清理变量

When a value is shorter than 256 bit, in some cases the remaining bits must be cleaned. The Solidity compiler is designed to clean such remaining bits before any operations that might be adversely affected by the potential garbage in the remaining bits. For example, before writing a value to memory, the remaining bits need to be cleared because the memory contents can be used for computing hashes or sent as the data of a message call. Similarly, before storing a value in the storage, the remaining bits need to be cleaned because otherwise the garbled value can be observed.

On the other hand, we do not clean the bits if the immediately following operation is not affected. For instance, since any non-zero value is considered true by JUMPI instruction, we do not clean the boolean values before they are used as the condition for JUMPI.

In addition to the design principle above, the Solidity compiler cleans input data when it is loaded onto the stack.

Different types have different rules for cleaning up invalid values:

Type	Valid Values	Invalid Values Mean
enum of n members	0 until n - 1	exception
bool	0 or 1	1
signed integers	sign-extended word	currently silently wraps; in the future exceptions will be thrown
unsigned integers	higher bits zeroed	currently silently wraps; in the future exceptions will be thrown