[Draft] EIP-2442 - LOGQUERYn opcodes

EIPS/eip-2442.md

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+---
+eip: 2442
+title: LOGQUERY(n) opcodes
+author: pinkiebell (@pinkiebell)
+discussions-to: https://ethereum-magicians.org/t/rfc-eip-2442-logqueryn-opcodes/3867
+status: Draft
+type: Standards Track
+category: Core
+created: 2019-12-19
+---
+
+## Abstract
+
+This EIP specifies new opcodes `LOGQUERY(x)`, which mirrors the semantics of the `LOG(n)` opcodes to gain the support
+for querying/filtering the most recent log event of the current contract/EVM context.
+
+## Motivation
+
+To better support stateless(light)-clients, layer-2 scaling solutions and similar applications,
+gaining the ability to query the last emitted log event and having the log-data available inside a
+contract has a huge benefit to an overall simpler system/application design.
+
+In the context of optimistic-rollup solutions, one could imagine to use `LOG` events, corresponding `topics` as storage keys and log-data
+as a way to signal state changes to light clients and simultaneously using it as a storage backend for the on-chain contract itself.
+
+A rudimentary example:
+```
+// Layer-2 Bridge contract transfers AMOUNT of TOKEN from `Alice` to `Bob`.
+
+let storageTopic = keccak256(Alice, Token)
+// query Alice
+let success = logquery1(memoryPtr, 0, 32, storageTopic)
+
+// we had a past event
+if (success) {
+  // the data of the log contains the uint256 balance
+  let aliceBalance = mload(memoryPtr)
+
+  if (aliceBalance >= AMOUNT) {
+    mstore(memoryPtr, aliceBalance - AMOUNT)
+    // update Alice's new balance
+    log1(memoryPtr, 32, storageTopic)
+
+    // Bob's new (pre) balance
+    mstore(memoryPtr, AMOUNT)
+    let storageTopic = keccak256(Bob, Token)
+    // query Bob
+    success = logquery1(memoryPtr, 0, 32, storageTopic)
+    if (success) {
+      // update Bob's balance
+      mstore(memoryPtr, mload(memoryPtr) + AMOUNT)
+    }
+    // log Bob's new balance
+    log1(memoryPtr, 32, storageTopic)
+  } else {
+    // Alice has not enough AMOUNT
+    revert()
+  }
+
+} else {
+  // no event for (Alice, Token) emitted in the past - who is Alice?
+  revert()
+}
+```
+
+Other solutions like validating log events with past block-hashes are inconvienent and also of limited use.
+Additionally, there is no trustless way of verifying if any given log was the latest/most recent  one.
+
+## Specification
+
+```
+GAS_BASE = 100
+GAS_BYTE = 3
+
+```
+
+`(t, f, s)` = copy `s` bytes from log's data at position `f` to mem at position `t`.
+
+| Opcode                                             | Hex  | Gas cost                          | Notes                                          |
+| -------------------------------------------------- | ---- | --------------------------------- | ---------------------------------------------- |
+| logquery0(t, f, s)                                 | 0xb0 | `GAS_BASE + (s * GAS_BYTE)`       | No topic filter - fetch most recent log        |
+| logquery1(t, f, s, topic1)                         | 0xb1 | `(GAS_BASE * 2) + (s * GAS_BYTE)` | Get last log with 1 topic and topic = `topic1` |
+| logquery2(t, f, s, topic1, topic2)                 | 0xb2 | `(GAS_BASE * 3) + (s * GAS_BYTE)` | ...                                            |
+| logquery3(t, f, s, topic1, topic2, topic3)         | 0xb3 | `(GAS_BASE * 4) + (s * GAS_BYTE)` | ...                                            |
+| logquery4(t, f, s, topic1, topic2, topic3, topic4) | 0xb4 | `(GAS_BASE * 5) + (s * GAS_BYTE)` | ...                                            |
+
+
+Common behaviour for all `logqueryX` opcodes:
+- If there is no matching log
+  - Push `0` to stack and do not touch memory(`t`, `s`).
+- If there is a matching log
+  - Push `1` to the stack and fill memory(`t` = memory pointer, `s` = size) with the log's `data` starting at `f` and size `s` bytes.
+  - If `f + s` is greater than the log's `data`, then pad the remaining length with `0`'s.
+
+## Rationale
+
+As described in the motivation section, this opcode is widely useful and should help the ongoing Eth1.x efforts.
+
+## Backwards Compatibility
+
+There are no backwards compatibility concerns.
+
+## Test Cases
+
+TBD
+
+## Implementation
+
+TBD
+
+## Copyright
+
+Copyright and related rights waived via [CC0](https://creativecommons.org/publicdomain/zero/1.0/).