More accurate value for log2(10)
[libav.git] / doc / optimization.txt
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a552591f 1optimization Tips (for libavcodec):
5e123bd3 2===================================
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3
4What to optimize:
5e123bd3 5-----------------
c5a44f57 6If you plan to do non-x86 architecture specific optimizations (SIMD normally),
a6493a8f 7then take a look in the x86/ directory, as most important functions are
8ea9ce41 8already optimized for MMX.
a552591f 9
8ea9ce41 10If you want to do x86 optimizations then you can either try to finetune the
a6493a8f 11stuff in the x86 directory or find some other functions in the C source to
8ea9ce41 12optimize, but there aren't many left.
a552591f 13
5e123bd3 14
a552591f 15Understanding these overoptimized functions:
5e123bd3 16--------------------------------------------
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17As many functions tend to be a bit difficult to understand because
18of optimizations, it can be hard to optimize them further, or write
6609f9e2 19architecture-specific versions. It is recommended to look at older
1c1b5a40 20revisions of the interesting files (for a web frontend try ViewVC at
0fa5f24c 21http://svn.ffmpeg.org/ffmpeg/trunk/).
0a46c933 22Alternatively, look into the other architecture-specific versions in
a6493a8f 23the x86/, ppc/, alpha/ subdirectories. Even if you don't exactly
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24comprehend the instructions, it could help understanding the functions
25and how they can be optimized.
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26
27NOTE: If you still don't understand some function, ask at our mailing list!!!
2b165e29 28(http://lists.mplayerhq.hu/mailman/listinfo/ffmpeg-devel)
a552591f 29
5e123bd3 30
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31When is an optimization justified?
32----------------------------------
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33Normally, clean and simple optimizations for widely used codecs are
34justified even if they only achieve an overall speedup of 0.1%. These
35speedups accumulate and can make a big difference after awhile. Also, if
36none of the following factors get worse due to an optimization -- speed,
37binary code size, source size, source readability -- and at least one
38factor improves, then an optimization is always a good idea even if the
39overall gain is less than 0.1%. For obscure codecs that are not often
40used, the goal is more toward keeping the code clean, small, and
41readable instead of making it 1% faster.
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42
43
8ea9ce41 44WTF is that function good for ....:
5e123bd3 45-----------------------------------
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46The primary purpose of this list is to avoid wasting time optimizing functions
47which are rarely used.
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48
49put(_no_rnd)_pixels{,_x2,_y2,_xy2}
8ea9ce41 50 Used in motion compensation (en/decoding).
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51
52avg_pixels{,_x2,_y2,_xy2}
8ea9ce41 53 Used in motion compensation of B-frames.
c5a44f57 54 These are less important than the put*pixels functions.
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55
56avg_no_rnd_pixels*
38aca760 57 unused
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58
59pix_abs16x16{,_x2,_y2,_xy2}
8ea9ce41 60 Used in motion estimation (encoding) with SAD.
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61
62pix_abs8x8{,_x2,_y2,_xy2}
8ea9ce41 63 Used in motion estimation (encoding) with SAD of MPEG-4 4MV only.
c5a44f57 64 These are less important than the pix_abs16x16* functions.
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65
66put_mspel8_mc* / wmv2_mspel8*
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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.
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70
71mpeg4_qpel* / *qpel_mc*
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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.
38aca760 77
a552591f 78qpel{8,16}_mc??_old_c / *pixels{8,16}_l4
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79 Just used to work around a bug in an old libavcodec encoder version.
80 Don't optimize them.
a552591f 81
7d67aa9b 82tpel_mc_func {put,avg}_tpel_pixels_tab
8ea9ce41 83 Used only for SVQ3, so only optimize them if you need fast SVQ3 decoding.
7d67aa9b 84
a552591f 85add_bytes/diff_bytes
8ea9ce41 86 For huffyuv only, optimize if you want a faster ffhuffyuv codec.
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87
88get_pixels / diff_pixels
8ea9ce41 89 Used for encoding, easy.
38aca760 90
a552591f 91clear_blocks
8ea9ce41 92 easiest to optimize
38aca760 93
a552591f 94gmc
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95 Used for MPEG-4 gmc.
96 Optimizing this should have a significant effect on the gmc decoding
2e1ad4a2 97 speed.
a552591f 98
143cc725 99gmc1
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100 Used for chroma blocks in MPEG-4 gmc with 1 warp point
101 (there are 4 luma & 2 chroma blocks per macroblock, so
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102 only 1/3 of the gmc blocks use this, the other 2/3
103 use the normal put_pixel* code, but only if there is
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104 just 1 warp point).
105 Note: DivX5 gmc always uses just 1 warp point.
143cc725 106
a552591f 107pix_sum
8ea9ce41 108 Used for encoding.
38aca760 109
8c55915b 110hadamard8_diff / sse / sad == pix_norm1 / dct_sad / quant_psnr / rd / bit
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111 Specific compare functions used in encoding, it depends upon the
112 command line switches which of these are used.
113 Don't waste your time with dct_sad & quant_psnr, they aren't
114 really useful.
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115
116put_pixels_clamped / add_pixels_clamped
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117 Used for en/decoding in the IDCT, easy.
118 Note, some optimized IDCTs have the add/put clamped code included and
119 then put_pixels_clamped / add_pixels_clamped will be unused.
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120
121idct/fdct
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122 idct (encoding & decoding)
123 fdct (encoding)
124 difficult to optimize
125
a552591f 126dct_quantize_trellis
8ea9ce41 127 Used for encoding with trellis quantization.
38aca760 128 difficult to optimize
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129
130dct_quantize
8ea9ce41 131 Used for encoding.
38aca760 132
a552591f 133dct_unquantize_mpeg1
8ea9ce41 134 Used in MPEG-1 en/decoding.
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135
136dct_unquantize_mpeg2
8ea9ce41 137 Used in MPEG-2 en/decoding.
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138
139dct_unquantize_h263
8ea9ce41 140 Used in MPEG-4/H.263 en/decoding.
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141
142FIXME remaining functions?
8ea9ce41 143BTW, most of these functions are in dsputil.c/.h, some are in mpegvideo.c/.h.
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144
145
38aca760 146
a552591f 147Alignment:
8ea9ce41 148Some instructions on some architectures have strict alignment restrictions,
c5a44f57 149for example most SSE/SSE2 instructions on x86.
8ea9ce41 150The minimum guaranteed alignment is written in the .h files, for example:
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151 void (*put_pixels_clamped)(const DCTELEM *block/*align 16*/, UINT8 *pixels/*align 8*/, int line_size);
152
153
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154General Tips:
155-------------
156Use asm loops like:
be449fca 157__asm__(
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158 "1: ....
159 ...
160 "jump_instruciton ....
8144dff0 161Do not use C loops:
7b8c3aed 162do{
be449fca 163 __asm__(
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164 ...
165}while()
166
be449fca 167Use __asm__() instead of intrinsics. The latter requires a good optimizing compiler
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168which gcc is not.
169
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170
171Links:
5e123bd3 172======
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173http://www.aggregate.org/MAGIC/
174
8ea9ce41 175x86-specific:
5e123bd3 176-------------
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177http://developer.intel.com/design/pentium4/manuals/248966.htm
178
38aca760 179The IA-32 Intel Architecture Software Developer's Manual, Volume 2:
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180Instruction Set Reference
181http://developer.intel.com/design/pentium4/manuals/245471.htm
182
183http://www.agner.org/assem/
184
185AMD Athlon Processor x86 Code Optimization Guide:
186http://www.amd.com/us-en/assets/content_type/white_papers_and_tech_docs/22007.pdf
187
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188
189ARM-specific:
5e123bd3 190-------------
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191ARM Architecture Reference Manual (up to ARMv5TE):
192http://www.arm.com/community/university/eulaarmarm.html
193
194Procedure Call Standard for the ARM Architecture:
195http://www.arm.com/pdfs/aapcs.pdf
196
197Optimization guide for ARM9E (used in Nokia 770 Internet Tablet):
198http://infocenter.arm.com/help/topic/com.arm.doc.ddi0240b/DDI0240A.pdf
199Optimization guide for ARM11 (used in Nokia N800 Internet Tablet):
200http://infocenter.arm.com/help/topic/com.arm.doc.ddi0211j/DDI0211J_arm1136_r1p5_trm.pdf
201Optimization guide for Intel XScale (used in Sharp Zaurus PDA):
202http://download.intel.com/design/intelxscale/27347302.pdf
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203Intel Wireless MMX2 Coprocessor: Programmers Reference Manual
204http://download.intel.com/design/intelxscale/31451001.pdf
20e570c8 205
2c2b3130 206PowerPC-specific:
5e123bd3 207-----------------
a1d0b6a2 208PowerPC32/AltiVec PIM:
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209www.freescale.com/files/32bit/doc/ref_manual/ALTIVECPEM.pdf
210
a1d0b6a2 211PowerPC32/AltiVec PEM:
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212www.freescale.com/files/32bit/doc/ref_manual/ALTIVECPIM.pdf
213
214CELL/SPU:
215http://www-01.ibm.com/chips/techlib/techlib.nsf/techdocs/30B3520C93F437AB87257060006FFE5E/$file/Language_Extensions_for_CBEA_2.4.pdf
216http://www-01.ibm.com/chips/techlib/techlib.nsf/techdocs/9F820A5FFA3ECE8C8725716A0062585F/$file/CBE_Handbook_v1.1_24APR2007_pub.pdf
20e570c8 217
277bb936 218SPARC-specific:
5e123bd3 219---------------
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220SPARC Joint Programming Specification (JPS1): Commonality
221http://www.fujitsu.com/downloads/PRMPWR/JPS1-R1.0.4-Common-pub.pdf
222
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223UltraSPARC III Processor User's Manual (contains instruction timings)
224http://www.sun.com/processors/manuals/USIIIv2.pdf
225
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226VIS Whitepaper (contains optimization guidelines)
227http://www.sun.com/processors/vis/download/vis/vis_whitepaper.pdf
277bb936 228
a552591f 229GCC asm links:
5e123bd3 230--------------
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231official doc but quite ugly
232http://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html
233
8ea9ce41 234a bit old (note "+" is valid for input-output, even though the next disagrees)
8c55915b 235http://www.cs.virginia.edu/~clc5q/gcc-inline-asm.pdf