将 n 个最低有效位设置为 1 [英] Set n least significant bits to 1

问题描述

我想将寄存器的前 N ​​位设置为 1.例如，如果 N 是 3，那么寄存器将是:...00000111.是否有说明或简短的方法来做到这一点?我目前的做法是:

I want to set the first N bits of a register to 1. For example, if N is 3 then the register would be: ...00000111. Is there an instruction or short-form way to do this? The way I'm currently doing it is:

   mov $0, %eax
   test %esi, %esi
   jz end
 loop:
   add $0b1, %eax
   dec %esi
   jz end_loop
   shl %eax
   jmp loop
  exit:
    ...
    

推荐答案

一个有用的数学事实是，具有 N 个低位 1 位的值比 2 的幂小 1，您可以通过移位得到.所以你可以简单地将计数加载到 %cl 中并执行

A useful mathematical fact is that a value with N low-order 1 bits is 1 less than a power of two, which you can get by shifting. So you can simply load the count into %cl and do

mov $1, %eax
shl %cl, %eax
dec %eax

当然，如果计数 N 是在汇编时已知的常数，那么让汇编器进行数学运算并加载结果:

Of course if the count N is a constant known at assembly time, then let the assembler do the math and just load the result:

mov $((1 << N) - 1), %eax

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