summaryrefslogtreecommitdiff
path: root/lib/math/log-impl.myr
blob: f2ff68e8843675e77309474229199c585ee6a0d8 (plain)
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
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
use std

use "fpmath"
use "util"

/*
    See [Mul16] (6.2.2) and [Tan90].
 */
pkg math =
	pkglocal const log32 : (x : flt32 -> flt32)
	pkglocal const log64 : (x : flt64 -> flt64)

	pkglocal const log1p32 : (x : flt32 -> flt32)
	pkglocal const log1p64 : (x : flt64 -> flt64)

	/* Constants from [Tan90], note that [128] contains accurate log(2) */
	pkglocal const accurate_logs32 : (uint32, uint32)[129]
	pkglocal const accurate_logs64 : (uint64, uint64)[129]
;;

extern const horner_polyu32 : (f : flt32, a : uint32[:] -> flt32)
extern const horner_polyu64 : (f : flt64, a : uint64[:] -> flt64)

type fltdesc(@f, @u, @i) = struct
	explode : (f : @f -> (bool, @i, @u))
	assem : (n : bool, e : @i, s : @u -> @f)
	horner : (f : @f, a : @u[:] -> @f)
	tobits : (f : @f -> @u)
	frombits : (u : @u -> @f)
	sgnmask : @u
	sig8mask : @u
	sig8last : @u
	emin : @i
	emax : @i
	precision : @u
	inf : @u
	ninf : @u
	nan : @u

	/* For log */
	logT1 : @u
	logT2 : @u

	/* For log1p */
	log1pT1 : @u
	log1pT2 : @u
	T3exp : @u

	/* For procedure 1 */
	C : (@u, @u)[:]
	Ai : @u[:]

	/* For procedure 2 */
	Bi : @u[:]
	Mtruncmask : @u
;;

/* Accurate representations for log(1 + j/2^7), all j */
const accurate_logs32 = [
	(0000000000, 0000000000),
	(0x3bff0000, 0x3429ac42),
	(0x3c7e0000, 0x35a8b0fc),
	(0x3cbdc000, 0x368d83eb),
	(0x3cfc2000, 0xb6b278c4),
	(0x3d1cf000, 0x3687b9ff),
	(0x3d3ba000, 0x3631ec66),
	(0x3d5a1000, 0x36dd7119),
	(0x3d785000, 0x35c30046),
	(0x3d8b2800, 0x365bba8e),
	(0x3d9a1000, 0xb621a791),
	(0x3da8d800, 0x34e7e0c3),
	(0x3db78800, 0xb635d46a),
	(0x3dc61800, 0x368bac63),
	(0x3dd49000, 0x36da7496),
	(0x3de2f000, 0x36a91eb8),
	(0x3df13800, 0x34edc55e),
	(0x3dff6800, 0xb6dd9c48),
	(0x3e06bc00, 0xb44197b9),
	(0x3e0db800, 0x36ab54be),
	(0x3e14ac00, 0xb6b41f80),
	(0x3e1b9000, 0xb4f7f85c),
	(0x3e226800, 0x36adb32e),
	(0x3e293800, 0xb650e2f2),
	(0x3e2ff800, 0x36c1c29e),
	(0x3e36b000, 0x35fe719d),
	(0x3e3d5c00, 0x3590210e),
	(0x3e43fc00, 0x36819483),
	(0x3e4a9400, 0xb6958c2f),
	(0x3e511c00, 0x36f07f8b),
	(0x3e57a000, 0xb6dac5fd),
	(0x3e5e1400, 0x354e85b2),
	(0x3e648000, 0xb5838656),
	(0x3e6ae000, 0x3685ad3f),
	(0x3e713800, 0x356dc55e),
	(0x3e778400, 0x36b72f71),
	(0x3e7dc800, 0x36436af2),
	(0x3e820200, 0xb6d35a59),
	(0x3e851a00, 0xb6d8ec63),
	(0x3e882c00, 0x363f9ae5),
	(0x3e8b3a00, 0x36e55d5d),
	(0x3e8e4400, 0x36c60b4d),
	(0x3e914a00, 0x34fde7bd),
	(0x3e944a00, 0x36d09ef4),
	(0x3e974800, 0xb6ea28f7),
	(0x3e9a3e00, 0x36ecd4c4),
	(0x3e9d3200, 0x36455694),
	(0x3ea02200, 0xb6779796),
	(0x3ea30c00, 0x363c21c6),
	(0x3ea5f200, 0x36fcabbc),
	(0x3ea8d600, 0xb693c690),
	(0x3eabb400, 0xb60e8baa),
	(0x3eae8e00, 0xb51029fe),
	(0x3eb16400, 0x353cae72),
	(0x3eb43600, 0x3601e9b1),
	(0x3eb70400, 0x366aa2ba),
	(0x3eb9ce00, 0x36bfb5df),
	(0x3ebc9600, 0xb6d50116),
	(0x3ebf5800, 0xb5f88faa),
	(0x3ec21600, 0x368ed0f4),
	(0x3ec4d200, 0xb64793ec),
	(0x3ec78800, 0x36f439b3),
	(0x3eca3c00, 0x36a0e109),
	(0x3eccec00, 0x36ac08bf),
	(0x3ecf9a00, 0xb6e09a03),
	(0x3ed24200, 0x3410e5bb),
	(0x3ed4e800, 0xb69b2b30),
	(0x3ed78a00, 0xb6b66dc4),
	(0x3eda2800, 0xb6084337),
	(0x3edcc200, 0x36c4b499),
	(0x3edf5a00, 0x3659da72),
	(0x3ee1ee00, 0x36bd3e6d),
	(0x3ee48000, 0xb6038656),
	(0x3ee70e00, 0xb687a3d0),
	(0x3ee99800, 0xb4c0ff8a),
	(0x3eec2000, 0xb6c6d3af),
	(0x3eeea400, 0xb6afd9f2),
	(0x3ef12400, 0x3601a7c7),
	(0x3ef3a200, 0x351875a2),
	(0x3ef61c00, 0x36ce9234),
	(0x3ef89400, 0x3675faf0),
	(0x3efb0a00, 0xb6e02c7f),
	(0x3efd7a00, 0x36c47bc8),
	(0x3effea00, 0xb68fbd40),
	(0x3f012b00, 0xb6d5a5a3),
	(0x3f025f00, 0xb444adb2),
	(0x3f039200, 0xb551f190),
	(0x3f04c300, 0x36f4f573),
	(0x3f05f400, 0xb6d1bdad),
	(0x3f072200, 0x36985d1d),
	(0x3f085000, 0xb6c61d2b),
	(0x3f097c00, 0xb6e6a6c1),
	(0x3f0aa600, 0x35f4bd35),
	(0x3f0bcf00, 0x36abbd8a),
	(0x3f0cf700, 0x36568cf9),
	(0x3f0e1e00, 0xb67c11d8),
	(0x3f0f4300, 0xb4a18fbf),
	(0x3f106700, 0xb5cb9b55),
	(0x3f118a00, 0xb6f28414),
	(0x3f12ab00, 0xb6062ce1),
	(0x3f13cb00, 0xb576bb27),
	(0x3f14ea00, 0xb68013d5),
	(0x3f160700, 0x369ed449),
	(0x3f172400, 0xb6bc91c0),
	(0x3f183f00, 0xb68ccb0f),
	(0x3f195900, 0xb6cc6ede),
	(0x3f1a7100, 0x3689d9ce),
	(0x3f1b8900, 0xb684ab8c),
	(0x3f1c9f00, 0x34d3562a),
	(0x3f1db400, 0x36094000),
	(0x3f1ec800, 0x359a9c56),
	(0x3f1fdb00, 0xb60f65d2),
	(0x3f20ec00, 0x36fe8467),
	(0x3f21fd00, 0xb368318d),
	(0x3f230c00, 0x36bc21c6),
	(0x3f241b00, 0xb6c2e157),
	(0x3f252800, 0xb67449f8),
	(0x3f263400, 0xb64a0662),
	(0x3f273f00, 0xb67dc915),
	(0x3f284900, 0xb6c33fe9),
	(0x3f295100, 0x36d265bc),
	(0x3f2a5900, 0x360cf333),
	(0x3f2b6000, 0xb6454982),
	(0x3f2c6500, 0x36db5cd8),
	(0x3f2d6a00, 0x34186b3e),
	(0x3f2e6e00, 0xb6e2393f),
	(0x3f2f7000, 0x35aa4906),
	(0x3f307200, 0xb6d0bb87),
	(0x3f317200, 0x35bfbe8e), /* Note C[128] is log2 */
]

const accurate_logs64 = [
	(000000000000000000, 000000000000000000),
	(0x3f7fe02a6b200000, 0xbd6f30ee07912df9),
	(0x3f8fc0a8b1000000, 0xbd5fe0e183092c59),
	(0x3f97b91b07d80000, 0xbd62772ab6c0559c),
	(0x3f9f829b0e780000, 0x3d2980267c7e09e4),
	(0x3fa39e87ba000000, 0xbd642a056fea4dfd),
	(0x3fa77458f6340000, 0xbd62303b9cb0d5e1),
	(0x3fab42dd71180000, 0x3d671bec28d14c7e),
	(0x3faf0a30c0100000, 0x3d662a6617cc9717),
	(0x3fb16536eea40000, 0xbd60a3e2f3b47d18),
	(0x3fb341d7961c0000, 0xbd4717b6b33e44f8),
	(0x3fb51b073f060000, 0x3d383f69278e686a),
	(0x3fb6f0d28ae60000, 0xbd62968c836cc8c2),
	(0x3fb8c345d6320000, 0xbd5937c294d2f567),
	(0x3fba926d3a4a0000, 0x3d6aac6ca17a4554),
	(0x3fbc5e548f5c0000, 0xbd4c5e7514f4083f),
	(0x3fbe27076e2a0000, 0x3d6e5cbd3d50fffc),
	(0x3fbfec9131dc0000, 0xbd354555d1ae6607),
	(0x3fc0d77e7cd10000, 0xbd6c69a65a23a170),
	(0x3fc1b72ad52f0000, 0x3d69e80a41811a39),
	(0x3fc29552f8200000, 0xbd35b967f4471dfc),
	(0x3fc371fc201f0000, 0xbd6c22f10c9a4ea8),
	(0x3fc44d2b6ccb0000, 0x3d6f4799f4f6543e),
	(0x3fc526e5e3a20000, 0xbd62f21746ff8a47),
	(0x3fc5ff3070a80000, 0xbd6b0b0de3077d7e),
	(0x3fc6d60fe71a0000, 0xbd56f1b955c4d1da),
	(0x3fc7ab8902110000, 0xbd537b720e4a694b),
	(0x3fc87fa065210000, 0xbd5b77b7effb7f41),
	(0x3fc9525a9cf40000, 0x3d65ad1d904c1d4e),
	(0x3fca23bc1fe30000, 0xbd62a739b23b93e1),
	(0x3fcaf3c94e810000, 0xbd600349cc67f9b2),
	(0x3fcbc286742e0000, 0xbd6cca75818c5dbc),
	(0x3fcc8ff7c79b0000, 0xbd697794f689f843),
	(0x3fcd5c216b500000, 0xbd611ba91bbca682),
	(0x3fce27076e2b0000, 0xbd3a342c2af0003c),
	(0x3fcef0adcbdc0000, 0x3d664d948637950e),
	(0x3fcfb9186d5e0000, 0x3d5f1546aaa3361c),
	(0x3fd0402594b50000, 0xbd67df928ec217a5),
	(0x3fd0a324e2738000, 0x3d50e35f73f7a018),
	(0x3fd1058bf9ae8000, 0xbd6a9573b02faa5a),
	(0x3fd1675cabab8000, 0x3d630701ce63eab9),
	(0x3fd1c898c1698000, 0x3d59fafbc68e7540),
	(0x3fd22941fbcf8000, 0xbd3a6976f5eb0963),
	(0x3fd2895a13de8000, 0x3d3a8d7ad24c13f0),
	(0x3fd2e8e2bae10000, 0x3d5d309c2cc91a85),
	(0x3fd347dd9a988000, 0xbd25594dd4c58092),
	(0x3fd3a64c55698000, 0xbd6d0b1c68651946),
	(0x3fd4043086868000, 0x3d63f1de86093efa),
	(0x3fd4618bc21c8000, 0xbd609ec17a426426),
	(0x3fd4be5f95778000, 0xbd3d7c92cd9ad824),
	(0x3fd51aad872e0000, 0xbd3f4bd8db0a7cc1),
	(0x3fd5767717458000, 0xbd62c9d5b2a49af9),
	(0x3fd5d1bdbf580000, 0x3d4394a11b1c1ee4),
	(0x3fd62c82f2ba0000, 0xbd6c356848506ead),
	(0x3fd686c81e9b0000, 0x3d54aec442be1015),
	(0x3fd6e08eaa2b8000, 0x3d60f1c609c98c6c),
	(0x3fd739d7f6bc0000, 0xbd67fcb18ed9d603),
	(0x3fd792a55fdd8000, 0xbd6c2ec1f512dc03),
	(0x3fd7eaf83b828000, 0x3d67e1b259d2f3da),
	(0x3fd842d1da1e8000, 0x3d462e927628cbc2),
	(0x3fd89a3386c18000, 0xbd6ed2a52c73bf78),
	(0x3fd8f11e87368000, 0xbd5d3881e8962a96),
	(0x3fd947941c210000, 0x3d56faba4cdd147d),
	(0x3fd99d9581180000, 0xbd5f753456d113b8),
	(0x3fd9f323ecbf8000, 0x3d584bf2b68d766f),
	(0x3fda484090e58000, 0x3d6d8515fe535b87),
	(0x3fda9cec9a9a0000, 0x3d40931a909fea5e),
	(0x3fdaf12932478000, 0xbd3e53bb31eed7a9),
	(0x3fdb44f77bcc8000, 0x3d4ec5197ddb55d3),
	(0x3fdb985896930000, 0x3d50fb598fb14f89),
	(0x3fdbeb4d9da70000, 0x3d5b7bf7861d37ac),
	(0x3fdc3dd7a7cd8000, 0x3d66a6b9d9e0a5bd),
	(0x3fdc8ff7c79a8000, 0x3d5a21ac25d81ef3),
	(0x3fdce1af0b860000, 0xbd48290905a86aa6),
	(0x3fdd32fe7e010000, 0xbd542a9e21373414),
	(0x3fdd83e7258a0000, 0x3d679f2828add176),
	(0x3fddd46a04c20000, 0xbd6dafa08cecadb1),
	(0x3fde24881a7c8000, 0xbd53d9e34270ba6b),
	(0x3fde744261d68000, 0x3d3e1f8df68dbcf3),
	(0x3fdec399d2468000, 0x3d49802eb9dca7e7),
	(0x3fdf128f5faf0000, 0x3d3bb2cd720ec44c),
	(0x3fdf6123fa700000, 0x3d645630a2b61e5b),
	(0x3fdfaf588f790000, 0xbd49c24ca098362b),
	(0x3fdffd2e08580000, 0xbd46cf54d05f9367),
	(0x3fe02552a5a5c000, 0x3d60fec69c695d7f),
	(0x3fe04bdf9da94000, 0xbd692d9a033eff75),
	(0x3fe0723e5c1cc000, 0x3d6f404e57963891),
	(0x3fe0986f4f574000, 0xbd55be8dc04ad601),
	(0x3fe0be72e4254000, 0xbd657d49676844cc),
	(0x3fe0e44985d1c000, 0x3d5917edd5cbbd2d),
	(0x3fe109f39e2d4000, 0x3d592dfbc7d93617),
	(0x3fe12f7195940000, 0xbd6043acfedce638),
	(0x3fe154c3d2f4c000, 0x3d65e9a98f33a396),
	(0x3fe179eabbd88000, 0x3d69a0bfc60e6fa0),
	(0x3fe19ee6b467c000, 0x3d52dd98b97baef0),
	(0x3fe1c3b81f714000, 0xbd3eda1b58389902),
	(0x3fe1e85f5e704000, 0x3d1a07bd8b34be7c),
	(0x3fe20cdcd192c000, 0xbd64926cafc2f08a),
	(0x3fe23130d7bec000, 0xbd17afa4392f1ba7),
	(0x3fe2555bce990000, 0xbd506987f78a4a5e),
	(0x3fe2795e1289c000, 0xbd5dca290f81848d),
	(0x3fe29d37fec2c000, 0xbd5eea6f465268b4),
	(0x3fe2c0e9ed448000, 0x3d5d1772f5386374),
	(0x3fe2e47436e40000, 0x3d334202a10c3491),
	(0x3fe307d7334f0000, 0x3d60be1fb590a1f5),
	(0x3fe32b1339120000, 0x3d6d71320556b67b),
	(0x3fe34e289d9d0000, 0xbd6e2ce9146d277a),
	(0x3fe37117b5474000, 0x3d4ed71774092113),
	(0x3fe393e0d3564000, 0xbd65e6563bbd9fc9),
	(0x3fe3b6844a000000, 0xbd3eea838909f3d3),
	(0x3fe3d9026a714000, 0x3d66faa404263d0b),
	(0x3fe3fb5b84d18000, 0xbd60bda4b162afa3),
	(0x3fe41d8fe8468000, 0xbd5aa33736867a17),
	(0x3fe43f9fe2f9c000, 0x3d5ccef4e4f736c2),
	(0x3fe4618bc21c4000, 0x3d6ec27d0b7b37b3),
	(0x3fe48353d1ea8000, 0x3d51bee7abd17660),
	(0x3fe4a4f85db04000, 0xbd244fdd840b8591),
	(0x3fe4c679afcd0000, 0xbd61c64e971322ce),
	(0x3fe4e7d811b74000, 0x3d6bb09cb0985646),
	(0x3fe50913cc018000, 0xbd6794b434c5a4f5),
	(0x3fe52a2d265bc000, 0x3d46abb9df22bc57),
	(0x3fe54b2467998000, 0x3d6497a915428b44),
	(0x3fe56bf9d5b40000, 0xbd58cd7dc73bd194),
	(0x3fe58cadb5cd8000, 0xbd49db3db43689b4),
	(0x3fe5ad404c358000, 0x3d6f2cfb29aaa5f0),
	(0x3fe5cdb1dc6c0000, 0x3d67648cf6e3c5d7),
	(0x3fe5ee02a9240000, 0x3d667570d6095fd2),
	(0x3fe60e32f4478000, 0x3d51b194f912b417),
	(0x3fe62e42fefa4000, 0xbd48432a1b0e2634),
]

const desc32 : fltdesc(flt32, uint32, int32) = [
	.explode = std.flt32explode,
	.assem = std.flt32assem,
	.horner = horner_polyu32,
	.tobits = std.flt32bits,
	.frombits = std.flt32frombits,
	.sgnmask = (1 << 31),
	.sig8mask = 0xffff0000, /* Mask to get 8 significant bits */
	.sig8last = 16, /* Last bit kept when masking */
	.emin = -126,
	.emax = 127,
	.precision = 24,
	.inf = 0x7f800000,
	.ninf = 0xff800000,
	.nan = 0x7fc00000,
	.logT1 = 0x3f707d5f, /* Just smaller than e^(-1/16) ~= 0.939413 */
	.logT2 = 0x3f88415b, /* Just larger than e^(1/16) ~= 1.06449 */
	.log1pT1 = 0xbd782a03, /* Just smaller than e^(-1/16) - 1 ~= -0.0605869 */
	.log1pT2 = 0x3d8415ac, /* Just larger than e^(1/16) - 1 ~= 0.06449445 */
	.T3exp = 26, /* Beyond 2^T3exp, 1 + x rounds to x */
	.C = accurate_logs32[:],
	.Ai = [ 0x3daaaac2 ][:], /* Coefficients for log(1 + f/F) */
	.Bi = [ /* Coefficients for log(1 + f) in terms of a = 2f/(2 + f) */
		0x3daaaaa9,
		0x3c4d0095,
	][:],
	.Mtruncmask = 0xfffff000, /* Mask to get 12 significant bits */
]

const desc64 : fltdesc(flt64, uint64, int64) = [
	.explode = std.flt64explode,
	.assem = std.flt64assem,
	.horner = horner_polyu64,
	.tobits = std.flt64bits,
	.frombits = std.flt64frombits,
	.sgnmask = (1 << 63),
	.sig8mask = 0xffffe00000000000, /* Mask to get 8 significant bits */
	.sig8last = 45, /* Last bit kept when masking */
	.emin = -1022,
	.emax = 1023,
	.precision = 53,
	.inf = 0x7ff0000000000000,
	.ninf = 0xfff0000000000000,
	.nan = 0x7ff8000000000000,
	.logT1 = 0x3fee0fabfbc702a3, /* Just smaller than e^(-1/16) ~= 0.939413 */
	.logT2 = 0x3ff1082b577d34ee, /* Just larger  than e^(1/16) ~= 1.06449 */
	.log1pT1 = 0xbfaf0540428fd5c4, /* Just smaller than e^(-1/16) - 1 ~= -0.0605869 */
	.log1pT2 = 0x3fb082b577d34ed8, /* Just larger than e^(1/16) - 1 ~= 0.06449445 */
	.T3exp = 55, /* Beyond 2^T3exp, 1 + x rounds to x */
	.C = accurate_logs64[:],
	.Ai = [
		0x3fb5555555550286,
		0x3f8999a0bc712416,
	][:],
	.Bi = [
		0x3fb55555555554e6,
		0x3f89999999bac6d4,
		0x3f62492307f1519f,
		0x3f3c8034c85dfff0,
	][:],
	.Mtruncmask = 0xfffffffff0000000, /* Mask to get 24 significant bits */
]

const log32 = {x : flt32
	-> loggen(x, desc32)
}

const log64 = {x : flt64
	-> loggen(x, desc64)
}

generic loggen = {x : @f, d : fltdesc(@f, @u, @i) :: numeric,floating @f, numeric,integral @u, numeric,integral @i, roundable @f -> @i
	var b = d.tobits(x)
	var n : bool, e : @i, s : @u
	(n, e, s) = d.explode(x)

	/*
	   Special cases for NaN, +/- 0, < 0, inf, and 1. There are
	   certain exceptions (inexact, division by 0, &c) that
	   should be flagged in these cases, which we do not honor
	   currently. See [Tan90].
	 */
	if std.isnan(x)
		-> d.frombits(d.nan)
	elif (b & ~d.sgnmask == 0)
		-> d.frombits(d.ninf)
	elif n
		-> d.frombits(d.nan)
	elif (b == d.inf)
		-> x
	elif std.eq(x, (1.0 : @f))
		-> (0.0 : @f)
	;;

	/* If x is close to 1, polynomial log1p(x - 1) will be sufficient */
	if (d.logT1 < b && b < d.logT2)
		-> procedure_2(x - (1.0 : @f), d)
	;;

        /*
	   Reduce x to 2^m * (F + f), with (F + f) in [1, 2), so
	   procedure_2's tables work. We also require that F have
	   only 8 significant bits.
	 */
	var m : @i, Y : @f, F : @f, f : @f

	if e < d.emin
		/* Normalize significand */
		var first_1 = find_first1_64((s : uint64), (d.precision : int64))
		var offset = (d.precision : @u) - 1 - (first_1 : @u)
		s = s << offset
		e = d.emin - offset
	;;

	m = e
	Y = d.assem(false, 0, s)
	if need_round_away(0, (s : uint64), (d.sig8last : int64))
		F = d.frombits((d.tobits(Y) & d.sig8mask) + (1 << d.sig8last))
	else
		F = d.frombits(d.tobits(Y) & d.sig8mask)
	;;

	f = Y - F

	-> procedure_1(m, F, f, Y, d)
}

const log1p32 = {x : flt32
	-> log1pgen(x, desc32)
}

const log1p64 = {x : flt64
	-> log1pgen(x, desc64)
}

generic log1pgen = {x : @f, d : fltdesc(@f, @u, @i) :: numeric,floating @f, numeric,integral @u, numeric,integral @i, roundable @f -> @i
	var b = d.tobits(x)
	var n, e, s
	(n, e, s) = d.explode(x)

	/*
	   Special cases for NaN, +/- 0, < 0, inf, and 1. There are
	   certain exceptions (inexact, division by 0, &c) that
	   should be flagged in these cases, which we do not honor
	   currently. See [Tan90].
	 */
	if std.isnan(x)
		-> d.frombits(d.nan)
	elif (b & ~d.sgnmask == 0)
		-> x
	elif std.eq(x, (-1.0 : @f))
		-> d.frombits(d.nan | d.sgnmask)
	elif x < (-1.0 : @f)
		-> d.frombits(d.nan)
	elif (b == d.inf)
		-> x
	;;

	/* If x is small enough that 1 + x rounds to 1, return x */
	if e < (-d.precision : @i)
		-> x
	;;

	/* If x is close to 0, use polynomial */
	if (n && b < d.log1pT1) || (!n && b < d.log1pT2)
		-> procedure_2(x, d)
	;;

        /*
	   Reduce x m, F, f as in log case. However, since we're
	   approximating 1 + x, more care has to be taken (for
	   example: 1 + x might be infinity).
	 */
	var Y, m, F, f
	if e > d.T3exp
		Y = x
	else
		Y = (1.0 : @f) + x
	;;

	/*
	   y must be normal, otherwise x would have been -1 +
	   (subnormal), but that would round to -1.
	 */
	var ny, ey, sy
	(ny, ey, sy) = d.explode(Y)
	m = ey
	Y = d.assem(ny, 0, sy)
	if need_round_away(0, (sy : uint64), (d.sig8last : int64))
		F = d.frombits((d.tobits(Y) & d.sig8mask) + (1 << d.sig8last))
	else
		F = d.frombits(d.tobits(Y) & d.sig8mask)
	;;

	/*
	   f is trickier to compute than in the exp case, because
	   the scale of the 1 is unknown near x.
	 */
	if m <= -2
		f = Y - F
	elif m <= d.precision - 1
		f = (d.assem(false, -m, 0) - F) + scale2(x, -m)
	else
		f = (scale2(x, -m) - F) + d.assem(false, -m, 0)
	;;

	-> procedure_1(m, F, f, Y, d)
}

/* Approximate log(2^m * (F + f)) by tables */
generic procedure_1 = {m : @i, F : @f, f : @f, Y : @f, d : fltdesc(@f, @u, @i) :: numeric,floating @f, numeric,integral @u, numeric,integral @i, roundable @f -> @i
	/*
	   We must compute log(2^m * (F + f)) = m log(2) + log(F)
	   + log(1 + f/F). Only this last term need be approximated,
	   since log(2) and log(F) may be precomputed.

	   For computing log(1 + f/F), [Tan90] gives two alternatives.
	   We choose step 3', which requires floating-point division,
	   but allows us to save approximately 2.5 KiB of precomputed
	   values.

	   F is some 1 + j2^(-7), so first we compute j. Note that
	   j could actually be 128 (Ex: x = 0x4effac00.)
	 */
	var j
	var nF, eF, sF
	(nF, eF, sF) = d.explode(F)
	if eF != 0
		j = 128
	else
		j = 0x7f & (((d.sig8mask & sF) >> d.sig8last) - 0x80)
	;;

	var Cu_hi, Cu_lo, log2_hi, log2_lo
	(Cu_hi, Cu_lo) = d.C[j]
	(log2_hi, log2_lo) = d.C[128]
	
	var L_hi = (m : @f) * d.frombits(log2_hi) + d.frombits(Cu_hi)
	var L_lo = (m : @f) * d.frombits(log2_lo) + d.frombits(Cu_lo)

	var u = ((2.0 : @f) * f)/(Y + F)
	var v = u * u
	var q = u * v * d.horner(v, d.Ai)

	-> L_hi + (u + (q + L_lo))
}

/* Approximate log1p by polynomial */
generic procedure_2 = {f : @f, d : fltdesc(@f, @u, @i) :: numeric,floating @f, numeric,integral @u, numeric,integral @i, roundable @f -> @i
	var g = (1.0 : @f)/((2.0 : @f) + f)
	var u = (2.0 : @f) * f * g
	var v = u * u
	var q = u * v * d.horner(v, d.Bi)

	/*
	   1 / (2 + f) in working precision was good enough for the
	   polynomial evaluation, but to complete the approximation
	   we need to add 2f/(2 + f) with higher precision than
	   working. So we go back and compute better, split u.
	 */
	var u1 = d.frombits(d.Mtruncmask & d.tobits(u))
	var f1 = d.frombits(d.Mtruncmask & d.tobits(f))
	var f2 = f - f1
	var u2 = (((2.0 : @f) * (f - u1) - u1 * f1) - u1 * f2) * g
	-> u1 + (u2 + q)
}