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diff --git a/ffmpeg/doc/optimization.txt b/ffmpeg/doc/optimization.txt new file mode 100644 index 0000000..5a66d6b --- /dev/null +++ b/ffmpeg/doc/optimization.txt @@ -0,0 +1,288 @@ +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 int16_t *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 |