Ultimately, all values in the EVM are stored in 256 bit words. Thus, in some cases, when the type of a value has less than 256 bits, it is necessary to clean the remaining bits. The Solidity compiler is designed to do such cleaning 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.
Note that access via inline assembly is not considered such an operation: If you use inline assembly to access Solidity variables shorter than 256 bits, the compiler does not guarantee that the value is properly cleaned up.
Moreover, 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.
The following table describes the cleaning rules applied to different types,
where higher bits refers to the remaining bits in case the type has less than 256 bits.
| Type | Valid Values | Cleanup of Invalid Values |
|---|---|---|
| enum of n members | 0 until n - 1 | throws exception |
| bool | 0 or 1 | results in 1 |
| signed integers | higher bits set to the sign bit | currently silently signextends to a valid value, i.e. all higher bits are set to the sign bit; may throw an exception in the future |
| unsigned integers | higher bits zeroed | currently silently masks to a valid value, i.e. all higher bits are set to zero; may throw an exception in the future |
Note that valid and invalid values are dependent on their type size.
Consider uint8, the unsigned 8-bit type, which has the following valid values:
0000...0000 0000 0000
0000...0000 0000 0001
0000...0000 0000 0010
....
0000...0000 1111 1111
Any invalid value will have the higher bits set to zero:
0101...1101 0010 1010 invalid value
0000...0000 0010 1010 cleaned value
For int8, the signed 8-bit type, the valid values are:
Negative
1111...1111 1111 1111
1111...1111 1111 1110
....
1111...1111 1000 0000
Positive
0000...0000 0000 0000
0000...0000 0000 0001
0000...0000 0000 0010
....
0000...0000 1111 1111
The compiler will signextend the sign bit, which is 1 for negative and 0 for
positive values, overwriting the higher bits:
Negative
0010...1010 1111 1111 invalid value
1111...1111 1111 1111 cleaned value
Positive
1101...0101 0000 0100 invalid value
0000...0000 0000 0100 cleaned value