X-Git-Url: https://git.sesse.net/?a=blobdiff_plain;f=doc%2Foptimization.txt;h=2b8c51b4c9ce183e7a9d2368b41a52bcd99c1d34;hb=72ad96c8544cff11b66fa3412023c6c535703f74;hp=c54eaf8bad3ec7c2909a6a4e7cd7b5e81a6d4e38;hpb=14c2634bcc6efd274cd41c31b597aad43c203dd2;p=ffmpeg diff --git a/doc/optimization.txt b/doc/optimization.txt index c54eaf8bad3..2b8c51b4c9c 100644 --- a/doc/optimization.txt +++ b/doc/optimization.txt @@ -1,33 +1,50 @@ 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 +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 i386 directory or find some other functions in the C source to +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 recommened to look at older -revisions of the interesting files (for a web frontend try ViewVC at -http://svn.mplayerhq.hu/ffmpeg/trunk/). +architecture-specific versions. It is recommended to look at older +revisions of the interesting files (web frontends for the various Libav +branches are listed at http://libav.org/download.html). Alternatively, look into the other architecture-specific versions in -the i386/, ppc/, alpha/ subdirectories. Even if you don't exactly +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) +(https://lists.libav.org/mailman/listinfo/libav-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 that list is to avoid wasting time to optimize functions -which are rarely used +----------------------------------- +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). @@ -131,14 +148,85 @@ 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); - + 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 Libav. 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: @@ -152,7 +240,7 @@ http://www.amd.com/us-en/assets/content_type/white_papers_and_tech_docs/22007.pd ARM-specific: - +------------- ARM Architecture Reference Manual (up to ARMv5TE): http://www.arm.com/community/university/eulaarmarm.html @@ -165,13 +253,15 @@ 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: +----------------- +PowerPC32/AltiVec PIM: www.freescale.com/files/32bit/doc/ref_manual/ALTIVECPEM.pdf -PowerPC32/Altivec PEM: +PowerPC32/AltiVec PEM: www.freescale.com/files/32bit/doc/ref_manual/ALTIVECPIM.pdf CELL/SPU: @@ -179,6 +269,7 @@ http://www-01.ibm.com/chips/techlib/techlib.nsf/techdocs/30B3520C93F437AB8725706 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 @@ -189,6 +280,7 @@ 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