optimization Tips (for libavcodec):===================================What to optimize:-----------------If you plan to do non-x86 architecture specific optimizations (SIMD normally),then take a look in the i386/ directory, as most important functions arealready optimized for MMX.If you want to do x86 optimizations then you can either try to finetune thestuff in the i386 directory or find some other functions in the C source tooptimize, but there aren't many left.Understanding these overoptimized functions:--------------------------------------------As many functions tend to be a bit difficult to understand becauseof optimizations, it can be hard to optimize them further, or writearchitecture-specific versions. It is recommened to look at olderrevisions of the interesting files (for a web frontend try ViewVC athttp://svn.mplayerhq.hu/ffmpeg/trunk/).Alternatively, look into the other architecture-specific versions inthe i386/, ppc/, alpha/ subdirectories. Even if you don't exactlycomprehend the instructions, it could help understanding the functionsand how they can be optimized.NOTE: If you still don't understand some function, ask at our mailing list!!!(http://lists.mplayerhq.hu/mailman/listinfo/ffmpeg-devel)When is an optimization justified?----------------------------------Normally, clean and simple optimizations for widely used codecs arejustified even if they only achieve an overall speedup of 0.1%. Thesespeedups accumulate and can make a big difference after awhile. Also, ifnone of the following factors get worse due to an optimization -- speed,binary code size, source size, source readability -- and at least onefactor improves, then an optimization is always a good idea even if theoverall gain is less than 0.1%. For obscure codecs that are not oftenused, the goal is more toward keeping the code clean, small, andreadable instead of making it 1% faster.WTF is that function good for ....:-----------------------------------The primary purpose of that list is to avoid wasting time to optimize functionswhich are rarely usedput(_no_rnd)_pixels{,_x2,_y2,_xy2}    Used in motion compensation (en/decoding).avg_pixels{,_x2,_y2,_xy2}    Used in motion compensation of B-frames.    These are less important than the put*pixels functions.avg_no_rnd_pixels*    unusedpix_abs16x16{,_x2,_y2,_xy2}    Used in motion estimation (encoding) with SAD.pix_abs8x8{,_x2,_y2,_xy2}    Used in motion estimation (encoding) with SAD of MPEG-4 4MV only.    These are less important than the pix_abs16x16* functions.put_mspel8_mc* / wmv2_mspel8*    Used only in WMV2.    it is not recommended that you waste your time with these, as WMV2    is an ugly and relatively useless codec.mpeg4_qpel* / *qpel_mc*    Used in MPEG-4 qpel motion compensation (encoding & decoding).    The qpel8 functions are used only for 4mv,    the avg_* functions are used only for B-frames.    Optimizing them should have a significant impact on qpel    encoding & decoding.qpel{8,16}_mc??_old_c / *pixels{8,16}_l4    Just used to work around a bug in an old libavcodec encoder version.    Don't optimize them.tpel_mc_func {put,avg}_tpel_pixels_tab    Used only for SVQ3, so only optimize them if you need fast SVQ3 decoding.add_bytes/diff_bytes    For huffyuv only, optimize if you want a faster ffhuffyuv codec.get_pixels / diff_pixels    Used for encoding, easy.clear_blocks    easiest to optimizegmc    Used for MPEG-4 gmc.    Optimizing this should have a significant effect on the gmc decoding    speed.gmc1    Used for chroma blocks in MPEG-4 gmc with 1 warp point    (there are 4 luma & 2 chroma blocks per macroblock, so    only 1/3 of the gmc blocks use this, the other 2/3    use the normal put_pixel* code, but only if there is    just 1 warp point).    Note: DivX5 gmc always uses just 1 warp point.pix_sum    Used for encoding.hadamard8_diff / sse / sad == pix_norm1 / dct_sad / quant_psnr / rd / bit    Specific compare functions used in encoding, it depends upon the    command line switches which of these are used.    Don't waste your time with dct_sad & quant_psnr, they aren't    really useful.put_pixels_clamped / add_pixels_clamped    Used for en/decoding in the IDCT, easy.    Note, some optimized IDCTs have the add/put clamped code included and    then put_pixels_clamped / add_pixels_clamped will be unused.idct/fdct    idct (encoding & decoding)    fdct (encoding)    difficult to optimizedct_quantize_trellis    Used for encoding with trellis quantization.    difficult to optimizedct_quantize    Used for encoding.dct_unquantize_mpeg1    Used in MPEG-1 en/decoding.dct_unquantize_mpeg2    Used in MPEG-2 en/decoding.dct_unquantize_h263    Used in MPEG-4/H.263 en/decoding.FIXME remaining functions?BTW, most of these functions are in dsputil.c/.h, some are in mpegvideo.c/.h.Alignment:Some instructions on some architectures have strict alignment restrictions,for example most SSE/SSE2 instructions on x86.The minimum guaranteed alignment is written in the .h files, for example:    void (*put_pixels_clamped)(const DCTELEM *block/*align 16*/, UINT8 *pixels/*align 8*/, int line_size);Links:======http://www.aggregate.org/MAGIC/x86-specific:-------------http://developer.intel.com/design/pentium4/manuals/248966.htmThe IA-32 Intel Architecture Software Developer's Manual, Volume 2:Instruction Set Referencehttp://developer.intel.com/design/pentium4/manuals/245471.htmhttp://www.agner.org/assem/AMD Athlon Processor x86 Code Optimization Guide:http://www.amd.com/us-en/assets/content_type/white_papers_and_tech_docs/22007.pdfARM-specific:-------------ARM Architecture Reference Manual (up to ARMv5TE):http://www.arm.com/community/university/eulaarmarm.htmlProcedure Call Standard for the ARM Architecture:http://www.arm.com/pdfs/aapcs.pdfOptimization guide for ARM9E (used in Nokia 770 Internet Tablet):http://infocenter.arm.com/help/topic/com.arm.doc.ddi0240b/DDI0240A.pdfOptimization guide for ARM11 (used in Nokia N800 Internet Tablet):http://infocenter.arm.com/help/topic/com.arm.doc.ddi0211j/DDI0211J_arm1136_r1p5_trm.pdfOptimization guide for Intel XScale (used in Sharp Zaurus PDA):http://download.intel.com/design/intelxscale/27347302.pdfPowerPC-specific:-----------------PowerPC32/AltiVec PIM:www.freescale.com/files/32bit/doc/ref_manual/ALTIVECPEM.pdfPowerPC32/AltiVec PEM:www.freescale.com/files/32bit/doc/ref_manual/ALTIVECPIM.pdfCELL/SPU:http://www-01.ibm.com/chips/techlib/techlib.nsf/techdocs/30B3520C93F437AB87257060006FFE5E/$file/Language_Extensions_for_CBEA_2.4.pdfhttp://www-01.ibm.com/chips/techlib/techlib.nsf/techdocs/9F820A5FFA3ECE8C8725716A0062585F/$file/CBE_Handbook_v1.1_24APR2007_pub.pdfSPARC-specific:---------------SPARC Joint Programming Specification (JPS1): Commonalityhttp://www.fujitsu.com/downloads/PRMPWR/JPS1-R1.0.4-Common-pub.pdfUltraSPARC III Processor User's Manual (contains instruction timings)http://www.sun.com/processors/manuals/USIIIv2.pdfVIS Whitepaper (contains optimization guidelines)http://www.sun.com/processors/vis/download/vis/vis_whitepaper.pdfGCC asm links:--------------official doc but quite uglyhttp://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.htmla bit old (note "+" is valid for input-output, even though the next disagrees)http://www.cs.virginia.edu/~clc5q/gcc-inline-asm.pdf