Why Does GCC Produce Undefined Behavior in Right Shift Operations with Excessive Shift Counts?

Undefined Behavior in Right Shift Operations with Excessive Shift Count

In the C standard, the behavior of right shift operations is well-defined for non-negative shift counts. However, when the shift count exceeds the width of the type being shifted, the behavior is considered undefined.

Consider the following code:

<code class="cpp">unsigned int val = 0x0FFFFFFF;
unsigned int res = val >> 34; // res should be 0 by C++ standard</code>

According to the C standard, since 34 is not a negative number, the resulting value res should be 0. However, GCC raises a warning for this code snippet and produces a non-zero result.

GCC's behavior in this case can be explained by the following excerpt from the draft C standard section 5.8 Shift operators:

The type of the result is that of the promoted left operand. The behavior is undefined if the right operand is negative, or greater than or equal to the length in bits of the promoted left operand.

In this case, if unsigned int is 32 bits or less, then the shift count of 34 exceeds the width of the promoted left operand. Therefore, the behavior is undefined, and GCC's warning is justified.

It is important to note that the undefined behavior in this context does not refer to undefined values. Instead, it means that the behavior is implementation-defined, and may vary across different compilers and platforms. In this case, GCC's behavior on Intel platforms is not in line with the C standard's expectations.

The above is the detailed content of Why Does GCC Produce Undefined Behavior in Right Shift Operations with Excessive Shift Counts?. For more information, please follow other related articles on the PHP Chinese website!