Add EIP: Decrease base cost of TLOAD/TSTORE

EIPS/eip-7609.md

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+---
+eip: 7609
+title: Decrease base cost of TLOAD/TSTORE
+description: Improve the efficiency of TLOAD/TSTORE by decreasing the base cost and introducing a superlinear pricing model.
+author: Charles Cooper (@charles-cooper), James Prestwich (@prestwich), brockelmore (@brockelmore)
+discussions-to: https://ethereum-magicians.org/t/eip-7609-reduce-transient-storage-pricing/18435
+status: Draft
+type: Standards Track
+category: Core
+created: 2024-02-01
+requires: 1153
+---
+
+## Abstract
+
+Decrease the base cost of TLOAD/TSTORE while introducing a superlinear pricing model. This increases the efficiency of TLOAD/TSTORE for common use cases, while providing a pricing model to prevent DoS vectors.
+
+## Motivation
+
+[EIP-1153](./eip-1153.md) introduces a new storage region, termed "transient storage". It behaves like storage (word-addressed and persists between call frames), but unlike storage it is wiped at the end of each transaction. During development of EIP-1153, the pricing was set to be the same as warm storage loads and stores. This was for two reasons: conceptual simplicity of the EIP, and it also addressed concerns about two related DoS vectors: being able to allocate too much transient storage, and the cost of rolling back state in the case of reverts.
+
+One of the most important use cases that EIP-1153 enables is cheap reentrancy protection. In fact, if transient storage is cheap enough for the first few slots, reentrancy protection can be enabled by default at the language level without too much burden to users, while simultaneously preventing the largest—and most expensive—class of smart contract vulnerabilities.
+
+Furthermore, it seems that transient storage is fundamentally overpriced. Its pricing does not interact with refunds, it only requires a new allocation on contract load (as opposed to memory, which requires a fresh allocation on every call), and has no interaction with the physical database.
+
+This EIP proposes a pricing model which charges additional gas per allocation, which is cheaper for common cases (fewer than 33 slots are written per contract), while making DoS using transient storage prohibitively expensive.
+
+## Specification
+
+The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 and RFC 8174.
+
+The gas cost for `TLOAD` is proposed to be 5 gas. The gas cost for `TSTORE` is proposed to be 8 gas + `expansion_cost`, where `expansion_cost` is calculated as `3 gas * len(transient storage mapping)` if the key is not yet in the transient storage mapping, and otherwise 0 gas.
+
+In pseudo-code:
+
+```python
+G_LOW = 5
+G_MID = 8
+
+SLOPE = 3
+
+def gas_tload(_key):
+    return G_LOW
+
+def gas_tstore(key, transient_mapping):
+    cost = G_MID
+    if key not in transient_mapping:
+        cost += SLOPE * transient_mapping.size()
+    return cost
+```
+
+## Rationale
+
+### Gas
+
+In benchmarking, `TLOAD` was found to cost a similar amount of CPU time as `MUL`, while `TSTORE` was found to cost about 1.5x that. The values `G_low` and `G_mid` were therefore chosen for `TLOAD` and `TSTORE`, respectively.
+
+## Backwards Compatibility
+
+No backward compatibility issues found.
+
+## Security Considerations
+
+The maximum number of transient slots which can be allocated on a single contract given 30m gas is approximately 4,470 (solution to `x(x-1)/2*3 + 8*x = 30_000_000`), which totals 143KB.
+
+The maximum number of transient slots which can be allocated in a transaction if you use the strategy of calling a new contract (also designed to maximize transient storage allocation) once the cost of `TSTORE` is more than the cost of calling a cold contract (2600 gas) is roughly 23,068, which totals 722KB.
+
+## Copyright
+
+Copyright and related rights waived via [CC0](../LICENSE.md).