optimization Tips (for libavcodec):

What to optimize:
if u plan to do non-x86 architecture specific optimiztions (SIMD normally) then
take a look in the i386/ directory, as most important functions are allready
optimized for MMX

if u want to do x86 optimizations then u can either try to finetune the stuff in the
i386 directory or find some other functions in the c source to optimize, but there
arent many left

Understanding these overoptimized functions:
as many functions, like the c ones tend to be a bit unreadable currently becouse 
of optimizations it is difficult to understand them (and write arichtecture 
specific versions, or optimize the c functions further) it is recommanded to look
at older CVS versions of the interresting files (just use CVSWEB at 
(http://cvs.sourceforge.net/cgi-bin/viewcvs.cgi/ffmpeg/ffmpeg/libavcodec/))
or perhaps look into the other architecture specific versions in i386/, ppc/, 
alpha/, ...; even if u dont understand the instructions exactly it could help
understanding the functions & how they can be optimized

NOTE:!!! if u still dont understand some function then ask at our mailing list!!!
(http://lists.sourceforge.net/lists/listinfo/ffmpeg-devel)



wtf is that function good for ....:
the primary purpose of that list is to avoid wasting time to optimize functions
which are rarely used

put(_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 then the put*pixels functions

avg_no_rnd_pixels*
	unused

pix_abs16x16{,_x2,_y2,_xy2}
	used in motion estimation (encoding) with SAD

pix_abs8x8{,_x2,_y2,_xy2}
	used in motion estimation (encoding) with SAD of MPEG4 4MV only
	these are less important then the pix_abs16x16* functions

put_mspel8_mc* / wmv2_mspel8*
	used only in WMV2
	it is not recommanded that u waste ur time with these, as WMV2 is a
	ugly and relativly useless codec

mpeg4_qpel* / *qpel_mc*
	use in MPEG4 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 workaround a bug in old libavcodec encoder
        dont optimze them

add_bytes/diff_bytes
	for huffyuv only, optimize if u want a faster ff-huffyuv codec

get_pixels / diff_pixels
	used for encoding, easy
        
clear_blocks
	easiest, to optimize
        
gmc
	used for mpeg4 gmc
        optimizing this should have a significant effect on the gmc decoding speed but
        its very likely impossible to write in SIMD

gmc1
	used for chroma blocks in mpeg4 gmc with 1 warp point
	(there are 4 luma & 2 chroma blocks per macrobock, 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
        dont waste ur time with dct_sad & quant_psnr they arent really usefull

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 optimize
        
dct_quantize_trellis
	used for encoding with trellis quantization
	difficult to optimize 

dct_quantize
	used for encoding
        
dct_unquantize_mpeg1
	used in mpeg1 en/decoding

dct_unquantize_mpeg2
	used in mpeg2 en/decoding

dct_unquantize_h263
	used in mpeg4/h263 en/decoding

FIXME remaining functions?
btw, most of these 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 inctructios on X86
the minimum guranteed alignment is writen 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.htm

The IA-32 Intel Architecture Software Developer's Manual, Volume 2: 
Instruction Set Reference
http://developer.intel.com/design/pentium4/manuals/245471.htm

http://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.pdf

GCC asm links:
official doc but quite ugly
http://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html

a bit old (note "+" is valid for input-output, even though the next says its not)
http://www.cs.virginia.edu/~clc5q/gcc-inline-asm.pdf