| 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 |
| http://svn.ffmpeg.org/ffmpeg/trunk/). |
| 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.mplayerhq.hu/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_instruciton .... |
| Do not use C loops: |
| do{ |
| __asm__( |
| ... |
| }while() |
| |
| Use __asm__() instead of intrinsics. The latter requires a good optimizing compiler |
| which gcc is not. |
| |
| |
| 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 MMX2 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 |