Commit | Line | Data |
---|---|---|
2ba45a60 DM |
1 | optimization Tips (for libavcodec): |
2 | =================================== | |
3 | ||
4 | What to optimize: | |
5 | ----------------- | |
6 | If you plan to do non-x86 architecture specific optimizations (SIMD normally), | |
7 | then take a look in the x86/ directory, as most important functions are | |
8 | already optimized for MMX. | |
9 | ||
10 | If you want to do x86 optimizations then you can either try to finetune the | |
11 | stuff in the x86 directory or find some other functions in the C source to | |
12 | optimize, but there aren't many left. | |
13 | ||
14 | ||
15 | Understanding these overoptimized functions: | |
16 | -------------------------------------------- | |
17 | As many functions tend to be a bit difficult to understand because | |
18 | of optimizations, it can be hard to optimize them further, or write | |
19 | architecture-specific versions. It is recommended to look at older | |
20 | revisions of the interesting files (web frontends for the various FFmpeg | |
21 | branches are listed at http://ffmpeg.org/download.html). | |
22 | Alternatively, look into the other architecture-specific versions in | |
23 | the x86/, ppc/, alpha/ subdirectories. Even if you don't exactly | |
24 | comprehend the instructions, it could help understanding the functions | |
25 | and how they can be optimized. | |
26 | ||
27 | NOTE: If you still don't understand some function, ask at our mailing list!!! | |
28 | (http://lists.ffmpeg.org/mailman/listinfo/ffmpeg-devel) | |
29 | ||
30 | ||
31 | When is an optimization justified? | |
32 | ---------------------------------- | |
33 | Normally, clean and simple optimizations for widely used codecs are | |
34 | justified even if they only achieve an overall speedup of 0.1%. These | |
35 | speedups accumulate and can make a big difference after awhile. Also, if | |
36 | none of the following factors get worse due to an optimization -- speed, | |
37 | binary code size, source size, source readability -- and at least one | |
38 | factor improves, then an optimization is always a good idea even if the | |
39 | overall gain is less than 0.1%. For obscure codecs that are not often | |
40 | used, the goal is more toward keeping the code clean, small, and | |
41 | readable instead of making it 1% faster. | |
42 | ||
43 | ||
44 | WTF is that function good for ....: | |
45 | ----------------------------------- | |
46 | The primary purpose of this list is to avoid wasting time optimizing functions | |
47 | which are rarely used. | |
48 | ||
49 | put(_no_rnd)_pixels{,_x2,_y2,_xy2} | |
50 | Used in motion compensation (en/decoding). | |
51 | ||
52 | avg_pixels{,_x2,_y2,_xy2} | |
53 | Used in motion compensation of B-frames. | |
54 | These are less important than the put*pixels functions. | |
55 | ||
56 | avg_no_rnd_pixels* | |
57 | unused | |
58 | ||
59 | pix_abs16x16{,_x2,_y2,_xy2} | |
60 | Used in motion estimation (encoding) with SAD. | |
61 | ||
62 | pix_abs8x8{,_x2,_y2,_xy2} | |
63 | Used in motion estimation (encoding) with SAD of MPEG-4 4MV only. | |
64 | These are less important than the pix_abs16x16* functions. | |
65 | ||
66 | put_mspel8_mc* / wmv2_mspel8* | |
67 | Used only in WMV2. | |
68 | it is not recommended that you waste your time with these, as WMV2 | |
69 | is an ugly and relatively useless codec. | |
70 | ||
71 | mpeg4_qpel* / *qpel_mc* | |
72 | Used in MPEG-4 qpel motion compensation (encoding & decoding). | |
73 | The qpel8 functions are used only for 4mv, | |
74 | the avg_* functions are used only for B-frames. | |
75 | Optimizing them should have a significant impact on qpel | |
76 | encoding & decoding. | |
77 | ||
78 | qpel{8,16}_mc??_old_c / *pixels{8,16}_l4 | |
79 | Just used to work around a bug in an old libavcodec encoder version. | |
80 | Don't optimize them. | |
81 | ||
82 | add_bytes/diff_bytes | |
83 | For huffyuv only, optimize if you want a faster ffhuffyuv codec. | |
84 | ||
85 | get_pixels / diff_pixels | |
86 | Used for encoding, easy. | |
87 | ||
88 | clear_blocks | |
89 | easiest to optimize | |
90 | ||
91 | gmc | |
92 | Used for MPEG-4 gmc. | |
93 | Optimizing this should have a significant effect on the gmc decoding | |
94 | speed. | |
95 | ||
96 | gmc1 | |
97 | Used for chroma blocks in MPEG-4 gmc with 1 warp point | |
98 | (there are 4 luma & 2 chroma blocks per macroblock, so | |
99 | only 1/3 of the gmc blocks use this, the other 2/3 | |
100 | use the normal put_pixel* code, but only if there is | |
101 | just 1 warp point). | |
102 | Note: DivX5 gmc always uses just 1 warp point. | |
103 | ||
104 | pix_sum | |
105 | Used for encoding. | |
106 | ||
107 | hadamard8_diff / sse / sad == pix_norm1 / dct_sad / quant_psnr / rd / bit | |
108 | Specific compare functions used in encoding, it depends upon the | |
109 | command line switches which of these are used. | |
110 | Don't waste your time with dct_sad & quant_psnr, they aren't | |
111 | really useful. | |
112 | ||
113 | put_pixels_clamped / add_pixels_clamped | |
114 | Used for en/decoding in the IDCT, easy. | |
115 | Note, some optimized IDCTs have the add/put clamped code included and | |
116 | then put_pixels_clamped / add_pixels_clamped will be unused. | |
117 | ||
118 | idct/fdct | |
119 | idct (encoding & decoding) | |
120 | fdct (encoding) | |
121 | difficult to optimize | |
122 | ||
123 | dct_quantize_trellis | |
124 | Used for encoding with trellis quantization. | |
125 | difficult to optimize | |
126 | ||
127 | dct_quantize | |
128 | Used for encoding. | |
129 | ||
130 | dct_unquantize_mpeg1 | |
131 | Used in MPEG-1 en/decoding. | |
132 | ||
133 | dct_unquantize_mpeg2 | |
134 | Used in MPEG-2 en/decoding. | |
135 | ||
136 | dct_unquantize_h263 | |
137 | Used in MPEG-4/H.263 en/decoding. | |
138 | ||
139 | ||
140 | ||
141 | Alignment: | |
142 | Some instructions on some architectures have strict alignment restrictions, | |
143 | for example most SSE/SSE2 instructions on x86. | |
144 | The minimum guaranteed alignment is written in the .h files, for example: | |
145 | void (*put_pixels_clamped)(const int16_t *block/*align 16*/, UINT8 *pixels/*align 8*/, int line_size); | |
146 | ||
147 | ||
148 | General Tips: | |
149 | ------------- | |
150 | Use asm loops like: | |
151 | __asm__( | |
152 | "1: .... | |
153 | ... | |
154 | "jump_instruction .... | |
155 | Do not use C loops: | |
156 | do{ | |
157 | __asm__( | |
158 | ... | |
159 | }while() | |
160 | ||
161 | For x86, mark registers that are clobbered in your asm. This means both | |
162 | general x86 registers (e.g. eax) as well as XMM registers. This last one is | |
163 | particularly important on Win64, where xmm6-15 are callee-save, and not | |
164 | restoring their contents leads to undefined results. In external asm (e.g. | |
165 | yasm), you do this by using: | |
166 | cglobal functon_name, num_args, num_regs, num_xmm_regs | |
167 | In inline asm, you specify clobbered registers at the end of your asm: | |
168 | __asm__(".." ::: "%eax"). | |
169 | If gcc is not set to support sse (-msse) it will not accept xmm registers | |
170 | in the clobber list. For that we use two macros to declare the clobbers. | |
171 | XMM_CLOBBERS should be used when there are other clobbers, for example: | |
172 | __asm__(".." ::: XMM_CLOBBERS("xmm0",) "eax"); | |
173 | and XMM_CLOBBERS_ONLY should be used when the only clobbers are xmm registers: | |
174 | __asm__(".." :: XMM_CLOBBERS_ONLY("xmm0")); | |
175 | ||
176 | Do not expect a compiler to maintain values in your registers between separate | |
177 | (inline) asm code blocks. It is not required to. For example, this is bad: | |
178 | __asm__("movdqa %0, %%xmm7" : src); | |
179 | /* do something */ | |
180 | __asm__("movdqa %%xmm7, %1" : dst); | |
181 | - first of all, you're assuming that the compiler will not use xmm7 in | |
182 | between the two asm blocks. It probably won't when you test it, but it's | |
183 | a poor assumption that will break at some point for some --cpu compiler flag | |
184 | - secondly, you didn't mark xmm7 as clobbered. If you did, the compiler would | |
185 | have restored the original value of xmm7 after the first asm block, thus | |
186 | rendering the combination of the two blocks of code invalid | |
187 | Code that depends on data in registries being untouched, should be written as | |
188 | a single __asm__() statement. Ideally, a single function contains only one | |
189 | __asm__() block. | |
190 | ||
191 | Use external asm (nasm/yasm) or inline asm (__asm__()), do not use intrinsics. | |
192 | The latter requires a good optimizing compiler which gcc is not. | |
193 | ||
194 | Inline asm vs. external asm | |
195 | --------------------------- | |
196 | Both inline asm (__asm__("..") in a .c file, handled by a compiler such as gcc) | |
197 | and external asm (.s or .asm files, handled by an assembler such as yasm/nasm) | |
198 | are accepted in FFmpeg. Which one to use differs per specific case. | |
199 | ||
200 | - if your code is intended to be inlined in a C function, inline asm is always | |
201 | better, because external asm cannot be inlined | |
202 | - if your code calls external functions, yasm is always better | |
203 | - if your code takes huge and complex structs as function arguments (e.g. | |
204 | MpegEncContext; note that this is not ideal and is discouraged if there | |
205 | are alternatives), then inline asm is always better, because predicting | |
206 | member offsets in complex structs is almost impossible. It's safest to let | |
207 | the compiler take care of that | |
208 | - in many cases, both can be used and it just depends on the preference of the | |
209 | person writing the asm. For new asm, the choice is up to you. For existing | |
210 | asm, you'll likely want to maintain whatever form it is currently in unless | |
211 | there is a good reason to change it. | |
212 | - if, for some reason, you believe that a particular chunk of existing external | |
213 | asm could be improved upon further if written in inline asm (or the other | |
214 | way around), then please make the move from external asm <-> inline asm a | |
215 | separate patch before your patches that actually improve the asm. | |
216 | ||
217 | ||
218 | Links: | |
219 | ====== | |
220 | http://www.aggregate.org/MAGIC/ | |
221 | ||
222 | x86-specific: | |
223 | ------------- | |
224 | http://developer.intel.com/design/pentium4/manuals/248966.htm | |
225 | ||
226 | The IA-32 Intel Architecture Software Developer's Manual, Volume 2: | |
227 | Instruction Set Reference | |
228 | http://developer.intel.com/design/pentium4/manuals/245471.htm | |
229 | ||
230 | http://www.agner.org/assem/ | |
231 | ||
232 | AMD Athlon Processor x86 Code Optimization Guide: | |
233 | http://www.amd.com/us-en/assets/content_type/white_papers_and_tech_docs/22007.pdf | |
234 | ||
235 | ||
236 | ARM-specific: | |
237 | ------------- | |
238 | ARM Architecture Reference Manual (up to ARMv5TE): | |
239 | http://www.arm.com/community/university/eulaarmarm.html | |
240 | ||
241 | Procedure Call Standard for the ARM Architecture: | |
242 | http://www.arm.com/pdfs/aapcs.pdf | |
243 | ||
244 | Optimization guide for ARM9E (used in Nokia 770 Internet Tablet): | |
245 | http://infocenter.arm.com/help/topic/com.arm.doc.ddi0240b/DDI0240A.pdf | |
246 | Optimization guide for ARM11 (used in Nokia N800 Internet Tablet): | |
247 | http://infocenter.arm.com/help/topic/com.arm.doc.ddi0211j/DDI0211J_arm1136_r1p5_trm.pdf | |
248 | Optimization guide for Intel XScale (used in Sharp Zaurus PDA): | |
249 | http://download.intel.com/design/intelxscale/27347302.pdf | |
250 | Intel Wireless MMX 2 Coprocessor: Programmers Reference Manual | |
251 | http://download.intel.com/design/intelxscale/31451001.pdf | |
252 | ||
253 | PowerPC-specific: | |
254 | ----------------- | |
255 | PowerPC32/AltiVec PIM: | |
256 | www.freescale.com/files/32bit/doc/ref_manual/ALTIVECPEM.pdf | |
257 | ||
258 | PowerPC32/AltiVec PEM: | |
259 | www.freescale.com/files/32bit/doc/ref_manual/ALTIVECPIM.pdf | |
260 | ||
261 | CELL/SPU: | |
262 | http://www-01.ibm.com/chips/techlib/techlib.nsf/techdocs/30B3520C93F437AB87257060006FFE5E/$file/Language_Extensions_for_CBEA_2.4.pdf | |
263 | http://www-01.ibm.com/chips/techlib/techlib.nsf/techdocs/9F820A5FFA3ECE8C8725716A0062585F/$file/CBE_Handbook_v1.1_24APR2007_pub.pdf | |
264 | ||
265 | GCC asm links: | |
266 | -------------- | |
267 | official doc but quite ugly | |
268 | http://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html | |
269 | ||
270 | a bit old (note "+" is valid for input-output, even though the next disagrees) | |
271 | http://www.cs.virginia.edu/~clc5q/gcc-inline-asm.pdf |