how does contract cost less gas in structs?

Question

I have a couple of questions.

1. we have int types as uint8, uint16, uint32, etc. I read that :

Normally there's no benefit to using these sub-types because Solidity reserves 256 bits of storage regardless of the uint size. For example, using uint8 instead of uint (uint256) won't save you any gas.

So If I type directly in the contract as state variable something like this:

• uint8 test1;
• uint32 test2;
• uint test3;

Why won't uint8 test have less gas consumption wherever I use it than uint32 test2 ? I am asking because with uint8 test1 , I use less storage space.

2. Let's say I use struct differently.

// way1
struct Person {
  uint8 test1;
  uint32 test2;
  uint8 test3;
}


// way2
struct Person {
  uint8 test1;
  uint8 test2;
  uint32 test3;
}

Why would the second way consume less gas than the first one ? If we look at the both struct, they both consume the same amount of storage, but still it's said way2 is gonna be much better for less gas consumption . Why ?

Answer 1

This is about how the variables are packed together and about how smart the compiler is (hint: not very smart). The compiler only tries to pack subsequent variables together but it can't pack variables which have something else between them.

In your first case the variables could all fit inside a single 32 byte slot, but if I'm not mistaken the compiler doesn't do that because they are of different types. In your second case the first two variables are of the same type and can be tightly packed inside one slot, therefore saving in gas.

You can read more about variable packing for example here: https://fravoll.github.io/solidity-patterns/tight_variable_packing.html