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 (web frontends for the various FFmpeg
 branches are listed at http://ffmpeg.org/download.html).

 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!!!

 (http://lists.ffmpeg.org/mailman/listinfo/ffmpeg-devel)

 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.

 
 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.

 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);
 
 

 General Tips:
 -------------
 Use asm loops like:

 __asm__(

     "1: ....
     ...

     "jump_instruction ....

 Do not use C loops:

 do{

     __asm__(

         ...
 }while()
 

 For x86, mark registers that are clobbered in your asm. This means both
 general x86 registers (e.g. eax) as well as XMM registers. This last one is
 particularly important on Win64, where xmm6-15 are callee-save, and not
 restoring their contents leads to undefined results. In external asm (e.g.
 yasm), you do this by using:
 cglobal functon_name, num_args, num_regs, num_xmm_regs
 In inline asm, you specify clobbered registers at the end of your asm:
 __asm__(".." ::: "%eax").

 If gcc is not set to support sse (-msse) it will not accept xmm registers
 in the clobber list. For that we use two macros to declare the clobbers.
 XMM_CLOBBERS should be used when there are other clobbers, for example:
 __asm__(".." ::: XMM_CLOBBERS("xmm0",) "eax");
 and XMM_CLOBBERS_ONLY should be used when the only clobbers are xmm registers:
 __asm__(".." :: XMM_CLOBBERS_ONLY("xmm0"));

 
 Do not expect a compiler to maintain values in your registers between separate
 (inline) asm code blocks. It is not required to. For example, this is bad:
 __asm__("movdqa %0, %%xmm7" : src);
 /* do something */
 __asm__("movdqa %%xmm7, %1" : dst);
 - first of all, you're assuming that the compiler will not use xmm7 in
    between the two asm blocks.  It probably won't when you test it, but it's
    a poor assumption that will break at some point for some --cpu compiler flag
 - secondly, you didn't mark xmm7 as clobbered. If you did, the compiler would
    have restored the original value of xmm7 after the first asm block, thus
    rendering the combination of the two blocks of code invalid
 Code that depends on data in registries being untouched, should be written as
 a single __asm__() statement. Ideally, a single function contains only one
 __asm__() block.
 
 Use external asm (nasm/yasm) or inline asm (__asm__()), do not use intrinsics.
 The latter requires a good optimizing compiler which gcc is not.
 
 Inline asm vs. external asm
 ---------------------------
 Both inline asm (__asm__("..") in a .c file, handled by a compiler such as gcc)
 and external asm (.s or .asm files, handled by an assembler such as yasm/nasm)

 are accepted in FFmpeg. Which one to use differs per specific case.

 
 - if your code is intended to be inlined in a C function, inline asm is always
    better, because external asm cannot be inlined
 - if your code calls external functions, yasm is always better
 - if your code takes huge and complex structs as function arguments (e.g.
    MpegEncContext; note that this is not ideal and is discouraged if there
    are alternatives), then inline asm is always better, because predicting
    member offsets in complex structs is almost impossible. It's safest to let
    the compiler take care of that
 - in many cases, both can be used and it just depends on the preference of the
    person writing the asm. For new asm, the choice is up to you. For existing
    asm, you'll likely want to maintain whatever form it is currently in unless
    there is a good reason to change it.
 - if, for some reason, you believe that a particular chunk of existing external
    asm could be improved upon further if written in inline asm (or the other
    way around), then please make the move from external asm <-> inline asm a
    separate patch before your patches that actually improve the asm.

 
 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
 

 
 ARM-specific:

 -------------

 ARM Architecture Reference Manual (up to ARMv5TE):
 http://www.arm.com/community/university/eulaarmarm.html
 
 Procedure Call Standard for the ARM Architecture:
 http://www.arm.com/pdfs/aapcs.pdf
 
 Optimization guide for ARM9E (used in Nokia 770 Internet Tablet):
 http://infocenter.arm.com/help/topic/com.arm.doc.ddi0240b/DDI0240A.pdf
 Optimization guide for ARM11 (used in Nokia N800 Internet Tablet):
 http://infocenter.arm.com/help/topic/com.arm.doc.ddi0211j/DDI0211J_arm1136_r1p5_trm.pdf
 Optimization guide for Intel XScale (used in Sharp Zaurus PDA):
 http://download.intel.com/design/intelxscale/27347302.pdf

 Intel Wireless MMX 2 Coprocessor: Programmers Reference Manual

 http://download.intel.com/design/intelxscale/31451001.pdf

 PowerPC-specific:

 -----------------

 PowerPC32/AltiVec PIM:

 www.freescale.com/files/32bit/doc/ref_manual/ALTIVECPEM.pdf
 

 PowerPC32/AltiVec PEM:

 www.freescale.com/files/32bit/doc/ref_manual/ALTIVECPIM.pdf
 
 CELL/SPU:
 http://www-01.ibm.com/chips/techlib/techlib.nsf/techdocs/30B3520C93F437AB87257060006FFE5E/$file/Language_Extensions_for_CBEA_2.4.pdf
 http://www-01.ibm.com/chips/techlib/techlib.nsf/techdocs/9F820A5FFA3ECE8C8725716A0062585F/$file/CBE_Handbook_v1.1_24APR2007_pub.pdf

 SPARC-specific:

 ---------------

 SPARC Joint Programming Specification (JPS1): Commonality
 http://www.fujitsu.com/downloads/PRMPWR/JPS1-R1.0.4-Common-pub.pdf
 

 UltraSPARC III Processor User's Manual (contains instruction timings)
 http://www.sun.com/processors/manuals/USIIIv2.pdf
 

 VIS Whitepaper (contains optimization guidelines)
 http://www.sun.com/processors/vis/download/vis/vis_whitepaper.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