summaryrefslogtreecommitdiff
path: root/libavcodec/arm/simple_idct_arm.S
blob: bf9ee3d4e6bb9181c87b53dbe2ef71b79b57bd9a (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
/*
 * Copyright (C) 2002 Frederic 'dilb' Boulay
 *
 * Author: Frederic Boulay <dilb@handhelds.org>
 *
 * The function defined in this file is derived from the simple_idct function
 * from the libavcodec library part of the Libav project.
 *
 * This file is part of Libav.
 *
 * Libav is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2.1 of the License, or (at your option) any later version.
 *
 * Libav is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with Libav; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
 */

#include "libavutil/arm/asm.S"

/* useful constants for the algorithm */
#define W1  22725
#define W2  21407
#define W3  19266
#define W4  16383
#define W5  12873
#define W6  8867
#define W7  4520
#define MASK_MSHW 0xFFFF0000

#define ROW_SHIFT 11
#define ROW_SHIFT2MSHW (16-11)
#define COL_SHIFT 20
#define ROW_SHIFTED_1 1024 /* 1<< (ROW_SHIFT-1) */
#define COL_SHIFTED_1 524288 /* 1<< (COL_SHIFT-1) */


function ff_simple_idct_arm, export=1
        @@ void simple_idct_arm(int16_t *block)
        @@ save stack for reg needed (take all of them),
        @@ R0-R3 are scratch regs, so no need to save them, but R0 contains the pointer to block
        @@ so it must not be overwritten, if it is not saved!!
        @@ R12 is another scratch register, so it should not be saved too
        @@ save all registers
        stmfd sp!, {r4-r11, r14} @ R14 is also called LR
        @@ at this point, R0=block, other registers are free.
        add r14, r0, #112        @ R14=&block[8*7], better start from the last row, and decrease the value until row=0, i.e. R12=block.
        @@ add 2 temporary variables in the stack: R0 and R14
        sub sp, sp, #8          @ allow 2 local variables
        str r0, [sp, #0]        @ save block in sp[0]
        @@ stack status
        @@ sp+4   free
        @@ sp+0   R0  (block)


        @@ at this point, R0=block, R14=&block[56], R12=__const_ptr_, R1-R11 free


__row_loop:
        @@ read the row and check if it is null, almost null, or not, according to strongarm specs, it is not necessary to optimize ldr accesses (i.e. split 32bits in 2 16bits words), at least it gives more usable registers :)
        ldr r1, [r14, #0]        @ R1=(int32)(R12)[0]=ROWr32[0] (relative row cast to a 32b pointer)
        ldr r2, [r14, #4]        @ R2=(int32)(R12)[1]=ROWr32[1]
        ldr r3, [r14, #8]        @ R3=ROWr32[2]
        ldr r4, [r14, #12]       @ R4=ROWr32[3]
        @@ check if the words are null, if all of them are null, then proceed with next row (branch __end_row_loop),
        @@ if ROWr16[0] is the only one not null, then proceed with this special case (branch __almost_empty_row)
        @@ else follow the complete algorithm.
        @@ at this point, R0=block, R14=&block[n], R12=__const_ptr_, R1=ROWr32[0], R2=ROWr32[1],
        @@                R3=ROWr32[2], R4=ROWr32[3], R5-R11 free
        orr r5, r4, r3           @ R5=R4 | R3
        orr r5, r5, r2           @ R5=R4 | R3 | R2
        orrs r6, r5, r1          @ Test R5 | R1 (the aim is to check if everything is null)
        beq __end_row_loop
        mov r7, r1, asr #16      @ R7=R1>>16=ROWr16[1] (evaluate it now, as it could be useful later)
        ldrsh r6, [r14, #0]      @ R6=ROWr16[0]
        orrs r5, r5, r7          @ R5=R4 | R3 | R2 | R7
        beq __almost_empty_row

@@ __b_evaluation:
        @@ at this point, R0=block (temp),  R1(free), R2=ROWr32[1], R3=ROWr32[2], R4=ROWr32[3],
        @@     R5=(temp), R6=ROWr16[0], R7=ROWr16[1], R8-R11 free,
        @@     R12=__const_ptr_, R14=&block[n]
        @@ to save some registers/calls, proceed with b0-b3 first, followed by a0-a3

        @@ MUL16(b0, W1, row[1]);
        @@ MUL16(b1, W3, row[1]);
        @@ MUL16(b2, W5, row[1]);
        @@ MUL16(b3, W7, row[1]);
        @@ MAC16(b0, W3, row[3]);
        @@ MAC16(b1, -W7, row[3]);
        @@ MAC16(b2, -W1, row[3]);
        @@ MAC16(b3, -W5, row[3]);
        ldr r8, =W1              @ R8=W1
        mov r2, r2, asr #16      @ R2=ROWr16[3]
        mul r0, r8, r7           @ R0=W1*ROWr16[1]=b0 (ROWr16[1] must be the second arg, to have the possibility to save 1 cycle)
        ldr r9, =W3              @ R9=W3
        ldr r10, =W5             @ R10=W5
        mul r1, r9, r7           @ R1=W3*ROWr16[1]=b1 (ROWr16[1] must be the second arg, to have the possibility to save 1 cycle)
        ldr r11, =W7             @ R11=W7
        mul r5, r10, r7          @ R5=W5*ROWr16[1]=b2 (ROWr16[1] must be the second arg, to have the possibility to save 1 cycle)
        mul r7, r11, r7          @ R7=W7*ROWr16[1]=b3 (ROWr16[1] must be the second arg, to have the possibility to save 1 cycle)
        teq r2, #0               @ if null avoid muls
        itttt ne
        mlane r0, r9, r2, r0     @ R0+=W3*ROWr16[3]=b0 (ROWr16[3] must be the second arg, to have the possibility to save 1 cycle)
        rsbne r2, r2, #0         @ R2=-ROWr16[3]
        mlane r1, r11, r2, r1    @ R1-=W7*ROWr16[3]=b1 (ROWr16[3] must be the second arg, to have the possibility to save 1 cycle)
        mlane r5, r8, r2, r5     @ R5-=W1*ROWr16[3]=b2 (ROWr16[3] must be the second arg, to have the possibility to save 1 cycle)
        it    ne
        mlane r7, r10, r2, r7    @ R7-=W5*ROWr16[3]=b3 (ROWr16[3] must be the second arg, to have the possibility to save 1 cycle)

        @@ at this point, R0=b0,  R1=b1, R2 (free), R3=ROWr32[2], R4=ROWr32[3],
        @@     R5=b2, R6=ROWr16[0], R7=b3, R8=W1, R9=W3, R10=W5, R11=W7,
        @@     R12=__const_ptr_, R14=&block[n]
        @@ temp = ((uint32_t*)row)[2] | ((uint32_t*)row)[3];
        @@ if (temp != 0) {}
        orrs r2, r3, r4          @ R2=ROWr32[2] | ROWr32[3]
        beq __end_b_evaluation

        @@ at this point, R0=b0,  R1=b1, R2 (free), R3=ROWr32[2], R4=ROWr32[3],
        @@     R5=b2, R6=ROWr16[0], R7=b3, R8=W1, R9=W3, R10=W5, R11=W7,
        @@     R12=__const_ptr_, R14=&block[n]
        @@ MAC16(b0, W5, row[5]);
        @@ MAC16(b2, W7, row[5]);
        @@ MAC16(b3, W3, row[5]);
        @@ MAC16(b1, -W1, row[5]);
        @@ MAC16(b0, W7, row[7]);
        @@ MAC16(b2, W3, row[7]);
        @@ MAC16(b3, -W1, row[7]);
        @@ MAC16(b1, -W5, row[7]);
        mov r3, r3, asr #16      @ R3=ROWr16[5]
        teq r3, #0               @ if null avoid muls
        it    ne
        mlane r0, r10, r3, r0    @ R0+=W5*ROWr16[5]=b0
        mov r4, r4, asr #16      @ R4=ROWr16[7]
        itttt ne
        mlane r5, r11, r3, r5    @ R5+=W7*ROWr16[5]=b2
        mlane r7, r9, r3, r7     @ R7+=W3*ROWr16[5]=b3
        rsbne r3, r3, #0         @ R3=-ROWr16[5]
        mlane r1, r8, r3, r1     @ R7-=W1*ROWr16[5]=b1
        @@ R3 is free now
        teq r4, #0               @ if null avoid muls
        itttt ne
        mlane r0, r11, r4, r0    @ R0+=W7*ROWr16[7]=b0
        mlane r5, r9, r4, r5     @ R5+=W3*ROWr16[7]=b2
        rsbne r4, r4, #0         @ R4=-ROWr16[7]
        mlane r7, r8, r4, r7     @ R7-=W1*ROWr16[7]=b3
        it    ne
        mlane r1, r10, r4, r1    @ R1-=W5*ROWr16[7]=b1
        @@ R4 is free now
__end_b_evaluation:
        @@ at this point, R0=b0,  R1=b1, R2=ROWr32[2] | ROWr32[3] (tmp), R3 (free), R4 (free),
        @@     R5=b2, R6=ROWr16[0], R7=b3, R8 (free), R9 (free), R10 (free), R11 (free),
        @@     R12=__const_ptr_, R14=&block[n]

@@ __a_evaluation:
        @@ a0 = (W4 * row[0]) + (1 << (ROW_SHIFT - 1));
        @@ a1 = a0 + W6 * row[2];
        @@ a2 = a0 - W6 * row[2];
        @@ a3 = a0 - W2 * row[2];
        @@ a0 = a0 + W2 * row[2];
        ldr r9, =W4              @ R9=W4
        mul r6, r9, r6           @ R6=W4*ROWr16[0]
        ldr r10, =W6             @ R10=W6
        ldrsh r4, [r14, #4]      @ R4=ROWr16[2] (a3 not defined yet)
        add r6, r6, #ROW_SHIFTED_1 @ R6=W4*ROWr16[0] + 1<<(ROW_SHIFT-1) (a0)

        mul r11, r10, r4         @ R11=W6*ROWr16[2]
        ldr r8, =W2              @ R8=W2
        sub r3, r6, r11          @ R3=a0-W6*ROWr16[2] (a2)
        @@ temp = ((uint32_t*)row)[2] | ((uint32_t*)row)[3];
        @@ if (temp != 0) {}
        teq r2, #0
        beq __end_bef_a_evaluation

        add r2, r6, r11          @ R2=a0+W6*ROWr16[2] (a1)
        mul r11, r8, r4          @ R11=W2*ROWr16[2]
        sub r4, r6, r11          @ R4=a0-W2*ROWr16[2] (a3)
        add r6, r6, r11          @ R6=a0+W2*ROWr16[2] (a0)


        @@ at this point, R0=b0,  R1=b1, R2=a1, R3=a2, R4=a3,
        @@     R5=b2, R6=a0, R7=b3, R8=W2, R9=W4, R10=W6, R11 (free),
        @@     R12=__const_ptr_, R14=&block[n]


        @@ a0 += W4*row[4]
        @@ a1 -= W4*row[4]
        @@ a2 -= W4*row[4]
        @@ a3 += W4*row[4]
        ldrsh r11, [r14, #8]     @ R11=ROWr16[4]
        teq r11, #0              @ if null avoid muls
        it    ne
        mulne r11, r9, r11       @ R11=W4*ROWr16[4]
        @@ R9 is free now
        ldrsh r9, [r14, #12]     @ R9=ROWr16[6]
        itttt ne
        addne r6, r6, r11        @ R6+=W4*ROWr16[4] (a0)
        subne r2, r2, r11        @ R2-=W4*ROWr16[4] (a1)
        subne r3, r3, r11        @ R3-=W4*ROWr16[4] (a2)
        addne r4, r4, r11        @ R4+=W4*ROWr16[4] (a3)
        @@ W6 alone is no more useful, save W2*ROWr16[6] in it instead
        teq r9, #0               @ if null avoid muls
        itttt ne
        mulne r11, r10, r9       @ R11=W6*ROWr16[6]
        addne r6, r6, r11        @ R6+=W6*ROWr16[6] (a0)
        mulne r10, r8, r9        @ R10=W2*ROWr16[6]
        @@ a0 += W6*row[6];
        @@ a3 -= W6*row[6];
        @@ a1 -= W2*row[6];
        @@ a2 += W2*row[6];
        subne r4, r4, r11        @ R4-=W6*ROWr16[6] (a3)
        itt   ne
        subne r2, r2, r10        @ R2-=W2*ROWr16[6] (a1)
        addne r3, r3, r10        @ R3+=W2*ROWr16[6] (a2)

__end_a_evaluation:
        @@ at this point, R0=b0,  R1=b1, R2=a1, R3=a2, R4=a3,
        @@     R5=b2, R6=a0, R7=b3, R8 (free), R9 (free), R10 (free), R11 (free),
        @@     R12=__const_ptr_, R14=&block[n]
        @@ row[0] = (a0 + b0) >> ROW_SHIFT;
        @@ row[1] = (a1 + b1) >> ROW_SHIFT;
        @@ row[2] = (a2 + b2) >> ROW_SHIFT;
        @@ row[3] = (a3 + b3) >> ROW_SHIFT;
        @@ row[4] = (a3 - b3) >> ROW_SHIFT;
        @@ row[5] = (a2 - b2) >> ROW_SHIFT;
        @@ row[6] = (a1 - b1) >> ROW_SHIFT;
        @@ row[7] = (a0 - b0) >> ROW_SHIFT;
        add r8, r6, r0           @ R8=a0+b0
        add r9, r2, r1           @ R9=a1+b1
        @@ put 2 16 bits half-words in a 32bits word
        @@ ROWr32[0]=ROWr16[0] | (ROWr16[1]<<16) (only Little Endian compliant then!!!)
        ldr r10, =MASK_MSHW      @ R10=0xFFFF0000
        and r9, r10, r9, lsl #ROW_SHIFT2MSHW @ R9=0xFFFF0000 & ((a1+b1)<<5)
        mvn r11, r10             @ R11= NOT R10= 0x0000FFFF
        and r8, r11, r8, asr #ROW_SHIFT @ R8=0x0000FFFF & ((a0+b0)>>11)
        orr r8, r8, r9
        str r8, [r14, #0]

        add r8, r3, r5           @ R8=a2+b2
        add r9, r4, r7           @ R9=a3+b3
        and r9, r10, r9, lsl #ROW_SHIFT2MSHW @ R9=0xFFFF0000 & ((a3+b3)<<5)
        and r8, r11, r8, asr #ROW_SHIFT @ R8=0x0000FFFF & ((a2+b2)>>11)
        orr r8, r8, r9
        str r8, [r14, #4]

        sub r8, r4, r7           @ R8=a3-b3
        sub r9, r3, r5           @ R9=a2-b2
        and r9, r10, r9, lsl #ROW_SHIFT2MSHW @ R9=0xFFFF0000 & ((a2-b2)<<5)
        and r8, r11, r8, asr #ROW_SHIFT @ R8=0x0000FFFF & ((a3-b3)>>11)
        orr r8, r8, r9
        str r8, [r14, #8]

        sub r8, r2, r1           @ R8=a1-b1
        sub r9, r6, r0           @ R9=a0-b0
        and r9, r10, r9, lsl #ROW_SHIFT2MSHW @ R9=0xFFFF0000 & ((a0-b0)<<5)
        and r8, r11, r8, asr #ROW_SHIFT @ R8=0x0000FFFF & ((a1-b1)>>11)
        orr r8, r8, r9
        str r8, [r14, #12]

        bal __end_row_loop

__almost_empty_row:
        @@ the row was empty, except ROWr16[0], now, management of this special case
        @@ at this point, R0=block, R14=&block[n], R12=__const_ptr_, R1=ROWr32[0], R2=ROWr32[1],
        @@                R3=ROWr32[2], R4=ROWr32[3], R5=(temp), R6=ROWr16[0], R7=ROWr16[1],
        @@                R8=0xFFFF (temp), R9-R11 free
        mov r8, #0x10000         @ R8=0xFFFF (2 steps needed!) it saves a ldr call (because of delay run).
        sub r8, r8, #1           @ R8 is now ready.
        and r5, r8, r6, lsl #3   @ R5=R8 & (R6<<3)= (ROWr16[0]<<3) & 0xFFFF
        orr r5, r5, r5, lsl #16  @ R5=R5 | (R5<<16)
        str r5, [r14, #0]        @ R14[0]=ROWr32[0]=R5
        str r5, [r14, #4]        @ R14[4]=ROWr32[1]=R5
        str r5, [r14, #8]        @ R14[8]=ROWr32[2]=R5
        str r5, [r14, #12]       @ R14[12]=ROWr32[3]=R5

__end_row_loop:
        @@ at this point, R0-R11 (free)
        @@     R12=__const_ptr_, R14=&block[n]
        ldr r0, [sp, #0]         @ R0=block
        teq r0, r14              @ compare current &block[8*n] to block, when block is reached, the loop is finished.
        sub r14, r14, #16
        bne __row_loop



        @@ at this point, R0=block, R1-R11 (free)
        @@     R12=__const_ptr_, R14=&block[n]
        add r14, r0, #14        @ R14=&block[7], better start from the last col, and decrease the value until col=0, i.e. R14=block.
__col_loop:

@@ __b_evaluation2:
        @@ at this point, R0=block (temp),  R1-R11 (free)
        @@     R12=__const_ptr_, R14=&block[n]
        @@ proceed with b0-b3 first, followed by a0-a3
        @@ MUL16(b0, W1, col[8x1]);
        @@ MUL16(b1, W3, col[8x1]);
        @@ MUL16(b2, W5, col[8x1]);
        @@ MUL16(b3, W7, col[8x1]);
        @@ MAC16(b0, W3, col[8x3]);
        @@ MAC16(b1, -W7, col[8x3]);
        @@ MAC16(b2, -W1, col[8x3]);
        @@ MAC16(b3, -W5, col[8x3]);
        ldr r8, =W1              @ R8=W1
        ldrsh r7, [r14, #16]
        mul r0, r8, r7           @ R0=W1*ROWr16[1]=b0 (ROWr16[1] must be the second arg, to have the possibility to save 1 cycle)
        ldr r9, =W3              @ R9=W3
        ldr r10, =W5             @ R10=W5
        mul r1, r9, r7           @ R1=W3*ROWr16[1]=b1 (ROWr16[1] must be the second arg, to have the possibility to save 1 cycle)
        ldr r11, =W7             @ R11=W7
        mul r5, r10, r7          @ R5=W5*ROWr16[1]=b2 (ROWr16[1] must be the second arg, to have the possibility to save 1 cycle)
        ldrsh r2, [r14, #48]
        mul r7, r11, r7          @ R7=W7*ROWr16[1]=b3 (ROWr16[1] must be the second arg, to have the possibility to save 1 cycle)
        teq r2, #0               @ if 0, then avoid muls
        itttt ne
        mlane r0, r9, r2, r0     @ R0+=W3*ROWr16[3]=b0 (ROWr16[3] must be the second arg, to have the possibility to save 1 cycle)
        rsbne r2, r2, #0         @ R2=-ROWr16[3]
        mlane r1, r11, r2, r1    @ R1-=W7*ROWr16[3]=b1 (ROWr16[3] must be the second arg, to have the possibility to save 1 cycle)
        mlane r5, r8, r2, r5     @ R5-=W1*ROWr16[3]=b2 (ROWr16[3] must be the second arg, to have the possibility to save 1 cycle)
        it    ne
        mlane r7, r10, r2, r7    @ R7-=W5*ROWr16[3]=b3 (ROWr16[3] must be the second arg, to have the possibility to save 1 cycle)

        @@ at this point, R0=b0,  R1=b1, R2 (free), R3 (free), R4 (free),
        @@     R5=b2, R6 (free), R7=b3, R8=W1, R9=W3, R10=W5, R11=W7,
        @@     R12=__const_ptr_, R14=&block[n]
        @@ MAC16(b0, W5, col[5x8]);
        @@ MAC16(b2, W7, col[5x8]);
        @@ MAC16(b3, W3, col[5x8]);
        @@ MAC16(b1, -W1, col[5x8]);
        @@ MAC16(b0, W7, col[7x8]);
        @@ MAC16(b2, W3, col[7x8]);
        @@ MAC16(b3, -W1, col[7x8]);
        @@ MAC16(b1, -W5, col[7x8]);
        ldrsh r3, [r14, #80]     @ R3=COLr16[5x8]
        teq r3, #0               @ if 0 then avoid muls
        itttt ne
        mlane r0, r10, r3, r0    @ R0+=W5*ROWr16[5x8]=b0
        mlane r5, r11, r3, r5    @ R5+=W7*ROWr16[5x8]=b2
        mlane r7, r9, r3, r7     @ R7+=W3*ROWr16[5x8]=b3
        rsbne r3, r3, #0         @ R3=-ROWr16[5x8]
        ldrsh r4, [r14, #112]    @ R4=COLr16[7x8]
        it    ne
        mlane r1, r8, r3, r1     @ R7-=W1*ROWr16[5x8]=b1
        @@ R3 is free now
        teq r4, #0               @ if 0 then avoid muls
        itttt ne
        mlane r0, r11, r4, r0    @ R0+=W7*ROWr16[7x8]=b0
        mlane r5, r9, r4, r5     @ R5+=W3*ROWr16[7x8]=b2
        rsbne r4, r4, #0         @ R4=-ROWr16[7x8]
        mlane r7, r8, r4, r7     @ R7-=W1*ROWr16[7x8]=b3
        it    ne
        mlane r1, r10, r4, r1    @ R1-=W5*ROWr16[7x8]=b1
        @@ R4 is free now
@@ __end_b_evaluation2:
        @@ at this point, R0=b0,  R1=b1, R2 (free), R3 (free), R4 (free),
        @@     R5=b2, R6 (free), R7=b3, R8 (free), R9 (free), R10 (free), R11 (free),
        @@     R12=__const_ptr_, R14=&block[n]

@@ __a_evaluation2:
        @@ a0 = (W4 * col[8x0]) + (1 << (COL_SHIFT - 1));
        @@ a1 = a0 + W6 * row[2];
        @@ a2 = a0 - W6 * row[2];
        @@ a3 = a0 - W2 * row[2];
        @@ a0 = a0 + W2 * row[2];
        ldrsh r6, [r14, #0]
        ldr r9, =W4              @ R9=W4
        mul r6, r9, r6           @ R6=W4*ROWr16[0]
        ldr r10, =W6             @ R10=W6
        ldrsh r4, [r14, #32]     @ R4=ROWr16[2] (a3 not defined yet)
        add r6, r6, #COL_SHIFTED_1 @ R6=W4*ROWr16[0] + 1<<(COL_SHIFT-1) (a0)
        mul r11, r10, r4         @ R11=W6*ROWr16[2]
        ldr r8, =W2              @ R8=W2
        add r2, r6, r11          @ R2=a0+W6*ROWr16[2] (a1)
        sub r3, r6, r11          @ R3=a0-W6*ROWr16[2] (a2)
        mul r11, r8, r4          @ R11=W2*ROWr16[2]
        sub r4, r6, r11          @ R4=a0-W2*ROWr16[2] (a3)
        add r6, r6, r11          @ R6=a0+W2*ROWr16[2] (a0)

        @@ at this point, R0=b0,  R1=b1, R2=a1, R3=a2, R4=a3,
        @@     R5=b2, R6=a0, R7=b3, R8=W2, R9=W4, R10=W6, R11 (free),
        @@     R12=__const_ptr_, R14=&block[n]
        @@ a0 += W4*row[4]
        @@ a1 -= W4*row[4]
        @@ a2 -= W4*row[4]
        @@ a3 += W4*row[4]
        ldrsh r11, [r14, #64]    @ R11=ROWr16[4]
        teq r11, #0              @ if null avoid muls
        itttt ne
        mulne r11, r9, r11       @ R11=W4*ROWr16[4]
        @@ R9 is free now
        addne r6, r6, r11        @ R6+=W4*ROWr16[4] (a0)
        subne r2, r2, r11        @ R2-=W4*ROWr16[4] (a1)
        subne r3, r3, r11        @ R3-=W4*ROWr16[4] (a2)
        ldrsh r9, [r14, #96]     @ R9=ROWr16[6]
        it    ne
        addne r4, r4, r11        @ R4+=W4*ROWr16[4] (a3)
        @@ W6 alone is no more useful, save W2*ROWr16[6] in it instead
        teq r9, #0               @ if null avoid muls
        itttt ne
        mulne r11, r10, r9       @ R11=W6*ROWr16[6]
        addne r6, r6, r11        @ R6+=W6*ROWr16[6] (a0)
        mulne r10, r8, r9        @ R10=W2*ROWr16[6]
        @@ a0 += W6*row[6];
        @@ a3 -= W6*row[6];
        @@ a1 -= W2*row[6];
        @@ a2 += W2*row[6];
        subne r4, r4, r11        @ R4-=W6*ROWr16[6] (a3)
        itt   ne
        subne r2, r2, r10        @ R2-=W2*ROWr16[6] (a1)
        addne r3, r3, r10        @ R3+=W2*ROWr16[6] (a2)
@@ __end_a_evaluation2:
        @@ at this point, R0=b0,  R1=b1, R2=a1, R3=a2, R4=a3,
        @@     R5=b2, R6=a0, R7=b3, R8 (free), R9 (free), R10 (free), R11 (free),
        @@     R12=__const_ptr_, R14=&block[n]
        @@ col[0 ] = ((a0 + b0) >> COL_SHIFT);
        @@ col[8 ] = ((a1 + b1) >> COL_SHIFT);
        @@ col[16] = ((a2 + b2) >> COL_SHIFT);
        @@ col[24] = ((a3 + b3) >> COL_SHIFT);
        @@ col[32] = ((a3 - b3) >> COL_SHIFT);
        @@ col[40] = ((a2 - b2) >> COL_SHIFT);
        @@ col[48] = ((a1 - b1) >> COL_SHIFT);
        @@ col[56] = ((a0 - b0) >> COL_SHIFT);
        @@@@@ no optimization here @@@@@
        add r8, r6, r0           @ R8=a0+b0
        add r9, r2, r1           @ R9=a1+b1
        mov r8, r8, asr #COL_SHIFT
        mov r9, r9, asr #COL_SHIFT
        strh r8, [r14, #0]
        strh r9, [r14, #16]
        add r8, r3, r5           @ R8=a2+b2
        add r9, r4, r7           @ R9=a3+b3
        mov r8, r8, asr #COL_SHIFT
        mov r9, r9, asr #COL_SHIFT
        strh r8, [r14, #32]
        strh r9, [r14, #48]
        sub r8, r4, r7           @ R8=a3-b3
        sub r9, r3, r5           @ R9=a2-b2
        mov r8, r8, asr #COL_SHIFT
        mov r9, r9, asr #COL_SHIFT
        strh r8, [r14, #64]
        strh r9, [r14, #80]
        sub r8, r2, r1           @ R8=a1-b1
        sub r9, r6, r0           @ R9=a0-b0
        mov r8, r8, asr #COL_SHIFT
        mov r9, r9, asr #COL_SHIFT
        strh r8, [r14, #96]
        strh r9, [r14, #112]

@@ __end_col_loop:
        @@ at this point, R0-R11 (free)
        @@     R12=__const_ptr_, R14=&block[n]
        ldr r0, [sp, #0]         @ R0=block
        teq r0, r14              @ compare current &block[n] to block, when block is reached, the loop is finished.
        sub r14, r14, #2
        bne __col_loop




@@ __end_simple_idct_arm:
        @@ restore registers to previous status!
        add sp, sp, #8 @@ the local variables!
        ldmfd sp!, {r4-r11, r15} @@ update PC with LR content.



@@ kind of sub-function, here not to overload the common case.
__end_bef_a_evaluation:
        add r2, r6, r11          @ R2=a0+W6*ROWr16[2] (a1)
        mul r11, r8, r4          @ R11=W2*ROWr16[2]
        sub r4, r6, r11          @ R4=a0-W2*ROWr16[2] (a3)
        add r6, r6, r11          @ R6=a0+W2*ROWr16[2] (a0)
        bal __end_a_evaluation
endfunc