什么是可切换MSB的最佳方式？ [英] What's the best way to toggle the MSB?

问题描述

所以我想我的切换号码的最显著位。下面是一个例子：

So I want to toggle the most significant bit of my number. Here is an example:

x = 100101 then answer should be 00101

我有一个64位的机器，所以我没想到的答案是 100000 ..的51 0＆GT; .. 100101
我想到的一个办法是计算在我的电话号码的位数，然后切换MSB，但不知道如何来算。

I have a 64 bit machine and hence I am not expecting the answer to be 100000..<51 0's>..100101 One way I thought of was to count the number of bits in my number and then toggle the MSB, but not sure on how to count.

推荐答案

作弊是典当它关闭编译器：有大部分的CPU指令做工作，像这样

The cheat is to pawn it off to the compiler: There are instructions in most CPUs for doing work like this.

以下应该做你想要什么。

The following should do what you want.

i ^ (1 << (sizeof i * CHAR_BIT - clz(i) - 1))

这将转化为 CLZ 指令，该指令计数前导零。

This will translate into the CLZ instruction, which counts the leading zeros.

有关GCC，请参阅：的http：// GCC .gnu.org / onlinedocs / GCC-4.1.2 / GCC /其他-Builtins.html

For GCC, see: http://gcc.gnu.org/onlinedocs/gcc-4.1.2/gcc/Other-Builtins.html

有一件事情要小心的是，这会导致不确定的行为，如果我== 0 。

One thing to be careful of is that this results in undefined behavior if i == 0.

您应该替换 CLZ（）用正确的内在您的编译器，在海湾合作委员会这是 __ builtin_clz ;在Visual Studio C ++这是 _BitScanForward 。

You should replace clz() with the correct intrinsic for your compiler, In GCC this is __builtin_clz; in Visual Studio C++ this is _BitScanForward.

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