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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 x86/ 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 x86 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 tend to be a bit difficult to understand because
of optimizations, it can be hard to optimize them further, or write
architecture-specific versions. It is recommended to look at older
revisions of the interesting files (for a web frontend try ViewVC at
Alternatively, look into the other architecture-specific versions in
the x86/, ppc/, alpha/ subdirectories. Even if you don't exactly
comprehend 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!!!
When is an optimization justified?
Normally, clean and simple optimizations for widely used codecs are
justified even if they only achieve an overall speedup of 0.1%. These
speedups accumulate and can make a big difference after awhile. Also, if
none of the following factors get worse due to an optimization -- speed,
binary code size, source size, source readability -- and at least one
factor improves, then an optimization is always a good idea even if the
overall gain is less than 0.1%. For obscure codecs that are not often
used, the goal is more toward keeping the code clean, small, and
readable instead of making it 1% faster.
WTF is that function good for ....:
The primary purpose of this list is to avoid wasting time optimizing functions
which are rarely used.
Used in motion compensation (en/decoding).
Used in motion compensation of B-frames.
These are less important than the put*pixels functions.
Used in motion estimation (encoding) with SAD.
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.
For huffyuv only, optimize if you want a faster ffhuffyuv codec.
get_pixels / diff_pixels
Used for encoding, easy.
easiest to optimize
Used for MPEG-4 gmc.
Optimizing this should have a significant effect on the gmc decoding
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.
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 (encoding & decoding)
fdct (encoding)
difficult to optimize
Used for encoding with trellis quantization.
difficult to optimize
Used for encoding.
Used in MPEG-1 en/decoding.
Used in MPEG-2 en/decoding.
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.
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);
General Tips:
Use asm loops like:
"1: ....
"jump_instruciton ....
Do not use C loops:
Use __asm__() instead of intrinsics. The latter requires a good optimizing compiler
which gcc is not.
The IA-32 Intel Architecture Software Developer's Manual, Volume 2:
Instruction Set Reference
AMD Athlon Processor x86 Code Optimization Guide:
ARM Architecture Reference Manual (up to ARMv5TE):
Procedure Call Standard for the ARM Architecture:
Optimization guide for ARM9E (used in Nokia 770 Internet Tablet):
Optimization guide for ARM11 (used in Nokia N800 Internet Tablet):
Optimization guide for Intel XScale (used in Sharp Zaurus PDA):
Intel Wireless MMX2 Coprocessor: Programmers Reference Manual
PowerPC32/AltiVec PIM:
PowerPC32/AltiVec PEM:
SPARC Joint Programming Specification (JPS1): Commonality
UltraSPARC III Processor User's Manual (contains instruction timings)
VIS Whitepaper (contains optimization guidelines)
GCC asm links:
official doc but quite ugly
a bit old (note "+" is valid for input-output, even though the next disagrees)