问题描述
给定一个无符号整数，我必须执行以下操作:
Given an unsigned int, I have to implement the following operations :
- 计算设置为 1 的位数
- 查找最左边 1 位的索引
- 查找最右边 1 位的索引
(操作不应依赖于架构).
(the operation should not be architecture dependents).
我已经使用按位移位完成了此操作，但我必须遍历几乎所有位(es.32).例如，计数 1:
I've done this using bitwise shift, but I have to iterate through almost all the bits(es.32) . For example, counting 1's:
unsigned int number= ...;
while(number != 0){
if ((number & 0x01) != 0)
++count;
number >>=1;
}
其他操作类似
所以我的问题是:有没有更快的方法来做到这一点?
So my question is: is there any faster way to do that?
推荐答案
如果你想要最快的方式，你将需要使用不可移植的方法.
If you want the fastest way, you will need to use non-portable methods.
Windows/MSVC:
- _BitScanForward()
- _BitScanReverse()
- __popcnt()
GCC:
- __builtin_ffs()
- __builtin_ctz()
- __builtin_clz()
- __builtin_popcount()
这些通常直接映射到本机硬件指令.所以它不会比这些更快.
These typically map directly to native hardware instructions. So it doesn't get much faster than these.
但由于它们没有 C/C++ 功能，它们只能通过编译器内部函数访问.
But since there's no C/C++ functionality for them, they're only accessible via compiler intrinsics.
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