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https://github.com/ethereum/solidity
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48 lines
2.2 KiB
ReStructuredText
48 lines
2.2 KiB
ReStructuredText
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.. index: variable cleanup
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*********************
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Cleaning Up Variables
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*********************
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When a value is shorter than 256 bit, in some cases the remaining bits
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must be cleaned.
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The Solidity compiler is designed to clean such remaining bits before any operations
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that might be adversely affected by the potential garbage in the remaining bits.
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For example, before writing a value to memory, the remaining bits need
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to be cleared because the memory contents can be used for computing
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hashes or sent as the data of a message call. Similarly, before
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storing a value in the storage, the remaining bits need to be cleaned
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because otherwise the garbled value can be observed.
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On the other hand, we do not clean the bits if the immediately
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following operation is not affected. For instance, since any non-zero
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value is considered ``true`` by ``JUMPI`` instruction, we do not clean
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the boolean values before they are used as the condition for
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``JUMPI``.
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In addition to the design principle above, the Solidity compiler
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cleans input data when it is loaded onto the stack.
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Different types have different rules for cleaning up invalid values:
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+---------------+---------------+-------------------+
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|Type |Valid Values |Invalid Values Mean|
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+===============+===============+===================+
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|enum of n |0 until n - 1 |exception |
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|members | | |
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+---------------+---------------+-------------------+
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|bool |0 or 1 |1 |
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+---------------+---------------+-------------------+
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|signed integers|sign-extended |currently silently |
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| |word |wraps; in the |
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| | |future exceptions |
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| | |will be thrown |
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+---------------+---------------+-------------------+
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|unsigned |higher bits |currently silently |
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|integers |zeroed |wraps; in the |
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| | |future exceptions |
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| | |will be thrown |
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+---------------+---------------+-------------------+
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