C#:左移赋值运算符行为

Question

我运行的代码有时会产生这样的结果:

UInt32 current;  
int left, right;  

...

//sometimes left == right and no shift occurs
current <<= (32 + left - right);

//this works
current <<= (32 - right);
current <<= left;

对于任何 >= 32 的值，只有值 % 32 被移动。框架中是否发生了一些“优化”？

Answer 1

C# 3.0 语言规范，7.8“移位运算符”:

For the predefined operators, the number of bits to shift is computed as follows:

• When the type of x is int or uint, the shift count is given by the low-order five bits of count. In other words, the shift count is computed from count & 0x1F.
• When the type of x is long or ulong, the shift count is given by the low-order six bits of count. In other words, the shift count is computed from count & 0x3F.