GCC位扫描 - 转发找到下一个设置位？ [英] GCC Bit-scan-forward to find next set bit?

问题描述

我有一个 uint64_t ，我想找到第一个设置位的索引，将它重置为零并找到下一个设置位。

我如何知道何时终止？对所有零的BSF是未定义的...

  const uint64_t input = source; 
 
 if（0！= input）{
 
 int32_t setIndex = GCC_BSF_INTRINSIC（input）; 
 
 while（setIndex！= UNDEFINED ???）{
 
 //做我的逻辑
 
 //复位
 input [setIndex ] = 0; 
 
 setIndex = BSF_Variant（input）; 
} 
} 
  

有人请帮忙吗？

解决方案

最简单的方法就是检查输入：

<$ p $（input）{
int32_t index = __builtin_ffsll（input）; $ p> while（input）
//做东西
}


---

更复杂的是，根据文档文档：

- 内置函数： int __builtin_ffs（int x）

返回1加上x的最低有效1位的索引，或者如果x为零，则返回零

。

您可以这样做：

  for（int index = __builtin_ffsll（input）; 
 index; 
 index = __builtin_ffsll（input））
 {
 // do stuff 
} 
  

完成同样的事情，你只需重复 __ builtin_ffsll 调用，所以它更加冗长，在我看来并没有贡献澄清。

I have a uint64_t and I would like to find the index of the first set bit, reset it to zero and find the next set bit.

How do I know when to terminate? BSF on all zeros is undefined...

const uint64_t input = source;

if(0 != input){

    int32_t setIndex = GCC_BSF_INTRINSIC(input);

    while(setIndex != UNDEFINED???){

        //Do my logic

        //Reset
        input[setIndex] = 0;

        setIndex = BSF_Variant(input);
    }
}

Could somebody please help?

解决方案

The simplest would be to just check the input:

while (input) {
    int32_t index = __builtin_ffsll(input);
    // do stuff
}

---

More complicatedly, according to the docs the docs:

— Built-in Function: int __builtin_ffs (int x)
Returns one plus the index of the least significant 1-bit of x, or if x is zero, returns zero.

Which lets you do:

for (int index  = __builtin_ffsll(input); 
     index; 
     index = __builtin_ffsll(input))
{
    // do stuff
}

Which accomplishes the same thing, you just have to repeat the __builtin_ffsll call, so it's more verbose and in my opinion doesn't contribute to clarity.

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