Assembly code for optimized bitshifting of a vector

2018-06-21 12:02:39

i'm trying to write a routine that will logically bitshift by n positions to the right all elements of a vector in the most efficient way possible for the following vector types: BYTE->BYTE, WORD->WORD, DWORD->DWORD and WORD->BYTE (assuming that only 8 bits are present in the result). I would like to have three routines for each type depending on the type of processor (SSE2 supported, only MMX suppported, only standard instruction se supported). Therefore i need 12 functions in total.

I have already found by myself how to backup and restore the registers that i need, how to make a loop, how to copy data into regular registers or MMX registers and how to shift by 1 position logically.

Because i'm not familiar with assembly language that's about it. Which registers should i use for each instruction set? How will the availability of the large vector (an image) in L1 cache be optimized? How do i find the next element of the vector (a pointer kind of thing), i know i can make a mov by address and i assume i have to increment the address by 1, 2 or 4 depending on my type of data?

Although i have all the ideas, writing the code is a bit difficult at this point.

Thank you.

Arnaud.

Edit: Here is what i'm trying to do for MMX for a shift by 1 on a DWORD:

__asm("push mm"); // backup register
__asm("push cx"); // backup register
__asm("mov %cx, length"); // initialize loop
__asm("loopstart_shift1:"); // start label
__asm("movd %xmm0, r/m32"); // get 32 bits data
__asm("psrlq %xmm0, 1"); // right shift 32 bits data logically (stuffs 0 on the left) by 1
__asm("mov r/m32,%xmm0"); // set 32 bits data
__asm("dec %cx"); // decrement index
__asm("cmp %cx,0");
__asm("jnz loopstart_shift1");
__asm("pop cx"); // restore register
__asm("pop mm"); // restore register
__asm("emms"); // leave MMX state

I strongly suggest you pause and take a look at using intrinsics with C or C++ instead of trying to write raw asm - that way the C/C++ compiler will take care of all the register allocation, instruction scheduling and general housekeeping tasks and you can just focus on the important parts, eg instead of using psrlq see _m_psrlq in mmintrin.h . (Better yet, look at using 128 bit SSE intrinsics.)

Sounds like you'd benefit from either using or looking into BitMagic's source. its entirely intrinsics based too, which makes its far more portable (though from the looks of it your using GCC, so it might have to get an MSVC to GCC intrinics mapping).

链接地址: http://www.djcxy.com/p/60378.html

上一篇: 代码到MIPS汇编

下一篇: 用于优化矢量移位的汇编代码