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 are
already optimized for MMX.

If you want to do x86 optimizations then you can either try to finetune the
stuff in the i386 directory or find some other functions in the C source to
optimize, but there aren't many left.

Understanding these overoptimized functions:
As many functions, like the C ones tend to be a bit unreadable currently
because of optimizations it is difficult to understand them (and write
architecture specific versions, or optimize the C functions further) it is
recommended to look at older CVS versions of the interesting files (just use
ViewCVS at http://www1.mplayerhq.hu/cgi-bin/cvsweb.cgi/ffmpeg/?cvsroot=FFMpeg).
Alternatively, look into the other architecture-specific versions in the i386/,
ppc/, alpha/ subdirectories. Even if you don't exactly understand the
instructions, it could help understanding the functions and how they
can be optimized.

NOTE: If you still don't understand some function, 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 than 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 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 optimize

gmc
    Used for MPEG-4 gmc.
    Optimizing this should have a significant effect on the gmc decoding
    speed but it's very likely impossible to write in SIMD.

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 optimize

dct_quantize_trellis
    Used for encoding with trellis quantization.
    difficult to optimize

dct_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.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 disagrees)
http://www.cs.virginia.edu/~clc5q/gcc-inline-asm.pdf