在不截断的情况下撤消班次

Question

我对此有点困惑。值不应该在移位后截断吗？

有人知道为什么会这样吗？

long a, b, c, n;
//assign any value to a, set b and c to 0x000...0

n = 128; //any number works;
b = a << n;
c = b >> n;

a == (b >> n); // True
a == c; //True;

后记

我一直都明白，如果您在任何方向移动缓冲区，“落在”缓冲区大小之外的值都会被截断，并且除非您从原始缓冲区中获取它们，否则它们基本上会丢失。 对于我在汇编中使用过的每个平台都是如此，我认为这会扩展到更高级别的语言。

但是，在 c++ 中，我可以将缓冲区（int 或 long）移动比它可以容纳的更多位，如果我然后向相反方向移动，结果等于原始“缓冲区”。

Answer 1

查看Godbolt 的简化示例：

long left_shift(unsigned long a) {
    return (a << 128);
}

long right_shift(unsigned long a) {
    return ((a << 128) >> 128);
}

我们得到如下所示的程序集（gcc-7.3.0，-O2）：

left_shift(unsigned long):
  xor eax, eax
  ret
right_shift(unsigned long):
  xor eax, eax
  ret

正如 HolyBlackCat 所说，这是未定义的行为。 gcc 通过将移位的结果视为 0 来实现它，即使在 right_shift 函数中我们可以从逻辑上推断出结果。 clang 在left_shift 函数中什么都不做，这也是完美的cromulent。

Answer 2

看起来 C 编译器的工作方式有点奇怪，但它绝对合理
如果移位数大于操作数长度（以位为单位），则编译器将其除以操作数长度并使用提醒作为移位数

// A << N

L = length of A
if L < N
    then N = A % L;

因此，您可以为班次数设置这样的数字，使其撤消禁用！！！

N = 63 可能是您的答案，对于 A 上的任何值（甚至 A=1）（A=0 除外），它都不起作用（您的条件为 FALSE）（A=0 除外）

PUT [61 [60 ...

试试这段代码来了解发生了什么

#include <cstdlib>
#include <cstdio>

using namespace std;

void DisplayBinary(unsigned int n, int l)
{    
    for (int i = l - 1 ; i >= 0; i--)
    {
        printf("%x", (n & (1 << i)) >> i);
    }
}

int main(int argc, char** argv)
{
    // x = 340
    unsigned int x = 0x154;
    for (int numberOfShifts = 0; numberOfShifts < 100; numberOfShifts++) {
        printf("numberOfShifts = %d\tresult = ", numberOfShifts);
        DisplayBinary(x << numberOfShifts, 32);
        printf("\n");
    }

    return 0;
}

看输出：

numberOfShifts = 0      result = 00000000000000000000000101010100
numberOfShifts = 1      result = 00000000000000000000001010101000
numberOfShifts = 2      result = 00000000000000000000010101010000
numberOfShifts = 3      result = 00000000000000000000101010100000
numberOfShifts = 4      result = 00000000000000000001010101000000
numberOfShifts = 5      result = 00000000000000000010101010000000
numberOfShifts = 6      result = 00000000000000000101010100000000
numberOfShifts = 7      result = 00000000000000001010101000000000
numberOfShifts = 8      result = 00000000000000010101010000000000
numberOfShifts = 9      result = 00000000000000101010100000000000
numberOfShifts = 10     result = 00000000000001010101000000000000
numberOfShifts = 11     result = 00000000000010101010000000000000
numberOfShifts = 12     result = 00000000000101010100000000000000
numberOfShifts = 13     result = 00000000001010101000000000000000
numberOfShifts = 14     result = 00000000010101010000000000000000
numberOfShifts = 15     result = 00000000101010100000000000000000
numberOfShifts = 16     result = 00000001010101000000000000000000
numberOfShifts = 17     result = 00000010101010000000000000000000
numberOfShifts = 18     result = 00000101010100000000000000000000
numberOfShifts = 19     result = 00001010101000000000000000000000
numberOfShifts = 20     result = 00010101010000000000000000000000
numberOfShifts = 21     result = 00101010100000000000000000000000
numberOfShifts = 22     result = 01010101000000000000000000000000
numberOfShifts = 23     result = 10101010000000000000000000000000
numberOfShifts = 24     result = 01010100000000000000000000000000
numberOfShifts = 25     result = 10101000000000000000000000000000
numberOfShifts = 26     result = 01010000000000000000000000000000
numberOfShifts = 27     result = 10100000000000000000000000000000
numberOfShifts = 28     result = 01000000000000000000000000000000
numberOfShifts = 29     result = 10000000000000000000000000000000
numberOfShifts = 30     result = 00000000000000000000000000000000
numberOfShifts = 31     result = 00000000000000000000000000000000
numberOfShifts = 32     result = 00000000000000000000000101010100 <<<===
numberOfShifts = 33     result = 00000000000000000000001010101000
numberOfShifts = 34     result = 00000000000000000000010101010000
numberOfShifts = 35     result = 00000000000000000000101010100000
numberOfShifts = 36     result = 00000000000000000001010101000000
numberOfShifts = 37     result = 00000000000000000010101010000000
numberOfShifts = 38     result = 00000000000000000101010100000000
numberOfShifts = 39     result = 00000000000000001010101000000000
numberOfShifts = 40     result = 00000000000000010101010000000000
numberOfShifts = 41     result = 00000000000000101010100000000000
numberOfShifts = 42     result = 00000000000001010101000000000000
numberOfShifts = 43     result = 00000000000010101010000000000000
numberOfShifts = 44     result = 00000000000101010100000000000000
numberOfShifts = 45     result = 00000000001010101000000000000000
numberOfShifts = 46     result = 00000000010101010000000000000000
numberOfShifts = 47     result = 00000000101010100000000000000000
numberOfShifts = 48     result = 00000001010101000000000000000000
numberOfShifts = 49     result = 00000010101010000000000000000000
numberOfShifts = 50     result = 00000101010100000000000000000000
numberOfShifts = 51     result = 00001010101000000000000000000000
numberOfShifts = 52     result = 00010101010000000000000000000000
numberOfShifts = 53     result = 00101010100000000000000000000000
numberOfShifts = 54     result = 01010101000000000000000000000000
numberOfShifts = 55     result = 10101010000000000000000000000000
numberOfShifts = 56     result = 01010100000000000000000000000000
numberOfShifts = 57     result = 10101000000000000000000000000000
numberOfShifts = 58     result = 01010000000000000000000000000000
numberOfShifts = 59     result = 10100000000000000000000000000000
numberOfShifts = 60     result = 01000000000000000000000000000000
numberOfShifts = 61     result = 10000000000000000000000000000000
numberOfShifts = 62     result = 00000000000000000000000000000000
numberOfShifts = 63     result = 00000000000000000000000000000000
numberOfShifts = 64     result = 00000000000000000000000101010100 <<<===
numberOfShifts = 65     result = 00000000000000000000001010101000
numberOfShifts = 66     result = 00000000000000000000010101010000
numberOfShifts = 67     result = 00000000000000000000101010100000
numberOfShifts = 68     result = 00000000000000000001010101000000
numberOfShifts = 69     result = 00000000000000000010101010000000
numberOfShifts = 70     result = 00000000000000000101010100000000
numberOfShifts = 71     result = 00000000000000001010101000000000
numberOfShifts = 72     result = 00000000000000010101010000000000
numberOfShifts = 73     result = 00000000000000101010100000000000
numberOfShifts = 74     result = 00000000000001010101000000000000
numberOfShifts = 75     result = 00000000000010101010000000000000
numberOfShifts = 76     result = 00000000000101010100000000000000
numberOfShifts = 77     result = 00000000001010101000000000000000
numberOfShifts = 78     result = 00000000010101010000000000000000
numberOfShifts = 79     result = 00000000101010100000000000000000
numberOfShifts = 80     result = 00000001010101000000000000000000
numberOfShifts = 81     result = 00000010101010000000000000000000
numberOfShifts = 82     result = 00000101010100000000000000000000
numberOfShifts = 83     result = 00001010101000000000000000000000
numberOfShifts = 84     result = 00010101010000000000000000000000
numberOfShifts = 85     result = 00101010100000000000000000000000
numberOfShifts = 86     result = 01010101000000000000000000000000
numberOfShifts = 87     result = 10101010000000000000000000000000
numberOfShifts = 88     result = 01010100000000000000000000000000
numberOfShifts = 89     result = 10101000000000000000000000000000
numberOfShifts = 90     result = 01010000000000000000000000000000
numberOfShifts = 91     result = 10100000000000000000000000000000
numberOfShifts = 92     result = 01000000000000000000000000000000
numberOfShifts = 93     result = 10000000000000000000000000000000
numberOfShifts = 94     result = 00000000000000000000000000000000
numberOfShifts = 95     result = 00000000000000000000000000000000
numberOfShifts = 96     result = 00000000000000000000000101010100 <<<===
numberOfShifts = 97     result = 00000000000000000000001010101000
numberOfShifts = 98     result = 00000000000000000000010101010000
numberOfShifts = 99     result = 00000000000000000000101010100000