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274 lines
10 KiB
274 lines
10 KiB
optimization Tips (for libavcodec): |
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=================================== |
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What to optimize: |
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----------------- |
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If you plan to do non-x86 architecture specific optimizations (SIMD normally), |
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then take a look in the x86/ directory, as most important functions are |
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already optimized for MMX. |
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If you want to do x86 optimizations then you can either try to finetune the |
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stuff in the x86 directory or find some other functions in the C source to |
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optimize, but there aren't many left. |
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Understanding these overoptimized functions: |
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-------------------------------------------- |
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As many functions tend to be a bit difficult to understand because |
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of optimizations, it can be hard to optimize them further, or write |
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architecture-specific versions. It is recommended to look at older |
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revisions of the interesting files (web frontends for the various Libav |
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branches are listed at http://libav.org/download.html). |
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Alternatively, look into the other architecture-specific versions in |
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the x86/, ppc/, alpha/ subdirectories. Even if you don't exactly |
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comprehend the instructions, it could help understanding the functions |
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and how they can be optimized. |
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NOTE: If you still don't understand some function, ask at our mailing list!!! |
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(https://lists.libav.org/mailman/listinfo/libav-devel) |
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When is an optimization justified? |
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---------------------------------- |
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Normally, clean and simple optimizations for widely used codecs are |
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justified even if they only achieve an overall speedup of 0.1%. These |
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speedups accumulate and can make a big difference after awhile. Also, if |
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none of the following factors get worse due to an optimization -- speed, |
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binary code size, source size, source readability -- and at least one |
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factor improves, then an optimization is always a good idea even if the |
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overall gain is less than 0.1%. For obscure codecs that are not often |
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used, the goal is more toward keeping the code clean, small, and |
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readable instead of making it 1% faster. |
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WTF is that function good for ....: |
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----------------------------------- |
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The primary purpose of this list is to avoid wasting time optimizing functions |
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which are rarely used. |
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put(_no_rnd)_pixels{,_x2,_y2,_xy2} |
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Used in motion compensation (en/decoding). |
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avg_pixels{,_x2,_y2,_xy2} |
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Used in motion compensation of B-frames. |
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These are less important than the put*pixels functions. |
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avg_no_rnd_pixels* |
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unused |
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pix_abs16x16{,_x2,_y2,_xy2} |
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Used in motion estimation (encoding) with SAD. |
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pix_abs8x8{,_x2,_y2,_xy2} |
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Used in motion estimation (encoding) with SAD of MPEG-4 4MV only. |
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These are less important than the pix_abs16x16* functions. |
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put_mspel8_mc* / wmv2_mspel8* |
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Used only in WMV2. |
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it is not recommended that you waste your time with these, as WMV2 |
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is an ugly and relatively useless codec. |
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mpeg4_qpel* / *qpel_mc* |
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Used in MPEG-4 qpel motion compensation (encoding & decoding). |
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The qpel8 functions are used only for 4mv, |
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the avg_* functions are used only for B-frames. |
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Optimizing them should have a significant impact on qpel |
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encoding & decoding. |
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qpel{8,16}_mc??_old_c / *pixels{8,16}_l4 |
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Just used to work around a bug in an old libavcodec encoder version. |
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Don't optimize them. |
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add_bytes/diff_bytes |
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For huffyuv only, optimize if you want a faster ffhuffyuv codec. |
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get_pixels / diff_pixels |
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Used for encoding, easy. |
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clear_blocks |
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easiest to optimize |
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gmc |
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Used for MPEG-4 gmc. |
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Optimizing this should have a significant effect on the gmc decoding |
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speed. |
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gmc1 |
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Used for chroma blocks in MPEG-4 gmc with 1 warp point |
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(there are 4 luma & 2 chroma blocks per macroblock, so |
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only 1/3 of the gmc blocks use this, the other 2/3 |
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use the normal put_pixel* code, but only if there is |
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just 1 warp point). |
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Note: DivX5 gmc always uses just 1 warp point. |
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pix_sum |
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Used for encoding. |
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hadamard8_diff / sse / sad == pix_norm1 / dct_sad / quant_psnr / rd / bit |
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Specific compare functions used in encoding, it depends upon the |
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command line switches which of these are used. |
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Don't waste your time with dct_sad & quant_psnr, they aren't |
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really useful. |
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put_pixels_clamped / add_pixels_clamped |
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Used for en/decoding in the IDCT, easy. |
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Note, some optimized IDCTs have the add/put clamped code included and |
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then put_pixels_clamped / add_pixels_clamped will be unused. |
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idct/fdct |
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idct (encoding & decoding) |
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fdct (encoding) |
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difficult to optimize |
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dct_quantize_trellis |
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Used for encoding with trellis quantization. |
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difficult to optimize |
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dct_quantize |
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Used for encoding. |
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dct_unquantize_mpeg1 |
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Used in MPEG-1 en/decoding. |
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dct_unquantize_mpeg2 |
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Used in MPEG-2 en/decoding. |
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dct_unquantize_h263 |
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Used in MPEG-4/H.263 en/decoding. |
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FIXME remaining functions? |
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BTW, most of these functions are in dsputil.c/.h, some are in mpegvideo.c/.h. |
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Alignment: |
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Some instructions on some architectures have strict alignment restrictions, |
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for example most SSE/SSE2 instructions on x86. |
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The minimum guaranteed alignment is written in the .h files, for example: |
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void (*put_pixels_clamped)(const int16_t *block/*align 16*/, UINT8 *pixels/*align 8*/, int line_size); |
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General Tips: |
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------------- |
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Use asm loops like: |
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__asm__( |
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"1: .... |
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... |
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"jump_instruction .... |
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Do not use C loops: |
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do{ |
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__asm__( |
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... |
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}while() |
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For x86, mark registers that are clobbered in your asm. This means both |
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general x86 registers (e.g. eax) as well as XMM registers. This last one is |
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particularly important on Win64, where xmm6-15 are callee-save, and not |
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restoring their contents leads to undefined results. In external asm (e.g. |
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yasm), you do this by using: |
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cglobal functon_name, num_args, num_regs, num_xmm_regs |
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In inline asm, you specify clobbered registers at the end of your asm: |
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__asm__(".." ::: "%eax"). |
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If gcc is not set to support sse (-msse) it will not accept xmm registers |
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in the clobber list. For that we use two macros to declare the clobbers. |
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XMM_CLOBBERS should be used when there are other clobbers, for example: |
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__asm__(".." ::: XMM_CLOBBERS("xmm0",) "eax"); |
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and XMM_CLOBBERS_ONLY should be used when the only clobbers are xmm registers: |
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__asm__(".." :: XMM_CLOBBERS_ONLY("xmm0")); |
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Do not expect a compiler to maintain values in your registers between separate |
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(inline) asm code blocks. It is not required to. For example, this is bad: |
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__asm__("movdqa %0, %%xmm7" : src); |
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/* do something */ |
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__asm__("movdqa %%xmm7, %1" : dst); |
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- first of all, you're assuming that the compiler will not use xmm7 in |
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between the two asm blocks. It probably won't when you test it, but it's |
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a poor assumption that will break at some point for some --cpu compiler flag |
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- secondly, you didn't mark xmm7 as clobbered. If you did, the compiler would |
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have restored the original value of xmm7 after the first asm block, thus |
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rendering the combination of the two blocks of code invalid |
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Code that depends on data in registries being untouched, should be written as |
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a single __asm__() statement. Ideally, a single function contains only one |
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__asm__() block. |
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Use external asm (nasm/yasm) or inline asm (__asm__()), do not use intrinsics. |
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The latter requires a good optimizing compiler which gcc is not. |
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Inline asm vs. external asm |
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--------------------------- |
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Both inline asm (__asm__("..") in a .c file, handled by a compiler such as gcc) |
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and external asm (.s or .asm files, handled by an assembler such as yasm/nasm) |
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are accepted in Libav. Which one to use differs per specific case. |
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- if your code is intended to be inlined in a C function, inline asm is always |
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better, because external asm cannot be inlined |
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- if your code calls external functions, yasm is always better |
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- if your code takes huge and complex structs as function arguments (e.g. |
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MpegEncContext; note that this is not ideal and is discouraged if there |
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are alternatives), then inline asm is always better, because predicting |
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member offsets in complex structs is almost impossible. It's safest to let |
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the compiler take care of that |
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- in many cases, both can be used and it just depends on the preference of the |
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person writing the asm. For new asm, the choice is up to you. For existing |
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asm, you'll likely want to maintain whatever form it is currently in unless |
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there is a good reason to change it. |
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- if, for some reason, you believe that a particular chunk of existing external |
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asm could be improved upon further if written in inline asm (or the other |
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way around), then please make the move from external asm <-> inline asm a |
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separate patch before your patches that actually improve the asm. |
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Links: |
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====== |
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http://www.aggregate.org/MAGIC/ |
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x86-specific: |
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------------- |
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http://developer.intel.com/design/pentium4/manuals/248966.htm |
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The IA-32 Intel Architecture Software Developer's Manual, Volume 2: |
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Instruction Set Reference |
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http://developer.intel.com/design/pentium4/manuals/245471.htm |
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http://www.agner.org/assem/ |
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AMD Athlon Processor x86 Code Optimization Guide: |
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http://www.amd.com/us-en/assets/content_type/white_papers_and_tech_docs/22007.pdf |
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ARM-specific: |
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------------- |
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ARM Architecture Reference Manual (up to ARMv5TE): |
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http://www.arm.com/community/university/eulaarmarm.html |
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Procedure Call Standard for the ARM Architecture: |
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http://www.arm.com/pdfs/aapcs.pdf |
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Optimization guide for ARM9E (used in Nokia 770 Internet Tablet): |
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http://infocenter.arm.com/help/topic/com.arm.doc.ddi0240b/DDI0240A.pdf |
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Optimization guide for ARM11 (used in Nokia N800 Internet Tablet): |
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http://infocenter.arm.com/help/topic/com.arm.doc.ddi0211j/DDI0211J_arm1136_r1p5_trm.pdf |
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Optimization guide for Intel XScale (used in Sharp Zaurus PDA): |
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http://download.intel.com/design/intelxscale/27347302.pdf |
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Intel Wireless MMX 2 Coprocessor: Programmers Reference Manual |
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http://download.intel.com/design/intelxscale/31451001.pdf |
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PowerPC-specific: |
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----------------- |
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PowerPC32/AltiVec PIM: |
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www.freescale.com/files/32bit/doc/ref_manual/ALTIVECPEM.pdf |
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PowerPC32/AltiVec PEM: |
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www.freescale.com/files/32bit/doc/ref_manual/ALTIVECPIM.pdf |
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CELL/SPU: |
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http://www-01.ibm.com/chips/techlib/techlib.nsf/techdocs/30B3520C93F437AB87257060006FFE5E/$file/Language_Extensions_for_CBEA_2.4.pdf |
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http://www-01.ibm.com/chips/techlib/techlib.nsf/techdocs/9F820A5FFA3ECE8C8725716A0062585F/$file/CBE_Handbook_v1.1_24APR2007_pub.pdf |
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GCC asm links: |
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-------------- |
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official doc but quite ugly |
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http://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html |
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a bit old (note "+" is valid for input-output, even though the next disagrees) |
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http://www.cs.virginia.edu/~clc5q/gcc-inline-asm.pdf
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