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Merge pull request #951 from Denton-L/interesting-case
Document use of smaller storage variables
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@ -15,6 +15,23 @@ Statically-sized variables (everything except mapping and dynamically-sized arra
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- If an elementary type does not fit the remaining part of a storage slot, it is moved to the next storage slot.
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- Structs and array data always start a new slot and occupy whole slots (but items inside a struct or array are packed tightly according to these rules).
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.. warning::
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When using elements that are smaller than 32 bytes, your contract's gas usage may be higher.
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This is because the EVM operates on 32 bytes at a time. Therefore, if the element is smaller
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than that, the EVM must use more operations in order to reduce the size of the element from 32
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bytes to the desired size.
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It is only beneficial to use reduced-size arguments if you are dealing with storage values
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because the compiler will pack multiple elements into one storage slot, and thus, combine
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multiple reads or writes into a single operation. When dealing with function arguments or memory
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values, there is no inherent benefit because the compiler does not pack these values.
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Finally, in order to allow the EVM to optimize for this, ensure that you try to order your
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storage variables and ``struct`` members such that they can be packed tightly. For example,
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declaring your storage variables in the order of ``uint128, uint128, uint256`` instead of
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``uint128, uint256, uint128``, as the former will only take up two slots of storage whereas the
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latter will take up three.
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The elements of structs and arrays are stored after each other, just as if they were given explicitly.
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Due to their unpredictable size, mapping and dynamically-sized array types use a ``sha3``
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