Ordering of storage variables and struct members affects how 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.
- State Vars Ordering
- Declaration -> Packing
- Packing -> Gas Costs
- SLOADs & SSTOREs
uint128,uint128,uint256- Two Storage Slots
uint128,uint256,uint128- Three Storage Slots
Storage, Stack Memory, Storage, and Flow Operations, Storage Layout, Storage Packing, [Storage Layout & Inheritance], Storage Layout for Mappings & Dynamic Arrays