ffmpeg/doc/optimization.txt

   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