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Basic Gas Optimizations

  1. Order storage variables such that the least number of storage slots are used: This allows multiple variables to use the same storage slot.

For example, the contract FourSlots occupies 4 storage slots (considering each slot is 256 bits / 32 bytes in the evm)

contract FourSlots {
    uint256 a;
    uint256 b;
    bool c;
    bool d;
}

On the other hand, the contract ThreeSlots only occupies 3 slots. a and b are packed into storage slot 0', whereas canddtake up slots1and2`.

contract FourSlots {
    bool a;
    bool b;
    uint256 c;
    uint256 d;
}

A good example of this out in the wild can be found in UniswapV2 here. The .getReserves() call is used a lot, and all three variables below are storage on one storage slot 8.

uint112 private reserve0;           // uses single storage slot, accessible via getReserves
uint112 private reserve1;           // uses single storage slot, accessible via getReserves
uint32  private blockTimestampLast; // uses single storage slot, accessible via getReserves

If you add the size of each variable (112 + 112 + 32), you get 256 bits. Thats the maximum capacity of one storage slot.