Understanding Storage

You have heard stories of lost temples deep inside the Black Forest, but you never thought you’d actually find one. You’ve found what you were looking for, but the temple’s protector looks like they have no intention of allowing you to get out alive…

Equipped with the knowledge of Solidity's storage model, we can now execute some gas optimization tricks. Explore how the reordering of state variables and the use of memory caching can save some functions a considerable amount of gas.

Variable Packing

Variable Caching

As the treant’s branches raise over your heads, ready to strike, Cassilyn begins to sing. The music is surprisingly sorrowful but nevertheless enchanting. 

The treant slows, then retreats back to where it had been standing. As you watch, tears of pale sap run down its gnarled features. Is the creature mourning the memory of those who once inhabited this temple? How long has it been since it heard this music performed? 

What’s important is that it no longer shows any hostility towards you or your party. It simply watches as Cassilyn begins to gather the other scrolls. Nor does it move as you continue to explore the courtyard and, to your surprise, find a Node Stone. You’ve no idea how it came here, but you wrap it up to transfer to the nearest chapterhouse. 

Satisfied with your loot, you make to leave. Cassilyn, however, decides to stay. There is so much here to study, and she feels perfectly safe now she has the music required to calm the temple's protectors. Wishing her well, you begin your journey out of the Black Forest and back to civilization. 

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........intruuuuuuuudeeeeeeeeeeeerrrrrssss?

The Treant raises its branches, thicker and stronger than any human arms, and smashes them into the ground. You attempt to back away, only to discover thin vines winding around your ankles, holding you in place. Nature itself guards this place! You can’t escape, and you have no chance of facing a Treant head-on. You need to find a different solution quickly, or you’re all dead! 

Look at the songs here! If we can perform them, it might appease the Treant!

The bard hurriedly begins to pick through the tattered scrolls, but it seems like they are encrypted with some ancient script! 

Help decrypt the writing on the scrolls so that Cassilyn can attempt to calm the rampaging treant before it’s too late…

In this quest, we learned how the ordering of variables can impact gas efficiency.

By strategically ordering storage slots such that variables share the same slot, the EVM can combine multiple reads/writes on the same slot into a single operation, saving gas.

It is also wise to reduce datatype sizes (e.g. from <code class="richtext rt-inlinecode">uint256</code> to <code class="richtext rt-inlinecode">uint128</code>) if it allows for better packing and it still fits within your needed integer constraints.

Unlike in storage, data in memory are not packed into slots. So you do not have to worry about the order of variables in memory.

It might also be pointless squeezing memory variables into smaller datatype sizes. The EVM performs arithmetic operations on 32 byte units. It takes more gas for the EVM to operate on <code class="richtext rt-inlinecode">uint128</code> than <code class="richtext rt-inlinecode">uint256</code> since it takes gas for the EVM to convert the smaller datatype into 32 bytes first.

Reading and writing on storage is more gas expensive than doing the same on memory. By temporarily caching storage values into memory, we minimize the number of reads/writes on storage, resultantly saving gas.

Storage Efficiency

Here is an odd puzzle for any self-proclaimed EVM master. See if you can build an "Ouroboros", and prove your mastery over low-level bytecode sorcery.

Finding Ouroboros

Starknet contracts support state variables, but how they live in contract storage is generally abstracted away from the typical developer. In this quest, we will take a peek under the hood and see how state variables are stored exactly.

Starknet Storage Layout

Do you know how low-level calls work in Starknet? Or how calldata is encoded? This knowledge could be useful for on-chain sleuthing and some other advanced use cases. 

Learn about the inner workings of calls and calldata in this fun interface spoofing exercise!

Calldata Spoofing

Starknet contract addresses, like in many other blockchains, can be precomputed too. This allows for the creation of vanity addresses.

In this quest, see if you can deploy your smart contract under a specified address pattern...

Vanity Addresses

Can we build an EVM inside of an EVM? That seems like a strange question, but the answer is yes we can! In this quest, we will build a self-hosted EVM (sEVM), squeezed into a single smart contract. By rebuilding the EVM, develop a deeper appreciation of the machine's inner workings and EVM bytecode.