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optimization Tips (for libavcodec):
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What to optimize:
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if you plan to do non-x86 architecture specific optimiztions (SIMD normally) then
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take a look in the i386/ directory, as most important functions are allready
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optimized for MMX
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if you want to do x86 optimizations then u can either try to finetune the stuff in the
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i386 directory or find some other functions in the c source to optimize, but there
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arent many left
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Understanding these overoptimized functions:
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as many functions, like the c ones tend to be a bit unreadable currently becouse 
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of optimizations it is difficult to understand them (and write arichtecture 
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specific versions, or optimize the c functions further) it is recommanded to look
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at older CVS versions of the interresting files (just use CVSWEB at 
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http://www1.mplayerhq.hu/cgi-bin/cvsweb.cgi/ffmpeg/libavcodec/?cvsroot=FFmpeg)
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or perhaps look into the other architecture specific versions in i386/, ppc/, 
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alpha/, ...; even if u dont understand the instructions exactly it could help
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understanding the functions & how they can be optimized
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NOTE:!!! if u still dont understand some function then 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 then 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 MPEG4 4MV only
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	these are less important then 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 recommanded that u waste ur time with these, as WMV2 is a
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	ugly and relativly useless codec
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mpeg4_qpel* / *qpel_mc*
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	use in MPEG4 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 encoding & decoding
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qpel{8,16}_mc??_old_c / *pixels{8,16}_l4
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	just used to workaround a bug in old libavcodec encoder
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        dont optimze them
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tpel_mc_func {put,avg}_tpel_pixels_tab
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	used only for SVQ3, so only optimze them if u need fast SVQ3 decoding
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add_bytes/diff_bytes
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	for huffyuv only, optimize if u want a faster ff-huffyuv 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 mpeg4 gmc
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        optimizing this should have a significant effect on the gmc decoding speed but
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        its very likely impossible to write in SIMD
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gmc1
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	used for chroma blocks in mpeg4 gmc with 1 warp point
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	(there are 4 luma & 2 chroma blocks per macrobock, 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 command line
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        switches which of these are used
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        dont waste ur time with dct_sad & quant_psnr they arent really usefull
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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 then 
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        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 mpeg1 en/decoding
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dct_unquantize_mpeg2
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	used in mpeg2 en/decoding
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dct_unquantize_h263
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	used in mpeg4/h263 en/decoding
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FIXME remaining functions?
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btw, most of these 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 inctructios on X86
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the minimum guranteed alignment is writen in the .h files
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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 says its not)
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http://www.cs.virginia.edu/~clc5q/gcc-inline-asm.pdf
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