feat: store packing #343
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feat: store packing #343
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glihm
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Sep 10, 2023
Co-authored-by: glihm <dev@glihm.net>
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Thanks @glihm ! |
milancermak
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Sep 11, 2023
| When writing Cairo smart contracts, it is important to optimize storage usage to reduce gas costs. Indeed, most of the cost associated to a transaction is related to storage updates; and each storage slot costs gas to write to. | ||
| This means that by packing multiple values into fewer slots, you can decrease the gas cost incurred to the users of your smart contract. | ||
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| Cairo provides the `StorePacking` trait to enable packing struct fields into a fewer number of storage slots. For example, consider a `Sizes` struct with 3 fields of different types. The total size is 8 + 32 + 64 = 104 bits. This is less than the 128 bits of a single `u128`. This means we can pack all 3 fields into a single `u128` variable, taking up only one storage slot instead of 3. |
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This makes it sound like a storage slot is exactly 128 bits, not full 251.
| First, when packing: | ||
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| - `tiny` is a `u8` so we just convert it directly to a `u128` with `.into()`. This creates a `u128` value with the low 8 bits set to `tiny`'s value. | ||
| - `small` is a `u32` so we first shift it left by 8 bits (add 8 bits with the value 0 to the left) to make space for `tiny`. The value of `tiny` now takes bits 0-7 and the value of small takes bits 8-39. Then we add tiny to small to combine them into a single `u128` value. This takes bits 8-39. |
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The wording here makes it sound like the shift size (8 bits) is a function of the size of the next element (small, 32 bits), which of course it isn't.
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TYVM for the reviews @milancermak @glihm ! |
Utilitycoder
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Oct 2, 2023
* feat: store packing * Apply suggestions from code review Co-authored-by: glihm <dev@glihm.net> * fix: build * add suggestions * fmt --------- Co-authored-by: glihm <dev@glihm.net>
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Added a chapter on storage packing for efficient storage optimisation
Closes #324