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authorDenton Liu <liu.denton+github@gmail.com>2016-08-26 01:20:54 +0800
committerDenton Liu <liu.denton+github@gmail.com>2016-08-26 01:20:54 +0800
commit0268cbddc7dd413fb11be679e929d4c4be4ca46a (patch)
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Document use of smaller storage variables
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diff --git a/docs/miscellaneous.rst b/docs/miscellaneous.rst
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@@ -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``