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| -rw-r--r-- | docs/miscellaneous.rst | 17 |
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diff --git a/docs/miscellaneous.rst b/docs/miscellaneous.rst index 882a6002..98497872 100644 --- a/docs/miscellaneous.rst +++ b/docs/miscellaneous.rst @@ -15,6 +15,23 @@ Statically-sized variables (everything except mapping and dynamically-sized arra - If an elementary type does not fit the remaining part of a storage slot, it is moved to the next storage slot. - 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). +.. warning:: + When using elements that are smaller than 32 bytes, your contract's gas usage may be higher. + This is because the EVM operates on 32 bytes a a time. Therefore, if the element is smaller than + that, the EVM must use more operations in order to reduce the size of the element from 32 bytes + to the desired size. + + It is only beneficial to use reduced-size arguments if you are dealing with storage values + because the compiler will pack multiple elements into one storage slot. When dealing with + function arguments or memory values, there is no inherent benefit because the compiler does not + pack these values. + + Finally, in order to allow the EVM to optimize for this, ensure that you try to order your + storage variables such that they can be packed tightly. For example, declaring your storage + variables in the order of ``uint128, uint128, uint256`` instead of ``uint128, uint256, + uint128``, as the former will only take up two slots of storage whereas the latter will take up + three. + The elements of structs and arrays are stored after each other, just as if they were given explicitly. Due to their unpredictable size, mapping and dynamically-sized array types use a ``sha3`` |