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158 lines
5.1 KiB
158 lines
5.1 KiB
optimization Tips (for libavcodec): |
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What to optimize: |
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If you plan to do non-x86 architecture specific optimizations (SIMD normally), |
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then take a look in the i386/ directory, as most important functions are |
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already optimized for MMX. |
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If you want to do x86 optimizations then you can either try to finetune the |
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stuff in the i386 directory or find some other functions in the C source to |
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optimize, but there aren't many left. |
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Understanding these overoptimized functions: |
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As many functions tend to be a bit difficult to understand because |
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of optimizations, it can be hard to optimize them further, or write |
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architecture-specific versions. It is recommened to look at older |
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CVS versions of the interesting files (just use ViewCVS at |
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http://www1.mplayerhq.hu/cgi-bin/cvsweb.cgi/ffmpeg/?cvsroot=FFMpeg). |
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Alternatively, look into the other architecture-specific versions in |
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the i386/, ppc/, alpha/ subdirectories. Even if you don't exactly |
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comprehend the instructions, it could help understanding the functions |
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and how they can be optimized. |
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NOTE: If you still don't understand some function, ask at our mailing list!!! |
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(http://www1.mplayerhq.hu/mailman/listinfo/ffmpeg-devel) |
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WTF is that function good for ....: |
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The primary purpose of that list is to avoid wasting time to optimize functions |
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which are rarely used |
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put(_no_rnd)_pixels{,_x2,_y2,_xy2} |
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Used in motion compensation (en/decoding). |
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avg_pixels{,_x2,_y2,_xy2} |
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Used in motion compensation of B-frames. |
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These are less important than the put*pixels functions. |
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avg_no_rnd_pixels* |
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unused |
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pix_abs16x16{,_x2,_y2,_xy2} |
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Used in motion estimation (encoding) with SAD. |
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pix_abs8x8{,_x2,_y2,_xy2} |
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Used in motion estimation (encoding) with SAD of MPEG-4 4MV only. |
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These are less important than the pix_abs16x16* functions. |
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put_mspel8_mc* / wmv2_mspel8* |
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Used only in WMV2. |
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it is not recommended that you waste your time with these, as WMV2 |
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is an ugly and relatively useless codec. |
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mpeg4_qpel* / *qpel_mc* |
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Used in MPEG-4 qpel motion compensation (encoding & decoding). |
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The qpel8 functions are used only for 4mv, |
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the avg_* functions are used only for B-frames. |
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Optimizing them should have a significant impact on qpel |
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encoding & decoding. |
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qpel{8,16}_mc??_old_c / *pixels{8,16}_l4 |
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Just used to work around a bug in an old libavcodec encoder version. |
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Don't optimize them. |
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tpel_mc_func {put,avg}_tpel_pixels_tab |
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Used only for SVQ3, so only optimize them if you need fast SVQ3 decoding. |
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add_bytes/diff_bytes |
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For huffyuv only, optimize if you want a faster ffhuffyuv codec. |
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get_pixels / diff_pixels |
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Used for encoding, easy. |
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clear_blocks |
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easiest to optimize |
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gmc |
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Used for MPEG-4 gmc. |
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Optimizing this should have a significant effect on the gmc decoding |
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speed but it's very likely impossible to write in SIMD. |
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gmc1 |
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Used for chroma blocks in MPEG-4 gmc with 1 warp point |
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(there are 4 luma & 2 chroma blocks per macroblock, so |
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only 1/3 of the gmc blocks use this, the other 2/3 |
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use the normal put_pixel* code, but only if there is |
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just 1 warp point). |
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Note: DivX5 gmc always uses just 1 warp point. |
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pix_sum |
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Used for encoding. |
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hadamard8_diff / sse / sad == pix_norm1 / dct_sad / quant_psnr / rd / bit |
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Specific compare functions used in encoding, it depends upon the |
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command line switches which of these are used. |
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Don't waste your time with dct_sad & quant_psnr, they aren't |
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really useful. |
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put_pixels_clamped / add_pixels_clamped |
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Used for en/decoding in the IDCT, easy. |
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Note, some optimized IDCTs have the add/put clamped code included and |
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then put_pixels_clamped / add_pixels_clamped will be unused. |
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idct/fdct |
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idct (encoding & decoding) |
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fdct (encoding) |
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difficult to optimize |
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dct_quantize_trellis |
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Used for encoding with trellis quantization. |
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difficult to optimize |
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dct_quantize |
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Used for encoding. |
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dct_unquantize_mpeg1 |
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Used in MPEG-1 en/decoding. |
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dct_unquantize_mpeg2 |
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Used in MPEG-2 en/decoding. |
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dct_unquantize_h263 |
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Used in MPEG-4/H.263 en/decoding. |
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FIXME remaining functions? |
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BTW, most of these functions are in dsputil.c/.h, some are in mpegvideo.c/.h. |
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Alignment: |
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Some instructions on some architectures have strict alignment restrictions, |
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for example most SSE/SSE2 instructions on x86. |
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The minimum guaranteed alignment is written in the .h files, for example: |
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void (*put_pixels_clamped)(const DCTELEM *block/*align 16*/, UINT8 *pixels/*align 8*/, int line_size); |
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Links: |
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http://www.aggregate.org/MAGIC/ |
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x86-specific: |
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http://developer.intel.com/design/pentium4/manuals/248966.htm |
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The IA-32 Intel Architecture Software Developer's Manual, Volume 2: |
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Instruction Set Reference |
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http://developer.intel.com/design/pentium4/manuals/245471.htm |
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http://www.agner.org/assem/ |
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AMD Athlon Processor x86 Code Optimization Guide: |
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http://www.amd.com/us-en/assets/content_type/white_papers_and_tech_docs/22007.pdf |
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GCC asm links: |
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official doc but quite ugly |
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http://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html |
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a bit old (note "+" is valid for input-output, even though the next disagrees) |
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http://www.cs.virginia.edu/~clc5q/gcc-inline-asm.pdf
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