-
Notifications
You must be signed in to change notification settings - Fork 164
/
decode_arm64.s
494 lines (439 loc) · 12 KB
/
decode_arm64.s
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
// Copyright 2020 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// +build !appengine
// +build gc
// +build !noasm
#include "textflag.h"
// The asm code generally follows the pure Go code in decode_other.go, except
// where marked with a "!!!".
// func decode(dst, src []byte) int
//
// All local variables fit into registers. The non-zero stack size is only to
// spill registers and push args when issuing a CALL. The register allocation:
// - R2 scratch
// - R3 scratch
// - R4 length or x
// - R5 offset
// - R6 &src[s]
// - R7 &dst[d]
// + R8 dst_base
// + R9 dst_len
// + R10 dst_base + dst_len
// + R11 src_base
// + R12 src_len
// + R13 src_base + src_len
// - R14 used by doCopy
// - R15 used by doCopy
//
// The registers R8-R13 (marked with a "+") are set at the start of the
// function, and after a CALL returns, and are not otherwise modified.
//
// The d variable is implicitly R7 - R8, and len(dst)-d is R10 - R7.
// The s variable is implicitly R6 - R11, and len(src)-s is R13 - R6.
TEXT ·decode(SB), NOSPLIT, $56-56
// Initialize R6, R7 and R8-R13.
MOVD dst_base+0(FP), R8
MOVD dst_len+8(FP), R9
MOVD R8, R7
MOVD R8, R10
ADD R9, R10, R10
MOVD src_base+24(FP), R11
MOVD src_len+32(FP), R12
MOVD R11, R6
MOVD R11, R13
ADD R12, R13, R13
loop:
// for s < len(src)
CMP R13, R6
BEQ end
// R4 = uint32(src[s])
//
// switch src[s] & 0x03
MOVBU (R6), R4
MOVW R4, R3
ANDW $3, R3
MOVW $1, R1
CMPW R1, R3
BGE tagCopy
// ----------------------------------------
// The code below handles literal tags.
// case tagLiteral:
// x := uint32(src[s] >> 2)
// switch
MOVW $60, R1
LSRW $2, R4, R4
CMPW R4, R1
BLS tagLit60Plus
// case x < 60:
// s++
ADD $1, R6, R6
doLit:
// This is the end of the inner "switch", when we have a literal tag.
//
// We assume that R4 == x and x fits in a uint32, where x is the variable
// used in the pure Go decode_other.go code.
// length = int(x) + 1
//
// Unlike the pure Go code, we don't need to check if length <= 0 because
// R4 can hold 64 bits, so the increment cannot overflow.
ADD $1, R4, R4
// Prepare to check if copying length bytes will run past the end of dst or
// src.
//
// R2 = len(dst) - d
// R3 = len(src) - s
MOVD R10, R2
SUB R7, R2, R2
MOVD R13, R3
SUB R6, R3, R3
// !!! Try a faster technique for short (16 or fewer bytes) copies.
//
// if length > 16 || len(dst)-d < 16 || len(src)-s < 16 {
// goto callMemmove // Fall back on calling runtime·memmove.
// }
//
// The C++ snappy code calls this TryFastAppend. It also checks len(src)-s
// against 21 instead of 16, because it cannot assume that all of its input
// is contiguous in memory and so it needs to leave enough source bytes to
// read the next tag without refilling buffers, but Go's Decode assumes
// contiguousness (the src argument is a []byte).
CMP $16, R4
BGT callMemmove
CMP $16, R2
BLT callMemmove
CMP $16, R3
BLT callMemmove
// !!! Implement the copy from src to dst as a 16-byte load and store.
// (Decode's documentation says that dst and src must not overlap.)
//
// This always copies 16 bytes, instead of only length bytes, but that's
// OK. If the input is a valid Snappy encoding then subsequent iterations
// will fix up the overrun. Otherwise, Decode returns a nil []byte (and a
// non-nil error), so the overrun will be ignored.
//
// Note that on arm64, it is legal and cheap to issue unaligned 8-byte or
// 16-byte loads and stores. This technique probably wouldn't be as
// effective on architectures that are fussier about alignment.
LDP 0(R6), (R14, R15)
STP (R14, R15), 0(R7)
// d += length
// s += length
ADD R4, R7, R7
ADD R4, R6, R6
B loop
callMemmove:
// if length > len(dst)-d || length > len(src)-s { etc }
CMP R2, R4
BGT errCorrupt
CMP R3, R4
BGT errCorrupt
// copy(dst[d:], src[s:s+length])
//
// This means calling runtime·memmove(&dst[d], &src[s], length), so we push
// R7, R6 and R4 as arguments. Coincidentally, we also need to spill those
// three registers to the stack, to save local variables across the CALL.
MOVD R7, 8(RSP)
MOVD R6, 16(RSP)
MOVD R4, 24(RSP)
MOVD R7, 32(RSP)
MOVD R6, 40(RSP)
MOVD R4, 48(RSP)
CALL runtime·memmove(SB)
// Restore local variables: unspill registers from the stack and
// re-calculate R8-R13.
MOVD 32(RSP), R7
MOVD 40(RSP), R6
MOVD 48(RSP), R4
MOVD dst_base+0(FP), R8
MOVD dst_len+8(FP), R9
MOVD R8, R10
ADD R9, R10, R10
MOVD src_base+24(FP), R11
MOVD src_len+32(FP), R12
MOVD R11, R13
ADD R12, R13, R13
// d += length
// s += length
ADD R4, R7, R7
ADD R4, R6, R6
B loop
tagLit60Plus:
// !!! This fragment does the
//
// s += x - 58; if uint(s) > uint(len(src)) { etc }
//
// checks. In the asm version, we code it once instead of once per switch case.
ADD R4, R6, R6
SUB $58, R6, R6
MOVD R6, R3
SUB R11, R3, R3
CMP R12, R3
BGT errCorrupt
// case x == 60:
MOVW $61, R1
CMPW R1, R4
BEQ tagLit61
BGT tagLit62Plus
// x = uint32(src[s-1])
MOVBU -1(R6), R4
B doLit
tagLit61:
// case x == 61:
// x = uint32(src[s-2]) | uint32(src[s-1])<<8
MOVHU -2(R6), R4
B doLit
tagLit62Plus:
CMPW $62, R4
BHI tagLit63
// case x == 62:
// x = uint32(src[s-3]) | uint32(src[s-2])<<8 | uint32(src[s-1])<<16
MOVHU -3(R6), R4
MOVBU -1(R6), R3
ORR R3<<16, R4
B doLit
tagLit63:
// case x == 63:
// x = uint32(src[s-4]) | uint32(src[s-3])<<8 | uint32(src[s-2])<<16 | uint32(src[s-1])<<24
MOVWU -4(R6), R4
B doLit
// The code above handles literal tags.
// ----------------------------------------
// The code below handles copy tags.
tagCopy4:
// case tagCopy4:
// s += 5
ADD $5, R6, R6
// if uint(s) > uint(len(src)) { etc }
MOVD R6, R3
SUB R11, R3, R3
CMP R12, R3
BGT errCorrupt
// length = 1 + int(src[s-5])>>2
MOVD $1, R1
ADD R4>>2, R1, R4
// offset = int(uint32(src[s-4]) | uint32(src[s-3])<<8 | uint32(src[s-2])<<16 | uint32(src[s-1])<<24)
MOVWU -4(R6), R5
B doCopy
tagCopy2:
// case tagCopy2:
// s += 3
ADD $3, R6, R6
// if uint(s) > uint(len(src)) { etc }
MOVD R6, R3
SUB R11, R3, R3
CMP R12, R3
BGT errCorrupt
// length = 1 + int(src[s-3])>>2
MOVD $1, R1
ADD R4>>2, R1, R4
// offset = int(uint32(src[s-2]) | uint32(src[s-1])<<8)
MOVHU -2(R6), R5
B doCopy
tagCopy:
// We have a copy tag. We assume that:
// - R3 == src[s] & 0x03
// - R4 == src[s]
CMP $2, R3
BEQ tagCopy2
BGT tagCopy4
// case tagCopy1:
// s += 2
ADD $2, R6, R6
// if uint(s) > uint(len(src)) { etc }
MOVD R6, R3
SUB R11, R3, R3
CMP R12, R3
BGT errCorrupt
// offset = int(uint32(src[s-2])&0xe0<<3 | uint32(src[s-1]))
MOVD R4, R5
AND $0xe0, R5
MOVBU -1(R6), R3
ORR R5<<3, R3, R5
// length = 4 + int(src[s-2])>>2&0x7
MOVD $7, R1
AND R4>>2, R1, R4
ADD $4, R4, R4
doCopy:
// This is the end of the outer "switch", when we have a copy tag.
//
// We assume that:
// - R4 == length && R4 > 0
// - R5 == offset
// if offset <= 0 { etc }
MOVD $0, R1
CMP R1, R5
BLE errCorrupt
// if d < offset { etc }
MOVD R7, R3
SUB R8, R3, R3
CMP R5, R3
BLT errCorrupt
// if length > len(dst)-d { etc }
MOVD R10, R3
SUB R7, R3, R3
CMP R3, R4
BGT errCorrupt
// forwardCopy(dst[d:d+length], dst[d-offset:]); d += length
//
// Set:
// - R14 = len(dst)-d
// - R15 = &dst[d-offset]
MOVD R10, R14
SUB R7, R14, R14
MOVD R7, R15
SUB R5, R15, R15
// !!! Try a faster technique for short (16 or fewer bytes) forward copies.
//
// First, try using two 8-byte load/stores, similar to the doLit technique
// above. Even if dst[d:d+length] and dst[d-offset:] can overlap, this is
// still OK if offset >= 8. Note that this has to be two 8-byte load/stores
// and not one 16-byte load/store, and the first store has to be before the
// second load, due to the overlap if offset is in the range [8, 16).
//
// if length > 16 || offset < 8 || len(dst)-d < 16 {
// goto slowForwardCopy
// }
// copy 16 bytes
// d += length
CMP $16, R4
BGT slowForwardCopy
CMP $8, R5
BLT slowForwardCopy
CMP $16, R14
BLT slowForwardCopy
MOVD 0(R15), R2
MOVD R2, 0(R7)
MOVD 8(R15), R3
MOVD R3, 8(R7)
ADD R4, R7, R7
B loop
slowForwardCopy:
// !!! If the forward copy is longer than 16 bytes, or if offset < 8, we
// can still try 8-byte load stores, provided we can overrun up to 10 extra
// bytes. As above, the overrun will be fixed up by subsequent iterations
// of the outermost loop.
//
// The C++ snappy code calls this technique IncrementalCopyFastPath. Its
// commentary says:
//
// ----
//
// The main part of this loop is a simple copy of eight bytes at a time
// until we've copied (at least) the requested amount of bytes. However,
// if d and d-offset are less than eight bytes apart (indicating a
// repeating pattern of length < 8), we first need to expand the pattern in
// order to get the correct results. For instance, if the buffer looks like
// this, with the eight-byte <d-offset> and <d> patterns marked as
// intervals:
//
// abxxxxxxxxxxxx
// [------] d-offset
// [------] d
//
// a single eight-byte copy from <d-offset> to <d> will repeat the pattern
// once, after which we can move <d> two bytes without moving <d-offset>:
//
// ababxxxxxxxxxx
// [------] d-offset
// [------] d
//
// and repeat the exercise until the two no longer overlap.
//
// This allows us to do very well in the special case of one single byte
// repeated many times, without taking a big hit for more general cases.
//
// The worst case of extra writing past the end of the match occurs when
// offset == 1 and length == 1; the last copy will read from byte positions
// [0..7] and write to [4..11], whereas it was only supposed to write to
// position 1. Thus, ten excess bytes.
//
// ----
//
// That "10 byte overrun" worst case is confirmed by Go's
// TestSlowForwardCopyOverrun, which also tests the fixUpSlowForwardCopy
// and finishSlowForwardCopy algorithm.
//
// if length > len(dst)-d-10 {
// goto verySlowForwardCopy
// }
SUB $10, R14, R14
CMP R14, R4
BGT verySlowForwardCopy
makeOffsetAtLeast8:
// !!! As above, expand the pattern so that offset >= 8 and we can use
// 8-byte load/stores.
//
// for offset < 8 {
// copy 8 bytes from dst[d-offset:] to dst[d:]
// length -= offset
// d += offset
// offset += offset
// // The two previous lines together means that d-offset, and therefore
// // R15, is unchanged.
// }
CMP $8, R5
BGE fixUpSlowForwardCopy
MOVD (R15), R3
MOVD R3, (R7)
SUB R5, R4, R4
ADD R5, R7, R7
ADD R5, R5, R5
B makeOffsetAtLeast8
fixUpSlowForwardCopy:
// !!! Add length (which might be negative now) to d (implied by R7 being
// &dst[d]) so that d ends up at the right place when we jump back to the
// top of the loop. Before we do that, though, we save R7 to R2 so that, if
// length is positive, copying the remaining length bytes will write to the
// right place.
MOVD R7, R2
ADD R4, R7, R7
finishSlowForwardCopy:
// !!! Repeat 8-byte load/stores until length <= 0. Ending with a negative
// length means that we overrun, but as above, that will be fixed up by
// subsequent iterations of the outermost loop.
MOVD $0, R1
CMP R1, R4
BLE loop
MOVD (R15), R3
MOVD R3, (R2)
ADD $8, R15, R15
ADD $8, R2, R2
SUB $8, R4, R4
B finishSlowForwardCopy
verySlowForwardCopy:
// verySlowForwardCopy is a simple implementation of forward copy. In C
// parlance, this is a do/while loop instead of a while loop, since we know
// that length > 0. In Go syntax:
//
// for {
// dst[d] = dst[d - offset]
// d++
// length--
// if length == 0 {
// break
// }
// }
MOVB (R15), R3
MOVB R3, (R7)
ADD $1, R15, R15
ADD $1, R7, R7
SUB $1, R4, R4
CBNZ R4, verySlowForwardCopy
B loop
// The code above handles copy tags.
// ----------------------------------------
end:
// This is the end of the "for s < len(src)".
//
// if d != len(dst) { etc }
CMP R10, R7
BNE errCorrupt
// return 0
MOVD $0, ret+48(FP)
RET
errCorrupt:
// return decodeErrCodeCorrupt
MOVD $1, R2
MOVD R2, ret+48(FP)
RET