c - 负值左移 0 个位置？

Question

在 C 中，负值的左移是未定义的行为。我遇到过两个使用 Intel 的 ICC 编译的库，其中有问题的代码被删除了。同样的代码在 Clang、Comeau、GCC 和 MSVC 下运行良好。

标准是否提到将负值左移 0 位？它也是未定义的吗？

(我很好奇的细节是 0 大小的移位，实际上根本没有移位。所以我想知道语言是否含糊，以至于可能允许 0 大小的左移位)。

Answer 1

摘自 C99 with Technical corrigenda TC1, TC2, and TC3 included :

6.5.7 Bitwise shift operators

[...]

The integer promotions are performed on each of the operands. The type of the result is that of the promoted left operand. If the value of the right operand is negative or is greater than or equal to the width of the promoted left operand, the behavior is undefined.
The result of E1 << E2 is E1 left-shifted E2 bit positions; vacated bits are filled with zeros. If E1 has an unsigned type, the value of the result is E1 × 2^E2, reduced modulo one more than the maximum value representable in the result type. If E1 has a signed type and nonnegative value, and E1 × 2^E2 is representable in the result type, then that is the resulting value; otherwise, the behavior is undefined.

因此，始终未定义。