ReentrancyGuard vs ReentrancyGuardTransient gas saving

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1

So I tried to check how much gas I can save if I use ReentrancyGuardTransient instead of ReentrancyGuard and for most of the tests it seems like it's around ~2000 gas or less. Based on my understanding this should be a little more, considering the fact that ReentrancyGuard is writing into an already existing storage slot, so I would assume it should be around ~5000 gas saved.

In theory gas usage should be something like this:

ReentrancyGuard.nonReentrant gas usage:

  1. SLOAD 2100 gas (status == 2)
  2. SSTORE 2900 gas (status 2)
  3. SSTORE 100 gas (status 1)

ReentrancyGuardTransient.nonReentrant gas usage:

  1. TLOAD 100 gas (status == 2)
  2. TSTORE 100 gas (status 2)
  3. TSTORE 100 gas (status 1)

May I ask what is the reason for this? Is it because of TransientSlot is being used?

:1234: Code to reproduce

I created a small repo where this can be reproduced and here are the actual gas snapshots.

:computer: Environment

forge version: 1.0.0-stable
openzeppelin-solidity: 5.2.0

Thanks in advance

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3

I think I found the answer here: What is the gas overhead of ReentrancyGuard? - #6 by frangio

It is because of the 2800 refund at the second SSTORE.

So the correct breakdown is this:

ReentrancyGuard.nonReentrant gas usage:

  1. SLOAD 2100 gas (status == 2)
  2. SSTORE 2900 gas (status 2)
  3. SSTORE 100 gas and refunds 2800 (status 1)

ReentrancyGuardTransient.nonReentrant gas usage:

  1. TLOAD 100 gas (status == 2)
  2. TSTORE 100 gas (status 2)
  3. TSTORE 100 gas (status 1)

it's 2300 vs 300 gas so the 2000 gas saving checks out

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4

What is the fundamental difference between the ReentrancyGuardTransient contract and the classic ReentrancyGuard contract?

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6

ReentrancyGuardTransient utilises transient storage.

Check the official docs: https://docs.soliditylang.org/en/v0.8.28/contracts.html#transient-storage

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As far as I can see it is simply to use this kind of transient
variable with cheaper storage cost. But the saving is significant? we are talking about a variable of type value.

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Check my previous answer: ReentrancyGuard vs ReentrancyGuardTransient gas saving - #3 by gabkov

It saves around ~2000 gas/call in this case. Depending on the implementation you can save more.

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