1/* 2 * Copyright (C) 2010 The Android Open Source Project 3 * 4 * Licensed under the Apache License, Version 2.0 (the "License"); 5 * you may not use this file except in compliance with the License. 6 * You may obtain a copy of the License at 7 * 8 * http://www.apache.org/licenses/LICENSE-2.0 9 * 10 * Unless required by applicable law or agreed to in writing, software 11 * distributed under the License is distributed on an "AS IS" BASIS, 12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 13 * See the License for the specific language governing permissions and 14 * limitations under the License. 15 */ 16 17/* 18 * This is a fast-and-accurate implementation of inverse Discrete Cosine 19 * Transform (IDCT) for ARMv6+. It also performs dequantization of the input 20 * coefficients just like other methods. 21 * 22 * This implementation is based on the scaled 1-D DCT algorithm proposed by 23 * Arai, Agui, and Nakajima. The following code is based on the figure 4-8 24 * on page 52 of the JPEG textbook by Pennebaker and Mitchell. Coefficients 25 * are (almost) directly mapped into registers. 26 * 27 * The accuracy is achieved by using SMULWy and SMLAWy instructions. Both 28 * multiply 32 bits by 16 bits and store the top 32 bits of the result. It 29 * makes 32-bit fixed-point arithmetic possible without overflow. That is 30 * why jpeg_idct_ifast(), which is written in C, cannot be improved. 31 * 32 * More tricks are used to gain more speed. First of all, we use as many 33 * registers as possible. ARM processor has 16 registers including sp (r13) 34 * and pc (r15), so only 14 registers can be used without limitations. In 35 * general, we let r0 to r7 hold the coefficients; r10 and r11 hold four 36 * 16-bit constants; r12 and r14 hold two of the four arguments; and r8 hold 37 * intermediate value. In the second pass, r9 is the loop counter. In the 38 * first pass, r8 to r11 are used to hold quantization values, so the loop 39 * counter is held by sp. Yes, the stack pointer. Since it must be aligned 40 * to 4-byte boundary all the time, we align it to 32-byte boundary and use 41 * bit 3 to bit 5. As the result, we actually use 14.1 registers. :-) 42 * 43 * Second, we rearrange quantization values to access them sequentially. The 44 * table is first transposed, and the new columns are placed in the order of 45 * 7, 5, 1, 3, 0, 2, 4, 6. Thus we can use LDMDB to load four values at a 46 * time. Rearranging coefficients also helps, but that requires to change a 47 * dozen of files, which seems not worth it. In addition, we choose to scale 48 * up quantization values by 13 bits, so the coefficients are scaled up by 49 * 16 bits after both passes. Then we can pack and saturate them two at a 50 * time using PKHTB and USAT16 instructions. 51 * 52 * Third, we reorder the instructions to avoid bubbles in the pipeline. This 53 * is done by hand accroding to the cycle timings and the interlock behavior 54 * described in the technical reference manual of ARM1136JF-S. We also take 55 * advantage of dual issue processors by interleaving instructions with 56 * dependencies. It has been benchmarked on four devices and all the results 57 * showed distinguishable improvements. Note that PLD instructions actually 58 * slow things down, so they are removed at the last minute. In the future, 59 * this might be futher improved using a system profiler. 60 */ 61 62#ifdef __arm__ 63#include <machine/cpu-features.h> 64#endif 65 66#if __ARM_ARCH__ >= 6 67 68// void armv6_idct(short *coefs, int *quans, unsigned char *rows, int col) 69 .arm 70 .text 71 .align 72 .global armv6_idct 73 .func armv6_idct 74 75armv6_idct: 76 // Push everything except sp (r13) and pc (r15). 77 stmdb sp!, {r4, r5, r6, r7, r8, r9, r10, r11, r12, r14} 78 79 // r12 = quans, r14 = coefs. 80 sub r4, sp, #236 81 bic sp, r4, #31 82 add r5, sp, #224 83 add r12, r1, #256 84 stm r5, {r2, r3, r4} 85 add r14, r0, #16 86 87pass1_head: 88 // Load quantization values. (q[0, 2, 4, 6]) 89 ldmdb r12!, {r8, r9, r10, r11} 90 91 // Load coefficients. (c[4, 1, 2, 3, 0, 5, 6, 7]) 92 ldrsh r4, [r14, #-2] ! 93 ldrsh r1, [r14, #16] 94 ldrsh r2, [r14, #32] 95 ldrsh r3, [r14, #48] 96 ldrsh r0, [r14, #64] 97 ldrsh r5, [r14, #80] 98 ldrsh r6, [r14, #96] 99 ldrsh r7, [r14, #112] 100 101 // r4 = q[0] * c[0]; 102 mul r4, r8, r4 103 104 // Check if ACs are all zero. 105 cmp r0, #0 106 orreqs r8, r1, r2 107 orreqs r8, r3, r5 108 orreqs r8, r6, r7 109 beq pass1_zero 110 111 // Step 1: Dequantizations. 112 113 // r2 = q[2] * c[2]; 114 // r0 = q[4] * c[4] + r4; 115 // r6 = q[6] * c[6] + r2; 116 mul r2, r9, r2 117 mla r0, r10, r0, r4 118 mla r6, r11, r6, r2 119 120 // Load quantization values. (q[7, 5, 1, 3]) 121 ldmdb r12!, {r8, r9, r10, r11} 122 123 // r4 = r4 * 2 - r0 = -(r0 - r4 * 2); 124 // r2 = r2 * 2 - r6 = -(r6 - r2 * 2); 125 rsb r4, r0, r4, lsl #1 126 rsb r2, r6, r2, lsl #1 127 128 // r7 = q[7] * c[7]; 129 // r5 = q[5] * c[5]; 130 // r1 = q[1] * c[1] + r7; 131 // r3 = q[3] * c[3] + r5; 132 mul r7, r8, r7 133 mul r5, r9, r5 134 mla r1, r10, r1, r7 135 mla r3, r11, r3, r5 136 137 // Load constants. 138 ldrd r10, constants 139 140 // Step 2: Rotations and Butterflies. 141 142 // r7 = r1 - r7 * 2; 143 // r1 = r1 - r3; 144 // r5 = r5 * 2 - r3 = -(r3 - r5 * 2); 145 // r3 = r1 + r3 * 2; 146 // r8 = r5 + r7; 147 sub r7, r1, r7, lsl #1 148 sub r1, r1, r3 149 rsb r5, r3, r5, lsl #1 150 add r3, r1, r3, lsl #1 151 add r8, r5, r7 152 153 // r2 = r2 * 1.41421 = r2 * 27146 / 65536 + r2; 154 // r8 = r8 * 1.84776 / 8 = r8 * 15137 / 65536; 155 // r1 = r1 * 1.41421 = r1 * 27146 / 65536 + r1; 156 smlawt r2, r2, r10, r2 157 smulwb r8, r8, r10 158 smlawt r1, r1, r10, r1 159 160 // r0 = r0 + r6; 161 // r2 = r2 - r6; 162 // r6 = r0 - r6 * 2; 163 add r0, r0, r6 164 sub r2, r2, r6 165 sub r6, r0, r6, lsl #1 166 167 // r5 = r5 * -2.61313 / 8 + r8 = r5 * -21407 / 65536 + r8; 168 // r8 = r7 * -1.08239 / 8 + r8 = r7 * -8867 / 65536 + r8; 169 smlawt r5, r5, r11, r8 170 smlawb r8, r7, r11, r8 171 172 // r4 = r4 + r2; 173 // r0 = r0 + r3; 174 // r2 = r4 - r2 * 2; 175 add r4, r4, r2 176 add r0, r0, r3 177 sub r2, r4, r2, lsl #1 178 179 // r7 = r5 * 8 - r3 = -(r3 - r5 * 8); 180 // r3 = r0 - r3 * 2; 181 // r1 = r1 - r7; 182 // r4 = r4 + r7; 183 // r5 = r8 * 8 - r1 = -(r1 - r8 * 8); 184 // r7 = r4 - r7 * 2; 185 rsb r7, r3, r5, lsl #3 186 sub r3, r0, r3, lsl #1 187 sub r1, r1, r7 188 add r4, r4, r7 189 rsb r5, r1, r8, lsl #3 190 sub r7, r4, r7, lsl #1 191 192 // r2 = r2 + r1; 193 // r6 = r6 + r5; 194 // r1 = r2 - r1 * 2; 195 // r5 = r6 - r5 * 2; 196 add r2, r2, r1 197 add r6, r6, r5 198 sub r1, r2, r1, lsl #1 199 sub r5, r6, r5, lsl #1 200 201 // Step 3: Reorder and Save. 202 203 str r0, [sp, #-4] ! 204 str r4, [sp, #32] 205 str r2, [sp, #64] 206 str r6, [sp, #96] 207 str r5, [sp, #128] 208 str r1, [sp, #160] 209 str r7, [sp, #192] 210 str r3, [sp, #224] 211 b pass1_tail 212 213 // Precomputed 16-bit constants: 27146, 15137, -21407, -8867. 214 // Put them in the middle since LDRD only accepts offsets from -255 to 255. 215 .align 3 216constants: 217 .word 0x6a0a3b21 218 .word 0xac61dd5d 219 220pass1_zero: 221 str r4, [sp, #-4] ! 222 str r4, [sp, #32] 223 str r4, [sp, #64] 224 str r4, [sp, #96] 225 str r4, [sp, #128] 226 str r4, [sp, #160] 227 str r4, [sp, #192] 228 str r4, [sp, #224] 229 sub r12, r12, #16 230 231pass1_tail: 232 ands r9, sp, #31 233 bne pass1_head 234 235 // r12 = rows, r14 = col. 236 ldr r12, [sp, #256] 237 ldr r14, [sp, #260] 238 239 // Load constants. 240 ldrd r10, constants 241 242pass2_head: 243 // Load coefficients. (c[0, 1, 2, 3, 4, 5, 6, 7]) 244 ldmia sp!, {r0, r1, r2, r3, r4, r5, r6, r7} 245 246 // r0 = r0 + 0x00808000; 247 add r0, r0, #0x00800000 248 add r0, r0, #0x00008000 249 250 // Step 1: Analog to the first pass. 251 252 // r0 = r0 + r4; 253 // r6 = r6 + r2; 254 add r0, r0, r4 255 add r6, r6, r2 256 257 // r4 = r0 - r4 * 2; 258 // r2 = r2 * 2 - r6 = -(r6 - r2 * 2); 259 sub r4, r0, r4, lsl #1 260 rsb r2, r6, r2, lsl #1 261 262 // r1 = r1 + r7; 263 // r3 = r3 + r5; 264 add r1, r1, r7 265 add r3, r3, r5 266 267 // Step 2: Rotations and Butterflies. 268 269 // r7 = r1 - r7 * 2; 270 // r1 = r1 - r3; 271 // r5 = r5 * 2 - r3 = -(r3 - r5 * 2); 272 // r3 = r1 + r3 * 2; 273 // r8 = r5 + r7; 274 sub r7, r1, r7, lsl #1 275 sub r1, r1, r3 276 rsb r5, r3, r5, lsl #1 277 add r3, r1, r3, lsl #1 278 add r8, r5, r7 279 280 // r2 = r2 * 1.41421 = r2 * 27146 / 65536 + r2; 281 // r8 = r8 * 1.84776 / 8 = r8 * 15137 / 65536; 282 // r1 = r1 * 1.41421 = r1 * 27146 / 65536 + r1; 283 smlawt r2, r2, r10, r2 284 smulwb r8, r8, r10 285 smlawt r1, r1, r10, r1 286 287 // r0 = r0 + r6; 288 // r2 = r2 - r6; 289 // r6 = r0 - r6 * 2; 290 add r0, r0, r6 291 sub r2, r2, r6 292 sub r6, r0, r6, lsl #1 293 294 // r5 = r5 * -2.61313 / 8 + r8 = r5 * -21407 / 65536 + r8; 295 // r8 = r7 * -1.08239 / 8 + r8 = r7 * -8867 / 65536 + r8; 296 smlawt r5, r5, r11, r8 297 smlawb r8, r7, r11, r8 298 299 // r4 = r4 + r2; 300 // r0 = r0 + r3; 301 // r2 = r4 - r2 * 2; 302 add r4, r4, r2 303 add r0, r0, r3 304 sub r2, r4, r2, lsl #1 305 306 // r7 = r5 * 8 - r3 = -(r3 - r5 * 8); 307 // r3 = r0 - r3 * 2; 308 // r1 = r1 - r7; 309 // r4 = r4 + r7; 310 // r5 = r8 * 8 - r1 = -(r1 - r8 * 8); 311 // r7 = r4 - r7 * 2; 312 rsb r7, r3, r5, lsl #3 313 sub r3, r0, r3, lsl #1 314 sub r1, r1, r7 315 add r4, r4, r7 316 rsb r5, r1, r8, lsl #3 317 sub r7, r4, r7, lsl #1 318 319 // r2 = r2 + r1; 320 // r6 = r6 + r5; 321 // r1 = r2 - r1 * 2; 322 // r5 = r6 - r5 * 2; 323 add r2, r2, r1 324 add r6, r6, r5 325 sub r1, r2, r1, lsl #1 326 sub r5, r6, r5, lsl #1 327 328 // Step 3: Reorder and Save. 329 330 // Load output pointer. 331 ldr r8, [r12], #4 332 333 // For little endian: r6, r2, r4, r0, r3, r7, r1, r5. 334 pkhtb r6, r6, r4, asr #16 335 pkhtb r2, r2, r0, asr #16 336 pkhtb r3, r3, r1, asr #16 337 pkhtb r7, r7, r5, asr #16 338 usat16 r6, #8, r6 339 usat16 r2, #8, r2 340 usat16 r3, #8, r3 341 usat16 r7, #8, r7 342 orr r0, r2, r6, lsl #8 343 orr r1, r7, r3, lsl #8 344 345#ifdef __ARMEB__ 346 // Reverse bytes for big endian. 347 rev r0, r0 348 rev r1, r1 349#endif 350 351 // Use STR instead of STRD to support unaligned access. 352 str r0, [r8, r14] ! 353 str r1, [r8, #4] 354 355pass2_tail: 356 adds r9, r9, #0x10000000 357 bpl pass2_head 358 359 ldr sp, [sp, #8] 360 add sp, sp, #236 361 362 ldmia sp!, {r4, r5, r6, r7, r8, r9, r10, r11, r12, r14} 363 bx lr 364 .endfunc 365 366#endif 367