Details | Last modification | View Log | RSS feed
Rev | Author | Line No. | Line |
---|---|---|---|
6147 | serge | 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 | When debugging a x86 external asm compilation issue, if lost in the macro |
||
195 | expansions, add DBG=1 to your make command-line: the input file will be |
||
196 | preprocessed, stripped of the debug/empty lines, then compiled, showing the |
||
197 | actual lines causing issues. |
||
198 | |||
199 | Inline asm vs. external asm |
||
200 | --------------------------- |
||
201 | Both inline asm (__asm__("..") in a .c file, handled by a compiler such as gcc) |
||
202 | and external asm (.s or .asm files, handled by an assembler such as yasm/nasm) |
||
203 | are accepted in FFmpeg. Which one to use differs per specific case. |
||
204 | |||
205 | - if your code is intended to be inlined in a C function, inline asm is always |
||
206 | better, because external asm cannot be inlined |
||
207 | - if your code calls external functions, yasm is always better |
||
208 | - if your code takes huge and complex structs as function arguments (e.g. |
||
209 | MpegEncContext; note that this is not ideal and is discouraged if there |
||
210 | are alternatives), then inline asm is always better, because predicting |
||
211 | member offsets in complex structs is almost impossible. It's safest to let |
||
212 | the compiler take care of that |
||
213 | - in many cases, both can be used and it just depends on the preference of the |
||
214 | person writing the asm. For new asm, the choice is up to you. For existing |
||
215 | asm, you'll likely want to maintain whatever form it is currently in unless |
||
216 | there is a good reason to change it. |
||
217 | - if, for some reason, you believe that a particular chunk of existing external |
||
218 | asm could be improved upon further if written in inline asm (or the other |
||
219 | way around), then please make the move from external asm <-> inline asm a |
||
220 | separate patch before your patches that actually improve the asm. |
||
221 | |||
222 | |||
223 | Links: |
||
224 | ====== |
||
225 | http://www.aggregate.org/MAGIC/ |
||
226 | |||
227 | x86-specific: |
||
228 | ------------- |
||
229 | http://developer.intel.com/design/pentium4/manuals/248966.htm |
||
230 | |||
231 | The IA-32 Intel Architecture Software Developer's Manual, Volume 2: |
||
232 | Instruction Set Reference |
||
233 | http://developer.intel.com/design/pentium4/manuals/245471.htm |
||
234 | |||
235 | http://www.agner.org/assem/ |
||
236 | |||
237 | AMD Athlon Processor x86 Code Optimization Guide: |
||
238 | http://www.amd.com/us-en/assets/content_type/white_papers_and_tech_docs/22007.pdf |
||
239 | |||
240 | |||
241 | ARM-specific: |
||
242 | ------------- |
||
243 | ARM Architecture Reference Manual (up to ARMv5TE): |
||
244 | http://www.arm.com/community/university/eulaarmarm.html |
||
245 | |||
246 | Procedure Call Standard for the ARM Architecture: |
||
247 | http://www.arm.com/pdfs/aapcs.pdf |
||
248 | |||
249 | Optimization guide for ARM9E (used in Nokia 770 Internet Tablet): |
||
250 | http://infocenter.arm.com/help/topic/com.arm.doc.ddi0240b/DDI0240A.pdf |
||
251 | Optimization guide for ARM11 (used in Nokia N800 Internet Tablet): |
||
252 | http://infocenter.arm.com/help/topic/com.arm.doc.ddi0211j/DDI0211J_arm1136_r1p5_trm.pdf |
||
253 | Optimization guide for Intel XScale (used in Sharp Zaurus PDA): |
||
254 | http://download.intel.com/design/intelxscale/27347302.pdf |
||
255 | Intel Wireless MMX 2 Coprocessor: Programmers Reference Manual |
||
256 | http://download.intel.com/design/intelxscale/31451001.pdf |
||
257 | |||
258 | PowerPC-specific: |
||
259 | ----------------- |
||
260 | PowerPC32/AltiVec PIM: |
||
261 | www.freescale.com/files/32bit/doc/ref_manual/ALTIVECPEM.pdf |
||
262 | |||
263 | PowerPC32/AltiVec PEM: |
||
264 | www.freescale.com/files/32bit/doc/ref_manual/ALTIVECPIM.pdf |
||
265 | |||
266 | CELL/SPU: |
||
267 | http://www-01.ibm.com/chips/techlib/techlib.nsf/techdocs/30B3520C93F437AB87257060006FFE5E/$file/Language_Extensions_for_CBEA_2.4.pdf |
||
268 | http://www-01.ibm.com/chips/techlib/techlib.nsf/techdocs/9F820A5FFA3ECE8C8725716A0062585F/$file/CBE_Handbook_v1.1_24APR2007_pub.pdf |
||
269 | |||
270 | GCC asm links: |
||
271 | -------------- |
||
272 | official doc but quite ugly |
||
273 | http://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html |
||
274 | |||
275 | a bit old (note "+" is valid for input-output, even though the next disagrees) |
||
276 | http://www.cs.virginia.edu/~clc5q/gcc-inline-asm.pdf-> |