5e8d07f302, 0x3d16400000000000,
    0x3fea0c667b5de54d, 0xbcf5000000000000,
    0x3fea1e7aed8eb8f6, 0x3d09e00000000000,
    0x3fea309bec4a2e27, 0x3d2ad80000000000,
    0x3fea42c980460a5d, 0xbd1af00000000000,
    0x3fea5503b23e259b, 0x3d0b600000000000,
    0x3fea674a8af46213, 0x3d38880000000000,
    0x3fea799e1330b3a7, 0x3d11200000000000,
    0x3fea8bfe53c12e8d, 0x3d06c00000000000,
    0x3fea9e6b5579fcd2, 0xbd29b80000000000,
    0x3feab0e521356fb8, 0x3d2b700000000000,
    0x3feac36bbfd3f381, 0x3cd9000000000000,
    0x3fead5ff3a3c2780, 0x3ce4000000000000,
    0x3feae89f995ad2a3, 0xbd2c900000000000,
    0x3feafb4ce622f367, 0x3d16500000000000,
    0x3feb0e07298db790, 0x3d2fd40000000000,
    0x3feb20ce6c9a89a9, 0x3d12700000000000,
    0x3feb33a2b84f1a4b, 0x3d4d470000000000,
    0x3feb468415b747e7, 0xbd38380000000000,
    0x3feb59728de5593a, 0x3c98000000000000,
    0x3feb6c6e29f1c56a, 0x3d0ad00000000000,
    0x3feb7f76f2fb5e50, 0x3cde800000000000,
    0x3feb928cf22749b2, 0xbd04c00000000000,
    0x3feba5b030a10603, 0xbd0d700000000000,
    0x3febb8e0b79a6f66, 0x3d0d900000000000,
    0x3febcc1e904bc1ff, 0x3d02a00000000000,
    0x3febdf69c3f3a16f, 0xbd1f780000000000,
    0x3febf2c25bd71db8, 0xbd10a00000000000,
    0x3fec06286141b2e9, 0xbd11400000000000,
    0x3fec199bdd8552e0, 0x3d0be00000000000,
    0x3fec2d1cd9fa64ee, 0xbd09400000000000,
    0x3fec40ab5fffd02f, 0xbd0ed00000000000,
    0x3fec544778fafd15, 0x3d39660000000000,
    0x3fec67f12e57d0cb, 0xbd1a100000000000,
    0x3fec7ba88988c1b6, 0xbd58458000000000,
    0x3fec8f6d9406e733, 0xbd1a480000000000,
    0x3feca3405751c4df, 0x3ccb000000000000,
    0x3fecb720dcef9094, 0x3d01400000000000,
    0x3feccb0f2e6d1689, 0x3cf0200000000000,
    0x3fecdf0b555dc412, 0x3cf3600000000000,
    0x3fecf3155b5bab3b, 0xbd06900000000000,
    0x3fed072d4a0789bc, 0x3d09a00000000000,
    0x3fed1b532b08c8fa, 0xbd15e00000000000,
    0x3fed2f87080d8a85, 0x3d1d280000000000,
    0x3fed43c8eacaa203, 0x3d01a00000000000,
    0x3fed5818dcfba491, 0x3cdf000000000000,
    0x3fed6c76e862e6a1, 0xbd03a00000000000,
    0x3fed80e316c9834e, 0xbd0cd80000000000,
    0x3fed955d71ff6090, 0x3cf4c00000000000,
    0x3feda9e603db32ae, 0x3cff900000000000,
    0x3fedbe7cd63a8325, 0x3ce9800000000000,
    0x3fedd321f301b445, 0xbcf5200000000000,
    0x3fede7d5641c05bf, 0xbd1d700000000000,
    0x3fedfc97337b9aec, 0xbd16140000000000,
    0x3fee11676b197d5e, 0x3d0b480000000000,
    0x3fee264614f5a3e7, 0x3d40ce0000000000,
    0x3fee3b333b16ee5c, 0x3d0c680000000000,
    0x3fee502ee78b3fb4, 0xbd09300000000000,
    0x3fee653924676d68, 0xbce5000000000000,
    0x3fee7a51fbc74c44, 0xbd07f80000000000,
    0x3fee8f7977cdb726, 0xbcf3700000000000,
    0x3feea4afa2a490e8, 0x3ce5d00000000000,
    0x3feeb9f4867ccae4, 0x3d161a0000000000,
    0x3feecf482d8e680d, 0x3cf5500000000000,
    0x3feee4aaa2188514, 0x3cc6400000000000,
    0x3feefa1bee615a13, 0xbcee800000000000,
    0x3fef0f9c1cb64106, 0xbcfa880000000000,
    0x3fef252b376bb963, 0xbd2c900000000000,
    0x3fef3ac948dd7275, 0x3caa000000000000,
    0x3fef50765b6e4524, 0xbcf4f00000000000,
    0x3fef6632798844fd, 0x3cca800000000000,
    0x3fef7bfdad9cbe38, 0x3cfabc0000000000,
    0x3fef91d802243c82, 0xbcd4600000000000,
    0x3fefa7c1819e908e, 0xbd0b0c0000000000,
    0x3fefbdba3692d511, 0xbcc0e00000000000,
    0x3fefd3c22b8f7194, 0xbd10de8000000000,
    0x3fefe9d96b2a23ee, 0x3cee430000000000,
    0x3ff0000000000000, 0x0,
    0x3ff00b1afa5abcbe, 0xbcb3400000000000,
    0x3ff0163da9fb3303, 0xbd12170000000000,
    0x3ff02168143b0282, 0x3cba400000000000,
    0x3ff02c9a3e77806c, 0x3cef980000000000,
    0x3ff037d42e11bbca, 0xbcc7400000000000,
    0x3ff04315e86e7f89, 0x3cd8300000000000,
    0x3ff04e5f72f65467, 0xbd1a3f0000000000,
    0x3ff059b0d315855a, 0xbd02840000000000,
    0x3ff0650a0e3c1f95, 0x3cf1600000000000,
    0x3ff0706b29ddf71a, 0x3d15240000000000,
    0x3ff07bd42b72a82d, 0xbce9a00000000000,
    0x3ff0874518759bd0, 0x3ce6400000000000,
    0x3ff092bdf66607c8, 0xbd00780000000000,
    0x3ff09e3ecac6f383, 0xbc98000000000000,
    0x3ff0a9c79b1f3930, 0x3cffa00000000000,
    0x3ff0b5586cf988fc, 0xbcfac80000000000,
    0x3ff0c0f145e46c8a, 0x3cd9c00000000000,
    0x3ff0cc922b724816, 0x3d05200000000000,
    0x3ff0d83b23395dd8, 0xbcfad00000000000,
    0x3ff0e3ec32d3d1f3, 0x3d1bac0000000000,
    0x3ff0efa55fdfa9a6, 0xbd04e80000000000,
    0x3ff0fb66affed2f0, 0xbd0d300000000000,
    0x3ff1073028d7234b, 0x3cf1500000000000,
    0x3ff11301d0125b5b, 0x3cec000000000000,
    0x3ff11edbab5e2af9, 0x3d16bc0000000000,
    0x3ff12abdc06c31d5, 0x3ce8400000000000,
    0x3ff136a814f2047d, 0xbd0ed00000000000,
    0x3ff1429aaea92de9, 0x3ce8e00000000000,
    0x3ff14e95934f3138, 0x3ceb400000000000,
    0x3ff15a98c8a58e71, 0x3d05300000000000,
    0x3ff166a45471c3df, 0x3d03380000000000,
    0x3ff172b83c7d5211, 0x3d28d40000000000,
    0x3ff17ed48695bb9f, 0xbd05d00000000000,
    0x3ff18af9388c8d93, 0xbd1c880000000000,
    0x3ff1972658375d66, 0x3d11f00000000000,
    0x3ff1a35beb6fcba7, 0x3d10480000000000,
    0x3ff1af99f81387e3, 0xbd47390000000000,
    0x3ff1bbe084045d54, 0x3d24e40000000000,
    0x3ff1c82f95281c43, 0xbd0a200000000000,
    0x3ff1d4873168b9b2, 0x3ce3800000000000,
    0x3ff1e0e75eb44031, 0x3ceac00000000000,
    0x3ff1ed5022fcd938, 0x3d01900000000000,
    0x3ff1f9c18438cdf7, 0xbd1b780000000000,
    0x3ff2063b88628d8f, 0x3d2d940000000000,
    0x3ff212be3578a81e, 0x3cd8000000000000,
    0x3ff21f49917ddd41, 0x3d2b340000000000,
    0x3ff22bdda2791323, 0x3d19f80000000000,
    0x3ff2387a6e7561e7, 0xbd19c80000000000,
    0x3ff2451ffb821427, 0x3d02300000000000,
    0x3ff251ce4fb2a602, 0xbd13480000000000,
    0x3ff25e85711eceb0, 0x3d12700000000000,
    0x3ff26b4565e27d16, 0x3d11d00000000000,
    0x3ff2780e341de00f, 0x3d31ee0000000000,
    0x3ff284dfe1f5633e, 0xbd14c00000000000,
    0x3ff291ba7591bb30, 0xbd13d80000000000,
    0x3ff29e9df51fdf09, 0x3d08b00000000000,
    0x3ff2ab8a66d10e9b, 0xbd227c0000000000,
    0x3ff2b87fd0dada3a, 0x3d2a340000000000,
    0x3ff2c57e39771af9, 0xbd10800000000000,
    0x3ff2d285a6e402d9, 0xbd0ed00000000000,
    0x3ff2df961f641579, 0xbcf4200000000000,
    0x3ff2ecafa93e2ecf, 0xbd24980000000000,
    0x3ff2f9d24abd8822, 0xbd16300000000000,
    0x3ff306fe0a31b625, 0xbd32360000000000,
    0x3ff31432edeea50b, 0xbd70df8000000000,
    0x3ff32170fc4cd7b8, 0xbd22480000000000,
    0x3ff32eb83ba8e9a2, 0xbd25980000000000,
    0x3ff33c08b2641766, 0x3d1ed00000000000,
    0x3ff3496266e3fa27, 0xbcdc000000000000,
    0x3ff356c55f929f0f, 0xbd30d80000000000,
    0x3ff36431a2de88b9, 0x3d22c80000000000,
    0x3ff371a7373aaa39, 0x3d20600000000000,
    0x3ff37f26231e74fe, 0xbd16600000000000,
    0x3ff38cae6d05d838, 0xbd0ae00000000000,
    0x3ff39a401b713ec3, 0xbd44720000000000,
    0x3ff3a7db34e5a020, 0x3d08200000000000,
    0x3ff3b57fbfec6e95, 0x3d3e800000000000,
    0x3ff3c32dc313a8f2, 0x3cef800000000000,
    0x3ff3d0e544ede122, 0xbd17a00000000000,
    0x3ff3dea64c1234bb, 0x3d26300000000000,
    0x3ff3ec70df1c4ecc, 0xbd48a60000000000,
    0x3ff3fa4504ac7e8c, 0xbd3cdc0000000000,
    0x3ff40822c367a0bb, 0x3d25b80000000000,
    0x3ff4160a21f72e95, 0x3d1ec00000000000,
    0x3ff423fb27094646, 0xbd13600000000000,
    0x3ff431f5d950a920, 0x3d23980000000000,
    0x3ff43ffa3f84b9eb, 0x3cfa000000000000,
    0x3ff44e0860618919, 0xbcf6c00000000000,
    0x3ff45c2042a7d201, 0xbd0bc00000000000,
    0x3ff46a41ed1d0016, 0xbd12800000000000,
    0x3ff4786d668b3326, 0x3d30e00000000000,
    0x3ff486a2b5c13c00, 0xbd2d400000000000,
    0x3ff494e1e192af04, 0x3d0c200000000000,
    0x3ff4a32af0d7d372, 0xbd1e500000000000,
    0x3ff4b17dea6db801, 0x3d07800000000000,
    0x3ff4bfdad53629e1, 0xbd13800000000000,
    0x3ff4ce41b817c132, 0x3d00800000000000,
    0x3ff4dcb299fddddb, 0x3d2c700000000000,
    0x3ff4eb2d81d8ab96, 0xbd1ce00000000000,
    0x3ff4f9b2769d2d02, 0x3d19200000000000,
    0x3ff508417f4531c1, 0xbd08c00000000000,
    0x3ff516daa2cf662a, 0xbcfa000000000000,
    0x3ff5257de83f51ea, 0x3d4a080000000000,
    0x3ff5342b569d4eda, 0xbd26d80000000000,
    0x3ff542e2f4f6ac1a, 0xbd32440000000000,
    0x3ff551a4ca5d94db, 0x3d483c0000000000,
    0x3ff56070dde9116b, 0x3d24b00000000000,
    0x3ff56f4736b529de, 0x3d415a0000000000,
    0x3ff57e27dbe2c40e, 0xbd29e00000000000,
    0x3ff58d12d497c76f, 0xbd23080000000000,
    0x3ff59c0827ff0b4c, 0x3d4dec0000000000,
    0x3ff5ab07dd485427, 0xbcc4000000000000,
    0x3ff5ba11fba87af4, 0x3d30080000000000,
    0x3ff5c9268a59460b, 0xbd26c80000000000,
    0x3ff5d84590998e3f, 0x3d469a0000000000,
    0x3ff5e76f15ad20e1, 0xbd1b400000000000,
    0x3ff5f6a320dcebca, 0x3d17700000000000,
    0x3ff605e1b976dcb8, 0x3d26f80000000000,
    0x3ff6152ae6cdf715, 0x3d01000000000000,
    0x3ff6247eb03a5531, 0xbd15d00000000000,
    0x3ff633dd1d1929b5, 0xbd12d00000000000,
    0x3ff6434634ccc313, 0xbcea800000000000,
    0x3ff652b9febc8efa, 0xbd28600000000000,
    0x3ff6623882553397, 0x3d71fe0000000000,
    0x3ff671c1c708328e, 0xbd37200000000000,
    0x3ff68155d44ca97e, 0x3ce6800000000000,
    0x3ff690f4b19e9471, 0xbd29780000000000,
];

// exp2(x): compute the base 2 exponential of x
//
// Accuracy: Peak error < 0.503 ulp for normalized results.
//
// Method: (accurate tables)
//
//   Reduce x:
//     x = k + y, for integer k and |y| <= 1/2.
//     Thus we have exp2(x) = 2**k * exp2(y).
//
//   Reduce y:
//     y = i/TBLSIZE + z - eps[i] for integer i near y * TBLSIZE.
//     Thus we have exp2(y) = exp2(i/TBLSIZE) * exp2(z - eps[i]),
//     with |z - eps[i]| <= 2**-9 + 2**-39 for the table used.
//
//   We compute exp2(i/TBLSIZE) via table lookup and exp2(z - eps[i]) via
//   a degree-5 minimax polynomial with maximum error under 1.3 * 2**-61.
//   The values in exp2t[] and eps[] are chosen such that
//   exp2t[i] = exp2(i/TBLSIZE + eps[i]), and eps[i] is a small offset such
//   that exp2t[i] is accurate to 2**-64.
//
//   Note that the range of i is +-TBLSIZE/2, so we actually index the tables
//   by i0 = i + TBLSIZE/2.  For cache efficiency, exp2t[] and eps[] are
//   virtual tables, interleaved in the real table tbl[].
//
//   This method is due to Gal, with many details due to Gal and Bachelis:
//
//      Gal, S. and Bachelis, B.  An Accurate Elementary Mathematical Library
//      for the IEEE Floating Point Standard.  TOMS 17(1), 26-46 (1991).

/// Exponential, base 2 (f64)
///
/// Calculate `2^x`, that is, 2 raised to the power `x`.
#[inline]
#[cfg_attr(all(test, assert_no_panic), no_panic::no_panic)]
pub fn exp2(mut x: f64) -> f64 {
    let redux = f64::from_bits(0x4338000000000000) / TBLSIZE as f64;
    let p1 = f64::from_bits(0x3fe62e42fefa39ef);
    let p2 = f64::from_bits(0x3fcebfbdff82c575);
    let p3 = f64::from_bits(0x3fac6b08d704a0a6);
    let p4 = f64::from_bits(0x3f83b2ab88f70400);
    let p5 = f64::from_bits(0x3f55d88003875c74);

    // double_t r, t, z;
    // uint32_t ix, i0;
    // union {double f; uint64_t i;} u = {x};
    // union {uint32_t u; int32_t i;} k;
    let x1p1023 = f64::from_bits(0x7fe0000000000000);
    let x1p52 = f64::from_bits(0x4330000000000000);
    let _0x1p_149 = f64::from_bits(0xb6a0000000000000);

    /* Filter out exceptional cases. */
    let ui = f64::to_bits(x);
    let ix = ui >> 32 & 0x7fffffff;
    if ix >= 0x408ff000 {
        /* |x| >= 1022 or nan */
        if ix >= 0x40900000 && ui >> 63 == 0 {
            /* x >= 1024 or nan */
            /* overflow */
            x *= x1p1023;
            return x;
        }
        if ix >= 0x7ff00000 {
            /* -inf or -nan */
            return -1.0 / x;
        }
        if ui >> 63 != 0 {
            /* x <= -1022 */
            /* underflow */
            if x <= -1075.0 || x - x1p52 + x1p52 != x {
                force_eval!((_0x1p_149 / x) as f32);
            }
            if x <= -1075.0 {
                return 0.0;
            }
        }
    } else if ix < 0x3c900000 {
        /* |x| < 0x1p-54 */
        return 1.0 + x;
    }

    /* Reduce x, computing z, i0, and k. */
    let ui = f64::to_bits(x + redux);
    let mut i0 = ui as u32;
    i0 = i0.wrapping_add(TBLSIZE as u32 / 2);
    let ku = i0 / TBLSIZE as u32 * TBLSIZE as u32;
    let ki = ku as i32 / TBLSIZE as i32;
    i0 %= TBLSIZE as u32;
    let uf = f64::from_bits(ui) - redux;
    let mut z = x - uf;

    /* Compute r = exp2(y) = exp2t[i0] * p(z - eps[i]). */
    let t = f64::from_bits(TBL[2 * i0 as usize]); /* exp2t[i0] */
    z -= f64::from_bits(TBL[2 * i0 as usize + 1]); /* eps[i0]   */
    let r = t + t * z * (p1 + z * (p2 + z * (p3 + z * (p4 + z * p5))));

    scalbn(r, ki)
}

#[test]
fn i0_wrap_test() {
    let x = -3.0 / 256.0;
    assert_eq!(exp2(x), f64::from_bits(0x3fefbdba3692d514));
}