java - Java 移位操作实现背后的逻辑

Question

根据 JLS 第 15.19 条:

If the promoted type of the left-hand operand is int, only the five lowest-order bits of the right-hand operand are used as the shift distance. It is as if the right-hand operand were subjected to a bitwise logical AND operator & (§15.22.1) with the mask value 0x1f (0b11111). The shift distance actually used is therefore always in the range 0 to 31, inclusive.

If the promoted type of the left-hand operand is long, then only the six lowest-order bits of the right-hand operand are used as the shift distance. It is as if the right-hand operand were subjected to a bitwise logical AND operator & (§15.22.1) with the mask value 0x3f (0b111111). The shift distance actually used is therefore always in the range 0 to 63, inclusive.

我们可以在 C# 中找到同样的东西。在 Python 中，它将是 6 位而不是 5 位，但逻辑保持不变。

其背后的原因是什么？对我来说，拥有以下内容会更自然:

x << 32 = 0 

而不是

x << 32 = x

此外，对于负整数使用移位运算永远不会得到 0。

Answer 1

摘自 Joshua Bloch 和 Neal Gafter 所著的《Java Puzzlers: Traps, Pitfalls and Corner Cases》一书:

Language designers should perhaps consider restricting shift distances to the range from 0 to the type size in bits and changing the semantics of shifting a value by the type size to return 0. Although this would avoid the confusion …, it could have negative performance consequences; Java's semantics for the shift operators are those of the shift instructions on many processors.