ffmpeg/doc/optimization.txt

optimization Tips (for libavcodec):

What to optimize:
if you 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 you 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://www1.mplayerhq.hu/cgi-bin/cvsweb.cgi/ffmpeg/libavcodec/?cvsroot=FFmpeg)
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://www1.mplayerhq.hu/mailman/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

tpel_mc_func {put,avg}_tpel_pixels_tab
    used only for SVQ3, so only optimze them if u need fast SVQ3 decoding

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